another prop problem please help

jmans72 · Apr 18, 2011

Hi guys i have a 1987 17ft sunbird with a 120 mercruiser i think its a 2.5liter gm. Anyway here is the problem we went out on it today and with 3 people at say 200lbs each the boat took forever to plain out we had to get on the bow to get it down.When it did plain i was seeing 27 to 30 mph and about 3500rpm it looks to be a 19 p prop (if im reading it properly)any help would be great.

Bondo · Apr 19, 2011

Re: another prop problem please help

Ayuh,... My guess is you have 2 problems...
The motor is down on power, 'n the prop is too big...

Start with a Compression test, 'n tune up on the motor...
If that don't bring back the power, 'n increase the rpms at wot...
Put a 17" or 15" prop on it...

jmans72 · Apr 19, 2011

Re: another prop problem please help

I will check but it just had a head gasket replaced last summer so it should have ok compression.Didnt think to check that.Should the rpms be higher?If so how high?

Silvertip · Apr 19, 2011

Re: another prop problem please help

Yes the rpms should be higher but the prop is not likely the problem -- especially if the boat performed ok previously. Props don't just suddenly stop performing unless it is damaged. Because you had a head gasket replaced is a sign that something else went on at that time. Whoever did the work may not have properly tuned the engine, it may be running on three cylinders, it may have water in the fuel, it may be running on bad gas. Diagnose, diagnose, diagnose!!!!!

jmans72 · Apr 19, 2011

Re: another prop problem please help

I just bought the boat so not sure about the prop but it runs good or seems to.Im going to check everything out today and see what the compression is and we will go from there thanks for the info.

steelespike · Apr 19, 2011

Re: another prop problem please help

While it might not be a rocket out of the hole your struggle is unusual.
It should plane fairly easily and run around 40 at about 4600 rpm.
Don't panic yet,it could just be the throttle not opening all the way or the timing set wrong
or not advancing all the way,a plug wire off.Let us know what you find.

stylesabu · Apr 19, 2011

Re: another prop problem please help

SO YOU DIDN'T OWN THE BOAT LAST SUMMER? SORRY about caps. has thwe boat ever performed better for you? believe what see not what someone says, old owner.

jmans72 · Apr 20, 2011

Re: another prop problem please help

No didnt own the boat last summer sorry for the confusion there.I went out and checked the compression i have 110 on all but the first one and its 65 im going to try adjusting the valves and see what that does.

steelespike · Apr 20, 2011

Re: another prop problem please help

If you squirt some oil in the cylinder it will tell you if its the rings.

jmans72 · Apr 22, 2011

Re: another prop problem please help

ok adjusted the valves didnt see a change.Did a second wet test still no change so i tried firing it up with the gauge on it and it shows the same as the other 3.

steelespike · Apr 22, 2011

Re: another prop problem please help

It's been a long time since I did a compression test but 110 seems a little low to me.
I've gotten 100 psi cold dry hand cranked on a number of 50+ year small old outboards.
5-30 hp. And my experience has been that these generally are lower than a typical 4 stroke.

another prop problem please help

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