Motorcycle Repair: Honda 71 scrambler 350 points, honda cb350, rotor stator

Question
Hey. So I've been working on this bike for quite some time, and I can't seem to fix it's backfiring problem. I was told it was because of the points, and have used a gauge to readjust them several times, and also cleaned them off with the special soft sand paper. The bike still seems to backfire. I also thought it might have been the carbs, but I clean them both, and the bike runs pretty strong, just won't stop backfiring. Any ideas? thanks!

-Dan

Answer
Dan, setting the ignition timing requires several steps.... I recommend removing the point plate, removing the spark advancer and checking it for proper advance/retard function. Lube it up and reinstall it all.

Turn the motor over until the points are at the widest gap possible. Measure that and adjust to about .016" on both sides.

Then turn the motor over until the right side points just OPEN and see if that corresponds with the F mark on the rotor/stator marks. If not, move the points plate around until you get proper firing. Then check the left side, so those points just open at the LF mark alignment. Move the point backing plate for that side points, NOT the whole outside backing plate. Once you have the points opening at F and LF marks, you are done with the timing.

Backfiring can come from many causes, usually due to exhaust leaks, modified mufflers with no backpressure, carburetors with holes in the diaphragms, air leaks at the manifold to head and the actual intake manifolds themselves, when the crack in the recesses where the carb spigot rests.

If you watch the points while the engine is running, you should see only a random little spark at the contacts. If you see HEAVY, CONSTANT arcing, then the condenser for that set of points is bad or the wire connectors have worked loose.

When you clean the carbs, be sure to remove the emulsion tubes up behind the primary main jets.

http://www.cmsnl.com/honda-cb350-super-sport-350-k0-us_model430/partslist/E++19....  part with no number next to #18.

Also, check your valve clearances and compression readings. Tight or leaky valves will never be cured by carburetor adjusting. Spark plug caps can also fail... should have 5k ohms resistance.

Bill Silver