165 Mercruiser power loss

[im4bernstein]

I recently purchased a 1977 Four Winns with the GM inline 6 blocked 165 mercruiser. When I take the boat out it runs almost wide open for about 25 minutes and then falls on its face, so I hurry up put it in neutral and let it idle for a minute or so then it will only run about 10 minutes wide open until finally all it will do is idle. Anybody who could possibly suggest anything, I'm ready to try.

 

[DJ]

Re: 165 Mercruiser power loss

Check these items.<br /><br />1. Make sure the fuel tank is venting. You should have an external hull vent for the fuel tank. You can try running it with the fuel cap loosened to see if that makes a difference.<br /><br />2. Your coil could be breaking down-overheat. The coil should not have 12 volts running to it in the "run" key position. It should be about 9.5V.<br /><br />If you have 12 V, you need a ballast resistor in the coil lead wire. They are available at most auto parts stores. Automotive is OK for this part.

 

[im4bernstein]

Re: 165 Mercruiser power loss

Thanks for the tip, I'll test the coil as soon as the weather breaks. I've already replaced everything in the fuel department and since you mentioned the venting I almost certain it's venting because when I fuel it up I can hear a kinda sucking noise out the vent your talking about. I've tried almost everything to try and make this thing work, I even tore the motor down and replaced all the gaskets and rebuilt the carb. I changed all the parts in the dist. the only thing I didn't do was replace the coil or check it like you said so I'll cross my fngers and give it a shot.

 

[Blue Thunder]

Re: 165 Mercruiser power loss

What is the water temp getting to after a long hard run? Extend WOT runs are very hard on the valve train and cause excessive heat. If the valve springs are getting weak, and you are running at WOT a lot, the valves do not get enough "seat time" to cool. Try not running at WOT or try a different prop to get the WOT rpm down. You didn't say what your WOT rpm was. If this is the problem, valve train service is in order or a lower cruise rpm.<br /><br />BT

 

[im4bernstein]

Re: 165 Mercruiser power loss

I've watched the water temp. before and I know it wasn't getting to hot. I'm running a 21-pitch prop right now at approximatly 4400 rpm at WOT although it will go to about 5000. All try all of your tips and hopefully when I get it back out I can make something happen. Thanks.

 

[Doug Durako]

Re: 165 Mercruiser power loss

I replaced the coil twice in 10 years of running my former boat with that motor.<br /><br />I thought it was only supposed to turn 4200 rpm at max?

 

[DJ]

Re: 165 Mercruiser power loss

I'm with ddboater. That engine does NOT like anything over 4300.<br /><br />You may want to check the accuracy of your tach.<br /><br />If it's correct, you need to go up in pitch on the prop. About 3-4" at least.<br /><br />The engine has loads of torque so it can handle the higher pitch prop.

 

[magster65]

Re: 165 Mercruiser power loss

I agree, tank vent likely or the coil maybe.

 

[daveswaves]

Re: 165 Mercruiser power loss

I had the same thing happen to my inline 6 2 years ago, turned out to be water in the fuel bowl (and sediment) I rebuilt the carb and it has been fine since. I agree, you are reving too high if you can hit 5000. Spec for this engine is 3900 to 4200 RPM at WOT. Either prop up or throttle back