Calling Mikuni Tuning Gurus!!!!!!

Finnigan

Finnigan

Over 1,000 Posts
Hey all

Started my bike for the FIRST TIME yesterday and I couldnt be happier!

I have vm30's? on her and running stacks. When she fired up one cylinder was running crazy fast compared to the other and the tach read 4000rpm (chokes were off)

I know I'm supposed to be able to adjust each carb via the adjusting screws but don't know which to fiddle with and what they do.

Oh and the bike wants to die after 5000 rpm...please help me get her purring!!!

Thanks!
 
You have a carb sync tool? Gonna need one of those to get started.

Get the bike warmed up and the idle running around 1000 RPM and the carbs synced.

After that, tweak the air screws to get the highest RPM possible and dial the idle back down to 1000 RPM.

Let us know when you get that far and we can work on the pilot jets.
 
spaced the slides on each carb like the manual said, made sure the throttle cable had slack and took it outside to start it up. started on first kick and was idling high so i took it to the gas station on the corner where i'd have more light and could make noise. I must have messed with the idle screws and air screws for 2-3 hours and although i got a nice smooth idle she died whenever i put her into first gear or tried to pull away. Ended up pushing her home for 11 city blocks on a friday...fml

Someone please help me figure this out... :'(
 
what kind of bike?

was it running right before you installed the carbs?

can you post a pic of the install
 
Assuming the timing is correct and your idle is good, it's time to start working on the pilot jets. Pilot jets control the fueling from idle to around 1/4 throttle setting. If the bike dies or hesitates when you blip the throttle from idle, it's time to go bigger on the pilot jets.

When you think you have the pilot jets set correctly, take the bike out for a spin. You should be able to hold 20 mph in 3rd gears WITHOUT having to move your throttle position. If find yourself having to adjust the throttle a bit to hold a steady speed, you still need to go bigger on the pilots.
 
cxman said:
what kind of bike?

was it running right before you installed the carbs?

can you post a pic of the install
Click to expand...

It's a CB360 with velocity stacks, i can post a pic if that will help

I got them from Sonreir, they are vm28s or 30s with 176 needles
 
Sonreir said:
Assuming the timing is correct and your idle is good, it's time to start working on the pilot jets. Pilot jets control the fueling from idle to around 1/4 throttle setting. If the bike dies or hesitates when you blip the throttle from idle, it's time to go bigger on the pilot jets.

When you think you have the pilot jets set correctly, take the bike out for a spin. You should be able to hold 20 mph in 3rd gears WITHOUT having to move your throttle position. If find yourself having to adjust the throttle a bit to hold a steady speed, you still need to go bigger on the pilots.
Click to expand...

I can't really get it so that I can pull away in 1st gear so I'm starting there
 
Pic of the setup
 

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Not sure about the VM28's or 30's, but on some VM's the slide can be installed backwards, worth checking, especially on the fast cylinder.
 
Finnigan said:
I can't really get it so that I can pull away in 1st gear so I'm starting there
Click to expand...

Sounds like you may need to go up on the pilot jets (again, assuming ignition timing is good). Where are you at on the pilots right now? Also... start over with your air screws one turn out.
 
Not sure about the jets, I can pull them out if they can be measured. I didn't change anything out since you mailed them to me.

BTW I have the tuning manual but its pretty confusing, can i take the jet out without removing the carbs?
 
I forgot to mention that unless the air screws are almost all the way out the rpms stay at 4000 and there is white exhaust coming out like crazy, I'm a little nervous of overheating
 
I think you're fighting an air leak. Everything between the carbs and the engine (e.g. the manifolds) need to be air tight. No leaks allowed.
 
You can test for this by spraying starter fluid or WD40 at the intake boots while it's running - the former will cause the bike to rev up; the latter will bog it down, if there's a leak. You can also use the flame of a match or lighter to see if air is getting sucked in. And where.
 
The boots are new and the gaskets are new, if there is a leak what should I use to patch it?

Thanks guys
 
A bit of RTV will do the trick, if needed. Hopefully you don't need it though. Are the boots the correct size for the new carbs? They should be a pretty tight fit and I usually use a bit of grease when putting them on, it helps to avoid leaks and makes the installation a bit easier.

Also, don't torque down the manifolds too much. Having things too tight can cause just as many problems as having things too tight.
 
Finnigan said:
I forgot to mention that unless the air screws are almost all the way out the rpms stay at 4000 and there is white exhaust coming out like crazy, I'm a little nervous of overheating
Click to expand...

Are you referring to the air screw or the idle screw? The idle screw is the one that will lower the idle, turn this counter clockwise to get it down to around 1200 and then start tuning the air screw. If after turning the idle screw out and the rpms still down lower you either have a air leak, or the slides are stuck open too far for some reason.
 
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