1989 Mercury 90hp backfires during idle

[rjames47]

I have a 1989 Mercury 90hp it’s high idling and backfiring. It’s only doing this during idling. When running the motor it runs great. I’ve cleaned the carbs, new fuel pump, new fuel hoses and new spark plugs. I’m looking for my next move. I’ve been reading vacuum hoses will do this. I’m just not sure, a little help please

 

[rjames47]

I also replaced the throttle links but I made sure to mark it to get same adjustment as it was. Could the idle jets be clogged again? This boat hasn’t been run at least 6 years

 

[roscoe]

Backfire thru the carbs or out the exhaust?

Gonna bet it is running lean, explains the backfire and the high idle.

Yes, the carbs could clog up in a minute or two if there is debris getting into them.

Were all fuel lines replaced, from the tank, to the carbs? They do break down over time, faster if exposed to gas with ethanol.

And its time to check the timing.

 

[rjames47]

I believe I found the problem. Pulled the carbs full of sand / grime. I think the problem is with the fuel tank. It’s plastic and I’m afraid it’s deteriorating. I didn’t replace the fuel lines to the tank though

 

[rjames47]

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[rjames47]

[No message]

 

[rjames47]

Carbs were clean and looked good. I ran a gallon of fuel with sea foam to decarbon it and it ran well. I hooked it up to the fuel tank this morning and that’s what they look like.

 

[rjames47]

Is there a way to check if my fuel tank can be used with ethanol fuel? The residue left in the carbs looks like a powdery substance. The tank is a 1989 original with the boat.

 

[roscoe]

likely that the tank is fine, but the lines from the tank are breaking down.

Open up the fuel line and check the inside. If they are the original fuel lines, 29 years old, its past time to replace them.

 

[rjames47]

Thanks roscoe I was thinking the same thing. I'm sure the hoses are original to the boat because I don't remember my father doing much work to the boat