Fuel petcock - Yes, the dreaded topic again

I HATE to post yet another question about petcocks but after reading numerous (and I DO mean numerous) searched thread topics about the care and feeding of the XJ fuel petcock I've yet to find another post similar to my problem.

Some time over the winter my XJ700 decided it didn't want to idle smoothly or run well with the petcock in the "on" position. It runs like a champ in the "prime" position though but I don't think it's smart to operate the bike in that manner.

Am I a candidate for a petcock rebuild kit or is this really a carb issue?

 

While I am far from an expert on anything pertaining to motorcycles, from what I have read during the past few days I think that I may be able to at least give you an idea of where to start.

My understanding is that in the ON and RESERVE positions, the petcock is vacuum operated; that is, it only permits fuel to flow when there is a vacuum from the engine present to pull fuel through it. The PRIME position, on the other hand, simply lets fuel flow wherever and whenever it wants to; whether the engine wants/needs it or not. This latter fact resulted in fuel in my oil when I transported my "new" XJ700 in a trailer with the petcock in the PRIME position.

Given that your bike runs fine in the PRIME position, it would seem, and I'm certain someone will correct me if I'm wrong, that there is one of two possibilities:

1) There is no vacuum present to pull fuel through the petcock.

2) The petcock is, for some reason, not responding to the vacuum that should be drawing fuel.

Again, I must plead ignorance in how to resolve either of these conditions, assuming that one of them is in fact the problem. It might give you something to investigate though until someone who actually knows something about these bikes can give you a more informed opinion.

Good luck!

 

Rasputin's reasoning seems sound enough to me.

 

Rasputin,

Thanks for the response. I read several posts by you and others who responded to a previous inquiry about petcocks. I'm semi-intelligent and walk upright, hell, I even made it through Grad school but damned if I can figure out this little bugger!

 

that's funny Spankster, my 81' 650 has been doing that same thing since I have had it(2 yrs)
I've been really lazy aboot changing it: but now that it's 1) leaked fuel into my chamber twice, and 2) leaked fuel around the petcock switch, I guess it's time to fork out the HUGE $23.00 to rebuild it.

Good luck amigo!
(me too)

Rich

 

Check the diaphraghm in the petcock for a hole and also when you remove the petcock to do this check the tube that goes up in the tank for crub that could block it.