Atomic 4 won't rev under forward load - SailNet Community

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  #1  
Old 06-29-2009
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Atomic 4 won't rev under forward load

This is our first season with our new-to-us 1979 Catalina 30.
The old Atomic 4 was running great Ė until this weekend! It started right up and went into forward gear ok, but when I want to accelerate out of the slip, the engine would bog down as if under a load. It would only rev to about 1,000 RPM while laboring heavily. The boat was moving slowly, but the engine just would not rev up to speed. It sounded like an old, overloaded dump truck laboring up a steep hill.

So I tried reverse, and it ran just fine! It would rev up and power the boat like it always did!

Iím pretty knowledgeable with gasoline engines, so I went through the fuel, spark, air thing and saw nothing obvious. Iím not very knowledgeable about boat drivelines. Could this be a problem with the transmission? Shaft bearings? Prop? I saw that the shaft was turning, and I didnít notice any excessive heat anywhere.

Any and all help is greatly appreciated!
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Old 06-29-2009
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Line wrapped around prop? Tightens around shaft one way, but not the other way. Also, check oil in transmission. I'm the proud owner of a transmission with a hole in the side for not following that advice.
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Prop Foul?

I'd couldn't tell by your profile, if you were in salt/brackish water, but regardless, I'd first check for growth on the prop (barnacles especially) or some fouling of some sort. My Yanmar behaves similarly when the barnacles get thick on my prop-same with dock neighbors A4. It just won't spin up to speed and lugs out.

Take a swim with a mask, gloves and a scraper and see what you find. It might be as simple as that.
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Old 06-29-2009
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Also make sure that your cable is running smoothly and that you are positively in gear. It make sound obvious, but the forward "detente" on an A4 needs a push to fully engage, and cable tweaking is often necessary, as is cable lubrication.

Of course, if you have crew, they can physically pull the gear lever with their hands (pick a crew with short hair!) pretty easily. If the problem persists, you need a transmission adjustment. If it doesn't, check the cable linkages.
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Fouled prop, seems to happen faster in the summer.
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A-4 won't rev

These guys are stating the obvious but likely your issue is not so obvious. Actually had the exact problem you are describing a few years ago. Engine started and went into gear. As soon as throttle would be applied, engine would stall.

The good news is that it was dirt and/or debris in the carburetor. The carb is pretty easy to remove with a couple of wrenches. Blow it out with some compressed air and some carb cleaner, reinstall and you should be good to go. These carburetors are pretty sensitive to dirt.

Good luck.

Ps- I always carried a spare rebuilt carburetor after that. Figured I could switch it out in a hurry if I had carb problems again.
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So if it is the carb then why does the motor run normal in reverse???

Quote:
Originally Posted by Sanduskysailor View Post
These guys are stating the obvious but likely your issue is not so obvious. Actually had the exact problem you are describing a few years ago. Engine started and went into gear. As soon as throttle would be applied, engine would stall.

The good news is that it was dirt and/or debris in the carburetor. The carb is pretty easy to remove with a couple of wrenches. Blow it out with some compressed air and some carb cleaner, reinstall and you should be good to go. These carburetors are pretty sensitive to dirt.

Good luck.

Ps- I always carried a spare rebuilt carburetor after that. Figured I could switch it out in a hurry if I had carb problems again.
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There's a 2:1 reducing gear on the A4 reverse, so 2,000 rpms in reverse is the same as 1,000 in forward, as far as prop rpms go. Sounds more like prop blockage of some kind to me.

Still, have you checked your compression yet? Timing?
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Check your fuel filter, lines and the carb for clogging and debris. It could be junk in the tank. My 30 year old A4 only stalls going into forward gear but not reverse when the fuel filter is shot. Change the filter media and it works like a charm.
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