backfiring through the carburetor???
 

The motor is a 508 with a B&M 250 blower. My dad was cruising in the boat at 3800 rpms a little over 3 years ago and all the sudden it started popping through the carb so he put it in storage. I got the boat out a month ago and started working on it replace plug,wires,cap and rotor looked good,upgraded fuel lines and pumps and rebuilt the carb. I took the boat out to test it last week and still popping it will cruise at 2800rpms and it doesn't pop but if you try to go any fast it starts popping. If you try to hammer it starts popping bad and will die. So I took the boat home and pulled the valve cover and though for sure I would find a broke spring but I was wrong so I am lost... ANYONE??? :confused:

The first thing any professional mechanic will do is a compression test.
Dennis Moore

Try changing the coil...its cheap and easy to do. Could also be the pickup in the dist. :drink:

It has all MSD stuff the dizzy is was only a year old. Haven't changed the coil yet but have a new one I checked the old coil vs new coil all checked the same. The compression check is what i was going to do tonight. I was thinking maybe a head gasket between cyl's

What ignition are you using? Was the motor just installed or proven and the problem started suddenly? It sounds like a timing advance
problem to me. If you had low compression you would see it on
the plugs or crank case pressure. Give me all the particulars on the set up and I will try to help. Cant make any promises!

Flat cam lobe or valve train problem.

MSD pro billet dis,Blaster2 coil, Msd 6M-2 with soft touch timing is set at 28 total . Motor was run for about 2 years with no changes when it started popping. Pier 57 is the one that built the motor so not sure all the spec on the motor..

Crap fuel.

Fixx
 

Low fuel pressure,,,running out of fuel...Blower motors will pop like h e l l if they dont have enough fuel pressure..7 psi at idle and +7 on boost when running..dont keep running it like this,, it will only run lein and pull the valves out of the heads..

If the compression is fine I strongly suggest installing a reliable ignition and throwing the MSD (junk) in the trash.
The MSD ignition is too weak for an engine with a blower.
The single spark of the MSD CD ignition (after 3000 RPM the MSD is single spark) has a very short spark duration.
The short duration of the spark will result in fouled plugs and poor performance.
I suggest a Mercruiser Thunderbolt IV distributor (a standard non-CD ignition) with a 20 degree advance module. Set the initial timing at 8 degrees, giving you a total advance of 28 degrees.
This will give you a long spark duration from idle to 5500 rpm and increase your performance. If you open the spark plug gap to .045 inches you will increase the amount of voltage buildup at the coil and delivered to the spark plugs.

Dennis Moore