Ok need to bounce some things off you guys.Had my Re-powered 1990 502/Bravo out this Saturday and my motor started popping and shut down. Cranked and trying to restart it was popping bad out the carb. Put it on the trailer got it home did some checking Sunday and found the it seem to be coming from the right bank only. Pulled the plugs did find that #4’s ground electrode was bent close to the center electrode. No evidents that any thing hit it. Did a compression check all is good. Will be doing a leak down tonight (tool at work). Pulled valve cover nothing bent or miss adjusted. I am thinking that the Ignition module took a dump and is advancing on it own. (Merc. Thunderbolt). Any other thoughs?

New cam in rebuild? Check for flat exhaust cam lobe/pushrod/rocker problem too. Wont necessarily show up as compression or leakdown problem but will have the symptoms you described.
Also check the spark plug wires are correct.
Gary

A bent ground electrode is a bad sign. Usually means something bouncing around the combustion chamber hit it. Either some thing went down the carburater or something broke off. If it's popping, I'd look for a piece of a valve. Do a leak down test.

Just reread your first post. A broken valve would show up on a compression test. Still seems valve related. Good luck.

Check the basics, is the distributor tight and is the timing correct? base and full.

Check cap, rotor and wires and firing order.

What plugs did you go with?

Could also be water in the fuel.

Note the spark plug is not crushed nor is there any thing showing a hit on the electrode or the edge or the plug. electrodes are not push together. almost like a droped plug before installed. I did not put them in so I an guessing this is what has happened. Cam is a rollor and I also pulled the rockers and check it checked. this also came on way to fast to be a cam. Running 100% great then slall then no restart/ popping. my thought is this. Since Merc dist has no advance and it is all in the module I am thing the module failed. is this symptom commen for a module?

Could be the module, but I don't think so. I would check the distributor cap and rotor, rotor alignment, distributor shaft and drive gear.

Dave

I had a module go bad and the engine would not start.

Don't forget to check the ground wire coming from the ingition module. They can break at the spade connector or up in the plug to the module. Give it a yank to make sure it is solid.

BT

We have a winner. Drive gear.

Motors is out a getting flushed as we speak.