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Rough idle, 1988 Mercury 90hp 3 cyl 2 stroke

donaldboyer

Contributing Member
1988 Mercury 90hp 3 cyl 2 stroke
s/n 0b312461

We took out our boat for the first time in several months for the first trip after winter. I had fogged the engine for the winter, and had started it on the muffs to burn out the fogging oil, and also used Power Tune to "blow out the cooties".

Starting was a little hard, but not a big concern It idled rough and stalled a few times which is unusual for this engine - it idles fine usually.

I spoke to the previous owner who told me he hadn't ever changed the spark plugs! So the ones that are in it now are the ones that it came with from the factory.

I need to sort this rough idle, is changing the spark plugs the best option?

Also.... the next time I'll have it running is when we will be away on holiday with no internet..... so it the new plugs don't fix this problem, what should I try next? Adjust low speed screws or something? I'd like to have a couple of options that I can try instead of having a trial and error over several trips away in the boat!

Thanks!
 
Re: Rough idle

Trouble shooting on a 2 stroke starts with a compression test !--------no point in starting / guessing on anything else.-----Post the results.
 
P107078222.jpg
Here is a pic of the plugs. They look oily to me but this might be normal. Do they look like they need replaced? (I am tempted to throw a new set in)
 
You could try different plugs ----------last one I did had one carburetor that was not adjusted properly.--------Your motor may look pristine but you need to do a compression test---Trust me.
 
if those plugs are 20 years old, you should change them no matter how you think they look, besides, they're already out, right? That said, Raceone is right about the compression test, but in the mean time, swap out the plugs, and see if she runs better.
 
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