Carburetor question

[Pmt133]

Long and short, I am working on a 4.3l with a carb that is running what I would call excessively rich. Boat ran for about an hour and while idling around produced more black soot and smoke on the transom than my 33 with twin diesels... Running on the hose results in eye watering and fuel smell as well as a slick from the exhaust.... That being said it does run reasonably well. I did do the base idle and idle mixture adjustment but it continues to run that rich at idle... I'm also getting a pretty strong fuel odor from the oil so I want to refrain from running it too much as I don't want to wash out piston walls or anything.

So here is where my question comes in, I intend to yank the carb and pull it apart and check everything over. Now, I also have a known good carb off a 3.0l. From what little info I could find, the primary jetting is the same on the two. I have listed the casting # for each below. What I am wondering is if the primary jets are the same, is there anything preventing me from popping on that carb and seeing if everything clears up? With what limited info I can find I am guessing it probably won't work as I am sure the power jetting is different and maybe the venturi as well... I just can't really find anything definitive. But I am assuming it is a no go and there is a reason why the castings are numbered different but figured I'd ask before I go pulling apart a carb.

Thanks in advance.

4.3l: 807764
3.0l: 864940

 

[Scott Danforth]

Ignore the 3.0 carb. Won't help you at all

Have you checked the choke on the carb?

 

[Bt Doctur]

check your fuel pump pressure first

 

[stresspoint]

why not just rebuild the carb you have , chances are its just the float level has been nudged somehow or the accelerator pump check valve is worn or has something lodged under it .

 

[Scott Danforth]

BT has a point. Check the clear tygon tube going from the fuel pump to the carb

If there is fuel in there, your fuel pump has failed

If no fuel, check pressure. If over 9psi, it will push the needle open

 

[Pmt133]

Ignore the 3.0 carb. Won't help you at all

Have you checked the choke on the carb?

I actually zip tied the choke open just to be sure, but it appears to be opening all the way. That was my first thought though

check your fuel pump pressure first

Not a bad idea. It's one of the low pressure electric ones from merc (Its a later vortec block without the mechanical provisions) but I suppose something could be up. I have everything to check it.

why not just rebuild the carb you have , chances are its just the float level has been nudged somehow or the accelerator pump check valve is worn or has something lodged under it .

I had intentions on this and the carb kit is actually sitting on top of my ultrasonic tank on my work bench with a float. Originally the engine was bought used from a mechanic. I test ran it and it ran like... well dog $***. But it idled fine. I figured for a bay test if I could get it running okay I would do it after. Then it cleared out and ran fine. I suspect you are correct and some crap from the bowl got sucked through and is causing the issue... At this moment, I just don't have the time to sit down and work on the carb. Been getting a lot of OT at work so the good thing is I can afford to use the boat. Bad thing is I don't have the time to use the boat. I figured 5 nuts to swap on a good one would be quick enough to get me out and then when I have the time, service the one that is supposed to be on it.

My other issue is I suspect the carb on the engine was sent out or exchanged for a rebuilt unit from some where at one point and it may not have the proper jetting for the engine in it anymore... Hence the rich condition. But I won't know that until I pull it apart and measure everything. I've seen quite a few times those rebuilds don't always get the parts they are supposed to from friends... hence why I do most the work I can myself, at least it'll be done as close to right as possible. My clue is the paint on the carb has been redone to flat black and nothing else on the engine is and the casting number is pretty heavily covered...

 

[kenny nunez]

Is the carburetor in question a Mercarb with 1 idle mixture screw or a Rochester with 2 idle mixture screws? Also does it have a nipple where the clear hose connects to the air horn?

 

[Pmt133]

Is the carburetor in question a Mercarb with 1 idle mixture screw or a Rochester with 2 idle mixture screws? Also does it have a nipple where the clear hose connects to the air horn?

Mercarb. No clear hose as it's a factory electric fuel pump engine.

 

[kenny nunez]

You are probably right about the carburetor being re cycled and it has the wrong parts in it. Maybe someone on the site has a extra correct one to sell that is rebuildable.

 

[Pmt133]

You are probably right about the carburetor being re cycled and it has the wrong parts in it. Maybe someone on the site has a extra correct one to sell that is rebuildable.

Yeah, maybe I'll pull it apart when I have the time and see if the jets are stamped the right size and go from there.

 

[JustJason]

Black soot is often times a sign of low engine compression. So if you run out of ideas, check that compression.

 

[Pmt133]

Black soot is often times a sign of low engine compression. So if you run out of ideas, check that compression.

I actually checked it when I bought it... about 155 across the board.

 

[JustJason]

I actually checked it when I bought it... about 155 across the board.

Were you still having this problem when you bought it? Or did the problem occur after you bought it/checked the compression?

 

[Pmt133]

Were you still having this problem when you bought it? Or did the problem occur after you bought it/checked the compression?

I bought the engine off a mechanic for a repower. Story is he sent it out and reinstalled the engine in the customers boat. Boat went to hell and sat for a season or two then they junked it and he resold the engine to me a couple months ago... had about 50 hours or so on it. Far as I/he knew it ran fine in customers boat and runs fine in mine other than being really rich. I ran compression test along with checking other ancillary things when purchasing.

 

[Pmt133]

Think what I'll do is throw in a set of clean plugs, run it hard then pull the plugs to get a read on exactly how rich it is then go from there... I fell onto a 4 barrel carb and manifold cheap and may just do that if the jetting is off. That carb has all the correct parts in it still. Hopefully though a rebuild is all that is needed for this one...

 

[rustybronco]

When rich running is suspected and all the usual suspects check out, needle and seat, float height etc. suspect a bad power valve piston.

 

[Pmt133]

When rich running is suspected and all the usual suspects check out, needle and seat, float height etc. suspect a bad power valve piston.

Yeah. I intend to check the fuel pressure... I'm not seeing any fuel dribbling into the carb from the vent or venturi like I had on my old engine when I rebuilt the carb... but you never know. Can't check anything until it's apart.

 

[rustybronco]

Mercarb, Mercruiser Power Piston 131-69

www.carburetor-parts.com

 

[Pmt133]

Mercarb, Mercruiser Power Piston 131-69

www.carburetor-parts.com

Yeah that's the one.

 

[Pmt133]

The carb has made it from the boat to the work bench next to the rebuild kit... If I get home tonight after work and it magically rebuilt itself I wouldn't complain. We'll see what happens when I pull it apart. Looking at the under side where the gasket/spacer are, it hasn't been gone through in a while.