90 hp 2 stroke 1998 Merc Poor Performance

[LynP]

I bought a 1989 Invader 172 (17') Center Console Bay Boat listed at 750 lbs. less motor. It had a 1998 90ELPTO s/n 0G677xxx. It had not been run for 8 years. I check compressor after two days of Sea Foaming cylinders, all cylinders were at 130 PSI. Clean tank (no water). Removed and installed carb kit, carbs were very clean. Installed new plugs. Started engine, would only run using primer bulb. Rebuilt fuel pump. Engine very, very rough idle. Took it to a nearby lake at 3000 ft. elevation. With the motor, battery, 25 gallons fuel, and my fat body I estimate total weight of 2000 lbs. The motor had a rebuild 14 x 15 pitch prop. The engine at WOT 4700 - 4800 RPM, GPS speed was at a very disappointing 22 - 23 MPH. I have ordered 13-3/4 x 13 pitch. I do use the boat up to 7500 feet elevation. So ordering that prop is not a loss, if engine is not producing full HP. The only thing I have not tested is the reeds. Can reeds cause engine not reaching full power, plus cause idle issue?.

 

[racerone]

Yes , it could be a broken reed valve or one stuck open.

 

[Dave1027]

Have you done a sync and link? What max timing spec is set?
4800 RPM and barely on plane? I would definitely try another prop. I don't trust "rebuilt" props. I always had prop problems till I went SS.

 

[LynP]

Have you done a sync and link? What max timing spec is set?
4800 RPM and barely on plane? I would definitely try another prop. I don't trust "rebuilt" props. I always had prop problems till I went SS.

I do not know what sync and link is. Timing was checked by marine shop, they did not why it was so bad of idle. The boat at 4000 RPM was on a plane at 16 MPH.

 

[Dave1027]

With a 15 pitch prop and 4800 rpm I would expect at least 30 mph. Could the prop hub be slipping?

 

[LynP]

Yes , it could be a broken reed valve or one stuck open.

I may be a little paranoid now. However I ordered a set of Boyesen reeds. they are about double the price as OEM. I am buried in this boat now, so what is another fifty bucks. If I can gain a little HP at 7500 feet elevation it will be worth it. As of now I am sure it will not even plane at that elevation.

 

[LynP]

With a 15 pitch prop and 4800 rpm I would expect at least 30 mph. Could the prop hub be slipping?

How do I tell if it is slipping?

 

[Dave1027]

How do I tell if it is slipping?

Take a center punch and a hammer and put a punch mark on the hub and on the prop so they line up. Then you can take the boat out and run it as you usually would. If afterward the punch marks no longer line up that confirms slippage.
The other possibility is the prop is flattening out. Aluminum props are inherently more likely to flatten out. I could see this happening especially on a repaired prop that has hairline cracks. That's another reason why I like SS props.

 

[LynP]

Thanks guys, I will give a report after reeds and prop have been replaced.
Lyn Perry