Thats what popped into my mind too, while you have the valve covers off fire motor up and check for a rocker arm/valve that's not moving as much as the rest. If motor is old enough it could have been built with a flat tappet hyd before roller cams became all the rage and with the lack of zinc/zddp in past few years in the same brands oil everyone has always bought the possibility of a cam lobe wiping out is a common occurence!!! Smitty

Is it the 8560 distributor? If so probably the magnetic pickup or the redactor wheel. Their distributors are crap, figure on a year or two at most. Just went through the same thing. Tossed it and put a whole new one in and it runs perfect again. I swapped the distributor out from my other side and the problem went away.

On a timing light you will probably see the skipping around.

Fixx
 

At least ck the fuel pressure before you replace anything,,if the carb is lacking fuel weather its FI of a carb it will pop through the carb,,usually when the msd dist goes they go and the engine usually wont fire,,some will and some just dont..

here is a similar problem which turned out to be a bad connection on the injector which in return with the injector not pulsing not enough fuel made the engine back fire under load..

reads this..http://www.offshoreonly.com/forums/g...ting-back.html

I ran all new 1/2 fuel line from both takes and removed fuel selector (pumps where sucking valves closed) two new Holley Blue fuel pump with two regulator one to each bowl with a cross over line in case one fuel pumps fails. Fuel pressure is set at 7psi on both lines. I was think of removing the electric fuel pump and replacing them with a Holley 130+ mechanical fuel pump just to keep it simple would this pump be ok?

What Dennis said. I had virtually the same symptoms. Engine would idle just fine, got up on plane with no problem and would cruise all day at 3500 RPM. As soon as I hit it hard and the secondaries started to open, it would start backfiring thru the carb. Started off thinking it was a fuel pump or carb issue, started switching things out, and then turned my attention to ignition. Decided to replace plugs. They all looked fine, but decided to do compression check while they were out. Number 6 was 80 PSI off from the rest. Pulled head, and had a hole in an intake valve - hence no compression. What caused that, you ask? Why, the big chunk of piston ring land that broke off and got into it! Number 5 on the other bank had the same problem. Detonation puts another engine six feet under. Hope it didn't get yours too, but take a shortcut and start with that compression test!

+1 seems like valve train instability.

Fixx
 

where are the fuel pumps mounted? and d oyou have fuel pressure gauges on the dash to monitor the fuel pressure? or are the gauges on the carb? fuel pressure will sometime drop when under way..did you use boost referance regulators?

Dennis with the timming that low would the exhaust temps be high?

Ok I just got around to doing a compression test all the cyl where 121 but one and it was 126.. I am guessing that it's low for a 508 so it looks like it need rings!! Would low compression be making it pop?

I think you posted a wrong number??????

If one cylinder is low, it is probably a valve not opening all the way or not closing tightly.
aka: valve train or cam lobe