petcock/fuel filter question

[shadowfist]

finally figured out my problem (kind of..) i threw in new plugs and still couldn't get my bike started. after cranking on it for a while I pulled the plugs out and it looked as though they were completely untouched and absent was any sort of fuel smell. but i looked at the filter several times and there was definitely fuel there. anyway, long story short i either have
A: a problem with my petcock where it only sometimes gets fuel through when in the 'on" position.
B: A problem with my filter (possibly not a 'gravity fed' one)
or C: somehow my engine isn't 'sucking' enough on that vacuum line

how do i test these possibilities?
here is what i have done so far.
i pulled the tank off and when cranking the vacuum line sucks away on my finger so it is doing something at least, whether there is enough suck remains a question.
i grabbed a straw from the kitchen and stuck it on the vacuum port on the petcock and made little suck/blow actions on the straw, and gas came out the petcock (again, enough gas remains a question)
so how do i narrow this down? is it my petcock diaphragm? is this classic 'you have the wrong type of fuel filter smart-donkey'...?


bike runs great as long as i leave it in 'pri' while running (and yes i put it back to on while parked) but it seems to run fine for a while if i leave it to 'on' (i got about 3/4 to a full mile up the road before i remembered that i didnt put it back to 'pri' and it hadn't died yet. so whats the deal?

[GQROD]

A lot of fuel should flow when vacuum is applied with the straw.

If you don't see a lot of fuel it is blocked and you may have to replace or rebuild the petcock.

If you have an aftermarket fuel filter the concensus by other members is to remove it since the petcock already has a filter and any additional filter causes restrictions to fuel flow.

Lastly vacuum hose is cheap, replace it to ensure there are no cracks leaking vacuum.

Hope this helps.

[Nessism]

If the bike runs well on PRI but not in the ON position, you must have a petcock problem.

[shadowfist]

so, i have seen some petcock diaphrams on ebay.Like This
should i rebuild it with a new diaphram or just replace the whole unit? and is there a good place to get petcocks these days? (like is there a 3rd party unit that works great?) I am a bit learry of getting a used one as mine SEEMS to work at times.

[hikermikem]

Buy a new OEM petcock, they cost around $50. The consensus here is the rebuild kits don't work (or work occasionally).

I have replaced all my bikes' petcocks with shiny new ones & haven't had any problems since. Replace the vacuum & fuel lines while you're at it.

Also get new bolt washers/seals - these are the "rubberized" flat washers used on the bolts that attach the petcock to the tank

For what it's worth, I use a paper lawn mower fuel filter on my bikes - yes the petcock has a screen, but it doesn't stop the very fine particulate like "rust dust". One of my tanks is extremely rusted on the inside & the carbs would be plugged if I didn't use a filter...

A new petcock will last you another 20 years, good investment,