Mikuni Carb sync/balancing question

[steveg_nh]

I'm riding a Yamaha Attak 4 stroke now, but still have a couple of 2 strokes in the stable.

I finally got around to learning how to clean carbs myself on these two strokes. Both are round slide Mikuni VM carbs. Boy am I kicking myself for not learning this sooner. So easy.

Anyway, the carbs (on both machines) were all screwed up and not adjusted properly at all. Screws and cables (slides). So I started from scratch. Got the slides adjusted so they open at the same time and are both full open at WOT (even with top of carb). Also used the drill bit method to space the slides for the idle screw. And have the air scews at 1.5 turns out for starters. And choke properly adjusted too.

While they are both running good, I thought that was the extent of the adjustments, but someone taught me to take the balancing (syncing) a step further with a carb syn gauge. He did one sled and it showed that even though I had them dead even in terms of adjustments at idle, one cylinder was drawing a wee bit more vaccum, so we adjusted the idle screw of that one carb to balance it out. Is that correct?

Everything I've read has said to have both carbs the same, air screw and idle screw, but not much more detail on tweaking each carb's idle screw to get a matching vaccum. So now the carbs on this sled have the slides at a slightly different point at idle. Since cables are balanced and slides are wide open at same time, I would think it's just at idle.

I'm curious about this. Is it just about getting to perfection, and not really necessary if were that close already? None of the service manuals mention doing this or show use of a sync gauge at all.

Is there any reason it's bad to have the idle adjustment slightly different on each carb, with all the above being equal (cable free play, slide travel, opening at same time, etc)? If you do this, do you need to them tweak the throttle cable on that carb, since there's likely a tiny but of slack given the slide sits higher at idle?

Just trying to learn and connect all the dots.

Thanks!

 

[MrSled]

I'm riding a Yamaha Attak 4 stroke now, but still have a couple of 2 strokes in the stable.

I finally got around to learning how to clean carbs myself on these two strokes. Both are round slide Mikuni VM carbs. Boy am I kicking myself for not learning this sooner. So easy.

Anyway, the carbs (on both machines) were all screwed up and not adjusted properly at all. Screws and cables (slides). So I started from scratch. Got the slides adjusted so they open at the same time and are both full open at WOT (even with top of carb). Also used the drill bit method to space the slides for the idle screw. And have the air scews at 1.5 turns out for starters. And choke properly adjusted too.

While they are both running good, I thought that was the extent of the adjustments, but someone taught me to take the balancing (syncing) a step further with a carb syn gauge. He did one sled and it showed that even though I had them dead even in terms of adjustments at idle, one cylinder was drawing a wee bit more vaccum, so we adjusted the idle screw of that one carb to balance it out. Is that correct?

Everything I've read has said to have both carbs the same, air screw and idle screw, but not much more detail on tweaking each carb's idle screw to get a matching vaccum. So now the carbs on this sled have the slides at a slightly different point at idle. Since cables are balanced and slides are wide open at same time, I would think it's just at idle.

I'm curious about this. Is it just about getting to perfection, and not really necessary if were that close already? None of the service manuals mention doing this or show use of a sync gauge at all.

Is there any reason it's bad to have the idle adjustment slightly different on each carb, with all the above being equal (cable free play, slide travel, opening at same time, etc)? If you do this, do you need to them tweak the throttle cable on that carb, since there's likely a tiny but of slack given the slide sits higher at idle?

Just trying to learn and connect all the dots.

Thanks!

For the most part they dont have to be done on 2 strokes from what I have seen, they can be pretty close but if you have the tools to Sync them , why not?

 

[steveg_nh]

For the most part they dont have to be done on 2 strokes from what I have seen, they can be pretty close but if you have the tools to Sync them , why not?

Agreed. What just wasn't clear was the process and how the cable relates. Seems like the best is to loosen cables, adjust idle screw using drill bit method, then adjust cables by getting both slides even (flush) at WOT. Then double check with sync tool. If they are off and I adjust idle on one carb though, aren't I changing the cable pull on that carb, since I'm moving the slide after the WOT adjustment? Is that ok? That's what's not clear to me. Seems like cable on the carb that is adjusted could be tighter or looser than the other one.

I'm likely overthinking it, but just not seeing how tweaking the idle screw based on carb sync tool won't mess with the cable adjustment.