35 Big twin - high speed problem

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Richard L. Paquette
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35 Big twin - high speed problem

Postby Richard L. Paquette » Tue Aug 07, 2018 11:25 pm

I have what is basically a 1957/59/59 OMC 35 with a high speed problem. After building the motor out of spare parts, it ran fine for a year them mysteriously will only run at what seems to be about throttle saving speed.

I checked all the linkages and it is fully opening the throttle.

I replaced the plugs and now boots with new ones.

I took apart the carb but found no problems. I ended up swapping the carb for a known good carb that I opened up to be sure and repacked the needles, however still the same thing.

It feels like the high speed is just out of adjustment and if I could just find the right spot..., it is planing the boat but not developing full rpm and is running rough at all speeds.

I am pretty sure I have ruled out the carb as the problem.

Next I will pull the mag plate...

I have not run into this before and am stumped... it is putting out too much power to be dropping a cylinder...

XR55
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Re: 35 Big twin - high speed problem

Postby XR55 » Tue Aug 07, 2018 11:30 pm

Is the compression relief valve closing completely ?

Chris_P
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Re: 35 Big twin - high speed problem

Postby Chris_P » Wed Aug 08, 2018 6:19 am

There may be a certain mag setting where the plug wires are wore through in a spot and are grounding 1 cylinder?

FrankR
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Re: 35 Big twin - high speed problem

Postby FrankR » Wed Aug 08, 2018 7:59 am

Compression? Maybe it swallowed something?

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Mumbles
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Re: 35 Big twin - high speed problem

Postby Mumbles » Wed Aug 08, 2018 10:46 am

You'll need new or clean and polished points, NGK B8S plugs, good coils and new condensers to get good and reliable spark.

fleetwin
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Re: 35 Big twin - high speed problem

Postby fleetwin » Wed Aug 08, 2018 4:27 pm

I'm a little cornfused...Are you saying that the engine runs OK until you advance the throttle past the point where maximum advance is achieved, but the throttle is not completely open yet? Sounds like you have verified that the fuel saver linkage is set up OK. The engine ran OK before, so your collection of different pieces/parts is not an issue.
How does the engine misbehave when the throttle is open all the way? Misfire? Bogging?

outbdnut2
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Re: 35 Big twin - high speed problem

Postby outbdnut2 » Wed Aug 08, 2018 11:37 pm

Interesting - I'm just guessing here, but I'm wondering if you have a reed problem and it only flexes right at a particular speed ? or some other crankcase leak?
Dave

BillW
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Re: 35 Big twin - high speed problem

Postby BillW » Thu Aug 09, 2018 5:36 am

He did say this:

"It feels like the high speed is just out of adjustment and if I could just find the right spot..., it is planing the boat but not developing full rpm and is running rough at all speeds."

I'm wondering about a small fuel pump diaphragm leak...Might the problem get worse when you pump the primer bulb hard?

Richard L. Paquette
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Re: 35 Big twin - high speed problem

Postby Richard L. Paquette » Tue Aug 14, 2018 9:19 am

Reading the Seloc manual I note they seems to think compression tests are of paramount importance in diagnosing whatever. Well..., I borrowed one after going through the mag and swapping the carb again to only get the same, though worse results. Top was only 70 and lower didn't register will just a pull start, Had to use the starter and it didn't even get to to get to 30.

Lesson learned and a compression tested added to the Christmas list.

Will check the head gasket first...

Richard L. Paquette
VP Publications AOMCI
Posts: 87
Joined: Tue Mar 17, 2015 8:24 am
Location: Azilda Ontario
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Re: 35 Big twin - high speed problem

Postby Richard L. Paquette » Tue Aug 14, 2018 9:40 am

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