What causes an increase in RPM [from 1100 to 3500]

[roundogg]

I have a 80 model Special--carbs been cleaned many times--and still, when the engine gets warm, the RPM will jump from 1100 to 3500 at the drop of the hat. Not all the time, but more often than not. Could this be in the spark advance?

[crunchbird]

This often means you need to synch your carbs. If you are at a red light, racing at 3500, see if you can bump it back down to 1000 by letting out the clutch in first gear with the brake on. If you have this high-or-low idle situation you need to synch, in my experience.

To synch carbs I got the CarbTune II from England, it has stainless steel rods rather than mercury and has worked well for me.

[roundogg]

Yeah, did the mercury stick thing everytime the carbs were worked on. I have tried the 1st gear and out a little out trick, rpm does drop but within a few seconds, right back up. Am thinking I might have a air leak or the hose on the spark advance is crappy. I have put 352 miles on the thing since I got it running again--it sat for almost 10 years--14,560 total on odometer-- still has the factory rear tire. May just be full of creeping crud and needs to be run very fast with some high octane fuel. Will keep tinkering and talking to folks that can help and more experience with the imports. Yes, I am a ex Hogger and trike man. It just takes a little time to smarten up sometimes. Thank You for the input.

[crunchbird]

I think your diagnosis of air problem is good since you are synching but still have the problem. I had a loose carb boot on the xs11 so the synching didn't work until I found that, though the problem wasn't biphasic idle, it was just rough. On another bike (xs650) the synching didn't help either, it turned out the tube from the airbox to the carb was partially crushed. I couldn't balance those carbs, just found a compromise.

So I know unequal air defeats synching.

[Dragonrider]

Yep, sounds like an air leak. Check the rubbers between the carbs and the engine. Check for cracks, and make sure they are properly attatched/sealed to the engine and firmly clamped to the carbs. Also check the sync nipples . . . I have seen those loosen up and leak around them when hot . . . as well as the rubber caps on them. Does not take much . . .

[roundogg]

Sounds like I have some more tinkering to do. Thanks to everyone that has put me on the right track. Keep eating bugs and loving them!

[reddog]

high idle

An easy test for a vacuum leak is to take a can of starting fluid and spray small shots around the rubber manifolds between carb & heads while the motor is at idle, if it is a vacuum leak the RPMs will pick up. Next spray boots between air cleaner & carbs though this is not as criticle it can still cause problems. Oh, while working with starting fluid you should stay away from the spark plugs & wires if you have any bad wires things could get REAL exciting.javascript:smilie('')
eek!