I have a 1969 CB350. I just got done repairing it but I encountered a big problem. When I started it instantly jumped to 8,000 RPMs. I can't figure it out. Valves are all set up properly, new plugs, Mikuni racing carbs.
Odds are your throttle is held open for some reason. First thing I'd do is have a peek through your carbs (while still mounted) to see if the slides are returning properly. If they aren't, I'd check the cable at the carbs (adjusted too high?) and at the 1-2 cable linkage (cable popped partially out?). Other possibilities - return springs not mounted? Choke on?
The beauty of mikunis is that increased air pressure through the venturi won't cause the slides to rise (or fail to drop), so runaway revs are usually caused by direct mechanical action upon the slides (as opposed to, say, an air leak around the manifold).
For an engine to run at those sort of revs its got to get air from some where,If its got a manifold leak between carb and head, it would run lean, as it cant pull much fuel from the carbs.running lean can add to the higher revs.It would have to be a big leak for it to run with the chokes closed so do a manual test by blocking the carb intakes with your hand when its running, No air, engine should die.If still runs its serious leak.If it dies, then its mostly an adjustment problem,so check your idle screws are wound out so the slides are all the way down to the bottom of the carb throat,then start re adjusting from there. There's also needle hight to consider, if thats been played with at all, it should be almost fully down in the main jet when the slide is down.when thats checked the idle mixture will need setting may be 1.5 to 2.5 turns out from closed (usually the fully screwed in position is closed) Sorry i cant give you specifics for your correct settings, this is general carb info.someone may have more detail for you. Its been a long time since ive worked on minuki carbs and It may not be very acurate description of the adjustments. Steve.
If you're a mikuni expert then forgive this post, I don't in any way mean to condescend but I just simply don't know what you know.
First off, when you say "mikuni racing carbs", which ones do you mean? VMs (round slide), or did you mount up some flat slides?
Odds are you've got VMs on there, in which case (and I don't know if you realize this), the slide should almost entirely block the venturi at idle - as in, you look inside it and you can't see through it because the slide's in the way. It's possible that your idle screws are just set WAY too high.
In order for a manifold air leak to cause this problem, you'd have to somewhere be providing a ridiculous amount of excess fuel to mix with it - hence my previous post, higher revs don't lift the slide (no butterfly valve), hence they don't open the needle jet to provide more fuel.... so how's an air leak going to drive it to 8k?
Watch, having said that, your next post will say it was in air leak.