Bike was running great. Previous owner rebuilt carbs, new intake boots, and sealed airbox within 3,000 miles prior to my purchase. Upon acquiring the bike I immediately did valve adjustment, inspected the cam chain (ok), both oils changed, then paid a mechanic to adjust and sync carbs (he used a colortune and vacuum carb sync tool). I've put 1,000 miles on it since then and it felt great, fine tuned. Then last week things started going wrong.
I know there are divided opinions on this, but I've had good experiences running seafoam once a year on my 83 KZ750. It smoothed out the idle and sounded cleaner, so I figure I'll do the same on the 81' GS850L. I ran 4-6oz on a full tank of gas and on that tank during a round trip from Oakland to Sacramento, my petcock fails, badly. I limp home getting about 20mpg because it's dumping out the petcock as well as feeding back through the vacuum tube to the #2 carburetor. I assume coincidence because seafoam shouldn't destroy a petcock right? I'm still a novice mechanic so if anyone has information otherwise I'd be happy to be corrected.
I replace the petcock and spark plugs gaped within spec. Now I have a loss of power between 2500 and 4000rpms, and at speeds in 4th and 5th gear it comes and goes when the throttle control is consistent. I also have backfiring when decelerating.
I ran the bike at about 30mph at 3,000 rpms in fourth gear for a few blocks after warming up and then clutch and kill switch. Coast to a stop and pull the plugs. Links to the pictures below, but I'll save you the time cause they're all pretty much chalk white. I'm running lean, which I think is consistent with the midrange loss of power and backfiring.
So my ideas:
1. My petcock has been failing for a while, so when the mechanic adjusted the fuel and air it was compensating for a small leak? Once I replaced the petcock I would be running lean? Can I fix this with a turn of a screw? What would be the procedure for that?
2. I'm having some sort of blockage or other problem in the main jet, which would explain why my power loss is in the rpm band that it is. I really don't want to pull my carbs because I just had them balanced and I don't have the tool, nor possibly the skills, to put them back together and sync them. I'll do it if I have to though. I bought this to be my commuter bike for school between Oakland and Sacramento, and here on week two I'm taking the greyhound again!
3. I have an airleak somewhere in the system that showed itself after the seafoam and petcock events? I don't know if either of those would cause this but maybe?
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