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Problem solved, I think: Petcock/tank malfunction

tas850g

tas850g

Forum Mentor
Its been atleast 3-4 weeks trying to figure out what the heck was causing my 1979 GS850G not to even idle, at best. After going through the carbs/cam adjustments it ran worse than before. I didn't think it was supposed to do this but Murphy's law seems to catch up to me everytime.

After installing my second set of fuel o-rings and seting my air/fuel screws back to what i counted pre-cleaning (air 1 - 1 1/2 turn and fuel 1 1/2 turn, i know the specs and heard everyone's advise but wanted to go to the extreme) I fired it up this morning and to no avail it did the same thing as post cleaning. No idle except with full choke and after approx. 2 mins it stalls out and can not get it going again. Whenever i give it easy throttle it just boggs out along with full choke.

The fix, i think, i switch the petcock to prime and leave it there and almost instantly after i fire it up it stays idling!!! I had to shut off the choke and turn down the idle screw from 5k to about 2500 just to prove/show me that it will maintain that idle. And sure enough it does, YEAH!!!

After 10mins of idling i switch the petcock back to the On position and it stays idling for 2-3 mins but throttle response isn't quiet the same and it eventually stalls again. Switch back to Pri. and voila nice throttle response and idles again.

The interesting thing i did not do anything to the tank/petcock during the cleaning so i just assumed, I know what kind of trouble that will get you in, this should not be the problem.

All this aggravation for such a superficial/possibly easy fix. I have always believed in this saying but majorly overlooked it, it's the small things that make the difference.

Now i can move on to the next task to get it back on the road.
 
There is a piece under the barrel valve in the petcock that pushes on the diaphragm that allows fuel to flow. yours has probably shortened itself over the years due to a loop of spring built into that push piece. It probably needs to be lengthened a tiny bit to allow fuel to flow better. You have to be careful not to go too far or fuel will flow all the time even with no vacuum present.
 
tas850g said:
The fix, i think, i switch the petcock to prime and leave it there...
Click to expand...

Just a cautionary note, DON'T LEAVE THE PETRCOCK ON PRIME when the bike's not running.

Fuel will continue to gravity feed into the carbs & may leak past the needle valve and find its way into your crankcase. This is a BAD thing. remove your oil fill cap & sniff... if it smells like fuel, DO NOT run the bike - drain the oil & re-fill with fresh oil.

As others here point out, petcock replacement is pretty much a given after 25-30 yrs of use. Buy yourself an OEM petcock & use it for another 30 yrs - don't waste your money on a rebuild kit, most here report lousy results with those.

Good luck,

mike
 
I was thinking about and was going to buy a new petcock at some point. I want to take the old one apart to see how it works so i can get a better understanding of it. Thanks for the help.
 
If it's like my stock unit what you'll find under the lever is a small diameter spring maybe 3/4" long. Then comes the barrel valve with the pushrod/half loop next. That pushes on the diaphragm. On the other side of the diaphragm is another spring that returns the diaphram to a closed position when no vacuum is present. Just do it over something where you don't lose parts.
 
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