Problem solved, I think: Petcock/tank malfunction

[tas850g]

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.

[Billy Ricks]

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]

Thanks for the information. I will take it apart and check it out.

[hikermikem]

The fix, i think, i switch the petcock to prime and leave it there...

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

[tas850g]

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.

[Billy Ricks]

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.

[tas850g]

i have never taken it apart so i'll see what i find within the next couple of days.