Zero to 6000 rpm....Scared me to death!!!

[jmacsuga]

Hello All: Weather is getting warmer, and started on my '78. I have been starting it and running for about 1/2 hour every so often all winter. I couldn't get the blinkers to work, so I have been following your advice and cleaning all the connections, grounds, etc. Well, I took the gas tank off, substituted an auxiliary tank, blocked off all the carb vacuum lines, and started the bike. Was running OK, and when I shut it off for troubleshooting, I shut off the gas. The last time I started it though, it ran for about a minute, then the engine speed went to 6000 rpm!!!! Well, shut it down, of course. Tried again, same result. My question: Can anyone tell me what happened and will it do it again when I get the tank back on and hook up the vacuum lines to the petcocks??? I drained all the gas out of the carbs, and it has just been sitting ever since. I get the impression from some posts that I might have screwed up the vacuum advance or got too much gas in the carbs and locked the floats. Hope to hear from all you really good mechanics!!!! This site is invaluable!!! By the way, it has ran Ok without this trouble all winter. It didn't do this for probably an hour running off and on.

[DGXSER]

Well, it is tough to say for sure if it will happen again. The most likely causes of the engien speed climbing on its own like that is a vacuum leak, a poor or no synching of the carbs, or a stuck throttle linkage. When it climbs slowly it is typically a vacuum leak, or synch issue.

If it ran fine for 30 minutes or more without doing that, perhaps you inadvertently fixed the vacuum leak.

[jimbyjimb]

I could be off on this but I feel like a vacuum leak wouldn't cause the motor to speed up unless there was a fuel supply excess along with it. Stranger things have happened but the only vacuum related issues I've ever faced have only made the motor run like garbage/die. My bike will get to 4k no problem on full choke so if it's warm enough and there were a vacuum leak and the choke was applied it might get up there.

Stuck or sticky throttle linkage is a distinct possibility. I don't see the vacuum advance causing this kind of problem though, it has to be fuel/air because the engine can't rev if the fuel/air isn't there.

Sometimes the little black caps on the carb boots will loosen up on me and I need to replace them every few months for fear of losing one. Check the caps out and if it revs like this again try reducing to half or no choke and see what happens. W/O choke a vacuum leak will make the bike run like garbage and possibly pop back through the exhaust. It can also make idle erratic, though multiple things could cause any one of these problems.

[DGXSER]

On this bike, I have had a vacuum leak cause the idle to climb.

I have read experiences on here on more than a few occasions where a bad synch will cause this issue to happen. Basically, one cylinder draws hard and pulls the others along for the ride, increasing engine speed, increasing vacuum, making that one stronger again and on up the revs go.

With this internet method of advanced technological bike mechanic diagnosis, nothing is ever the absolute right answer until it fixes the problem. But it is a possibility I would not ignore.

[James England]

Possibly a stuck/sticking slide itself, as opposed to the linkage.

[BA80]

I'm betting that when you were fooling with the tank you hooked the throttle cable and pulled it up out of it's seat and it's holding the carbs open.

[Brad]

Had something like your situation happen to me last year. Turned out to be my clutch cable was getting stuck between the back side of the carbs linkage and motor. This would happen when I would rev up the motor and sometimes when I turned my bars.

[jmacsuga]

To answer your questions:

1. The throttle linkage is functioning normally, and there was no control of the throttle when this happened. Engine speed went to basically full throttle, and there was no control. It had been running normally for at least an hour with no problems.
2. There was no vacuum leak, as all the vacuum vents on the carbs were blocked off.
3. The engine was warm, has been working fine, and there was no choke on. The only thing is that with the aux tank that I was using, there was maybe too much gas pressure, and it locked up the slides/floats, and that is what caused it. Is that possible???


I will be taking the carbs off (maybe today or tomorrow) and let you know if I find anything.

[jimbyjimb]

Not to over analyze but just because the vacuum ports are sealed or appear to be doesn't mean there can't be a vacuum leak. The boots could be cracked and sucking air or incorrectly sealedor some such. The only way to be positive there isn't a vacuum leak would be to run the bike and spray something like brake cleaner on the boots/intake seals and see if that affects the engine speed or exhaust color.

I'm still trying to think of what else could cause this and I don;t see how it could be electrical, only mechanical or fuel/air. You could certainly be onto something thinking that too much fuel is getting shoved into the cylinders but I would expect to see black or wet plugs and black smoke out of the pipes if this were the case, particularly if the bike was hot at the time and not needing a ridiculously rich mixture to run.

[TopCatGr58]

2. There was no vacuum leak, as all the vacuum vents on the carbs were blocked off.
3. The engine was warm, has been working fine, and there was no choke on. The only thing is that with the aux tank that I was using, there was maybe too much gas pressure, and it locked up the slides/floats, and that is what caused it. Is that possible???


I will be taking the carbs off (maybe today or tomorrow) and let you know if I find anything.

AHA.....the carb vents are NOT to be blocked, they are to be OPEN to vent, otherwise, the float bowl pressure can not equalize, and they will FLOOD the carbs, and that may be the source of your sudden increase in fuel to rev the engine!? The fuel supply is just gravity feed, and the floats and needle valves/seats are more than adequate to control the flow....provided the bowl is properly vented!

The Intake boots synch ports are to be capped, but folks have had them dry/crack, or the heat stiffened them up, and they actually leaked around the base. Also the intake boots where they bolt to the engine can leak...the rubber again hardens, cracks, etc.!

T.C.

[jmacsuga]

Problem Solved

Hi, All: Ok, my running wild mystery has been solved. I took the carb assembly off and found that a hose clamp had been lodged between the throttle stop arm and the idle adjustment screw. Couldn't be seen when the carbs were mounted. I also found that the vacuum advance hose to the vacuum advance cylinder had split, and that two other vacuum hoses had split at the carb mounts. This is a blessing in disguise because it could have happened when I was riding, and that might have been a real trip!!! Thanks for all your suggestions!!! Gave me some areas to investigate and that was a real help. I put all back together yesterday, and it runs GREAT!!! Have a great day!!!