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RPM engine problems?

G

gen-st

New member
Hello all, have a question. In my 26 ft cruiser the i/o engine (Mercruiser) acting strange. Sometimes I can't reach speed of more than 10-14 MPH having RPM of not more than 3500 and it doesn't go any higher with gas trottle up to the max. In other occasions, I have no problems at all to have 33 MPH with 4500-4700 RPM.... Anybody have any idea where to look to fix that issue?
 
FIRST....Check that the jacket of the throttle cable is firmly anchored to the engine near the carb... Also that the jacket is firmly anchored at the throttle end. Then check ( engine off) that the throttle lever at the carb is all the way against the limit stop when the lever at the helm is "wide open".... and no slop or loose fastenings.
 
no positive results unfortunately. Put on neutral, put throttle full gas, engine easily gives 5500 rpm, but when I'm back in real drive, the very same story: no more than 3500 rpm = no more than 10-12 MPH...any suggestion is greatly appreciated, the summer is too short......
 
You need to determine is you have a fuel issue or an ignition issue or a slipping prop type issue.

Is this a EFI/MPI or carburated engine?

Any history before this occurred? Overheating? hitting a submerged object?

Ran out of fuel?
Water in the fuel?
 
Further timing check.... if mechanical advance, check that the weights aren't hanging up.

We could use some info on your engine so as to not waste time with irrelevant suggestions... Make/model/ S/N if possible. Carb vs which variant of FI...etc. Also did this engine ever run OK for you??? MAX WOT RPM when it did run right????
 
Thank you guys for advise. Two things was performed:
1. In addition to 100 of regular gasoline which I had in tank, I added 50 liters of '91 gas.
2. Engine compartment was not fully closed, the latches was worn out, just fixed them, so, engine compartment is fully sealed now.
Did test drive: she giving me speed 25-30 MPH, no problem.
Any idea, what solved the issue: 1 or 2?
 
Ayuh,... #1,... The symptoms are classic poor fuel or poor fuel flow,....

I'd pull the fuel filter, 'n inspect it's contents for anything but clean fresh gasoline, then throw on a new one,....
 
RPM engine problems due to the oil in the gas tank

OK, it was discovered that the reason my cruiser couldn't do more than 15 mph, having RPM of no more than 3000-3500 because there is oil in the gas tank which was gradually clogged gas pipes and gas filter.
Is anybody has any idea how this happens and how to resolve it?
Thank you!
 
Somebody refueled with diesel fuel?
Dont think so, gasoline is made from oil anyway. oil would be diluted by the gasoline too and oil does not simply clog things up.
 
microbes and algea can thrive in ethanol laced fuels, especially at the water to fuel boundary in a stagnant tank. Its a common, well known problem in diesel and not unknown in gasoline.

Also... RPM in neutral virtually meaningless. In a gasoline powered engine, the throttle controls the horsepower that an engine can produce, not RPM as in a diesel. Since it take a negligible amount of horsepower to overcome the frictional losses of a engine (in neutral), the engine is free to rev up until bad things happen...or a rev limiter, if equipped takes over, or the valves float in a hydraulic lifter engine.
 
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