Let us know about any modifications to the bike. Aftermarket exhaust? filter? pods? jetting? ignition? etc..
If everything is stock....
Look on eBay for a
new petcock, new diaphragm, or petcock rebuild kit for your 850, get that fixed and running without using the prime position.
If the problem persists....
It sounds like you've got a little bit of work to do. Here's where to start:
1. Buy or review a Clymers maintenance manual for your bike.
2. Review or print-out the
Carb Cleanup Series instructions.
3. Get a set of o-rings from Robert Barr at
www.cycleorings.com. Robert is a GSR member and a lot of us get o-rings from him.
4. Tear down your carbs completely and clean them using Berryman Chem Dip and by following the Carb Cleanup Series. Make sure
not to dip any rubber parts, gaskets, or diaphragms!
5. Inspect your slides/diaphragms for proper assembly by referencing the parts breakdown
HERE. I have a feeling that the one slide with the longer needle is assembled incorrectly of has something broken.
6. Re-assemble everything, set the pilot air screws to 1.5 turns out from seated, and perform a bench sync by following the instructions
HERE.
7. Install the carbs, clean the air filter, replace plugs with the correct heat rage plugs, charge the battery, check grounds, ect..
8. Get the bike to idle at ~1100 RPM and adjust each pilot air screw in or out for maximum RPM, one at a time, and set the idle back down to 1100 RPM between adjusting each carb using the idle adjuster knob.
9. Vacuum sync the carbs!
10. Check plug readings and let us know.
You might not have to/want to go this in depth, but I suggest you do. If you find the problem right away and fix it, then that's it. This is just a rough plan of action as you may have other issues as well. Ask questions and research the GSR using the search feature. Good luck!
-dean