Mikuni BS34 CV idle mixture screws

Hey guys. I have the Mikuni BS34 carbs and I am about to install them after cleaning and rebuilding them.

I don't remember what position the idle mixture screws were in when I took them apart, so I was wondering if there was a common position for these.

At this time, I have screwed them in all the way, and then backed them all out 5 turns...not really knowing what position they should be in.

Or am I supposed to just install them, run the engine and adjust according to the performance of the engine?

Anything else I should be doing before I install these back on my bike that you guys can think of would be very helpful as well.

Thanks guys.

1-1/2 to 2 turns from lightly seated will get you running, but each bike is a bit different with the final adjustment.

Motor Head - How do you know when you've got it just right, what way does the engine behave?

Sorry for the thread hi-jack, i have no experience on setting up carbs but i cleaned mine, serviced them & replaced the jets

in = less air? Correct?

750steve wrote: Motor Head - How do you know when you've got it just right, what way does the engine behave?

Sorry for the thread hi-jack, i have no experience on setting up carbs but i cleaned mine, serviced them & replaced the jets

in = less air? Correct?

Depends on the carbs, if the screw is behind the throttle slide. Then it is an Air screw, so in less air= richer. But if in front, then it is either a fuel or a volume of mixture screw. In =lean on fuel, etc.

Ok, since they're nearest the cylinder head they're air screws?

So what do i need to look for to get that perfect setting?

I either didn't explain it right or you've misunderstood. If it is up against the head, in front of the slide valve, then it is an mixture screw. This will be In Lean, Out Rich. So these you have are the BS38 Mikunis on your twin? I'd say start about 2 turns out, with stock airbox and stock jets. With Pod's, you will need richer Pilot jets, and then still be around 2-3 turns out.
That's just the Pilot circuit.
To find the best setting is trail and err. you need to turn the screw in till the Idle speed drops, then back out about 1/4 to 1/2 turns and then run it. It might be farther out, end up at 2-1/2 turns or 3 turns, maybe 3-1/2. If any farther I would suspect vacuum leak, or still to lean a Pilot jet, with Pod's.

Yip, that was me, i misunderstood!

I have the BS38's too, started off 2 turns out & balanced the carbs........i've done nothing to the mixture screws since.

I'm running #50 pilots (or whatever is 1 up from stock) as im running pod's (yes i know , but its a cafe build & the stk airbox wouldn't look right), needle on clip #4 & 135 mains



I'll play with the screws tomorrow night & see what they do. I get a slight unburnt fuel sting in my eyes when the bike is ran in my garage for a while (but this might just be normal in an enclosed space) so i was going to start off going leaner but i'll keep an eye on the throttle response & judge it by that

Thanks Motor Head.

Much appreciated!