Petcocks ... again !

[steve111]

I know the 80-83 vacuum petcock markings & function has been discussed before.

Q. Why do the carbs flood, gas leaks in the PRIme position... some have said "because the fuel is constantly flowing, vacuum or not. " I don't buy this, before I put my bike in two year storage the carbs never flooded/leaked no matter what position the valve was in, running or not !

To reiterate it has 3 positions: On, Reserve & Prime.

In the On and Reserve positions, the flow is stopped until the engine is running & (hose going to cylinder heads) creates vacuum . The vacuum opens a diaphragm & allows the fuel to flow.

Now, if I understand carbs correctly, fuel continues to flow into the bowl raising the float until it pushes a little needle/seat & stops; thus keeping the fuel in the carb bowl at a constant level (necessary for proper venturi & fuel distribution)

So.... I am wondering in the PRIme position, why are the carbs flooding all of a sudden ? If it were a bad needle/seat (or float) I would think the carbs would flood once the bike started running (yeah great a stream of napalm gas trailing from the bike).... in fact this is what I though was going on until I turned the valve from "PRIme to ON".... the thing ran fine, didn't flood/leak & didn't run out of gas when I cruised it !

I know it's a moot point since I am going to change to the earlier non vacuum petcock anyway (with simple, true On Off and Reserve).... but has me a bit confounded !

 

[5twins]

Your float needle seats may be a little varnished up from sitting which may be causing the needle to not seal 100%. There's a valve in the petcock that gets sucked open by the vacuum signal, but the vacuum signal "pulses" and that makes the valve do the same. It's not sucked fully open and staying that way while in operation. It's constantly bouncing open and closed. In the Prime position, you bypass that and get a full, constant flow. Your not quite 100% sealing float needle and seat probably can't handle that. Fuel flow is adequate in the vacuum operated positions, but not as strong as on Prime. You're probably able to burn off enough fuel riding so that the slower vacuum feed rate doesn't cause an over-fill condition or leaks.

 

[xjwmx]

Most of the pulsing of the vacuum is smoothed out by a jet that's in the petcock. Otherwise the diaphragm would wear out in about 5 min.

 

[steve111]

You're probably able to burn off enough fuel riding so that the slower vacuum feed rate doesn't cause an over-fill condition or leaks.

Not sure about this; our bikes sip fuel at a slow rate. In the PRIme position gas is pouring out. I can't imagine that when running it's using gas just enough & at exact rate to make up for the overflow ? Thanks for both replies.