Extremely vexxing idle/low RPM issue

1971 Norton Command. I'm at my wits end. I've cleaned, sync'd and replaced every possible part on my Amal's. Ive checked and triple checked timing, points gap, and the advance mechanism.

I'm trying to address two issues.

1. Inconsistent idle. Sometimes it will idle at 2000 RPM, then I'll ride the bike for 20 seconds and it drops down to 900-1000 and eventually stalls. Restart its at 2000 RPM again. I checked for air leaks, throttle cable issues, float levels (actually set a bit below recommended just to be sure it wasn't overflowing). All o-rings, and gaskets replaced and torqued. Usually there is a pop from a carb right before it dies.

2. Part throttle miss. When I accelerate the bike runs amazingly. ...but below around 3500 RPM it stumbles with a miss at steady throttle. It will run from 1000-6000 with even light acceleration without any miss, but if I'm not accelerating, it pops and stumbles consistently.

Any tips greatly appreciated.

Compression test results?
Valve clearances checked?
Did it previously run well?
What do the spark plugs look like?
Have you installed new ones to rule out?
Did you mechanically clean all carburetor passages & then verify flow with aerosol solvent?

Symptoms sound lean mixture.
Could be from partially blocked fuel passages.
Or, from worn out carbs. How many miles on them?

My bike had the vari-idle feature, until I replaced the carbs with new premiers.

Since you are running points, have you looked at the centrifugal advance weights? It is not uncommon for the pins to wear a groove and hang intermittently in the advance position rather than return to its original timing.

Different kind of answer.

Coils can do that before they completely crap out. I've had it happen with oils that static test good with a DVM, but crap out under use like running out of fuel except more bucking just before they die.

Old air cooled engines can eat plugs much sooner than a modern motorcycle would. Might want to try swapping out plugs.

Point is it could be ignition related. Ron beat me to it more or less.

I test everything myself, but don't use points or any of the stock ignition parts.

1. Sticky advance.
2. Sticky throttle slides.

How old are the condensers, people replace points and all but forget about the condensers, but I say more a pilot jet passage slightly blocked is my opinion, but then carb problems could be electrical and viser/verser, go through one thing at a time to sort it out.

Intermittent vacuum leak?

Thanks for all the ideas. THe bike and carbs have only 8k miles on them. Its nearly new. Compression and leakdown was outstanding as I checked this all before i started investing in a restore.

This bike belonged to my stepdad and he parked it in 1975 because a mechanic told him the exhaust nut was stripped. (it wasn't) and it sat for 48 years. I inherited it after he passed and sat on it for 15 years until I decided last year that "It is time".

I've been through many of these after reading a lot of suggestions. Its never idled "well" and this weird low/high idle has always been there. Today I had a small breakthrough. After pulling the carbs, cleaning all passages with carb cleaner (for the umpteenth time) , blowing out passages with 140psi shop air, and checking the float levels/sync, etc.. for the 10th time It I noticed the left exhaust was much "softer". I screwed in the air bypass 1/2 turn and suddenly the left side got more robust and idle increased 1000RPM. So I think I have an intermittent lean issue. I also had some binding in the throttle cable splitter that was causing higher RPM's when the bars were at full right lock.

Mechanical advance is smooth and no evidence of sticking or binding.
Checked the timing 4-5 times. It is spot on at 28-30 degrees at 3k RPM
Used propane around all carb areas looking for vacuum leak - no joy there.
I feel like slides probably aren't worn to the point of leakage. When I rebuilt the carbs they were tight and smooth.

I checked the plugs (Which have < 200 miles on them) and they are a nice medium brown color. However given the stumble I think these older machines like things a little richer so I moved the needles out a bit this evening and that seemed to help. Need to go through and fiddle some more.

This has been maddening. I had dual triple throat Weber 46 IDA's on my old Porsche - and they were easier than these bone simple Amals haha.

One other though - at certain idle speeds some gas comes out of the ticklers. Could it also be spurting into the carbs due to the vibration giving me a temporary rich condition? Seems unlikely but after many hours of fiddle-farting around with these, I'm going way out there on ideas.

cstreit911 said:

One other thought - at certain idle speeds some gas comes out of the ticklers.

Click to expand...

That's not right. It sounds like your fuel height is too high... With the old white floats you can't bend the float needle tab down to lower the fuel height. You can do that if you have the new "stay up" floats since they have a metal tab to hold the float needle...

But if you have the old style floats, you need to pull the bowl and drift the float needle seat upwards slightly and that lowers the fuel height.

Where do you live.....?? Maybe someone here lives nearby....

First, go through the cables, twist grip and splitter. Check condition, routing, and lubricate them.

If still not good… After sitting for THAT long… get the carbs professionally ultra sonic cleaned.

I assume you have already put in new, correct, spark plugs ?

8 k. ! strewth , theyre usually clapped out at 5.000 . Tho thats without filters . Not dissimilar Metals , so scouring & rattle , Yr donfor .

The orrible cable wotsit . PLASTIC . the cable joiner ROCKS , so uneven opperation . Wotcher Wants a Dual Cable Quater Turn twist Grip .

Tho Olde triumph cable slides are Chormed steel with a Brass Slider . the short ones STILL ROCK , so ya wanna LONG ONE if you must have a overloaded primary cable ! .

But'cher wanna Alloy DUAL cable - loads HALF from handlebar to undertank . THEN . Settem OFF taut .

as in , Trottle on stop , and Slides dissapeared at top . Then let go . NOT set from idle / closed . Initially . On any System .



You could throw some of those horrible japanese carbs on . But anyone with class , would use a 40 or 50 ( 42 / 48 ) D C O E Weber . or a dellorto carb , if they couldnt handle a Weber .
32 or 40 pumpers of a BM or Guzzi cetra . or D H L A .


On a real manifold . Not the kinkey stock ones . a CURVE . Stacks up against cross brace . Yr chokes wanna be 32 plus ! ! . work backfromthere .

Similar internal dimn.s to a 250 F , so work back from that . maybe .

cstreit911 said:

1971 Norton Command. I'm at my wits end. I've cleaned, sync'd and replaced every possible part on my Amal's. Ive checked and triple checked timing, points gap, and the advance mechanism.

I'm trying to address two issues.

1. Inconsistent idle. Sometimes it will idle at 2000 RPM, then I'll ride the bike for 20 seconds and it drops down to 900-1000 and eventually stalls. Restart its at 2000 RPM again. I checked for air leaks, throttle cable issues, float levels (actually set a bit below recommended just to be sure it wasn't overflowing). All o-rings, and gaskets replaced and torqued. Usually there is a pop from a carb right before it dies.

2. Part throttle miss. When I accelerate the bike runs amazingly. ...but below around 3500 RPM it stumbles with a miss at steady throttle. It will run from 1000-6000 with even light acceleration without any miss, but if I'm not accelerating, it pops and stumbles consistently.

Any tips greatly appreciated.

Click to expand...

I know you won't want to read this but my commando was doing this after about 8000 miles from new Amal carbs
The only thing mine didn't have was a misfire at part throttle
I changed to a single mikuni and the problem went away
But as others have said make sure you mechanically clean all the carb passages especially the pilot jet bush
You may end up having to remove the welch plug in top of the float chamber to get to the pilot mixing chamber as debris can sometimes move backwards and forward trapped inside
Good luck with it

The leaking tickler suggests too high a fuel level, carb cleaner does not clean out pilot jets properly, only a 16 thou #78 drill will clean out a pilot jet and leave it back at its original size. And 8000 miles on the factory fitted carbs means they are probably worn out.

cstreit911 said:

One other though - at certain idle speeds some gas comes out of the ticklers.

Click to expand...

Another thought....
After a carb rebuild, I had a similar situation. Much head scratching and studying later, I found that the aftermarket carb bowl gaskets protruded slightly too much into the float bowl, stopping the float from rising fully. Cut the gaskets back by about 1mm at the pivot area and problem went away.

On mine, I found a casting burr on the brass float spindle. Caused my float to hang up on it. All worked fine after filing it off.

8k miles is essentially irrelevant if carbs were left sitting decades with old fuel residue congeleing within passages.
Have you confirmed good spray lut the two tiny ports in the floor of carb throat?
Review the pdf Bush.ans Guide to Amal Carbs....shows a way to drill out blanked off air screw port to allow a good poke through amd clean out, tapping a thread to accept a cut off idle screw ro plug it off.

Bike a 1971? So glass/resin tank? Could be the notorious dissolving of resin, getting into carbs and or inlet valves.
Could be why your ticklers runbeth over at idle...

All that is confirmed by spraying carb cleaner through the passages is that SOME carb cleaner is getting through… not HOW MUCH is getting through.

You could EASILY still have a partially blocked pilot jet, and as mentioned above, due to the time spent sitting, this is very likely. Plus you could have semi loose crud rolling around in the passages.

Which is why I believe (as the carbs have done relatively low miles) it’s worth having them professionally ultrasonically cleaned.

In the past, I have actually had good results with a DIY method of boiling the stripped down carbs in a large pan of water and dish washing detergent. That may be worth a try if you’re reluctant to spend on the ultra sonic cleaning.

Having said all of that… many have said in the past that these carbs can be clapped out at 10k miles. So a careful check of slide, and slide bore, condition would be wise in order to ascertain if it’s worth persisting with these carbs.

IMO your running symptoms describe a weak slide, which can be indicative of slide wear.

"All that is confirmed by spraying carb cleaner through the passages is that SOME carb cleaner is getting through… not HOW MUCH is getting through"


Having done this for 40+ years, on snowmobile, outboard, lawnmower, ATV, automotive, & motorcycle carbs, I would offer that your claim is inaccurate.
The stream volume, force and pattern is easily identified if there is partial blockage.

The OP did not respond regarding MECHANICAL cleaning.
A crucial step.

concours said:

"All that is confirmed by spraying carb cleaner through the passages is that SOME carb cleaner is getting through… not HOW MUCH is getting through"


Having done this for 40+ years, on snowmobile, outboard, lawnmower, ATV, automotive, & motorcycle carbs, I would offer that your claim is inaccurate.
The stream volume, force and pattern is easily identified if there is partial blockage.

The OP did not respond regarding MECHANICAL cleaning.
A crucial step.

Click to expand...

Well, that’s the beauty of being able to agree to disagree !

Carbs are a finely tuned instrument, and IMO the difference in flow between a 20 and a 25 jet (for example) whilst being important to carb function, would be difficult / impossible to judge via observing carb cleaner spray flow / pattern.

Ergo it would be equally difficult to observe whether or not a jet is partially obstructed.

Emphasis here on ‘partially’ of course a fully or nearly fully blocked jet would give a visible difference.