Petcock?

[Dogma]

Model airplane tanks are pressurized by the exhaust to limit/prevent frothing. Apparently, the vibration of the engine can turn the fuel into suds. But that's... what? methanol, nitromethane, and castor oil? Maybe it's the oil that would froth up. In that case, frothing shouldn't be a problem on a 4-stroke street bike.

I know my carbs aren't supported like that, and it's not a problem. I can't remember seeing any other like that either.

[80GS1000]

As robertbarr mentioned, a properly-operating stock petcock will empty a 5-gallon tank in about 2 minutes. From personal experience, I can tell you that semi trucks get over 5 miles per gallon, so it would take over 25 miles to go through 5 gallons. For most trucks, this would take about half an hour. You trying to tell us that your bike can burn gas at a rate exceeding 2.5 gallons per minute? You either have a severe jetting problem or are going incredibly fast.

Oh, and all this time I (and everybody else around here) have been operating on the (evidently mis-informed) idea that the purpose of the PRIME position was to bypass the need for vacuum for fuel flow. I have always understood that by placing the petcock in the PRIME position, you could fill the float bowls first, then hit the button to start the bike.

Just to clarify, it depends on what type of fuel tap you have. If it's the vacuum operated prime type I mentioned, fuel won't flow without cranking the motor. This type in my experience will not keep up with a modified motor and/or intake/exhaust modifications at freeway speeds. Check the last sentence from the service manual in this image:




If you have this type of petcock stock , consider yourself lucky as you can prime your carbs without cranking the motor and flattening the battery.

[Dave8338]

Model airplane tanks are pressurized by the exhaust to limit/prevent frothing. Apparently, the vibration of the engine can turn the fuel into suds. But that's... what? methanol, nitromethane, and castor oil? Maybe it's the oil that would froth up. In that case, frothing shouldn't be a problem on a 4-stroke street bike.

I know my carbs aren't supported like that, and it's not a problem. I can't remember seeing any other like that either.

And all this time I thought they were pressureized in order to run inverted flight?

HMMMMMM...a GS on nitromethane, methanol, and maybe a squirt or two of OH that do sound like fun for at least a 1000 feet!

[jmanz6]

The fuel in model airplanes, boats, and cars can and does "froth", the pressurisation is to keep the fuel flowing since the tanks are not normally mounted above the carb. In my boats the tank was mounted even with the carb or even below it and the pressure was to keep the fuel going to the carb as long as the engine was running. For planes it's to keep the fuel flowing when climbing, diving, inverted or rolling. The tank has positive pressure so the only way out is thru the fuel line going to the carb.

[Dogma]

Whatever I was reading (instructions that came with the tank, I think) talked about the frothing. I don't remember any mention of helping with fuel feed. It makes sense though. This was like 1989.

20 years ago! Almost.

[Spyder]

I read about frothing (foaming) here somewhere. The vibrations the carbs get without the airbox holding the up, can cause frothing, resulting in poor jetting.
As i said my old tap drained the tank VERY slow. So, I pulled the hose on the new one today, and gas is SHOOTING out. I believe this to be my whole problems.
Damn! ! ! Now I have to set my carbs up... AGAIN! ! !

[Spyder]

Main problem gone

New tap improved performance. I bought this bike with K&N pods, and a 4 into 1 D&D pipe. I'm pretty confident it wasn't jetted, or at least not properly. I'm still not getting full potential from it.

I was told to go with Dynojet stage 3 kit. Anyone disagree? Should I go bigger?

Thanx for all of your advice. This site ROX...

Spyder...