Need help: popping in exhaust

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

With reference to my 4.5 liter.

It has recently begun popping in the exhaust when I let off the accelerator. Makes an awful noise, and driving power does seem reduced. Starts and idles easily.

Mixture does not appear rich, and no changes have been made to it. Twin sloper SU carbs - no flooding apparent. Fuel system clean, fresh fuel.

Ignition timing correct. Runs on each or both magnetos. New spark plugs.

Oil pressure seems low at idle (10), but on driving is 20-30 when warm, 40-50 when cold.

I cannot appreciate an obvious exhaust leak. Compression on all cylinders is 90 psi.

Any advice appreciated.

John Murch · Posted: Sat Jul 27, 2019 8:28 pm Post subject:

As your compression figures are even, it seems unlikely to be a leaky exhaust valve, could you have tracking in a magneto cap causing an unwanted spark in a cylinder which has an open exhaust valve?

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Thanks John - I thought about that. No evidence of shorting in the caps, and I cleaned them out and went over the contacts with a light sandpaper.

Furthermore, as it occurs with each mag independently (turning the other off), I doubt it's a mag issue.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

OK, problem identified.

Exhaust downpipe completely fractured just below the flange joint where it attached to the exhaust manifold. I'll weld it up this week hopefully.