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My old '84 yamaha 40 hp 2 stroke is idling and running a bit rough.

Benji91

New member
Hello.

I have a Yamaha 40 hp which I have problems with the idling. When I start it (usually most when it's cold), it idles pretty rough and at low speed I feel it's not quite right. It coughs a lot.

When it gets up to speed. 8-22 knots it almost never misfire. It's very rare I have problems with it then. It also starts up pretty easily. Sometimes he's a bit greedy with the choke. But I've heard it's normal for the Yamaha engines that they need a bit of choke at cold start.

I've changed the spark plugs.
I've taken out the carburetors and cleaned them and adjusted the float bowl.

Is it maybe a carburetor adjustment problem here? I don't feel it's ignition related as it runs decent in high rpm.


It's also a very old engine. It has run for god knows how many hours. So could this maybe be mechanical? Since it works better when getting up to operating temp?

Thanks for every help I can get.

Best Regards
Benjamin
 
When you pulled the carbs did you break them all the way down and especially back out the brass fuel jets and make sure they were perfectly clear and clean? Often you can back out those brass fuel jets without even pulling the carbs depending on the engine. Even a little gumming in those jets can cause problems and blasting carb cleaner through them in place won't always get rid of that gum. I used to pull my old 2 stroke jet orifices and run a COPPER wire through them back and forth as copper is softer than brass and won't leave scrapes inside the jet that a harder wire could that can lead to more gumming up.
 
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