Why does my 1978 Honda cx500 backfire I had the carbs redone new plugs etc the engine boggs done backfires spits and sputters?

Answer 1

First off, let's make sure that you have a true "backfire," which literally means that the fuel is igniting back through your carburetor, rather than popping in the muffler. Huge difference. One is harmful to the bike, the other one basically harmless.

CX500's ran lean coming straight out of the factory, and have a tendency to pop under deceleration for several reasons:

1) After 25 years the air cut off valve developed pin holes. Very common, and not usually covered by the standard CX500 carburetor rebuilt kit. I recommend Randakk's kit for CX series rebuilds.

2) Any exhaust leak can cause popping under deceleration.
Neither of these cause true "backfires."

3) Valves must be set to spec. If they are much more than .001 off, your engine will have a very distinctive tick, and at .002 off spec will not run properly. If they are severely off specification, you can damage the valves and piston heads.

4) If you had your carburetors off and failed to replace the two o-rings that lead from the intake manifolds to the head casing, you may have an air leak right at the head. This is not a happy place to have a leak.

5) You could also be flooding or starving the engine if you did not set the float levels properly.