Right Size Prop ? ( Low RPM ) ( Newbie )

[happy_flyer]

Hi...new to boating, new to the forum. I inherited a 1981 20ft Sunrunner. It has an OMC engine / sterndrive, it's V6 so I think it's the 3.8L engine ? The prop is 15x17. When the boat has just me and my wife, and a full tank of gas, I get 3200rpm, maybe 3300 max according to the boat tach, but it's sluggish to get planing. With 4 people on board I can't get above 2200 and it wont plane. Is it possible that I have the wrong prop ?

Thanks!!

 

[BAYLINER185]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

HMMM

I cant imagine that a 17" pitch is too much prop?

Is this boat a pontoon? or a V hull?

I would think if its V hull the 17" Pitch would rev the snot out of the motor so Im not sure where the issue is. If its a pontoon I may not have a good answer as I dotn have experiance with them but the rules apply.....Too little RPM usually means the motor is lugging and the pitch is too much. To High RPM means the prop doesnt have enough pitch and is slipping.

 

[steelespike]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

Would be good to do a compression, check and be sure it is tuned correctly
double check plug heatrange ,be sure the throttle is opening all the way.

 

[happy_flyer]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

It's a V hull...I was hoping for a prop answer not a doomed engine

 

[happy_flyer]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

It's a V hull...I was hoping for a prop answer not a doomed engine When I open the throttle with a load it seems like it is dumping fuel as it smells like gas and runs rough. I think the throttle is opening - out of gear it revs freely ...

 

[steelespike]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

Reving in neutral doesn't mean anything.A compression check followed by a good tune up if indicated.Compression check doesn't indicate doom it is just a tool to eliminate causes for poor performance.If you can handle tools a compression check is simple.Any number of things could cause a gassy smell.Could be as simple as a missfire.

 

[happy_flyer]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

Compression check will be done tomorrow - I am thinking/hoping it might not be the rings - there's no smoke at all. I'll do the check, pull the filters, clean the carb etc....thanks for the help so far !

 

[stevens]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

... and probably good to check the tach while you're at it!

 

[happy_flyer]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

Well now I feel like a complete idiot... :redface: ... just went to take a real close look at the motor...noticed that for some reason there was a plug with no lead attached...so I've been running on 5 cylinders ! Anyway, took the boat out, it's a pretty rough day on our lake today, but was able to get 4200 out of it, 35mph ( gps ), and it was out of the hole in no time at all. I'll still get the compression checked tomorrow, as I think it should be capable of around 4600. Not bad though, it is a 1981 after all. I'm happy now !

And thanks for everyone's input anyway !

 

[steelespike]

Re: Right Size Prop ? ( Low RPM ) ( Newbie )

Way to go.I guess that was the skip we suspected. Compression check--
If you have a weak cylinder squirt some oil in and test again.If it goes up its rings if not its valves.