Merc 350 4bbl black smoke when cold start - disable choke?

[tmh]

I repowered with a 1990's (not sure of year) 350 w/ 4bbl carb (not sure which make). I had a young mechanic do the job and so far he's done well for me. First time out recently it ran very well but flooded and couldn't restart - he did a carb re-build kit as the engine had sat for 1 1/2 years before I bought it. 2nd time out ran great, lots of power, but it had a lot of black smoke for quite a while when starting cold and I had to leave throttle open a bit to keep from stalling. once running a while, idled perfect and re-started with a touch.

So my young mechanic (he works as an mechanic's assistant at a big marina during summers) is checking the engine again and mentioned he may disable the choke or something like that - unfortunately he's not available right now and i don't remember exactly why, other than it has to do with the black smoke when starting.

I know that when starting the engine in the spring it can smkpe a lot due to the fogging from winterizing, but in installing, testing, tuning and running it twice for several hours the engine fog ought to be out by now, right?

Anyways I'm just trying to educate myself on what's going on. Anyone know what I'm talking about even?

 

[MikDee]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

Yeah, it sounds like it's running too rich! Maybe your carb is getting gummed up again from sitting previously for a long time, it don't take much to clog up the needle valves on the floats, and the jets, thats what it sounds like to me. Don't mess with the choke, unless the butterflies are stuck on when the engine is fully warmed up, usually they work pretty good without being messed with.

 

[John_S]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

Yep, choke would be the first thing to check, along with overflow tube from mechanical fuel pump, and float level adjustment.

 

[JustJason]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

black smoke means way to much fuel. I doubt its the choke, but the overflow line would be a quick and easy check. Double check your timing.
Run the boat a bit to get it warm, then carefully pull off the flame arrestor and shut the engine down. The instant the engine is off look down the venturi with a flashlight and see if your dripping fuel

Maybe your carb is getting gummed up again from sitting previously for a long time, it don't take much to clog up the needle valves on the floats, and the jets

uhmm... wouldn't that cause a lean condition??

 

[tmh]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

Thanks guys. I googled my problem a bit more and triedto better remember what my mechanic said. I think the problem is the choke is remaining closed (or partially closed) too long as the engine heats up. it's supposed to be closed to start then open up as the engine warms up - fairly quickly, right?

For some reason sounds like my choke isn't responding properly to the engine heating up so choke closed too much. Is this a temp. sensor issue? The cleaning and rebuild kit was done a week ago, so i doubt it's gummed up already!

If he disables the choke (not sure if he meant temporary so i can use boat this weekend) will it be very difficult to start cold? It's 90 deg. here this weekend but often 60's when we boat. I always thought choke was needed for cold start, but not sure - I assume if it wasn't needed it wouldn't be there!

Thanks as always for the help. I'll have boat back tonight so i can check on whatever is needed.

 

[Silvertip]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

A cold engine needs the choke. There are two types. One has a heating coil in a round housing on the side of the carb. On a cold start, engine vacuum pulls the choke open just a tad and after that the heating coil gradually opens the choke the rest of the way. If the coil is not getting current, the choke will remain partially open. The other type of choke uses a metal tube from the intake manifold to the same type of round choke coil assembly on the side of the carb. As the manifold warms, heat passes up the tube to heat the bimetal spring inside thus opening the choke. NEVER adjust the choke on a warm engine. It must be adjusted at ambient temp (stone cold) to get proper cold operation.

 

[tmh]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

OK, thanks, Silver. I'll have to check, but the mechanic said mine is "mechanical" which sounds like the bimetal spring set up you mentioned. Seems the spring isn't opening fast enough so choke is partially closed too long. Maybe I need a new spring (assembly?). I'll discuss this with him later on today.

 

[MikDee]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

black smoke means way to much fuel. I doubt its the choke, but the overflow line would be a quick and easy check. Double check your timing.
Run the boat a bit to get it warm, then carefully pull off the flame arrestor and shut the engine down. The instant the engine is off look down the venturi with a flashlight and see if your dripping fuel



uhmm... wouldn't that cause a lean condition??

Just if your float needle valves are clogged, and your float needle doesn't close, you could have an erratic temporary flooding condition.

 

[Mkos1980]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

Choke may be needed when its very cold outside sub 50* but I ran my q jet with choke open all the time. Cold engine started right up. Tie open the choke and see what happens.

 

[tmh]

Re: Merc 350 4bbl black smoke when cold start - disable choke?

Choke may be needed when its very cold outside sub 50* but I ran my q jet with choke open all the time. Cold engine started right up. Tie open the choke and see what happens.

Just spoke with the kid working on it - he said exactly what Mkos just said. Basically he said the 350 Mag engine doesn't like much choke and his marina has set them "always open" before with no problems. He says it may idle a bit high for a short while, but starting should be fine.

I rarely if ever boat at 50 deg. or below, but I guess i'd like to be able to start it in any conditions! I may revisit this later on in the season, but for now it's 80 deg+ outside and i just want to ride my boat!