View Full Version : 351 not turning the rpms it should
07-17-2006, 04:21 PM
"I am having problems with my '89 351 not turning more than about 3400 rpm's with max speed of around 35-37 mph. I have replaced the fuel filter, fuel pump, new plugs, plug wires, and thoroughly cleaned the air filter. The engine is running a lot smoother now, but I feel like we should be getting more rpm's. Is there something else simple that I should check out. The guy I bought the boat from said it should run about 45 mph."
07-18-2006, 03:24 PM
Sounds like you may need to try a different pitch prop. Would like to see 4500 or more at WOT. Is it trimmed correctly?
07-19-2006, 03:11 PM
"Sounds like good advice. It's probably overpropped if it sounds good at WOT now. That engine ought to turn in the neighborhood of 4000-4400 WOT (perhaps more depending on the engine and carb/EFI package.
Another possibility is that your secondaries might be frozen shut. Mine did this trick a few years ago and of course that will rob quite a bit from the engine and yet it will still run well.
WIth the engine off, opent he throttle all the way and manually open the secondaries. The linkage will prevent you from opening them unless the primaries are wide open. They are pulled open by vacuum, and any crud can easily gum them up and lock them closed. Plenty of carb-x will help if they are sticking. Or, if you can easily see them underway, just jam the throttle open and see if they are functioning properly. On my yot, I am unable to see them but I can hear them open and feel the motor surge a bit as they are pulled open."
07-20-2006, 11:42 AM
I think I have found the problem. It is a carb issue. My linkage to my choke was not connected properly and it was not letting my choke flaps open up as the engine warmed which in turn was causing the engine not to get enough air and turn the rpms necessary to open my secondaries. I am doing a minor rebuild on the carb to clean it and replace the power control valve to ensure that wasn't an issue also. I will let you know.
07-31-2006, 03:50 PM
"Fixed the choke problem and this helped dramatically, but the secondaries are still not opening. They move freely at wot. This particular carb secondary vacuum comes from a port on the primary. I cleaned the port thinking maybe it was clogged up and this did not help. I have also replaced the secondary diaphram. My top rpm is still around 3500 at 40 m.p.h. Any ideas on what else I should check?"
07-31-2006, 06:49 PM
Your vacuum diaphram might have a pinhole in it. Hot rod places (Jegs and Summit on-line) sell replacement parts for Holleys (I assume that's what you have)along with a trick adjustable vacuum secondary setup.
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