Mikuni VM34 idle HELP!

I have recently acquired a beautiful '73 850 Commando with a VM34 carb. When I took delivery it was running very nicely, but the PO had stripped the threads in the pattern Doherty twist grip and it moved around on the bars with interesting results. I bought and fitted a genuine Doherty unit, but the throttle cable is much "tighter" using the new grip. The adjuster on the cable originally had about 1 1/2 inches screwed out (lengthening the cover) and now with the adjuster completely closed down the idle is still too high at about 1500 rpm. If I blip the throttle the idle will drop to about 800 rpm but will slowly climb to about 2000, when another blip will drop it again before it climbs again in about 30 seconds to 2000. I can't adjust the cable anymore, and the air- screw has no effect at all, where originally it was an efective way of adjusting the idle. The engine is basically new, with only about 160 miles on it, but I can't ride it until I can get a consistent idle at a reasonable speed.

I looked at the Mikuni USA site and it makes no sense at all to my non technical brain. A google search only provides mixture advice. I know some of you will be shaking your heads at my ignorance, but HELPPPPP,

Your cable is pinched / routed against other things. Pull gastank and try twistings and drops again. You should hear a clack sound as slide drops to bottom of carb sharply ? Reroute a bit ? :|

Diver3284 said:

I bought and fitted a genuine Doherty unit, but the throttle cable is much "tighter" using the new grip. The adjuster on the cable originally had about 1 1/2 inches screwed out (lengthening the cover) and now with the adjuster completely closed down the idle is still too high at about 1500 rpm.

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I'm guessing the bottom of the cable slipped part-way out of the bottom of the carb slide, removing free-play (putting my money on that one). Pull the top off the carb and check the bottom of the slide for a protruding cable end. If it's okay, take the twist grip apart, and compare where the cable end fits the twist, and where the cable jacket nests in the housing between the new and old.

Diver3284 said:

If I blip the throttle the idle will drop to about 800 rpm but will slowly climb to about 2000, when another blip will drop it again before it climbs again in about 30 seconds to 2000. I can't adjust the cable anymore, and the air- screw has no effect at all, where originally it was an efective way of adjusting the idle.

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When you say air-screw, are you referring to the spring-loaded idle-speed screw on the side of the slide (usually on the left side), or the air screw next to the air filter/inlet (right side)? If you've lost free play in the cable, the slide will never touch down on the idle-speed screw, so it won't have any effect.

Let us know what you find!

Nathan

I've checked the cable routing, tank off, and it seems as fair and short as possible. Lubed the cable too with a light spray oil (RP7 here in AUS).
The problem appeared when the cable was in exactly the same position as originally.

The problem seems to be the amount of cable wound onto the twist grip inside the grip housing. The old one was a plastic pattern part that looked externally like the Doherty, while the genuine part has a cast metal (zinc) barrel, and it seems that there is more cable inside the housing. I've already ground about 1/4 inch off the ferrule holding the end of the cable housing to the grip.

I'm a bit reluctant to take the carb apart, but if I unscrew the top can I adjust the amount of cable going through the slide? How is the end of the cable secured in/to the slide. I can get a new cable tomorrow, (I think) but if I can somehow adjust the length of the inner that would be good.

BTW should I get a variation in the revs at rest when I move the bars from side to side? ALways have but not a real problem until now, when a slight movement sends the revs zooming.

Thanks for your help guys; keep the love coming!

Diver3284 said:

The problem seems to be the amount of cable wound onto the twist grip inside the grip housing. The old one was a plastic pattern part that looked externally like the Doherty, while the genuine part has a cast metal (zinc) barrel, and it seems that there is more cable inside the housing. I've already ground about 1/4 inch off the ferrule holding the end of the cable housing to the grip.

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Well, there you go! As long as you have enough "meat" on the barrel, grind away, although I'd be more inclined to build a cable with enough free-play to accommodate the new twist grip.

Diver3284 said:

I'm a bit reluctant to take the carb apart, but if I unscrew the top can I adjust the amount of cable going through the slide?

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No need to take it apart, as the brass ferrule where the cable seats to the top of the carb "should" be threaded, allowing you even more cable free-play adjustment.

Diver3284 said:

How is the end of the cable secured into the slide. I can get a new cable tomorrow, (I think) but if I can somehow adjust the length of the inner that would be good.

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It's not secured precisely, but, rather, held in place by a keyholed slot in the bottom of the slide. If the cable is allowed to slide too far down into the carb, it can get hung up in this slot, effectively removing a bunch of much-needed slack, causing the very problem you're describing in your first post

Diver3284 said:

BTW should I get a variation in the revs at rest when I move the bars from side to side? ALways have but not a real problem until now, when a slight movement sends the revs zooming.

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Bad! As pointed out earlier by Torontonian, make sure your cable is routed in such a manner that this doesn't happen. Usually, you can clear this up by how it's routed or not routed around fork legs and the like.

Nathan

Is it possible you have the wrong upper throttle cable for your new grip? I believe there are 2 types of upper throttle twist grips
(standard and 1/4 turn).
I ran into a problem like this when I mistakenly ordered a "STD" twist upper cable for my "1/4" twist throttle.
In my case he inner cable was too long and the adjusters could not remove all the slack.
Old Britt's sells both types. Just a thought.

If the reve went up by truning the bars the cable always was too tight...it should not happen....bad routing will make it worse..always make teh run as wide a sweep as you can without interference and don't tie wrap it anywhere...

If the old plastic barrel is still in good condition and gives more free play....just reuse it....you said that wasn't why you changed it

OK, took thre whole shebang to pieces after convincing myself that N-P was probably right and the cable had come out of the keyhole in the slide raising it up too far. WRONG All as it should be. And there was no threaded ferrule at the top of the carb, just a slide in one.

Then I got the old twistgrip out and remembered that I had used the original metal (pattern) inner grip rather than new plastic one from Doherty. I did this because a) the metal one seems more solid and better made than the plastic one, and b) I couldn't get the rubber grip off the old one Anyway I compared the two and they were identical.

THEN - LIGHT BULB MOMENT :!: :!: :!: . On the old set-up there was no split ferrule at the twistgrip housing; the outer cable is so thick that it was just jammed up into the housing about 5mm. So I put it all back together without said split ferrule, et voila, all good with about 1 inch (!!!) of extension on the cable adjuster. Hopefully this will work until the bike goes to a local expert for a full fettle at about 500 miles.

Thanks for all your suggestions and advice; can't beat a community effort.

Diver3284 said:

...the outer cable is so thick that it was just jammed up into the housing about 5mm...

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I should have done my usual and asked for pictures. :|