Help deciding what direction to look into

[JTGS850GL]

OK, here's what has happened to my 1982 GS850 recently. All maintanance on the bike has been performed (valves adjusted, carbs cleaned, new plugs, all fluids changed and brake pads replaced) and the bike has run flawlessly for aprox. 450 miles. A couple of days ago I started to run into a strange problem. If I hold the throttle steady at around 5k rpms and in 4th or 5th gear I seem to loose a cylinder and I get a lot of flame and backfire out the exhaust (V&H 4 into 1). Carbs have been rejetted for the V&H and K&N stock filter. I put the bike on the center stand and run through the gears and the same happens. If I roll on the gas the engine revs clean. No idle issues. Smooth and steady at ~1300 RPMs. Usually requires little to no choke to start. I also have a lot of rapid backfire on closed throttle deceleraton but the bike has done that since purchased.

Edit: I've purchsed a new set of NGK plug wire caps and will be picking up a new coil set just to have around.

[tom203]

I'd check my petcock for correct operation- maybe it's leaking down vac hose, fouling #2 plug at times and dumping raw gas into exhaust.

[Dogma]

Interesting theory Tom. Old petcock is always suspect anyway.

I was thinking intermittent spark. The '82 850 has advance built into the ignitor box, and so can fail at particular rev ranges, if I'm not mistaken. A battery weakened by a failing charging system can exhibit similar symptoms, I think.

[JTGS850GL]

Checked the plugs and all were clean and slightly greyish in color. A little leaner then I would have expected, but not completely white. No signs of fouling for sure.

[JTGS850GL]

I remember when doing the valve adjustments that I couldn't rotate two of the shim buckets (two inner) on the exhaust side. I sure hope that this isn't related. I do have a spare head just in case though. Has anyone heard of an issue that could be related to the buckets not turning?

[chef1366]

I remember when doing the valve adjustments that I couldn't rotate two of the shim buckets (two inner) on the exhaust side. I sure hope that this isn't related. I do have a spare head just in case though. Has anyone heard of an issue that could be related to the buckets not turning?

check compression.

[DLBass]

Check the coils. One sounds as if it's starting to break up

[JTGS850GL]

Already purchased replacement coils since one of the coils has a melted wire. Also purchased a new set of NGK plug caps. Today I started it up and found that #3 cylinder wasn't getting hot. I opened up the throttle and reved it up a few times and then the cylinder got hot. I'll be checking compression when I get back to from a trip.

Going to be attending the Petit Lemans at Road Atlanta all next week so I'm having to get things together for that trip. I must be a gluten for old motor vehicles since I have a 1993 Airstream Land Yacht motorhome that I'm packing up. I also have several early 70's Datsun 240Z's and a couple of 1990 300zx Twin Turbos.

[JTGS850GL]

Looks like the coils fixed the problem. Found two of the coil wires were melted half way through. Also found that some PO must have tried to replace the plug wires since these were not OEM and were glued in with white and black RTV silicone glue. All four wires would come right out of the coil with little to no force. All 4 caps were badly cracked as well. Anyone heard of replacing the wires on these coils?

Received the "new" used coils and replaced all four caps with new NGK caps from Z1. The caps that were on the coils looked good but went ahead and replaced them anyway. Seems to have fixed the issue with the number 3 cylinder cutting out.

I'm slowly accumulating another bike piece by piece. Picked up a head in great condition from an 83 GS850 that had just under 13,000 miles on it. Only cost me $24. All 8 shim buckets turn freely as they should and no carbon buildup. Debating on weather to keep it whole or part it out along with a complete set of BS32SS carbs.

Anyway, at least the bike is back on the road.

[tom203]

Good news- thanks for update. Apparently we should inspect everything for signs of PO's unhandywork. Yes, some folks on here have "dug out" and replace plug wires on some coils- but as you have found out, it needs to be done properly. Good luck and enjoy your fall riding.