500 thunderbolt low rpms under load.

[grew153owner]

PLEASE HELP. I bought this boat last fall with a thunderbolt (67) 500. When I started it this year it was hard to start and had no power. I took it to a private marine mechanic who rebuilt both carbs, fuel pump, replaced plugs, and cleaned the fuel filter. It starts easily now and idles great. The problem is under load it only goes 5-7 mph. If you try to go to WOT slowly nothing happens, it does't bog down, it just keeps chugging along at 5-7 mph. If you try to go to WOT quickly it stalls. Compression is 130 in 3 cylinders, 125 in the fourth. The mechanic took it to the water 4 times and adjusted the carbs (idle and fuel mixture) but the result is the same. Does anyone have a suggestion of what else it could be? The mechanic has given up trying to fix it and I don't want to get another motor if this one can be fixed.

 

[coolguy147]

Re: 500 thunderbolt low rpms under load.

maybe needs to be synced? check spark might be running on one less cylinder