Low rpm at max throttle [Archive] - Moyer Marine Atomic 4 Community

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Unregistered

11-16-2005, 06:20 AM

I have a Atomic 4 motor in my Iwrin 34 (1974) sailboat.

The problem is when giving the motor gas/trottle in neutural it revs up fine. But once the transmission is put into gear (forward or reverse) and then given gas/trottle it only goes up to 1500 rpm no matter how far you adjust the trottle handle. You can 'floor' it and it stays only at 1500 rpm. I'm guessing the problem may be in the transmission? I've recently had the carb rebuilt, all plugs checked, and the cable for the trottle replaced. Any thoughts what could cause this loss of power?

Don Moyer

11-16-2005, 06:21 AM

It's very unlikely that the transmission is causing your problem.

Did the problem just start, or has the engine always been this way?

Many boat builders installed props of a size that didn't allow for RPM over 1800 or 1900. Once you reach the maximum RPM that the engine can produce, the throttle will have no effect as you keep moving it forward. If this is your case, you may have a bit of barnacle or other growth on your prop. It doesn't take much growth on the prop to reduce engine RPM from 1800 to 1500.

If the prop isn't the problem, was there any engine maintenance performed between the time the engine ran OK and now?

If something really did happen to the engine to cause the problem, there should be other symptoms like poor compression in one or more cylinders (below 80 psi), smoke out of the exhaust or from the oil fill tube over the flywheel housing.

Don

Paul_NY

12-12-2005, 09:20 AM

Have you recently installed a water/fuel separator? I had the same low rpm problem last spring after installing a water/fuel separator. Idled fine but lacked power in gear.

Turned out a large air bubble at the top of the separator filter was sufficient to prevent full fuel flow when I throttled up. I have a manual fuel pump which isn't strong enough to pull enough fuel into the separator to top it off.

Don suggested I install a fuel bulb between the tank and the separator. A few pumps on the bulb pushed the air bubble out of the line. I have had no rpm problems since.

Paul