No 2 plug is black

[dougv42]

Hi all
Re: 1987 GS850
Recently sorted out my fuel tank with a new diaphragm so that the tap works properly instead of flowing regardless of whether it was on Prime, Main or Reserve. But having done that the bikes started pinging big time.
Pulled all the plugs in turn and it would seem that no2 carb is running rich as the plug was black (the other three were tan). Now heres the challenge. It was working just fine before the vacuum was sorted on the petcock but since being done it's playing up.

Question: How/why does sorting out the vacuum cause the carb to suddenly run rich?

In light of the fact that the vacuum is taken from the second carb there's clearly a connection but I don't understand and would like both advise as to how to fix it and more importantly what is going on to cause the problem in the first place.

Any advise/wisdom eagerly accepted

Doug

[Suzuki_Don]

You need to replace the petcock. The petcocks are unrepairable in my experience. The kits you buy to repair them with don't work very well.The petcock is allowing excess fuel to run to number 2 cyl. via the vacuum hose. If you take it fora run on prime the symptom of richness on number 2 cyl. will go away I am sure.

[BassCliff]

Greetings and Salutations!!

Hi Mr. dougv42,

Do you really have a 1987 GS850? They stopped importing them to the States in 1983 but Europe continued to receive them for a few years after that.

I concur with the bad petcock advice. There is probably fuel running back down the vacuum line and fouling the #2 plug.

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BassCliff

[dougv42]

Hi BassCliff

Yep, I really have an '87 850. I've upped a couple of snaps. She's the third one I've had. Got to say I prefered my first back in 1984, it was a '79 GN model.

Thanks for your advice by the way... oh and I live in England hence the '87 plate as you'd suggested.

[dougv42]

It would seem that turning the cock to Prime only worked temporarily as the noise has come back as has the blacked plug.
Any ideas?

[BassCliff]

Hi,

Setting the petcock to the "PRI" (Prime) position allows the fuel to flow without the vacuum from the #2 carb. It was meant as a test. Your petcock is bad and must be replaced. Some have had success with a petcock rebuild kit, but many have not.


Thank you for your indulgence,

BassCliff

[DrImodium]

Check for rust in your tank. If rust particles are blocking the diaphragm from closing, then fuel is flowing at will. Also, check your petcock vac tube to see if it smells like fuel. Black plugs are a sign of it being too rich. If its your #2 plug that's fouling, then I would say that your problem is your petcock, allowing fuel to flow down the vac tube and flood your carb.

Replace your petcock and you should be good. Just remember, if your replacement petcock is NOT vacuum actuated, you NEED to plug your vac port on the carb rail.