I bought this motor off craigslist. great compression(145psi), carbs are clean, just rebuilt fuel pump(had a small hole in diaphragm), nice blue/white spark. At this point I am lost as to whats wrong. At WOT it is dumping too much fuel or something. The motor smokes quite a bit at idle, even with new plugs and the correct mixture for the fuel. The starting enrichment system is operating properly.

May need a good dose of sea foam and do a good decarb on the motor. That would be a good cheap starting point to work from.

Floats not adjusted right or floats are swelled causing them to stick. Had a 70hp acting like this that had the carb bowl vents closed off.

That forty is a sensitive little engine when it comes to fuel. Recheck your fuel pump and make sure that it is not leaking thru the pulse fitting. Also make sure that the air fuel mixture screws are adjusted correctly. And since you bought it off Craiglist, I am going to assume that you installed it on another boat. If you did, make sure that the linkage is opening the carb butterflies 100% and not over opening which in that case it would be restricting air flow.

This motor has air/fuel jets instead of screws. The original theory was that it was leaking through the pulse fitting but rebuilding the fuel pump did not fix it.

i have the exact same engine except its a 35hp. it was doing the same thing. could be a bad primer, gum in the carb, fuel leak in the lines, possible blown head gasket...the list goes on. i would definitely start with the primer line from your tank to the motor. it could have a bad bulb or could possibly have a bad fitting that is sucking in air. regardless, they are all cheap do-it-yourself fixes. and definitely run some Seafoam through it! let me know when you find the problem!

If I had a cracked line between the fuel pump and carbs I would have a fuel leak. If I had one between the tank and carbs it would suck air. The primer bulb pumps up pretty firm.

what does the bulb do when you are on the water giving it WOT? also, check to see if your fuel lines may be breaking down and causing the line to get clogged b/c of the ethanol. that's something that wouldn't affect idle too much, but would WOT...

The problem is too much fuel at WOT, not too little. If the lines were partially clogged the motor would run great until the bowls emptied.

About the last item that has not been mentioned is that the high speed jet could have backed out. It would not have anything to do with idle, but sure would mess things up at high speed.

I found the problem today with a timing light. It turns out that the top coil is bad, anything above idle and it stops working.

hmmmm...would have never thought that. good job!

it was pure luck. I hooked the timing light up to the number 1 cylinder and as soon as it was off idle the coil would quit.