Petcock or Vac Leak?

[Talonrcn]

Morning!!

This is my first post to this site after hours of perusing BassCliff's site worth of tons of good info. After cleaning/dipping carbs, boot rings on the way, plugs new, oil etc. etc., i have noticed that the #3 cylinder isnt firing off, 1,2, and 4 are. #3 plug is wet and clean. I pulled the vac line from the petcock off the #3 carb, and the cylinder fires off and all is well.
With the line back on, #3 goes cold again. Floats are ok and within spec, no overflowing at any of the carbs. I did notice a drop or two of fuel at the vac line from the petcock, but the#3 cylinder wasn't running rich, it just wasnt firing off, Spark present, plug ok. Does pulling the vac line from the #3 carb lean out the mixture and allow combustion? I would think if it was a typical petcock issue i would have overflow or hard starting on all cylinders? Any ideas would be greatly appreciated.

EDIT: Petcock has only On and RESERVE, no prime.

[MisterCinders]

Surprised that you have spark on that No. 3 plug, if it's cold and plug is wet. Is the spark weak?

You should also check the timing to see if you are getting a mix of weak/late spark that allows 2 to fire but 3 not to.

[Talonrcn]

Timing is ok, just found it strange to have the cylinder fire of when the vac line is pulled, but not when it is put back on. I still have several other maintenance issues to complete after bringing it home from the PO, but wanted to get the #3 cylinder firing b4 i moved on.

[BassCliff]

Hi Mr. Talonrcn,

It kind of sounds like you have a bad petcock. It's not good to have fuel in the vacuum line. That means the petcock is leaking and can drown the spark plug, leak gasoline through the cylinder into the crankcase, and dilute the oil. I know petcocks are not cheap, but you really do want to have one that works correctly.

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[tom203]

Recently my petcock failed; It leaked down vacuum line but only when bike ran, fouling plug and behaving badly at low rpms. I have a prime position so I was able to block the line off and keep going . In your case, you should opt for a new petcock, as the rebuild kits don't get rave reviews.