this may sound stupid but did you change the fuel filter,it does not take much water to impede the fuel flow and pressure.

mine has one, it also houses the 650 dollar fuel pump.......

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I just ran this thing on the trailer and it runs up to 4800 smooth as silk. Hits the rev limiter at 4900. Runs fine not under a load. I'm going to put a fuel pressure gauge on it and run it in the water to see exactly what I'm getting under load at WOT, also taking some spray to squirt in the intake to see if RPM's go up to try and narrow it down to fuel pump problem.

how did you get enough cooling water to the motor on the trailer? Curious because my engines pump more than the hose will give at anything over 1000 rpms.

Hose wide open with deep water pickup! Temp runs around 170. Plus just bounced the rev limiter, didn't hold it at a high RPM very long

My 350 mag did that when timing was off, under load it cough and sputtered?? On hose ran fine.

Youu said you had 80% and 60% leakdown on 2 cylinders and stopped after that......DING!!!! DING!!!!

No fuel issue is going to cause that. The engine needs to come apart.

Griff I agree about the rebuild!! I'm going to do my own leak down test to confirm results from service. If this is the case then I think that I'm just running out of HP because of loosing power from the week cylinders. 57 mph at 3800 with the equivalent of a 5.5L in a 4200 lb boat running a 23P prop isn't to bad. She should come up and be happy with all cylinders working again

If that motor doesn't run like a complete piece of $hit at all rpm then those results are wrong.

Yours is the older LX with the Lingenfelter intake. The L29's like his will not have that.