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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Backfiring Problem: Triumph TR650


Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject. 

Triumph TR650 backfiring problem

Q: I’m having a backfiring problem with my old Triumph TR650 single carb. It had been sitting for a few years since I was using my other bike. I found that it had a blown head gasket that was leaking between the cylinders and also a stuck intake valve. I had the jug milled to fix a dip between the cylinders, which is what caused the head gasket to blow. I had the valves reseated, but I messed up the valve timing when I was making repairs. I followed the manual for matching the dots on the timing gears, adjusted the timing and added a new Mikuni carburetor. It now starts right up, but it backfires on acceleration. I tried checking the valve timing. It appeared to be off, so I made some adjustments by moving the gears. I also replaced the spark plug wires and checked the spark by cranking the engine through. The magneto seems to be putting out lots of spark. Any recommendations? — Mike White/New Jersey 

A: This seems like a good question to follow the previous one. Now that you’ve repaired all the obvious mechanical faults it may be time to muck about with the carburetor. Before I jump to that conclusion, I have to assume that by backfiring you mean spitting back through the carburetor. If it does that at a certain RPM in any gear you may have to check your valve timing again. When I got my Norton it would idle just fine and run wide open, but it stumbled horribly just off idle and would top out at 5,000rpm. At about 2,500-3,000rpm, it would reliably spit back through the carburetors. I initially thought this was a carburetion issue, and I tried richening the midrange mixture. Opening up the timing chest showed the error of that thinking, as the cam was advanced one tooth from where it should have been set. Once I got that set properly it ran the way Norton intended. Looking at the shop manual for your bike, I really only see one setting that changes between your TR6 and the rest. On the intake valve side, make sure you have the long timing mark on the intermediate gear aligned with the dot on the intake timing gear. The single dot on the crankshaft nests between two dots on the intermediate gear, and the single dot on the intermediate gear aligns with a single dot on the exhaust timing gear. Assuming the valve timing is correct, it’s probable that your midrange is lean. Find out what needle and needle jet you are using. The recommended setup for a single carb 650 Triumph is a 240 main jet, between 25 and 30 for the pilot jet, a P2 or P4 needle jet, 6DH2 needle and 1.0 air jet. Check and see if you have the recommended 3.0 slide. There are subtle differences between the single carb and the twin carbs in the earlier Triumph carb question. MC