Mikuni BS34 Carbs Fast Idle

[fcbaker]

i bought a junkyard 80 Special and started rebuilding a few months ago. Carbs were a mess but completely disassembled, cleaned and replaced floats, needle and seats, butterfly seals and rejetted to spec recommended for aftermarket exhaust and air filters. I also made all valve adjustments and cam chain adjustments. I replaced the rubber carb manifolds and gaskets. Started the bike and it ran great. Synced the carbs with mercury gauges and took it for a ride. Didn't pushed it too hard because it hadn't run for probably 10 or 15 years. Let the bike sit for a few days and have been fighting a fast idle since. It starts fine and immediately goes up to 3000 rpm. Had the carbs off 5 times and disassembled looking for vacuum leaks. The diaphragms look fine and when I open the throttle both piston raise together. I also installed a new coil, wires, cap and plugs and a Pamco Pete ignition. Lots of sparks and timing is on. Any thoughts or ideas are welcome. I want to replace the stock exhaust but want to get it running right first.
The jetting is now: pilot #45, air #127.5, main #145. Idle mix screws are out 2.5 turns.

 

[5twins]

That 145 main could be a bit too much for your current mods, which are ???? You said you're still running the stock exhaust? What about air filters? Not sure why you're messing with the air jet, that's usually not required. It can sometimes help in extremely pesky situations where you can't get things exactly right with just pilot jet changes, or for fine, fine final tuning. It's something you do last, if normal pilot and main jet changes don't quite get you there.

So, some things you can try - the 45 pilots with the stock 135 air jets or the stock 42.5 pilots with the 127.5 air jets. Mix screws open more, somewhere in the 3 to 3.5 turns out range. And I'd reduce the mains, probably down to a 137.5 or 140.

 

[peanut]

I had exactly the same issue with my 79 ( although that has BS38s).
One thing I did notice was that the idle climbed higher the hotter the engine became which suggested a lean mixture or air leak

When the idle was high I found that pressing down firmly on the throttle shaft would bring the revs down.It seemed like once hot the butterflys were not returning fully to the stops. This persuaded me to check the throttle return springs which I strengthened by shortening them .

I discovered that thewhite plastic shims either side of the throttle shafts had worn allowing the throttle shafts to move sideways 1-2 mm and the butterfly discs were binding on one side of the venturi walls preventing the discs from completely shutting down . I stripped the throttle shafts and added some thin plastic shims until the throttle shafts no longer floated from side to side.

I then loosened the butterfly screws and centred the butterfly discs in the venturi.
These three things together cured my high idle issue completely.
You might find something helpful in this vid

 

[machstang]

You said you went pamco but did you do e advance? Is it possible that it could be an advance spring broke?

 

[fcbaker]

I appreciate your responses. I have the stock jets I can reinstall but when I first put it back together it ran fine. I took it for a ride and put it through all the gears with no problem. Idle always returned to normal. At that time I synced the carbs and readjust the idle because it raised slightly when the carbs were synced. Since that time it starts right up but goes right to 3000+ rpm. I think it's the left carb causing the problem because when the problem started the left pipe was cold compared to the right pipe. Now both pipes are hot but the high idle is still occurring. I did install the e advancer when I put in the Pamco system. When it idled correctly I verified that the timing was on and it advanced correctly. I'm going to watch the video to see if I'm overlooking anything. I did replace one throttle plate because I buggered it up getting it out when I disassembled the carbs for cleaning. Again it's driving me crazy because it did run perfectly fine after I rebuilt the carbs, then it started the problem described. Again appreciate all your advice.