Motorcycle Classics

Backfiring Problem: Triumph TR650

Reader Contribution by Keith Fellenstein

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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

  • Published on Apr 15, 2013
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