WOT down from 4500 to 3900 speed down also

A

adamfsunrunner

Cadet
Joined
Jun 5, 2006
Messages
17
Hi,<br /><br />I just had my engine serviced because the coil was bad. Before the service my WOT rpms were about 3800 and top speed was down from around 40 to 30. The coil was replaced and now the engine runs great. No more stalling, hard running etc. BUT..RPM's and WOT speed has not changed??? What else could it be? Fuel is good, trim is working.<br /><br />Marina mechanic says maybe the control box or throttle cables getting old and not letting me put enough throttle down?? Does that sound right. Any easy fixes? I don't want to replace all that stuff this season. Anyone know what this could be. This is a drop from one season to the next.<br />My motor is a Merc 5.7L V8 with Thunderbolt ignition. It is 260hp. The boat is a 22 foot Sunrunner <br /><br />Thanks!!
 
P

paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: WOT down from 4500 to 3900 speed down also

Well, first check that you are getting full throttle simply by depressing the control all the way and than go check the throttle shaft on the carb to make sure it all the way open (remove the flame arrestor and look inside the carb, the throttle plates should be fully vertical). Have a good look at the prop and make sure it isn't bent. Make sure the bottom is clean. Lastly you may need to do a compression test, just because it seems to run ok doesn’t mean its not missing a few hp since last year. Also any chance the boat is carrying around a lot of water in the bottom, wouldn’t be the first time that’s happened!! Good luck.
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: WOT down from 4500 to 3900 speed down also

There are a LOT of reasons for low WOT rpms.<br />Here is a list.<br />Also, it would be handy to know what year yours is. Merc has been using the 5.7 for almost 30 years, and a lot of things have changed.<br /><br />EDIT:<br />Oops, forgot the chart.<br /><br />
Low%20Engine%20WOT%20rpm.png
 
KaGee

KaGee

Admiral
Joined
Aug 14, 2004
Messages
7,069
Re: WOT down from 4500 to 3900 speed down also

Adam,<br /><br />Welcome to Iboats. Both Don and Paulie have given you a list of things to begin to investigate. <br /><br />There is no quick, simple answer cure for your ailment. Time to start checking then post the results back and they can offer a more direct opinion.
 
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