Why is my engine backfiring?

Question for Paul McBride, the General Aviation News engines expert: I have an engine that will backfire between idle and 1200 rpm. It runs great after that with plenty of power.

The bottom #4 cylinder plug is always wet and full of crap. I put a new plug in and same thing. I changed the #4 intake gasket, removed the mag and checked, and a new set of wires were put on.

Any ideas?

Erik Sharp

Paul’s Answer: Erik, I’ll give this one my best guess, so I hope I’m in the right ballpark.

First, I am assuming that you have an IO-320 series engines vs just a plain O-320 series engine.

From the information you provided, my guess is that you have a lean mixture issue.

The normal manifold pressure reading on your gauge should be approximately 10 to 11 inches of manifold pressure at a normal engine operating RPM of 650 to 700 rpm.

I’d like to know what happens when you lean the mixture while coming back to the idle cutoff position on the quadrant at this power setting. Typically, you should see about a 25 to 50 rise in RPM just before you reach the idle cutoff position.

If that is not happening, then it points to a lean mixture setting.

This may be overcome by adjusting the star wheel on the fuel injector body towards the rich direction (there is an arrow on the block that holds the star wheel).

I’d make the first adjustment by using two clicks on the wheel and run the engine and try the same test again.

It’s a good idea to run the engine RPM up a little to clear out the engine before trying the lean-out procedure again.

If this is of no help and the manifold pressure remains up near 13 or 14 inches, then I’d suspect there is an induction leak somewhere.

Since you’ve already replaced the intake gasket on the #4 cylinder, I’d closely inspect the remaining three cylinders.

Also, be sure to check where the intake pipe goes into the oil sump, making sure the rubber hoses are in good condition and the clamps are tight.

Another thing to check in that area is making sure the metal intake pipe where it’s swaged into the sump is secure.