Vacuum gauge

[Tryden]

2004 2400SE with 5.0l having issues above 3000 rpm, backfiring through the carb and loss of power. Changed plugs, wires, cap, rotor, coil. Checked timing and set air fuel mixture, same problem. Bought a $20.00 vacuum gauge and watched a few YouTube videos, should have bought the vacuum gauge first. 10-13 bouncing rapidly, checked compression. #8 150 #6 0 #4 0 #2 150 #7 150 #5 150 #3 150 #1 150. Took both heads off and had them rebuilt (replaced one head as it had a crack by the plug), put it all back together and will take it out tomorrow, vacuum is a steady 17in.HG I have had The boat 2 years and max throttle was 4100 rpm at 36mph, after reading possible cause for tulip valve seems like over-prop is a possibility (14-19 pitch), I will check and see what rpm is know and if not 4400-4800 I will change the prop. Anyways that’s my boating story, been reading the post for the last 2 years and it’s a wealth of info. Troy

[mmwjr]

Welcome aboard!

I had a tuliped valve on my previous boat when I bought it, the problem was caused by water ingestion due to leaky exhaust. When you had the heads off how did 5he exhaust manifolds and rises look? Have they ever been replaced?

[jrsick]

A cracked head, and possibly head gasket leak between #4 and 6, could have been the cause of your vacuum fluctuation. Have you checked compression on 4 and 6 after the repairs?
You are doing right thing on the prop. 19 pitch is big for a 5.0 with 1.47 drive ratio in my experience.

[Tryden]

https://www.dropbox.com/s/apsic8oi7v...0head.jpg?dl=0

https://www.dropbox.com/s/8cm112hp7r...0head.jpg?dl=0

Couple pictures, will check the compression before I take it out. Risers and manifold looked to be Ok, don't want to tear the motor apart any time soon so I will do a few more checks. Thanks for the suggestions.