Merc 40 bogging down

[bdreaper1234]

I have a 98 3 cylinder 40 merc that i just bought a few months ago for my boat i am restoring. I just installed it and ran it sunday. The engine ran great for the first hour or so then all the sudden it starts to miss a bit at WOT and then loses 1000 rpms and will not recover. After that it idles horribly and just will not go over 4000 rpms. I pull it out of the water and get some gas treatment thinkin maybe it's just a little gumed up. I put the gas treatment in the full tank of brand new gas. It does the same thing. Runs great for a while then bamm it starts running like crap. Could it be a bad coil wire or a bad coil? How can i find out which one if so? Or do i need to just replace all the coils and wires? Thanks.

 

[Laddies]

Re: Merc 40 bogging down

Ck it for spark on all 3 cyl. ck. compression if they are aright the you must have a dirty carb

 

[marinegrh]

Re: Merc 40 bogging down

try running the engine again and when it starts to die down, squeeze the primer bulb and see if it recovers.if so, fuel pump time....basically, you are acting as the fuel pump by squeezing....next try with someone else driving, putting your hand over each carb as it is dropping rpm, see if the engine picks up you will have to do this kinda fast as you dont want to flood the engine out...sounds like you may have some trash in the bowls of the carbs and they may need a soakin..but try that first and let us know your results

 

[bdreaper123]

Re: Merc 40 bogging down

i tried to pump the bulb while it was running rough. No difference. I might just pull the carbs off this weekend and go through em. I just don't understand how it could be the carbs if it consistently runs good for 30 mins to an hour then starts acting up. I would think that if it is clogged jets or something it would run like crap from start to finish.

 

[Elmer Fudge]

Re: Merc 40 bogging down

If it ain't dirt in the carbs it could be the fuel pump or the tank not properly venting.