Petcock question

Hey everyone first poster here and i have a slight issue (i think?) So i know the pet cock is vacuum operated but when i suck on the line going to the ON positions nipple it forces gas out the PRIME nipple instead? Im just unsure of how this thing actually works.

Also i cannot get the bike to start at all either. Does the petcock allow the fuel to flow straight to the cylinder at first to get the bike starter or what?

 

Prime is used to refill the float bowls. You have one vacuum line and one fuel line. The vacuum allows the fuel to flow when on main tank or reserve.

The petcock allows the fuel to flow to the carburettors where is is mixed with air for combustion.

 

I suspect that the fuel evaporates in the bowls after sitting for a period and that is the reason for using the prime, you sure don't want to forget to turn it off after using to fill the bowls or you can over fill the crankcase will fuel easily.

 

Which shouldnt haplen if your floats/float needles are operating in spec

 

So, to summarize, if the petcock is pointing to On (or Res) and fuel ONLY comes out of the bigger nipple when you apply vacuum to the smaller nipple, that part is seemingly functioning correctly. I say seemingly b/c if it was ever rebuilt with the wrong o-ring or spring, it might seem to operate correctly, but the engine might not provide the same/enough vac. as you sucking on it. And edge case, but ask me how I know.

The other functionality to check is that turning it to Prime should let fuel flow (out of the same, larger nipple), vacuum or no.

All gas, vacuum-fed (On or Res) or 'primed,' flows through the same fuel line into the carb bowls, as other have said. Nothing goes directly into the cylinder unless your floats and needles aren't up to snuff.

So it's OK to test/start on Prime, but as others mentioned remember to turn back to On (or Res. as the case may be) once running or when you give up.

The only time I've ever needed Prime is if I've drained the bowls for storage or was careless and let the bike sit a really long time, in which case, the carbs likely need cleaning, too.

Before suspecting the petcock, I'd wonder what shape the carbs are in and have they been cleaned/serviced/known good at all recently.

 

Sometimes the Float Needles and Seats even in good condition will not seat correctly as it just happens which is a another reason to never leave it on Prime, it makes a mess of things when it happens and can dilute the engine oil and that isn't good and you may not even know it happen if its a small event.

 

Have you tried getting it to run on starting fluid?

 

Only need to do that if you have a issue ?