1.1 Amal Filters and Tuning.

From: Brian Pankow
Date: 30 April 1998 03:49

Hello Harry,
I have had a K&N in the garage for years and have not put it on the MK III yet. Did you have to change the jetting to get it to not run too lean? The only real thing that has kept me from making the switch.

From: Richard Wilson
Date: Thu, 30 Apr 1998 08:57:43 +0100

Hello Brian,
I'm no expert on Amals but I have fitted hundreds of K&N filters to Mikuni and Keihin carbs on Jap bikes over the years. My experience is that although larger main jets seem to help reduce the lean running, they are only effective at large throttle openings, and most people go overboard on the mains. I have had good results using flow restrictors in the air feed to the spray tubes. The extra flow you get from K&N filters seems to cause lean running in the low and mid range, and using larger mains will only overcome this by making it over rich in the upper range. I had a large selection from a company called L.E.D.A.R. who called them air-correctors. I believe that the kits available from Dynojet include this type of restrictor, plus a whole bunch of other expensive bits like spray tubes and needles, which you might not need.

Has anyone tried air-correctors?

Hope this is of use.

Richard Wilson
'67 Dunstall Atlas
'59 Dominator
+ loads of Kawasakis!

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1.2 Amal Body Distortion

Date: Fri, 17 May 1996 20:53:01 -0500
From: John Goodpaster

> The tightness at the top is do to over-torque of the flange nuts.
>>It makes the barrel out-of-round.I have streightend these out (back to
>>round) If any one wants to know how, let me know and i'll relate how I
>>discovered and corrected this problem...............

The way to fix these things is quite simple, With soft jaws in the vice squeeze the throat at near the top parallel to the flange very softly. When you get a strain on it pull the slide and see if it comes up easily. If it does relax pressure and try again ( to fit the slide) if it hangs a bit squeeze it a little more with the slide all the way down in the body. Triel and error will will get you results. I have fixed lots of bodys this way. Then file the flange flat and use new "0" ring or a bit of silicon seal and torque lightly....

John Goodpaster

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1.3 Amal Concentric Mk2 Carbs.

Date: Sat, 30 Mar 1996 11:35:10 -0500
From: Mike Taglieri

>I've got two of the newer style Amals (look like Mikunis) from
>a oif Triumph. Has anyone ever converted from the older
>Amals to these carbs on a Commando?

If your carbs have a screw-off plastic top, they are Amal Mark II's, an excellent carb. Here is what Brian Slark said about them in the British Marketing catalog about 10 years ago:


This carb is a great improvement over the old concentric style. If it looks like a Mikuni copy, it's the other way around. Amal allow Mikuni to make carbs from their patents. Being narrower than the Mikuni, these carbs can be mounted parallel on a Norton, retaining the excellent inlet port design. A pair of special manifolds with rubber sleeves are required for installation.
Special features are:

If your carbs have these features but a metal top, I believe they are called MK I 1/2's, a modification of the MK II for people who didn't want the plastic tops. What Slark calls the "old concentric style" is the MK I, which most of us swear by [or "at"] on this list.

Mike Taglieri
Raul -- '72 Commando Interstate

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1.3.1 Amal Concentric Mk2 Jetting.

From: Ken Dubey

I now have the mk 2 carbs cleaned and disassembled, soon to be grafted onto the Commando. The carbs came off a later 70's OIF Bonneville, and the jets are a couple of sizes smaller than the ones in the Concentrics now on the bike.

Are the main and pilot jets the same for each model? They appear to be the same size and thread, although the tube that holds them is much shorter than in the mk 1 carbs. If they are the same I am going to use the ones in the current carbs to establish a baseline for jetting. Anyone know the ballpark numbers for what jet #'s work for mk 2's on Commandos? Can anyone recommend a particular air filter setup to fit this conversion?


Date: Mon, 29 Jul 1996 20:56:26 -0400
From: Gary Slabaugh

Ken Dubey,
In a message dated 96-07-29 18:38:37 EDT, you write:
>Are the main and pilot jets the same for each model?

The main jets for MK2 Amals are the same as MK1s. Not sure about the pilot. I have a set of 34mm MK2s on my Nort 750 roadracer and previously they were installed on my 850 Interstate. #250-270 works for the 750 racer. I think I had 280 when installed on the 850 which also had a lot of go-fast stuff in the motor. If your carbs are 32mm, you will probably need 220-240 mains. Just a guess.

They really are a great carb.

Gary Slabaugh

Date: 30 Jul 1996 09:39:52 +0000
From: Cornforth, Bob

In addition to Gary's post, I too can confirm the main jets are identical for the MkI and MkIIs, as are the pilots. In fact these are the only two items common to the pair. A common mistake is to assume that the needle jets are the same too. They aren't, the MkII jets have a groove cut around the upper part of the jet and are longer to match the MkII needle.

To give you an idea of jetting, on my 850 Triton which runs 34s, I run 320 mains, .106 needle jet, 25 pilot amd 50 choke, No3 cutaway. Another thing to watch is the needle itself, there are a few types. Some Bonnies had stainless needles with .105 jets. These can't be used with the brass ones. I think the one's I've got in are 2A1s, and the shorter stainless ones are 2C1s ( I think) if it's important I'll whip 'em off and check for you.



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1.4 Amal Dual vs Single Carb.

Date: Mon, 1 Jul 1996 08:55:39 -0500
From: John S. Morris

A quick subjective report on my Commando, switching from a single Mikuni back to dual Amals (or you _can_ go home again! 8-))

I've been returning my Mark III to some semblance of stock condition and last week had two new Amals installed. I had read that the Mikuni offered easier starts, better gas mileage, fewer problems/less maintenance and comparable performance. After a couple hundred miles I can say the bike starts as easily with the dual Amals (one kick -- or one blip of the electric start), but there is decidedly better performance with the dual setup.

Before the bike pulled strongly until perhaps 4500 RPM and then flattened out, now the bike pulls very strongly all the way to near redline. It's quite a rush to wack the throttle open at 5000 RPM and get the same surge of power as at 3000 RPM.

I'm happy to trade off the added maintenance of the Amals for the performance gain.


'92 BMW R100GS-PD Paris-Dakar
'75 Norton Commando Mark III - Roadster
AMA #6775910 BMWMOA #54010 DoD #316 INOA #10095 RA #14948

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1.5 Amal Intake Sealing.

Date: Fri, 17 May 1996 09:23:52 -0600
From: Bob Cram.

Re my own message yesterday that read in part:
> Anyway, if you are using rubber o-rings between your Amal and the manifold,
> then the gap between the carb and the manifold should be no smaller than 3/16
> of an inch. When I mounted my carbs again this spring I was really surprised
> to see how loose this was.

I was wrong. The correct distance is .030 inches or 1/32." I started thinking last night that 3/16" was way too much. So I went back and checked. Sorry about that. The information in Vintage Bike also said on this topic:

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1.6 Mixture, Form and Function.

Date: Fri, 17 May 1996 13:54:39 -0600
From: Bob Cram

I came across an older motorcycle maintenance book in a used bookstore, *The Compete Guide to Motorcycle Repair and Maintenance* by Neil Schultz. Anyway, it contains an interesting graph showing exactly how pilot jet, slide cutaway, needle and needle jet, and main jet affect mixture at different throttle settings on a *typical* slide carburetor, which means this may not be strictly accurate for specific models. However, as a general guide it should be good. I'd never seen anything this detailed before and thought I'd pass along the information. I've converted the pictoral, graphical information to text-based info for purposes of e-mailing.

It is surprising just how far into throttle ranges some things have an influence. For example, I would not have suspected that the pilot jet would have an influence up to 7/16 throttle, or the main jet as low as 1/4 throttle.

Idle - 100% pilot jet
1/16 throttle - 60% pilot, 40% cutaway
1/8 throttle - 46% pilot, 46% cutaway, 8% needle/needle jet
3/16 throttle - 49% cutaway, 37% pilot, 14% needle/needle jet
1/4 throttle - 45% cutaway, 27% pilot, 24% needle/needle jet, 4% main jet
5/16 throttle - 37% cutaway, 33% needle/needle jet, 21% pilot, 9% main jet
3/8 throttle - 44% needle/needle jet, 25% cutaway, 16% pilot 15% main jet
7/16 throttle - 55% needle/needle jet, 23% main jet, 13% cutaway, 8% pilot
1/2 throttle - 63% needle/needle jet, 31% main jet, 6% cutaway
5/8 throttle - 57% needle/needle jet, 43% main jet
3/4 throttle - 55% main jet, 45% needle/needle jet
7/8 throttle - 92% main jet, 8% needle/ needle jet
Full throttle - 100% main jet
Here is some other perhaps useful information from graphs in the Haynes Carburettor manual.
  The latter two points suggest that the needle affects mixture for higher throttle openings than does the needle jet. This would suggest that the needle/needle jet stage mentioned by Schultz is really two stages needle jet, and then at a higher throttle opening, needle. In other words, if your mixture problems seem to occur at throttle openings in that transition area between cutaway and needle/needle jet (3/16 to 1/2 throttle), then playing around with changes to the cutaway or needle jet may be most effective. However, if the problem occurs in the transition area between needle/needle jet and main jet (7/16 to 3/4 throttle), then playing around with needle position may be most effective.

If you examine the two sources of information carefully, you will see that they do not entirely agree. For example, Haynes suggests that at 1/2 throttle, a cutaway change of .5 can affect mixture by 4.5%, but Schultz suggests that the cutaway only controls about 6% of the mixture at 1/2 throttle, which would mean that any change in cutaway would have minimal influence at 1/2 throttle. I am also somewhat suspicious of the Haynes claim that a change in cutaway has more effect on mixture at 1/2 throttle than a change in needle jet.

What is really quite striking though is how the different components overlap. Most other information on the components' effect on mixture tends to say something like cutaway governs mixture between 1/8 and 3/8, needle/needle jet between 3/8 and 3/4, etc. They sometimes mention there is some overlap, but it seems to me that the degree of overlap is at least as important as the information on when a specific component has the greatest effect.

Anyway, food for thought.

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1.7 Needles.

Date: Wed, 31 Jul 1996 12:39:53 -0600
From: Bob Cram

Eric Goforth wrote:

>When I ordered needles for my bike I was told that the
>four ring ones were what I should fit with the drilled
>needle jets that came on my '73 Commando.
>I'm a little unsure on what the sectioned spray tube refers
>to, I've seen it mentioned several times. Is this the sleeve
>type thing that surrounds the needle at the bottom of the
>carburetor venturi? Was this different on earlier
>I've seen latter "round-bodied" Concentrics like the ones
>on my Commando refered to as Mk II's, but I understand
>that this isn't correct. Were the later Mk I's with the
>extended tickler referred to as Mk Ia's or was there any
>difference in the the models?
>As long as you match the needle to the needle jet would there
>be any problem fitting the earlier undrilled needle jets to
>my carbs? Wasn't the drilled needle jet supposed to give a
>little reservoir of gas to smooth out the throttle response
>when the throttle was opened quickly?"


I don't know much about Commandos, but I have been told that the needles with 4 scribe marks are for Commandos, with 2 scribe marks for Triumphs. Someone mentioned earlier that the 4 scribe mark ones are only used on later Commandos. Much the same is true of the sectioned spray tub, which is for Commandos too. I'm not sure if it was used on all models or just some. It is the little brass tube at the bottom of the venturi as you suspected. I've also heard there is a MK 1.5 but I don't know if they are what you describe, i.e. with the shrouded tickler. The Walridge Motors catalogue talks about the Mk 1.5.

The drilled needle jet is for exactly what you say. If you use the undrilled needle jets, you have to use a different needle and jet holder with it. If you really want to do this, I can dig out the part numbers from Nicholson's Modern Motorcyle Mechanics. However, Nicholson says that the newer stuff with the hole through the jet is better and that older Concentrics can be improved
by moving to the new parts. I did read once in the Classic Motorcycle that some bikes suffer from "fluffing" because of this cross-drilled hole.Fluffing isn't defined, but from the context of the article it seems to mean a bit of richness when opening the throttle. The article mentioned filling the cross-drilled holes with a bit of solder if you have the fluffing problem. It seems to me that would be a bit tricky to accomplish without also getting solder all over other parts of the jet, but theoretically it should work.

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1.8 The Pilot Jet.

Date: Sun, 19 May 1996 10:50:30 +0100 (BST)
From: Philip Pick

Jim Bush wrote about an extra screw in his Mk1 Concetric carbs
> >I feel you will find that a past owner (a clever one this time)
> >has put in the extra screw not to act as a metering screw, but just
> >to give access for cleaning the pilot jet.
> >I often do this, using a shortened pilot or throttle stop screw and
> >O ring to seal up again, to gain access to clear a pilot jet.
> >
> >Philip Pick, Triple Cycles.

>On Fri 17 May, Jim Bush wrote again :
> One thing remains unknown to me - do you have an idea what the size of the
> standard pilot jet orifice is ? (or the setting for a 77 - Triumph T140) so
> I can check that these ones have not been drilled out larger to accomodate
> the metering. TDPO (the dumb previous owner) had also taken a standard pilot
> screw and brazed and filed the end to a point (fairly crudely) to use as
> needle screw adjuster - so it would also follow that the pilot jet may have
> been made larger.
> Are the pilot jets changeable ? - I guess the metal plug (alum) on the
> underside of the body could be drilled out and the jet replaced ?
> I like your idea of using this modification for cleaning the pilot passages
> and the jet - I've noticed in a few adds for bikes for sale here that "carbs
> need resleeving - poor idle" - it may be more obvious that the pilot air/gas
> passages are blocked, not the slides worn, although an idle accompanied by a
> "casanet band" would indicate worn bodies

I've done a little digging, and have come up with the following information.

The pilot jet presed into 4 stroke Mk1 bodies post 1968 is part number 622-107. I do believe this is not normally available as a spare part. It is (by measuring some I do have here) 0.103" outside diameter, and .200" long

The air side of this brass jet is of larger diameter than the fuel side. I can not measure this hole, but estimate it at 0.7 to 0.8"

The fuel side, and therefore the side that blocks, is by paperwork detection 0.016". This is standard drill size #78

I have changed these jets in the past, if cleaning with solvents has not cleared the blockage, by drilling into the carb at the other
hand pilot screw position, and pushing out the jet with a drift of 0.060 diameter. This small diameter is needed because the jet
is pused into the body via the pilot adjusting screw passage, and stops against a shoulder. The drift must be smaller the the hole in the shoulder, or the body will be damaged.

Once the body has been drilled for this access I use another (throttle stop or mixture) adjusting screw, suitable shortened, with standard O ring, to seal the drilling.

I only obtained the jet size details recentlu, and have yet to just drill jets clear. I'm convinced blocked pilot petrol passages
are the cause of much poor running on low use bikes, and many carbs have been sleeved for no good reason, then to be trashed because "that didn't work....blxxdy Amal's".

Another train of thought here, once a body has been bored, and the slide sleeved, how does an owner go about tuning / slide changes? We know that all Amal standard settings are now too rich due to fuel changes, and this mainly affects the slide number. A sleeved slide can always be weakened by lifting the cut away, but I've yet to find the magic file to put back that metal!

Philip Pick, Triple Cycles, 228 Henley Road Ilford Essex IG1 2TW England
Telphone +44 181 478 4807 Fax +44 181 478 4807

Date: Sun, 19 May 1996 16:42:47 GMT
From: Michael A. Moore

I think I've read about some trials guys in England (possibly the v. trick B40 that was in Classic Bike several years ago) that are drilling out the pressed in pilot jet and tapping the carb body to accept a screw-in Mikuni/Kei'hin/Dell'Orto/other pilot jet. This not only allow you to clean things out, but actually makes the pilot circuit tunable. This sounds like a great idea to me, and shouldn't be that hard to do. If I ever run an AMAL on anything I own I'm planning on making this modification.

Michael Moore
Euro Spares, SF CA

Date: Sun, 19 May 1996 21:13:56 +0100 (BST)
From: Philip Pick

On Sun 19 May, Michael A. Moore wrote:
> I think I've read about some trials guys in England (possibly the v. trick
> B40 that was in Classic Bike several years ago) that are drilling out the
> pressed in pilot jet and tapping the carb body to accept a screw-in
> Mikuni/Kei'hin/Dell'Orto/other pilot jet. This not only allow you to clean
> things out, but actually makes the pilot circuit tunable. This sounds like
> a great idea to me, and shouldn't be that hard to do. If I ever run an
> AMAL on anything I own I'm planning on making this modification.

Hi Michael and List:

Nothing to stop an owner removing the pressed in jet, or just drilling it out and fitting the Amal pilot jet from the earlier Mk1 and other carbs (part # 124/026/## if I remember correctly, in the early position. This position is in the body, at the body / bowl junction, in the petrol passage. Some post 1968 bodies are still tapped in this position, and though the hole has normally been staked to damage the thread to stop people screwing in a jet when a pressed in jet is fited.

Note that 2 stroke Mk1 carbs kept this screw in jet for rather longer than 4 stroke versions. Also note that 2 stroke carbs have different arrangements and part numbers for the jet holder, spray tube, needle jet and needle.

You'll note the bowl gaskets are large enough around these passage drillings to accept the jet. (Jets in stock at 20, 25, 30, 35, 40, 45 and 50 ;-))

Philip Pick, Triple Cycles, 228 Henley Road Ilford Essex IG1 2TW England
Telphone +44 181 478 4807 Fax +44 181 478 4807

Date: Sun, 19 May 1996 20:05:34 -0700
From: Thomas H. Allen

Hi Phil and Brit-iron folks:

Regarding the pressed in pilot jets: I have heard two stories regarding the switch from screw-in pilot jets to pressed in. The first is that the pressed-in model is smoother coming off of idle and the change was made for this reason. The second is that the change was made as a cost saving measure. I tend to believe the cost story. Any comments?


Date: Mon, 20 May 1996 00:08:15 -0700
From: Jim Bush

Regarding my query about the extra pilot jet adjuster on my carbs and how to deal with this - I had written:

>On the pilot circuit there has been drilled an adjusting screw for the gas
>flow on the "off-side" and well as there being the "air" screw on the

Philip Pick, Triple Cycles wrote:

>I've done a little digging, and have come up with the following
>The pilot jet presed into 4 stroke Mk1 bodies post 1968 is part number
>622-107. I do believe this is not normally available as a spare part.
>It is (by measuring some I do have here) 0.103" outside diameter,
>and .200" long
>The air side of this brass jet is of larger diameter than the fuel side.
>I can not measure this hole, but estimate it at 0.7 to 0.8"
>The fuel side, and therefore the side that blocks, is by paperwork
>detection 0.016". This is standard drill size #78
>I have changed these jets in the past, if cleaning with solvents
>has not cleared the blockage, by drilling into the carb at the other
>hand pilot screw position, and pushing out the jet with a drift of
>0.060 diameter. This small diameter is needed because the jet
>is pused into the body via the pilot adjusting screw passage, and stops
>against a shoulder. The drift must be smaller the the hole in the
>shoulder, or the body will be damaged.
>Once the body has been drilled for this access I use another (throttle
>stop or mixture) adjusting screw, suitable shortened, with standard
>O ring, to seal the drilling.
>I only obtained the jet size details recently, and have yet to just
>drill jets clear. I'm convinced blocked pilot petrol passages
>are the cause of much poor running on low use bikes, and many
>carbs have been sleeved for no good reason, then to be trashed because
>'that didn't work....blxxdy Amal's'

Well I got my measuring equip.out and measured the size of the pilot jet oriface in my T140 carbs - they were approx. 0.045" - way way bigger than the 0.016" that Philip quoted above.=20

Wanting to confirm that the 0.016" was realistic I found that a wire brush bristle was 0.014" (close enough) - this is pretty tiny - something is amiss ?

Pulled out a set of old 930 Norton carbs, tossed caution to the wind and drilled out the blanked off pilot jet hole (will tap a thread later) and drove out the pilot jet using my 0.060" drift (a small nail with a ground off end) and there she is. On close inspection it appeared as though there was no hole at all - only after digging around with my wire brush bristle did I indeed find an accommodating orifice which had been solidly plugged up. It by all accounts is 0.016" as given by Philip Pick. Thanks, I will now salvage a couple of pilot jets from a set of throw away carbs and put my Bonnie back in action.

Looking at this itsey bitsey tiny hole they call a pilot jet, it's no wonder that these beggers won't idle. The insides of all the carbs I have pulled apart recently have been scaly - it would take only one tiny scale to block up the works. I think Philips suggestion of drilling out the blanked off side and plugging as a matter of course has a lot of merit - especially now that the new fuels that we use (in Canada) all tend to gum up when left standing over the winter - says a lot about draining the carbs and keeping everything spic and span.

Anyone thought about a little filter to screw into that empty threaded pilot jet hole they used initially or some sort of filter material inserted into the pickup line for the pilot jet in the float bowl ? It's obvious that the filters on the petcocks etc. are like the gauze used on velocity stacks - they only keep out the big things like birds and stones, but not the harmful stuff like grit.

Thanks to Philip for pitching in on this one - what a great tool this internet stuff can be - I had been wrestling with this carb problem for a couple of years - stumped all the locals here and now shes done (almost).



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1.8.1 Pilot Jet and Idle Circuit Cleaning.

Date: Mon, 29 Apr 96 13:35:36 -0500
From: Kenneth D Balfanz

Russell Brown writes:
> And speaking of pilot circuits, I can see that there is a hole
> downstream of the main jet and just under the trailing edge of the slide
> that lines up with the pilot circuit, but there's also an even smaller
> hole downstream of the slide. Any ideas that this is?


Take a spray can of carb clearner and poke the nozzle in the idle adjust screws' hole when the screw is out. When you spray in this hole, fluid should be coming out of THREE other places; one of the large holes on the intake face of the carb, and the two small pilot circuit holes you mention above on both sides of the slide . If not, something is still plugged. Try blowing the cleaner from both directions (back flushing). Often it takes soaking ovenight in a jar of acetone to work things loose. I use a REACH brand tooth brush to clean the body. It won't disolve in the acetone like other brands of toothbrushes will. Keep all rubber o-rings away from this stuff or you'll be buying new ones. WEAR SAFETY GLASSES. The carb cleaner CAN spray all over, but it usually just heads straight for your eyes and face. Make sure there is plenty of ventilation. Please don't try this in the furnace room.

It's been about a year since I've done this. I'm sure someone can (will) correct me if I've got something wrong. :-)

Hope this helps.


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1.9 Slides.

Date: Sun, 19 May 1996 20:06:13 -0700
From: Thomas H. Allen

This is an important and very illuminating thread. By the time the dust settles you all will have saved countless Amal carb bodies. This or some edited version should get saved to the FAQ.

Now for my two cents worth. Phil Pick asks (paraphrasing), "Where is the magic file to put the metal back?" The obvious answer is there isn't one; however, all is not lost. You can richen them up again but you have to be very careful with your measurements.

The flat bottom of the slide can be sanded or filed to reduce the cutaway. BUT, you have to change the needle position by the same amount and you will soon run out of needle grooves and have to start shimming the clip. You get a second and maybe a third chance to get it right.

Bear in mind that the difference between a #2 and #3 slide is 0.0625" (each whole number represents 1/16"). This just happens to be the distance between clip grooves on a needle I measured (2 ID grooves, 3 clip grooves). So, changing the slide from a #3-1/2 to a #3 demention is not be exactly one needle groove. It's half a groove. To make up the difference you have to use 0.030" shim stock and cut a suitable shim to go under the clip. In short, if you have to go 1/2 richer on the cutaway you will have to raise the needle by the amount you take off the bottom of the slide, in this case 0.030", which means that you shim by that amount. If you use a belt sander use a fine grit.

Grinding the bottom of the slide will change the position of the bottom of the cutaway relative to the centerline of the slide but when I have done this mod that didn't seem to matter......as far as I can tell.

Make sure you keep good records or you will become the next DPO when someone down the line tries to figure out what you did.

Tom Allen

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1.10 Troubleshooting.

Date: Wed, 19 Jun 1996 16:58:22 -0600
From: Bob Cram

Mark Smith wrote:

>My Commando is running really rough; and I could use some troubleshooting
>advice, please.

>The symptoms are: She sounds as if she's 'missing' at idle, or as if she
>has a really lumpy cam. The engine doesn't smooth out as the revs increase,
>so a lumpy cam seems doubtful. She has no power under acceleration, even
>when the gearbox is in neutral with no load on the engine.

>The exhaust is _very_ black. <snip, snip> In other
>words, the ignition system looks OK to me. (Did I miss anything?)

>I believe that she's running very rich, and that I need to dig into the
>carbs. Am I heading down the right path?

>I've followed the discussions on Amal Carbs for some time, and am confused
>about a most fundamental point of the discussion. How do I _know_ when my
>carbs need reboring / resleeving and / or replacement?

>I've heard that it's very common for a bike like mine (22,600 miles) to have
>worn main jets and needles. Could it be that I'm only lacking a couple of
>'rebuilt' carbs? Sould I be looking into resleeving / reboring? I'm pretty
>confident that I can get all of the passages clean... am I naive on this


I would say that very rich running is by far the most likely problem. Black and sooty plugs mean the bike is running too rich, especially if the black and sooty is on the centre electrode wire and the insulator. A wet plug like you have on one side can mean you have oil fouling, but a very rich mixture can get the plug wet too. Smell the wet stuff. Does it smell like gas or oil? Feel it too. Is it slippery like oil? In any case, for oil to get the plug really wet, you'd have to have a lot of oil getting in there? The first signs of oil fouling are usually a bit of blackness on one side of an otherwise brown spark plug insulator. (Incidentally, a richer mixture on one cylinder can be a sign that your carburetor slides are poorly synchronized and are not rising at the same time.)

Don't worry much about a resleeving and reboring. That causes lean running at idle that can make it impossible to get an idle. I'm not saying you don't necessarily need this work, just that you have a much different problem to address right now, which is too rich a mixture. You have to look for causes of too much fuel or too little air, or both. Once thing you didn't say is if this is a problem on a bike you have owned for awhile or not. Knowing whether it happened suddenly or gradually would help in the troubleshooting.

You should also be aware that many people have had problems with rich running on older bikes, and the general consensus has been that the original carburetor specs for our bikes are no longer correct when using modern fuels with all the additives that are so different from 20, 30, or whatever years ago. This is a bit of a crap shoot though, because different types of bikes seem to be affected differently, and every country, and even regions within countries, use different additives. It is my general impression that this issue of fuel additives is more of a problem in the UK and Canada than it is in the US, but that's just an impression from where the most complaints seem to originate.

Anyway, here is a list of the more likely possible causes. Others may think of some I've missed. And it could be a combination of some of these things causing the problem. I've listed them in order of most to least likely, IMHO.

If none of the above works, then you are probably suffering from the fuel additives problem. In that case, start by trying either a smaller needle jet or a larger slide cutaway than the factory recommends. Ideally, try to borrow such a needle jet or slide from a friend or a shop for a test, so you aren't buying a lot of parts that might not work. If you have to buy, try the needle jet test first. They are a lot cheaper than throttle slides. Be sure to start out a smaller needle jet on the richest needle setting, i.e. bottom groove on the needle. Then you can adjust upwards after test runs.

This should definitely improve things, but you still may not get perfect running. If that is the case, try out a brand new set of correct spark plugs, maybe all that carbon and cleaning has made the old plug less efficient. If after this the plugs are still showing signs of carbon fouling and you are getting misfiring when riding about town, then you probably need to go to a one size larger cutaway on your throttle slides. If you try this, after already having made a change to a smaller needle je, you may suddenly find you are getting too lean a mixture. Then you need to adjust your needle setting again, but this time to a richer setting. And if you find it is still too lean after going to the richest needle setting you can, then you may have to go back to your original size needle jet with the new larger cutaway, and play around some more with needle settings.

I should add here that Philip Pick, who knows a lot more about such things than most of us, advises trying a larger cutaway on throttle slide first when you suspect a modern fuel additives problem, but Philip owns a shop so throttle slides are easy to come by for him. My own opinion is that this is one of those things that varies from place to place. In the UK, maybe it's best to start with a throttle slide change. In Canada, though, I think it's more likely to be the needle jet.

Now this long diatribe has probably scared the hell out of you, but you will more than likely find that you fix the problem with one of the more likely causes and don't have to go through all of this. In my case, I have been going through all of this for many months, which is why I sound like I have experience with this, and I am only now getting to a satisfactory solution. Once you do get the right mixture, and you won't be fouling plugs so often, then get yourself a good set of platinum plugs, which last longer and resist carbon fouling more. They're more expensive though, so don't spend money on them until you've got the bike sorted out.

Finally, it will help to know that the pilot air and fuel system is the primary influence on the bike's mixture at idle, throttle slide cutaway is the main influence coming off idle and to about 1/4 throttle, the needle and needle jet together have the most influence on mixture from 1/4 to 3/4 throttle (with the needle tending to have a greater influence a bit higher up the throttle range than the needle jet), and the main jet has the most influence on mixture from 3/4 to full throttle. Most around town riding occurs in the 1/4 to 1/2 throttle area.

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1.11 Tuning and Balancing.

Date: Mon, 26 Feb 1996 07:47:26 -0600
From: Jack Goertz

Okay, folks, I need some great Brit-Iron advice today.

I finally got my carburettors back together and on my 1967 Bonneville. You'll remember that I had Lund's (Albany, OR) bore/sleeve my original monoblocs, and he did a gorgeous job!

Yesterday was the first day I had the opportunity to work on the Bonne again. Every thing went together perfectly, no problem. Bike started on the third kick (not too bad for first start in over a month) and idles perfectly. Tachometer doesn't drop below 1000 rpm, even when the bike's not running (just found something else to fix/repair -- my wife is so pleased that I found yet another way to spend money on this classic), so can't say for sure how fast it is really idling. My best guess, based on the Y*m*h* 650, is that the Bonne is idling about 800-1000 rpm.

Now the problem: There's a flat spot between idle and low-speed cruising. On the tach, it's occurring between 1200rpm & 1500rpm. When throttling up from idle, the bike may die. If I pass the 1500rpm mark, it will run fine on up to whatever speed I have the nerve to crank it too.

The carbs have standard jets (as spec'ed in the Triumph shop manual). The needle is in the middle position. Should I raise the needle? Is the mid-range not picking up soon enough, or is the pilot range running too lean? Jets and needles are new.


Jack Goertz
Birmingham, AL USA
'67 Bonneville 650
'95 Sportster 1200

Date: Wed, 20 Mar 1996 07:19:26 -0600
From: Jack Goertz

To all who answered my plea for help, thanks! The '67 Bonne is now running great!

To recap:

After rebuilding the Bonneville, I tackled the carbs. Lunds' Machine Shop (Albany, OR), bored and sleeved the original (about 22,000 miles) monoblocs. 10 day door-to-door turnaround. I reassembled the carbs and put them on the bike. The bike idled, then died when rolling the throttle open.

I asked for help from Brit-Iron and downloaded everything from the FAQ that applied to Amals and went to work.

First, a new set of air filters from Xanders (thanks, Art). Then re-reading the FAQ, the Triumph factory shop manual, and Clymer's book on Triumph twins, and I was ready to start.

While the air filters were off the bike, I balanced the throttle cables so that both slides were lifting at exactly the same time (at least they felt that way to me). I screwed the pilot air screw all the way into the body until it bottomed out (no pressure), then backed off 1 1/2 turns. I flooded the carbs with the ticklers (never had any air slides) and the bike started on the first kick. I then followed the FAQ steps for setting the idle speed with the idle stop screws on each carb until the bike was idling smoothly again. It still died when rolling on the throttle.

I re-installed the air cleaners and re-started the bike. Since the speed range in question seemed to be at the bottom of the rpm range, I believed the problems were all in the pilot system, so I centered my adjustments there. Symptoms were that the carbs were running a bit lean (according to Clymer's checklist). A 1/4 turn in on the air screws, a bit of adjustment on the throttle stop screws, and it was running again, but seemed a bit rich, so I backed out the screw 1/8" and all is now well! The key was new air filter elements and careful adjustment of the pilot air screws!

Last Sunday was the day to test the bike on the road, and it ran flawlessly! No hesitation, no sign of lean-ed out mixture, no signs of over-rich, just a good strong response throughout the throttle range! It was terrific. I managed to put more miles on the Bonne last Sunday than I'd logged on that bike in over 5 years!

Of course it is now 30 degrees as I write this, so it's going to be a few days before I take it out again. Spring has sprung in Alabama. Now if the gods of weather realize it .....

Jack Goertz
Birmingham, AL
'67 T120R Bonneville
'95 XLH1200 Sportster

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