Comp TS6M Top Speed

[Supra_Comp]

Folks

Seem to be struggling with top speed on my 85 Comp. Engine runs smooth and gets on plain without issues, but seems to top out around 35 mph. It will run at that speed at 3/4 throttle without an issue but when I go WOT you can hear the motor want to spin up faster but it seems to just "stall" and never gets past 35.

Going to check timing tomorrow and check engine speed on the lake but, any thoughts on what else could cause this? If I were to describe it, it doesn't sputter or buck as ifnit had too much fuel but seems like it is gasping. Aka, sounds like the throttle blades go WOT but engine won't speed up any faster.

It has an original 13 x 12 prop that was recent refinished. Stock Holley carb w/ vacuum secondary.

Any thoughts?

[michael hunter]

I run a 13 x 13 Prop on my 86 comp 351 1to1 trans . It tops out in the low 40s.

[cadunkle]

What RPM while maxed out at 35 MPH? With a 13x12 in that small light boat I'd imagine you should be getting 4500+ RPM. I think PCM recommended max RPM is 4400. At 10-20% slip your max MPH at 4400 RPM should be 40-45 MPH. I'd guess slip should be well under 15% in a small light hull with a modern large surface area CNC prop... So you're probably not getting full RPM, lack of power.

If RPM is low, check carb operation (fully opening, fuel in both bowl correct, etc.), initial timing, timing advancing correctly, cap/rotor/plug condition (dead or weak cylinder?), etc. Basically all the tune up stuff. Could be a damaged diaphragm in the carb causing the secondaries to not open. Could be low compression or a generally worn out engine. Check the easy stuff first.

[Supra_Comp]

Great advice guys.

I definitely think it is lack of rpm, and probably a carb tuning issue. I think this issue was here before I rebuilt the engine (has probably less than 100 hrs on it and runs very smooth throughout the rpm ramge) and also dealt with some bad wiring of the ignition system.

The carb hasn't been looked at in over 10 years and I'm sure I screwed around with the metering block and mixture screws when I was having bad performance before the engine was rebuilt. It has symptoms of the secondaries not opening because you don't hear that deep tone that you typically would from other cars of boats I've been in whenyou go from partial to WOT...I'll try the paperclip test.

I did throw a vacuum gauge on the engine in tbe driveway. At idle it's pulling around 15-17" and based on research that seems to lead towards a healthy motor but potentially also a timing issue for base. Any thoughts?