How does the Petcock operate at WOT?

cf_z · Post by **cf_z** » Wed Sep 03, 2008 4:19 pm

So last night i was wondering about my moped problems, and i went back to the fuel system. I was thinking, the petcock takes vacuum to open the valve and let fuel flow.
And i also know that at Wide open Throttle there is little or almost no vacuum. So wont the petcock shut off and stop fuel flow?

That would be why the bike can go for a while at WOT then slow down. It just goes until all the fuel from the bowl is gone then it stops. Once at idle it brings all the fuel back in and it can do it all over again.

So i got a really long vacuum line and hooked it to the petcock, and ran it to my mouth (...

). At WOT when i keep suction on it the bike runs fine, when i let it vent to the air it would do the same thing as usual.

So is my petcock bad? or is this just how they work?

Bear45-70 · Post by **Bear45-70** » Wed Sep 03, 2008 4:21 pm

Because there is always some vacuum, even at WOT or the engine wouldn't run.

cf_z · Post by **cf_z** » Wed Sep 03, 2008 4:26 pm

unless its boosted

but i mean, is it enough to open up the petcock?

Bear45-70 · Post by **Bear45-70** » Wed Sep 03, 2008 4:29 pm

cf_z wrote:unless its boosted

but i mean, is it enough to open up the petcock?

If the engine stays running at WOT, it must be.

cf_z · Post by **cf_z** » Wed Sep 03, 2008 4:33 pm

... but mine does not.

so im just gonna assume that my petcocks bad?

is this the way they usually go bad?... or?

Ziraya · Post by **Ziraya** » Wed Sep 03, 2008 4:38 pm

I've heard the 'usual' way is that when left alone they glue shut, but mine went bad in a way that it never closed all the way, so my carb would leak when the engine was off for a while, like over night. new one fixed that completely, and they're not that expensive (40$ for mine) so if you think it's broken you should just replace it, and do your tubing while your at it, 77 cents a foot for me (plus a 6$ special handeling charge (screw you too motorcity powersports))

Bear45-70 · Post by **Bear45-70** » Wed Sep 03, 2008 4:39 pm

cf_z wrote:... but mine does not.

so im just gonna assume that my petcocks bad?

is this the way they usually go bad?... or?

Are you sure the vacuum hose is good and fits tight? A vacuum leak would cause this same issue. You could try cleaning before buying a valve. You could go borrow a vacuum pump at the auto parts store or buy one at Harbor Freight and apply vacuum and check it that way. I'm pretty spoiled and have all those specialty tools.

Clivester · Post by **Clivester** » Wed Sep 03, 2008 6:04 pm

I would replace your vacuum tubing and make sure the tank filter is clear before replacing the petcock. Its unusual for one to not open properly - generally they seem to either stick closed (often after storage) or stay open (crud hung up on the valve or damaged diaphragm).

Clive.

Bear45-70 · Post by **Bear45-70** » Wed Sep 03, 2008 6:36 pm

Clivester wrote:I would replace your vacuum tubing and make sure the tank filter is clear before replacing the petcock. Its unusual for one to not open properly - generally they seem to either stick closed (often after storage) or stay open (crud hung up on the valve or damaged diaphragm).

Clive.

With his symptom of dying at WOT but running at lower speeds I agree, sounds like a bad vacuum hose.

cf_z · Post by **cf_z** » Wed Sep 03, 2008 10:20 pm

i will replace the hose tomorrow. But for some reason i think i put a new hose on there when i built it up.

thanks for the help though guys

cf_z · Post by **cf_z** » Wed Sep 03, 2008 10:21 pm

Clivester wrote:Its unusual for one to not open properly - generally they seem to stay open (crud hung up on the valve or damaged diaphragm).

Clive.

Also, What would be the harm if it always stayed open? Seems like if the floats were working right it wouldnt mess anything up

Dac · Post by **Dac** » Wed Sep 03, 2008 10:41 pm

cf_z wrote:
Clivester wrote:Its unusual for one to not open properly - generally they seem to stay open (crud hung up on the valve or damaged diaphragm).

Clive.
Also, What would be the harm if it always stayed open? Seems like if the floats were working right it wouldn't mess anything up

i dont know, but it seems i have seen people do this before.

i have a straight gas line on my old school moped i have have had no problems with that.

Dac · Post by **Dac** » Wed Sep 03, 2008 10:45 pm

im almost thinking that honda might have put that on there so if you crash the gas wont keep flowing out.

if you lay your bike down the floot would floop over and just let the gas flow and flow and flow and flow. if the petcock is on it, if the bike lays down, the motor dies, gas stop flowing.

cf_z · Post by **cf_z** » Wed Sep 03, 2008 10:56 pm

thats what i was thinking also. of that if the fuel lines accidently became disconnected that the gas just wouldnt flow out.

it shouldnt effect gas mileage so i think i'll just go ahead and do that if the new vac line doesnt work.

adark248 · Post by **adark248** » Thu Sep 04, 2008 1:11 pm

I'm not exactly sure, but my petcock was stuck slightly open until I just replaced it.

The old one let gas trickle all the time, which, even though it was a little amount coming out, added up and gas would drain out to overflow on the carb.

It wasn't a huge deal because I just unplugged the line to the carb and plugged the end with a screwdriver tip so it wouldn't do that. I had to hook it up every time I wanted to ride.

It did seem to affect my performance though. Once I got the new petcock it was much easier to tune the idle/air and get things working. It also seems to have solved a bogging issue that popped up every once in a while (I assume because the petcock still worked some times, other times it would get stuck closed for a while) So in my experience the faulty petcock did seem to have a negative impact on performance.