High RPM issue

[BlkMage]

Just ONE MORE issue here guys. My bike likes to rev all the way up on it's own sometimes. I can have it just sitting in Neutral, not touch the throttle, and it revs up and stays there. I just rebuilt and synced the carbs, and it's not an issue with the cable or idle screw (not even touching right now). The only way I can seem to slow it down is to have the bike in gear with the clutch out and apply the brakes to slow the bike. Anyone ever heard of anything like this?

[wa407mpp]

check your cold starting circuit it might be bleeding through.

[BlkMage]

Originally posted by wa407mpp
check your cold starting circuit it might be bleeding through.

???

EDIT: Wait, do you mean the choke? If so, my dad checked when he was rebuilding the carbs and said they're all tight.

[Ken Talbot]

Try checking for vacuum leaks, such as a cracked advance or petcock hose, a cap missing from one of the vacuum nipples on the intake boots, a leak at a boot/cylinder face, or a boot/carb leak. If you had a leak and managed to get it sync'd, then the leak reseals itself temporarily, you've suddenly go one cylinder pulling stronger because its butterfly is open more than the others.

[egsols]

I had the same problem with a 79 special I just picked up. The throttle stand off on the carbs was bent up slighty and wouldn't give the cable any free play. Check for free play on your throttle and if there isn't any your cable is probalbly pulling to tight.

Ernie

[dragxs11]

This may seem like a dumb ? but you do have the vac advance hose going to carb #2 not intake #2, correct?

[dragxs11]

Throttle cable linkage could be hanging up on an intake clamp on #2 or #3 too.

[BlkMage]

Not a throttle problem, it's clear. Carb boots are brand new, so I doubt the leak is there. I can't find any leaks in the vac systems (which are hooked up correctly). Oh, and the carbs weren't synced with a vacuum system, we miced the butterfly valves when it was off the bike with lockwire.

But just in case it's the vac, I'm going to look on a parts bike we have to try and find another vac hose to the fuel system. I doubt if it's the one for the timing advance, as the bike runs very well at the high RPMs.

[dragxs11]

Maybe pull off the timing advance cover, disconnect the hose and plug it to make sure it's not the advance unit. If it's not that, and your butterflies are closed, I'm gonna have to wait for another idea to come to me.

[BlkMage]

While we wait, I have a report. I changed the line from the fuel valve (where the 4 fuel lines come in) to the number 2 carb boot engine side. It did ok for a minute or so, but then backfired HARD a couple of times and I killed it. My dad said before, just before it started reving, it would click a couple of times almost sounding like a backfire. Hope this info helps.

[ratbyk]

Go back to Ken Talbots advice. He gives good info. Most likely a vacuum leak somewhere.

[dragxs11]

Good, brainstorming time.
The timing advance unit(pick-up coils) are on a spring loaded assembly. When no vac. is present, it should be pulled back to idle state, which is where we want it to stay for the time being.
you could check to make sure the unit moves freely and the spring works. With no vac advance, if the problem persists, we're looking at the carbs.
When the carbs where installed, do you remember if the butterflies were closed, or cracked opened, and if so how much?
it sounds a little like your creating just enough vacume to borderline the vac advance, and once it starts pulling on that diaphram, it builds on itself, so to speak.
just thinking out loud, sorry if some of that doesn't make a great deal of sense.

[BlkMage]

Originally posted by dragxs11
Good, brainstorming time.
The timing advance unit(pick-up coils) are on a spring loaded assembly. When no vac. is present, it should be pulled back to idle state, which is where we want it to stay for the time being.
you could check to make sure the unit moves freely and the spring works. With no vac advance, if the problem persists, we're looking at the carbs.
When the carbs where installed, do you remember if the butterflies were closed, or cracked opened, and if so how much?
it sounds a little like your creating just enough vacume to borderline the vac advance, and once it starts pulling on that diaphram, it builds on itself, so to speak.
just thinking out loud, sorry if some of that doesn't make a great deal of sense.

The timing device moves freely. We used a VERY small (I think it was .02 inch) piece of lockwire to sync, so there's that small gap open on the butterflies, but otherwise they work fine.

[dragxs11]

One hole sucking air and running lean is not gonna pull the whole engine up that much. It's a good thing you didn't sync with a mercury filled syncronizer or could have trashed your engine with this issue. I'm just wondering if it's possible that, when you adjusted the butterflies if you set them all opened too much. Not sure if thats even possilble w'out a set in front of me.

[dragxs11]

I don't know right now if thats good or not. Someone else may correct me here, but with the idle screw out I would set the butterflies completely closed and let the idle screw take care of the gap. You guys probably took that into account, just covering bases. Can't have you still working on this thing a week from tomorrow.