[jmskms]

I have a 50 horse 4 stroke Mercury that starts right up, idles fine but will not go past 2800 rpm's with a load on it. The engine starts sputter as if it were not getting gas. Carbs have been rebuilt, compression and spark are fine, ANY IDEAS???????????

[ronnieboy]

i gotta 40 2000 4 stroke and love it, you need to richen the carbs bout 1/4 turn, might have to drill out brass plugs , check manual to see where, betcha it works , ron

[jmskms]

Thanks! I talked to the mechanic and it has already been drilled out, he is going to give the 1/4 turn a try. He said that when he had it running he took his hand and partially covered the air intake and it started to speed up. Hopefully this 1/4 turn works I will repost after I hear back!

[ronnieboy]

good, i bet it fixes your problem, good luck ron

[jmskms]

Still no luck, tried to richen the carbs like you said and drill out brass plugs but still having same issue

[ronnieboy]

if rpm dont come up, could be the computer, it is built to limit rpm if too hot or no oil pressure, i think bout 1200 rpm is all it will do. thought sure carbs would do it, ron

[jmskms]

You are on the same track as my mechanic, On Sat he checked the oil pressure then also removed the temp sensor to see if it was over heating and that would be the cause.

Where to go from here?????


Thanks

[jmskms]

I saw on a similar post and someone said they were having similar problems and it turned out to be the Power pack. Does anyone know what exactly the powerpack does?

[ronnieboy]

still sounds like the carbs to me, power packs limit at bout 1200 rpm, so it aint that, and spark plugs are good right?, fuel pump and filter are open right?, bet float levels or or debris in jet area are the culprit, gotta be the carbs, or fuel pump, good luck ron

[wildbill59]

I've got a 99" 50hph and had the same problem. The jets were cleaned what I thought were clean but until they were replaced it didn't help. The idle jet is really small and clogs easy. The pilot jet passages are even smaller. You're gonna need a syncho setup after you get it back together. A Merc tech this spring said the carb setups are junk and unless absolutely clean fuel is kept in them they will gum up. Just sitting in the sun cooks the fuel in the bowls and gums them up as the fuel evaporates. Fuel today is junk compared to yesteryears and smog gas is really bad when it ages. I will not buy equipment from somebody that lives in an EPA mandated smog area unless new carbs are figured in when bought. I'm putting in a good fuel filter/water separator this year. I've stopped buying 10% blend alcohol gas for this motor. Pull the plugs on the pilot jets. The idle/ pilot passages play a big role in this motor up to mid rpm's. Figure $75 for each carb for parts to do it right. Another trick is to give the primer bulb a squeeze once a week to keep the bowls filled. Once the gas evaporates you're back to the problems. Run Sea Foam in each fill up.

[ronnieboy]

sounds like good advise to me, i use seafoam in every tank i get, no matter if it will sit or not, also in my ranger 150 mag II, and in my lawn mower, hahahha, in EVERYTHING that uses gas. still bet its in the carbs, do you have a factory service manual for the motor? solves lots of problems, got mine from merc parts express, hope it helps, ron

[jmskms]

Wildbill, where did you get your jets? I am also in WI and think that may be the ticket!

Thanks

[wildbill59]

Since my motor only had less than 25 hours on it when I bought it I didn't replace everything in the carbs, just the jets and bowl, top cover gasket. I got them off of Mercury parts Express thru a dealer in Fund Du Lac. Took almost 2 weeks as the jets weren't in stock. My motor had sat for a couple of years without draining the fuel. I just bought a Honda mower from a guy in Jersey. He only ran it last year once after buying it but didn't drain the fuel. I tried cleaning the carbs out but the parts would have killed me. A new carb only cost $16 complete. Got lucky as the idle passage is a bout as wide as a hair.
You have to dunk the carbs in carb cleaner. Spray cleaner just won't cut it.
Clean out the pilot jet passages with compressed air a few times.

[jmskms]

Still working through issues but will post if I find solution.

[vernkelly]

jmskms,

I have the exact same problem on a 1999 40HP 4 stroke merc. Starts fine and idles ok. Revs up to ~2800 RPM then looses power. Like a major flat spot. If I throttle through it will rev on up but start missing, surging. If I back off the throttle it will die. I think I proved it will run steady below 2800 RPM and behave nicely, but problems get triggered when I exceed this RPM. Have not taken it in yet to have it diagnosed. I suspect it is an electronic control problem since it always at a specific RPM.

I replaced the under cowl fuel filter and spark plugs with no real improvement. Operation was at ~5300 ft. Has been doing this last year but would go away after motor was warmed up. Now problem does not go away. Engine has low hours and has not been tampered with. I am in California.