Stuck RPM's

I’m hoping some of you experts recognize what the cause of my problem is; I’ve done everything I know of. The problem is my rpm’s get stuck if I rev the engine to 3,000 or above. Sometimes it comes back down very slowly, other times I have to briefly hit the kill switch to bring the rpm’s back down. It happens very frequently.

Here’s what I’ve done so far:
• Deep-cleaned the carbs (full disassembly/dip with all passages cleaned and air blown and with new gaskets/o-rings). Set the pilot screw at 1¼ out. Measured and adjusted floats to mid-range factory specs.
• Valve clearance with all clearances at 0.102 (specs are 0.05 – 0.15).
• Removed factory air switch components and bridged the two reed cap valves with single hose, plugged the air box top hole but kept the bottom L-hose into breather cover.
• Returned bike to air box configuration with new air ducts and carb holders and air filter.

The only thing I can think of is my throttle cable may not have enough slack in it? It does have a slight bend when at zero throttle, but may not have enough?

The last item I can think of is the bike used to have pods, and the main jets are #130’s with pilot jets at factory #37.5 (Mikuni’s BS-34). I do have a Supertrapp 4 / 1 exhaust, so I left the main jets at #130. Could that be the problem?

By the way, the problem seems to be exasperated when choke is engaged.

I really would appreciate some guidance on this, guys, I’m at a loss here.

Sounds like a air leak at the vacuum ports. The vacuum hoses to the air switch and the ports, What did you do with those?

If you can make the problem happen with the bike sitting still, it might be a good time to put on a set of vacuum gauges. Check your synchronization. You might have all or some carburetor slides not closing back down due to wear, dirt, bent needles, etc. If you don't have gauges, just pull off the rubber tubes between the carburetors and the air cleaner box. You might be able to see if the slides are physically closing down. Caution if you use mercury gauges, don't rev the engine up much with the gauges attached. You can suck the mercury into the engine and end up breathing it.

The vacuum ports on all four carbs are plugged with brand new 7/32" vacuum caps. I'm pretty confident on the seals.

I did not do a carb synch yet, and the three adjustment screws have never been adjusted - still has the factory yellow paint on screws/nuts that has not been broken.

I did have to do a gentle smoothing with very fine emery cloth on a couple of the slides at the bottom as the PO (or someone) had been in there with a metal object and caused some damage resulting in sticky slides. But I believe I fixed it okay and the slides mechanically moved free.

Is it possible that I have damage to the diaphrams? I visually checked them and they seemed fine with no pin holes or tears.

Post edited by: Norseman, at: 2007/05/25 00:29

Anyone else out there who has some insight on this problem? I just got back from a short ride, and it's only gotten worse. Now it races when I shift, and if I down-shift, it really revs up to the point the bike is unsafe.

I just removed the slides/needles and inspected very closely with a bright light. The diaphragms are in great shape with no pin holes or cracks, the slides move completely freely up and down, and needles look straight. The throttle cable is also kink-free and slides freely, with appropriate slack. The throttle springs back to zero from any position.

With brand new carb holders and vacuum caps on carb vacuum ports, what on earth could be the cause?

Please, if anyone has any ideas, I would be very grateful.

Post edited by: Norseman, at: 2007/05/26 01:20

Since you have likely removed the carbs, check the clamps that are on the carb holders. The longish phillips screws can come out and contact the sync mechanisms fairly easily after the screws are tightened all the way... another possible issue is that the mixture screw o-rings are missing or shot. Your throttle butterflies may be binding... if assembly was put together with the bracket screws contacting one of the mount brackets a bit, it can "warp" the bracket a tad and make the butterflies sticky. The rod that the butterflies pivot on could be rusty and need cleaning. Last thing I can think of is that one or more of the return springs have unwound a turn and were assembled in an unwound state and don't provide enough closing power.

Post edited by: wiredgeorge, at: 2007/05/26 12:56