TOPIC:

This is going to sound dumb but when should each position on the petcock be used? I have never had a prime position on a bike before so I figured I'd ask.

Prime is for filling up the carb fuel bowls when they are empty (like after rebuilding/cleaning carbs), or maybe after it has set for awhile.
The rest of the time it stays in the on position, until you run low on fuel and then you switch to reserve.

It's a real bummer when you forget and run it on reserve until it runs out of gas, then you have to use the prime postion to fill up the carbs again (after pushing the bike to the gas station)

Yeah nothin like blasting down the highway passin cars reachin down to turn on the reserve and realizing you forgot to switch it back when you filled up. Then listening to those same cars honk as you push your bike down the highway.

Here's a stupid question about petcocks:

Did you name yours?

"did you name yours?"

yup, named mine "stupid P.O.S." and replaced it!

KingKustomz wrote:

Yeah nothin like blasting down the highway passin cars reachin down to turn on the reserve and realizing you forgot to switch it back when you filled up. Then listening to those same cars honk as you push your bike down the highway.

hahahaha so that was you last weekend??

Where would be a good place to get a rebuild kit for the petcocok on my '78? I'm guessing it's dirty because the bike won't run when on the on position, it acts like it's starving. I runs fine in the prime position.

Don't be too quick to blame (or replace) your petcock. Make sure the vacuum line from the carb is operating correctly first, that opens the valve.

Guitar geek, you're a sick puppy! (Mine's Little Willie)

Quickest cure for Petcock Problems is neutering...

It'll improve your dog's breath, but your cat will *never* look you in the eye again...

Kaw was infatuated with "vacumn" (negative pressure diaphram, actually) petcocks for a number of years and models.

The basic idea is that a vent line from the carb bank applies suction by venturi principle against spring tension to a diaphram in the petcock body, puckering it. When the motor's off, there's no pucker, the 'phram lays flat (vertically) against the fuel passage, cutting off fuel flow. Crank the motor, the carbs suck, the 'phram puckers, and gas flows.

If everything is honky-dory with the 'phram, you can pop your fuel lines with the cock in the "On" position and no gas will pi$$ out.

If it does, there's a problem, usually with the 'phram. Dry rot, etc. If your needles and seats in the carbs are overdue for a little affection, it'll let you know by leaving a puddle under the bike to get your attention. You might also notice fowling on the plug in the jug that the carb line is hooked to.

About a $30 OEM part fix at a dealer. A little rubber 'phram, a screw and a sping, as I recall.

The "Prime" position on the cock bypasses the 'phram and lets gas flow straight through the lines to the carbs.

ahhhhhhhh, i've been wondering what the PRI stood for:blink: so i have the two line off the petcock and i don't know what the smaller one on the bottoms for but when i pull the tank, gas pours out no matter what position it's in. guess i need to rebuild that, huh.

Be sure that after you do the petcock rebuild there is NO fuel leaking out of the smaller nipple on the petcock! Or anywhere for that matter. I recently rebuilt my petcock, and wanted to be sure there were no leakage problems so I filled the tank about half way, then put a section of hose on the two nipples (Vacuum, and Fuel), ran them both to separate cans on the floor, set the lever to the "ON" position, and let the bike sit over night. I checked the cans for fuel in the morning, and was OK. I just didn't want to take a chance on fuel leakage. I originally discovered the need to re-build the petcock when my number 2 plug kept fouling with lots of gas. It was a ruptured diaphragm in the Automatic Petcock that was allowing fuel to leak down into the vacuum tap on the carb. So, check that out before it causes some big troubles. Also, if you do find leakage there...make sure the gas leakage didn't find it's way past the piston in that cylinder and into your crankcase oil! If your oil level suddenly looks too high it's time to do an oil change. Better safe than sorry.