questions about 500hp motor
 

Looking for ideas as to what is causing one of my motors to run poorly at times.It is a 500hp blue merc and once in a while when I come off plane or am just idling along it will begin to sputter, miss
and revs will go down and up and if I leave in gear it will usually stall.
It may do this for a few minutes and then after reving it up a few times it clears and will go fine.There does not seem to be a pattern as to when it will occur that I have figured out yet so it cant be bad gas I dont think and It seems to clear itself after a while so I dont think it is the filter but I really am not sure what is causing the problem any suggestions Thankfully accepted.

Possibly water in the tank, or incorrect minimum air rate. Try cleaning the throttle plate (carb cleaner on a rag), wipe the edges of the plate and the bore where the plate meets it. Also remove the IAC and clean its pintle and bore. Then cycle the key off to on, wait 15 seconds, repeat 3x. Start and let idle until warmed up.

Thanks Go 4 Broke for the help,we have checked for water and do not think that is the problem, would the incorrect minimum air rate also cause bad performance at cruising and full trottle which doesnt seem to be happening and it does not do it all the time usually after longer runs it shows up but not short ones 5-15 mins-no problem but more than that seems to be it happens.I wonder if it could be electrical and temperature sets it off. It is a tough one to figure?

Min air rate would only effect the idle. The engine coolant temp sensor could also be a culprit. You really need a scanner and someone that can use it.

I'm assuming these are EFI motors, if they are carbs, everything I told you is crap!

Vacuum leak at the intake can cause these symptoms in efi and carb motors/

Go for broke sorry I should have said it is carb not EFI,I think we have found the problem in that it appears to be a bad coil.There is oil around the boot and the coil wire is not very snug.Replacing it friday and will see if that is the culprit.I hope it is that simple with my luck it usually is more time and money.Thanks for the imput.

one of mine carb 500's was doing the same thing and found that my secondary float was sticking, causing fuel to run into the back secondarys causing the engine to run rich. Cleaned out the fuel bowl reset the float and it ran great after that. Also check your four corner idle screws and make sure they are not out fo adjustment.

Terry

Actually if you run in rough water your floats need to be set every so often. I do mine every other year. That would be a good thing to check and do some maintenance at the same time.

Thanks for the advise the coil wire was the problem it is running great now what a big improvment. I will try to check the float and the idle screws and again Thank you for the help.