86 mercruiser falls flat when underload

[Jopey]

Hello,

I have an 86 Mercruiser 5.7 260hp that will free rev just fine in the water, but when underload she will fall flat on its face. Wont get up on plan and wont rev pass 2500rpm. Just got the boat out for the first time this yr. Plan on doing a full tune up today. Just looking for input from all you guys,always have steered me in the right direction.

Also its a 23 nova xl

Thanks
joe

 

[6meter]

Re: 86 mercruiser falls flat when underload

Old gas? Carb crudded up inside? Did you run the carb out of gas last fall? See what happens after the tune up.

 

[Bondo]

Re: 86 mercruiser falls flat when underload

Plan on doing a full tune up today.

Ayuh,...
Throw in a compression test while yer at it to see what's goin' on..

 

[Fordiesel69]

Re: 86 mercruiser falls flat when underload

Try this test to save LOTS of time. Take a good gas can and a hose, connect direct to the fuel pump. If it still bogs, your fuel problem is the fuel pump or carb, in which you can then measure fuel pressure. If low = pump, if normal = carb. If the gas can fixes the problem, suspect a clogged pickup tube, or clogged vent.

 

[solar7647]

Re: 86 mercruiser falls flat when underload

It dosnt sound like a gas issue to me It sounds like you may not be getting spark on all your cylinders. I had the same issue once and it ended up being a wire arking off the block, I would say check plugs and wires first. If you got a could piston not getting spark it will run fine in free idea since the pistons getting spark will have the power to over come the pressure in those dead cylinders but once load is applied they won't have the power to put you on plane.