Bike not idling below 2K..does this make sense?

[Newbee]

Bike won't idle below 2K it was fine until I hooked up the gas tank.(I was running off a remote tank). I am thinking that my vacuum activated petcock needs to be rebuilt? Anything over 2K it will idle for ever. Under 2K it dies almost instantly? Or what else could it be?

[Nessism]

Sounds like a vacuum leak. Does the bike have intake boot O-rings, and if so did you replace them lately? I think if the petcock diaphram was compromised the petcock wouldn't work. Easy to test by sucking on a piece of tubing though.

[Newbee]

does sound like a vacuum leak...

Vacuum leak is what I checked for when I was synching carbs. Sprayed some carb cleaner around boots, airbox etc. Noted no change in RPM.

Maybe it's just a coincedence but that thing ran fine until I hooked up the gas tank?

[Nessism]

Vacuum leak is what I checked for when I was synching carbs. Sprayed some carb cleaner around boots, airbox etc. Noted no change in RPM.

Maybe it's just a coincedence but that thing ran fine until I hooked up the gas tank?

Spraying carb cleaner, WD-40, brake cleaner, etc, is an unreliable method to determine if you have vacuum leaks.

What kind of bike are we talking about anyway?

[Newbee]

Dang....

The bike is 82 GS750T. The carbs synched great and it would idle at 1200rpm real nicely.

I guess further inspection is the next step?

[Plooker]

I would suggest pilling a float bowl and checking for sediment, especially if you had the tank physically off the bike. If you feel ambitious you could also pull the petcock and see if the tank is filthy inside. Last time I had a bike do that on me it was sediment in the tank which plugged the pilot jets and also restricted the fuel flow so it would not run on the idle circuit but couldn't flow fuel fast enough to run much into the needle circuit. If you need to rebuild the petcock or anything else in the fuel system do yourself a huge favor and get OEM parts. I ordered a petcock rebuild kit for my 850 from parts unlimited and it was a joke, the o-rings were about 15% too small and the diaphragm was so poorly reproduced it would not physically open under vacuum because the metal valve bit was hitting the cover while it was still fully seated and of course I found this out the hard way.

If the bike will run ok with the choke out but dies without it you almost certainly have a fuel delivery problem.

Hope this helps.

[Nessism]

Your bike does not have intake boot O-rings so that possible source is eliminated.

I trust you have all the sync port screws installed? Also, did you adjust the pilot screws to the highest idle setting? A lean pilot mixture setting will cause a hanging idle. Try blipping the idle, if it tries to settle down very slowly, the mixture is too lean.

[chef1366]

Make sure you didn't kink the fuel line when lowering the tank.

[Newbee]

I'm removing the tank...

I'll take the tank off and see if it makes a difference. All the vaccum port screws are in. It doesn't need the choke to idle at 2krpm. I think when I remove the tank I'll also drain the fuel bowls and look for sediment. I am also contemplating putting on a manual petcock. I view the vacuum operated as a source for future and potential problems.

[Steve]

I view the vacuum operated as a source for future and potential problems.

Personally, I see the manually-operated petcock "as a source for future and potential problems".

Training and experience would have to weigh in here, but I am not in the habit of turning a petcock every time I get on or off the bike. Some of you who have been riding dirt bikes or really old bikes that were not equipped with automatic petcocks (could be vacuum or electric) are used to doing this, so having to do it to another one is no big deal. In MY case, I would forget it rather quickly, leading to what I am sure could be serious problems.

The only real problem with a vacuum-operated petcock is that the diaphragm tends to wear out every 15-20 years. If you feel lucky, you can try replacing the diaphragm, but many on this board have had rather dismal success in that endeavor. Replacing it with a new OEM unit will fix it nicely, then you don't have to worry about it for another 15-20 years.

.

[doctorgonzo]

I'm with Steve. I know many people chime in that in years they have NEVER forgotten to turn off the petcock, but you only have to forget once.

[tkent02]

Personally, I see the manually-operated petcock "as a source for future and potential problems".

Training and experience would have to weigh in here, but I am not in the habit of turning a petcock every time I get on or off the bike. Some of you who have been riding dirt bikes or really old bikes that were not equipped with automatic petcocks (could be vacuum or electric) are used to doing this, so having to do it to another one is no big deal. In MY case, I would forget it rather quickly, leading to what I am sure could be serious problems.

The only real problem with a vacuum-operated petcock is that the diaphragm tends to wear out every 15-20 years. If you feel lucky, you can try replacing the diaphragm, but many on this board have had rather dismal success in that endeavor. Replacing it with a new OEM unit will fix it nicely, then you don't have to worry about it for another 15-20 years.

.

I have way too many bikes to remember which ones have a petcock that needs to be turned off, I would hate to burn my house down (along with all the other bikes, pets, children, etc) because I forgot to turn it off one day in the future. The old vacuum petcocks really only fail if they sit for years. If you buy a new one it is fixed forever.