OMC 4.3 Low Rev's

[shussh]

Hello-<br />This is my first posting, about my first powerboat. I purchased the boat over the winter. The boat is a 1988 21' Sunbird. The Motor is OMC 4.3 with Cobra Drive. It currently has a 15 x 17 Prop. It has new Plugs, wires, coil. Fired up on the first crank and took it out on the lake. Ran nice and smooth. Idles fine.<br />My problem : When I opened the throttle up, my maximum RPM's were only around 3500 RPM. I checked my book and it should be up around 4400. The boat also came with a 14 1/2" x 19 prop. That prop would decrease my RPM's even further ( I think). Any help would be appreciated.<br /><br />Thank you,<br /><br />Ron<br />VT

 

[tmclendon]

Re: OMC 4.3 Low Rev's

It sounds like you aren't making the HP you should be. Some thoughts:<br /><br />Is the boat full of water in the bilge, or otherwise heavily loaded? That would do it.<br /><br />I would assume since it came with 17" and 19" props that these at one time ran with in the correct RPM band.<br /><br />Make sure the plug wires are all good and tight (both ends), and no plugs got cracked. <br /><br />Make sure your tach is accurate. I ran a V-6 that came (from the factory!) with the tach set to 8-cyl, so overrevved it for years. Check agains a good diagnostic tach if you can.<br /><br />How fast are you running at WOT?<br /><br />Check the compression on all 8...could be a dead cylinder, or worse.<br /><br />Carb OK? Vacuum leaks?<br /><br />Good luck!<br />Tom

 

[shussh]

Re: OMC 4.3 Low Rev's

Hi--<br /><br />I am running about 30 MPH at WOT with 2 people aboard. No water in the bilge.<br /><br />Thx<br />Ron

 

[Lark40]

Re: OMC 4.3 Low Rev's

I would expect that a 2-barrel Cobra V-6 could hit 40mph on a 21' boat.<br /><br />I can just hit 40 with mine.<br /><br />30 seems right with 3500 rpm. Well, that leaves spark, timing, compression, and gas to check out.<br /><br />Could be several causes, but I would check to see that the carb was opening fully.<br /><br />I assume that the hull is clear of any marine growth. That would also slow you down.