RPM Drops In Forward

[mhinnc]

'72 C&C 30 MKI, Atomic 4.


The last time we were able to go out the RPMs were 'normal', we could get 3000 RPM in forward, easy. A couple of months went by before we could take the boat out again. No changes to motor/drive train during that time, boat sitting in slip.

Next time out, 800 or 900 RPM was best we could do in forward, so we turned around & went back to the slip. Asking around, the collective knowledge all pointed to crusted up shaft/prop. Less than two months ago it was cleaned. (Little Creek, Va.)

This is the first time back at the boat and am having the same issue, mostly. Now I can get a little more (1100 RPM) in forward sitting in the slip, no more. In reverse, easy 2500 RPM, in neutral, 3000+. Any ideas? Thanks.

[Chuck34]

Yes, your running gear getting crusted up can have a big effect, but no where near that much in that short amount of time…especially in the middle of the winter.

[Brioche]

What kind of engine? How about in neutral or reverse?

[Chuck34]

Originally Posted by Brioche

What kind of engine? How about in neutral or reverse?

30’ sailboat with an Atomic 4

[Jim Cate]

Originally Posted by Brioche

What kind of engine? How about in neutral or reverse?

Umm... the OP answered these questions in the first post. Did you not read it?

Jim

[Bill O]

Originally Posted by Chuck34

Yes, your running gear getting crusted up can have a big effect, but no where near that much in that short amount of time…especially in the middle of the winter.

Agree with the prop being encrusted as the cause. A little too cold now to jump in to check it, but if you have an UW camera you may be able to see it.

[Scubaseas]

Drysuit on a diver would be my first move. Sounds like a prop issue but IF the prop was cleaned 2 months shouldn't be that bad for growth. But your symptoms point to prop being fouled. Fixed prop, Feathering or folding?

[Jim Cate]

Possibly a plastic bag on the prop, or other such fouling not dependent upon biology.

Jim

[mhinnc]

I just found #1 cylinder has a loss of compression. I understand this could be the cause of RPM drop in forward.



The loss of compression has stirred up a memory (last time out with okay RPMs was October last) that there was a brief clicking, sticky valve noise prior to the engine coming up to temperature. This would have just preceded the loss of power going forward. Now to find out if there's a proper way to attempt to free up the valve without disassembling the parts to expose the valves.



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FYI - Although the hull was cleaned fairly recently there is already growth becoming visible here and there. I understand the divers who clean hulls were unusually busy this past year on the Chesapeake and continue to be so, especially here on the southern-most part.

[Bill O]

Did you do a compression test to determine the #1 was low?

What did that plug #1 look like when you pulled it? Do you have electronic or the old points in the distributor?


Pulling the head on an A4 wasn't that bad of a job, then re-seat your valves or replace if the valves are burnt or the stems are bent

[mhinnc]

Thumb test, as per Moyer. Trying to find a simple rubber tipped compression guage at the auto stores proved fruitless. Averse to paying $45 for the extra stuff that comes with the current guages.



#1 no resistance to thumb held over plug hole. #s 2 through 4, no way to hold thumb over plug hole. Will get a guage measurement soon.

Plug...sorted, not very wet, but did smear as if not really dry.

[Jim Cate]

Sounds like a stuck valve to me. Long ago I had an Anemic 4 which had an exhaust valve that liked to stick in the open position. I made a little tool out of a medium size allen key, shortening the short arm to the point where I could stick it in through the spark plug hole. Then, positioning the short end over the stuck valve, I'd tap the other end with a small hammer and the valve would release and the cylinder would resume normal function.

The cause was water getting into the exhaust manifold from a poorly designed cooling system and getting to the valve stem (the "downhill cylinder", so the small amount of water collected there) and causing some surface rust which seized up the valve.

It is possible that the same treatment would work for you.

Jim

PS I eventually rebuilt the cooling system and the problem went away... with no discernable performance decrement due to my shade tree mechanicing!

[Wotname]

^^ impressive workaround Jim

[mhinnc]

Jim, many thanks for the idea. I'll give it a try.