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90HP Johnson varies RPMbs

K

K. Kasparek

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"It's a 1982 motor. Start

"It's a 1982 motor. Starts great, idles great, but when I give it the gas, at times it labors to get on plane, 3,000 RPMS, then "kicks in" up to 4900 RPMS. Runs fine for a minute or two then drops RPMS to 4000, then back to 4500 then up to 4900 back to 4300 up to 4500, etc. Have checked fuel filter and checked for water in fuel, both look fine. Am I loosing a cylinder...power pack? How do I test? Thanks in advance!"
 
"To check if You have an ignit

"To check if You have an ignition cut out:
Hook a timing light to each cylinder and check if the indication dissappears. Whatch out for the spark plug wires on this model, they may short circuit to the cowling locks.
However to me it sounds like a fuel problem. Try to hook a manometer (oil filled) into the fuel lines after the fuel pump."
 
"Thanks Morten. The motor nev

"Thanks Morten. The motor never does die, if that's what you mean by "ignition cut out". Good advice about the plug wires. And I'm not familiar with a manometer. Is this a do-it-yourself type test or for mechanics only? Thanks again."
 
"You would need a helper to re

"You would need a helper to read it off, but yes a simple and very usefull test for fault search of fuel system. A plastic T-connector and hook it in as close to your carburators as possible. If using an oil filled manometer you eliminate most of the pulsing from the pump and get a more even reading. There is another possibility to and that is bad gaskets at the air inntake cover and base. However with the age of your engine, the pump may be due to repair/change.
Ignition cut out means it priodically drops a cylinder from time to time. The engine will keep on running on the remaining ones, but with reduced power."
 
Many thanks for your response.

Many thanks for your response. A friend of mine has referred me to a retired boat mechanic who has agreed to look over my shoulder.
 
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