Well when the rebuilt carb was put on, it immediately started and idled properly. At one point it wouldnt idle was backfiring, etc. I wasn't with the mechanics on the last sea trial or else I would have said "take it up to full throttle" and noticed it still wasn't right. I picked the boat up based on him saying it was good and when I took it out a couple days later I discovered it would t go above 30. Its really really strange, just seems to run good up to 3000, no strange noises or anything like that. In neutral, no issues revving way up. Shows no signs of a wiped cam love so really struggling to figure out what the hell is going on.

If you're running the thunderbolt iv trying switching out the ignition module. I had a v8-hp module go bad and it was similar in symptoms. Ran good up to 3000 rpm and then it would start missing. Changed it out and all is good now.

Yeah we had that problem last year and changed it out but I don't know where he got the part from because they are no longer made correct?

I actually bought a marine HEI one wire distributor I was planning to try scrapping the thunderbolt system all together and going with that however I'm not sure that I have clearance to remove the distributor with the engine in the boat

Results of my own compression test, 145-150 on cylinders 2-8
#1 cylinder at 120. So something is going on but with that being said, I would think it would still be running better, I've heard of people having cylinder down at 90 and getting better performance than this.

So what would make the #1 cylinder go south?

Next question for the masses about simply checking the timing on a carb 454 with thunderbolt 4 ignition. I have the V8 24 module. Just now I idle the boat put the timing light on it and the timing mark was jumping all over the place. I had read something somewhere that at idle it will do that if you haven't put it into base timing mode. But what I was trying to see is if the timing was advancing when I gave it some throttle so I throttled up to 2000 RPM I didn't want to go much higher because I was running it on the hose. At 2,000 RPM I was still seeing the timing mark jump all over the place. I also fiddled around a little bit with the advanced timing light but not quite sure I know how to use it properly all I know was no matter what the timing mark was jumping all over. Would this be indicative of a thunderbolt module that is no good?

This usually happens to me if the pick up clamp for light is right next to another plug wire.

Edit in: there is no special timing mode on the tbolt iv set up. Nothing is advancing anywhere near idle speed. Therefore timing should be steady and stay put at any idle speed.

Edited my last response

There is definitely base timing mode.

Explain so we can discuss and get on same page