Just installed a low hour used Paul Phaff racing engine in a Force Offshore Cat. It is a carbed engine with an MSD 6AL and soft touch rev limiter. The distributor is a mercury optical type, I guess a thunderbolt style. First time at the dock it ran for 5 minutes at idle and then stalled, weak or no spark at all. Problem was a bad coil. Replaced it the next morning with an MSD blaster coil and started right back up.

Got it back on the lake yesturday and everything seems pretty good for awhile. Engine is a little rough, not as smooth as I would like. I am thinking gas and carb gunk from siting for a year prior to install at that point. I start running her up to full speed and notice my engine cuts out just for a micro second every so often. I look at my tack and it is for sure dropping out, then right back to RPM's. Slow down and seems to go away.

I take her back up to speed for another long run (5 minutes at 4500 RPM) and same thing, but this time she drops out and stalls at high speed. Turn her over and nothing. Pull the coil wire and plenty of spark. Check for gas in the carb throat and have plenty. We sit for a few minutes in the lake thinking about it and decide to pull a spark plug wire and check for spark there. She arcs and starts to fire right away. Plug her back in and off we go. We get about 2 miles at 3500 RPM and same damn thing, she just drops out. We go through the same steps and nothing. We sit for a bit now thinking it is a heat issue. When I try to fire her up again after a bit, nothing. I pull the same plug wire to check for spark and she fires back up and off we go. Coincidence....maybe. But I am not sure exactly what I am seeing yet.

Obviously we have something overheating or breaking down. Any ideas where I should be looking?? Has anyone possibly seen this type of issue from a mercury distributor??

Sorry for the long post, but I wanted to give as much detail as possible for technical troublshooting.

Jim C

I don't like to mix and match. Merc w/ msd. I bet the msd box is going bad. In nascar they have 2 boxes in every car they go bad so often. I'd consider going all merc, or all msd. Personally I don't like msd.

I would go thru the carb anyway, she might run even better. although it sounds like an electrical prob.

I hear ya on the mix match...but I have alot of respect on Paul Phaff builds and tend to lean on their experience on performance combos. I will troubleshoot some more and look a little closer at the thunderbolt distributor. I searched some threads and found some past issues with the electronics in them causing my type problems.

As far as the carb goes...you are probably right as well. I plan to rebuild her after I get the other bugs worked out. But I am 99.9 percent sure gas is not causing the problem I am troubleshooting right now. I really think ignition is where I need to concentrate.

If I'm reading your post correctly, you never lost spark after the coil went bad the first day? Is there a fuel pressure gauge? mech or elec fuel pump? I would want to know what fuel pressure is running at at speed when it cuts out. pump could be on the way out?

It is an electric fuel pump with pres reg. I do not have the fuel pres guage hooked up yet because the engine had an electric sending unit and the guage is mechanical. Was waiting for another project day to straighten out the guage issues.

You are correct, I never lost spark out of the coil from what I can troubleshoot after the stall....but I am not so sure about loosing spark from the distributor to the plugs. I am leaning towards a distributor electronics issue. But I am always open to listen and learn. I will get some fuel pressure readings hooked up and see what happens.

Since this is a recent install you might try checking all the engine grounds and MSD wiring for good connection.
I had a similar problem on an engine recently. Would run fine until about 4500 rpm then would start surging/missing and quit. After lots of troubleshooting found a loose connection on the main positive wire on the MSD. Tightened that wire and bingo, problem solved.

Fred