High Idle Causing Redlining and Excessive vibration.

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Jul 26, 2008
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Commando 850cc 1973 Roadster. Boyer ignition, clean Amal original carbs.

The bike starts right up (one kick) and idles at around 1500 rpm's cold. The Rpm's continue to climb at idle as the engine gets hotter.

I have made adjustments to the throttle screw and pilot screw. If i screw the pilot in any more the bike will not idle. Same with the throttle stop, it wont bring the idle down without a complete stop in the motor. This seems to be the "sweet spot", however at 1500 rpms its just to high.

Any suggestions? Anything else besides the carbs that can be adjusted to bring the idle down while maintaining a strng tick-over speed?

any help will be apprecaitesd

Vacuum Leak. I'm game and i have all day tomorrow and monday to fetter.

Any tips or suggestions on how to find the leak? Anything i should be on the look out for or quick tips?

clean Amal original carbs.

But what are the wear factors like in the slides/ bodies.. sounds typical of what I have experienced with flogged out slides ~

(I used to pull at a set of lights and the revs would surely climb .. but if I engaged first gear and then just let the clutch out to load the thing , the revs would return to low revs ~ ????? )

You can run successfully with worn slides ~ even the point of being able to hear them rattling IMO , but there is a point of critical mass ~ :shock:
The slides have never been "sleeved" so there may be an opportunity there.

However the RPM's get so high so quick i CANNOT keep up with highway traffic without exceeding redline.

I can feel lots of room in the throttle and the bike responds very strong. Good off the line and if i open the throttle up she moves quick. Just redlines so fast. when i'm in 4th gear i hit 7,000 RPM's at 50mph.
I'm a little confused from your last line (...if i open the throttle up she moves quick. Just redlines so fast. when i'm in 4th gear i hit 7,000 RPM's at 50mph.) I don't get what this has to do with a high idle problem. This sounds like you have a slipping clutch.

Or the rev counter is fast? Might explain the high tickover? :roll:

To find intake leaks, use starter spray (ether0.

Squirt some on a joint area, and if the idle increases, you found the spot.

However, you need to get it under control first, so start by snugging up EVERYTHING.

ALWAYS start with the bloody hard-to-reach manifold mounting bolts, then the equally fussy inner carb mounting nuts.
Doesn't seem to be a vacuum leak. I sprayed all the areas that could be effecting the idle and nothing changed.

I have have seen the idle lower to about 900 rpm's at startup. I went through and tightened everything, continued to adjust the pilot screw and it seems to be ok. It does want to live around 1100 - 1500 rpm's once it warms up and has be run a bit. It may be time to look at a more modern carb.

thanks again for everyones help. i think it was the combination of the tightening, incorrect tach and frustration that was the problem.
Before you start spending big money check the slides are falling onto the stops and it's not mal-adjusted cables and splitter box that's holding them up.

Great minds think alike Cash, I have previously bought brand new cables that have insufficient slack to allow the slides to bottom out in the carb body and also that were slightly different from each other in length thus requiring individual adjustment at the carb top, both cables need to allow the carbs to open exactly together and close together.
ah... ha!

One of my throttle adjustment screws is broken. they lift the same but this may be causing the slides to stay up once i get going.

I'll replace the cable which is still the original '73 and the screws. Hopefully this will do the trick.
So the low idle lasted about 30 miles.

I'm back to 1100 - 1500 rpm's with rattling slides and shaking arms.

I balanced the slides, cleared the pilot crews, re-set the carbs to an even tick over thats strong, the balance pipe and filters in good shape and spraying starter fluid on every possible leak made no difference.

It idles low (600) but uneven at first start. At the first stop light i'm back to 1500 rpm's?

Any other ideas?
Well I am right up here in Flint if you can make it 60 miles. I am betting you have a warped carb body. It gets warm it gets worse. I have the service tools to fix it. I will work on it for some doe if you like. PM me
You didn't mention what ignition set-up you have. If you are still using points sometimes an old mechanical advance unit can hang up at an advanced ignition setting.
I have the boyer electronic ignition.

I had it installed by a local british bike friend. Are there any adjustments that need to be made to it? Is it possible its sending a a fast signal?

Also, i have had a couple people tell me i should buy a carb meter? Anyone know of one that works well with our bikes?

This was Just a stab in the dark, if you have electronic ignition I think it unlikely that over advancing at low speed is the problem. I would be inclined to run the bike up the road without an air filter and all the associated hardware, then you might visually see if the slides are sticking, lifting together, falling together, bottoming out correctly and that the slide adjustment screws allow the slides the full range of adjustment.
So i tried everything i could replace or adjust. The bodys and slide must have been just too worn. I'm giving up and buying a Mikuni 34mm Single carb.

I will post again what the final outcome is once installed and run.

I appreciate the suggestions and help!
jsouthard said:
I'm giving up and buying a Mikuni 34mm Single carb.

smart man -

panics (victory library) tuning guide for mik's is great too if you need to fiddle a bit w/ them! :D
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