High Revving carbs (again)

[Anonymous]

I've replaced my carbs with rebuit ones. The bike starts, but when I give it the gas, the RPMs shoot up to 3000+ and hover there. Sometimes they come back down, sometimes they don't. When they did come back down, I tried to gradually raise the RPMs with the throttle stop screw. For a moment, the RPMs would slowly rise accordingly, but then they would shoot up as well.

The last time I posted about this problem with the original carbs, some suggested syncronizing the carbs. Unfortunately, the RPMs act erratically enough as to make that almost impossible.

Could this be a symptom of leaking intake boots? The intake O-rings are in excellent shape, so I'm ruling those out as the culprit. I have also inspected the intake boots, and the actual rubber looks fine with no cracks. But where the rubber meets the metal flange, there are small, hairline gaps that can grow and shrink with some pressure. Do I smell some new boots in my future?

Thanks in advance.

[Anonymous]

Sounds to me like a bad vacuum leak. The way I check for vacuum leaks is to use a can of carb cleaner with the straw type nozzle and shoot at the carb bases while the engine is running. If there is a vacuum leak, you will hear a change in rpm.

[Anonymous]

I've disconnected and removed the vacuum line for the original petcock and replaced it with a manual one from Pingel. I also plugged its vacuum barb on carb #2.

When you say base of the carbs, is this where the carbs join the intake boots?

Thanks.

[Anonymous]

revs

Check your throttle cable for enough slack and proper routeing, as well as the return cable, Iam betting that the cable may need to be replaced due to it fraying in the houseing and hanging up.

[KEITH KRAUSE]

(Not trying to show you up basaraj) When checking for intake leaks at the carbs, just use water from a fine mist bottle. If you have a leak, the rpm's will drop a moment when the water hits. Make sure the motor is warm before checking. Carb cleaner is not good for the rubber parts or painted parts. It's also flammable and not as safe around a hot motor.

[Anonymous]

You could also be in need of a carb sync.

Michael

[Anonymous]

(Not trying to show you up basaraj) When checking for intake leaks at the carbs, just use water from a fine mist bottle. If you have a leak, the rpm's will drop a moment when the water hits. Make sure the motor is warm before checking. Carb cleaner is not good for the rubber parts or painted parts. It's also flammable and not as safe around a hot motor.

OK, that's cool whatever works. I've always used carb cleaner, it's an old mechanics trick. I may be wrong but, I always thought the flash point on carb cleaner was well above the tempratures reached on the intake and carb surfaces.

[KEITH KRAUSE]

I'm pretty sure it is. Water is just safer though. My main reason for using water is because carb spray is not good for rubber.

[Anonymous]

Re: revs

Check your throttle cable for enough slack and proper routeing, as well as the return cable, Iam betting that the cable may need to be replaced due to it fraying in the houseing and hanging up.

Good point.

[KEITH KRAUSE]

The sticking/frayed throttle cable(s) idea makes sense. It makes more sense if you have two cables. If you have just one, then that test you did...gradually raising the rpm with the screwknob and then the rpm's suddenly jump, I don't think could be caused by a single cable set up. But maybe? Wrong cable routing, binding, could do it with either set up.
It's interesting that you've got the same problem using two sets of carbs.
Makes you think it's not the carbs themselves, but something related to their installation. The cables are a possibility, so are vacuum leaks. If you're able to check the synch, I would still at least check it, after you're sure about the cables and any leaks.
If you figure it out, please let us know so we can help the next person.

[Anonymous]

did you at least do a mechanical sync first before you installed the carbs

[Anonymous]

I did do a mechanical sync before installation. At least I did a closed mechanical sync. I couldn't make sense of the language describing the open mechanical sync.

I only have one throttle cable, but will check for binding. I'm pretty sure it's okay, but will check for sure.

A number of responses have mentioned vacuum leaks, but since I have removed the vacuum petcock and plugged its appropriate vacuum nipple on carb 2, I can rule that out. Could the vacuum leak responses also be referring to leaks around the intake boots? And/or are there other possible vacuum leaks that I could/should check?

Thanks.

[Billy Ricks]

Around the intake manifolds at both the carb and the head.

[Anonymous]

I concur on the diagnosis -- intake leaks or widely out-of-sync carbs. The problem is, things are too unstable to use spray to try to detect the leaks.

When you said the intake O-rings are in good shape, what did you mean? These are really short-lived little (&)('s, and should be changed if you don't know their age. I've seen 4 year old bikes with bad intake O-rings.

As for the initial pre-installation sync procedure, I just use daylight. Looking down the barrel each carb, with your eye at the same spot, hold the bank of carbs up to light and note the size of the crescent; you want these to be as close as possible to the same size.

The only other possible culprit are the boots themselves; did you examine these with a strong light?

Finally, whilst you're horsing around, don't forget that if you're running the engine much above idle for any length of time, you'll need some cooling.

[KEITH KRAUSE]

I would synch the carbs with a vacuum tool. You can't rule out synching unless you've done it correctly using a vacuum tool. But before synching, you have to be sure there are no intake leaks.