Just had two new motors built from scratch. 540ci 650hp
One motor perfect but having problems with the other. At 4200rpms power loss to 3000 then back to 4000 then motor quits??? It happens every time we reach around 4200 rpms?? any suggestions will be appreciated. Everything new except stock distributors out of my old 540 merc bull dogs. But they worked great in them. Don't this seem like something electrical?? Getting plenty of fuel. Help please

what kind of ignitions ? .... swap "ONE" part at a time and see if the problem moves to the other motor .....m

Not sure but its the stock ignition that came with the Mercury bulldogs. We changed out all we could with no results. Man this is driving us crazy.

Just thought i would post what the problem was maybe help someone else. The sea water pump line on the drive was not on all the way and was sucking air into the system. Which in turn would set off the knock sensor at high speed. Man what a relief and pain in the butt to find.

Glad you found it....and glad you posted the results of your search. I've got the first of (2) 540/650's at the dyno/programmers right now awaiting the first run on the machine.

The water hose was loose? The motor was getting hot then? Just not following the connection. It's always those simple things that cause the most problems.

At least you found it before the hose came off all together. That would make for a different thread.

Good deal!

ThunderBolt 4/5, bad dist. sensor.

Yea the hose clamp wasn't on far enough and when they tightened it, it actually closed the center of the hose off allowing the pump to suck air around the end. At low speed enough water to cool. at high speed to much air passed by the knock sensor shutting down the engine. Now to get the hose back on will be the tricky part. Also i think we need new props. We have 28s but i'm thinking 30s