What's causing NC24's backfire?

Exhausts

 22 March 2011 14:05

Q. I’ve got a 1987 Honda NC24 and I’m trying to decide if the problem is loose tappets or a holed exhaust manifold.

The symptoms are: backfiring on deceleration, however this has been severely reduced when I found a small crack where the collector box attaches to the end pipe. Now it will only backfire if I nail it down the motorway for a while, then suddenly close the throttle.

The audible clicking/tapping noise gets louder and a higher frequency as the engine revs up, and has got worse over time, starting as barely audible, now quite clear, and more noticeable when the bike is warm.
 
A.
If the valves haven’t been done for a while, they need checking as a matter of course, and then the carbs should be serviced. As for the backfires, any crack or hole, no matter how small, in an exhaust system will allow the system to draw air into the exhaust on engine overrun.

This air ingress allows any unburnt fuel in the exhaust system to reignite thus causing the backfire. It will also disrupt the back pressure. Exhaust back pressure is used to scavenge exhaust gases from the engine’s cylinders after combustion has taken place.

To identify the leaks you could smear some dishwashing solution/bubble bath over the exhaust joints when the engine is cold. Start up, blip throttle and see if you get any bubbles that will show you where any leaks are.

It's good enough to find punctures and should show up leaks in the exhaust. Just remember to rinse the dishwasher salt solution off thoroughly afterwards when it is cool enough.