Carbs get fuel *sometimes*

I'm having an issue where my carbs aren't getting fuel constantly while running. They'll get fuel to be able to run for a minute, then it dies. Sometimes it won't start at all because the carbs are dry.

If I set the fuel tap to PRI, then the bike will chug, but only at full throttle. As soon as I let up on the throttle, it dies.

Could this be a float issue? If so, which way do they need to be tuned? I set them to the stock height of 18mm the last time I rebuilt them (two weeks ago).

1982 GPz750 w/ Mikuni BS34s

Could be a bad petcock or a blocked air vent in the gas cap.

Could be a float/needle if they are sticking in the up (closed) position.

+1 petcock.

Is there a way to check the petcock?

Well, you can actually activate it by sucking on the vacuum line, but don't let anybody see you. :whistle:

The less embarrassing option is to go to any pharmacy and get a large, plastic syringe. Put that into the vacuum line and that should activate the fuel flow easily.

This is the one I use on manometers, but it also works for petcocks.


Are you using a car fuel filter? The see through ones with paper elements often will stop flowing. They are meant for fuel pumps. Gravity doesn't apply enough pressure.

I've posted this before, but I'll post it again in case others might not have seen it.

Should be able to unbolt and or unscrew check to be sure the screen for the petcock is not plugged up clean the petcock ports and inspect any rubber for damage. Sometimes plugged vent holes will cause problems Im not sure how your carbs vent but just a thought.

loudhvx wrote: Well, you can actually activate it by sucking on the vacuum line, but don't let anybody see you. :whistle:

The less embarrassing option is to go to any pharmacy and get a large, plastic syringe. Put that into the vacuum line and that should activate the fuel flow easily.


Are you using a car fuel filter? The see through ones with paper elements often will stop flowing. They are meant for fuel pumps. Gravity doesn't apply enough pressure.

I have been using a fuel filter, but thought it could be a culprit so it's currently not attached. After taking it off, fuel flowed slightly, but not nearly enough. Is there a filter that I can use?

I've taken apart and cleaned the petcock and there wasn't much gunk in there. Nothing changed when I reinstalled it.

I went through this with a bank of Keihin CR Specials a few years ago. Tried everything under the sun to get fuel to flow to them. All the meanwhile my wife was saying "is the fuel line clear?" After a complete weekend of dicking around, and listening to her ask the same question over and over, I decided to appease her and blow out the fuel line. Like magic, some lump of crap blew out and problem solved!

You can use the metal screen filters available at Z1 Enterprises. There are also ones that are in an aluminum body with a cleanable internal scintered brass/bronze filter.

So I tried sucking on the vacuum line attached to the petcock, and a little bit of fuel flowed, but stopped after a second. Is there maybe a diaphragm that's getting stuck when vacuum is applied?

I also tried this with the tap set to PRI and had the same results; just a little shot of fuel then nothing.

Sounds like the petcock needs rebuilding. Passages clogging and vacuum diaphram leaking perhaps. Clean and seal the tank too so it doesn't clog up the petcock again as well as make rust holes .

Yup, should have flowed continuously under both conditions. The normal position takes gas from one pipe, and the prime position takes gas from the reserve pipe. So if there's any gas, it should have flowed.

So the petcock will have to be inspected.

Clean the tank, but only use a tank liner if you absolutely, absolutely have to. It's mess if the liner goes bad.