[miko]

When my 403 warms up or I back off the gas, it pops. I can see the muffler vibrating with each pop. I was told to do a compression test to check for burnt valves. Each cylinder is about 120. One spark plug looks a little worse than the others but they all look about the same. What could it be? Thanks

[Stuckinda60s]

That's unburned fuel and a burned or sticking exhaust valve can cause it. Usually, though, it's caused by a lean mixture when you back off on the throttle. If it's happening at idle, try restricting the air through the carb by placing your hand over it and choking it. If the idle smooths out, you're lean. This might be caused be a manifold leak.

It can also be caused by a header leak drawing air in and allowing combustion in the exhaust.

By the way, you're more apt to have your question answered in the Street Section or the Helping Section.

[ronj1977]

change the wire and clean the plug on that side

[camerjeff]

X2 on most likely an ignition issue, Plug or Wires. But could also being Timing, rich Condition or lean condition. exaust manifold leak or coupling is also a possibility. But I would start with the ignition.

[ronj1977]

Bad plug will make that cylinder Rich. Bad plug will create a timing issue. FIX stArt the car in dark open hood look for arc none found look at each plug to see If it's wet with gas or oil. If you find wet plug chances are that is your problem .

[miko]

Thanks for the advice guys. It looks like it might actually be the muffler. When we hold a tube to the exhaust pipe it sounds ok. When we hold it to the muffler, it rattles dramatically.