'89 Mercruiser 4.3 Bogs in midrange, Runs great idle and WOT

[Tanker_01]

I have a '89 Merc 4.3 with the 3304-9565 Rochester Carb on it. The boat starts cold great and idles perfectly. Once warmed up if I slowly increase throttle it starts to bog and will die around 10-20% throttle. If I push it past 20% up to like 60% it will catch and rev up. It runs at WOT perfectly fine. Another possibly related issue to this is when the motor is hot it does not want to start right up, I have to give it a pump or two and as long as I dont flood it then it will stumble for a second then start up and idle. If I flood it then it absolutely will not start unless I let it air out or spray starter fluid. Does this just sound like I need a good carb cleaning? Clogged jet maybe? I have a rebuild kit on the way now for it but I wanted to pick the brains of the experts before I dug into this. I have extensive experience with motorcycle carbs but this is my first automotive style carb.

Also, side question, do I need to remove the carb fully to refresh it or can everything be done with it installed on the intake?

 

[alldodge]

Most likely its the accelerator pump not being able to pump enough gas as the throttle opens. Or could also be the accelerator pump is worn out. Channels in the carb which need to be cleaned out

Flooding is probably also happening do to needle seat or float

Yes, carb needs to come off so it can be dunked into carb cleaner for a few hours

 

[Tanker_01]

Update on this problem. Took it out a week ago and while running along the lake it shot the ceramic of one of the plugs out. Clearly I'm running too lean. The plug was old and I do not think the PO fully tightened them because the remaining portion of the plug was barely more than finger tight. I borescoped the cylinder to make sure there wasn't anything still floating around there or a hole in the piston and everything looked good, cylinder had no vertical scoring whatsoever. Compression check was good as were all other cylinders. I am going to replace all the plugs and use the Carb kit I got to replace all gaskets and jets and such and see if I can get it running correctly. I will have to do a couple runs on the trailer first to check the plug color before I am confident to bring it back out onto the lake again. I am guessing if anything I should err on the side of rich?

 

[alldodge]

. I am guessing if anything I should err on the side of rich?

Agree

 

[Bondo]

I will have to do a couple runs on the trailer first to check the plug color before

Ayuh,..... On the wagon ain't much of a test,..... No Load,.....

I do agree with yer diagnosis, 'n thought process on the cure though,.....

 

[Tanker_01]

Ayuh,..... On the wagon ain't much of a test,..... No Load,.....

I do agree with yer diagnosis, 'n thought process on the cure though,.....

What is the best way to tune this thing on the water? The only adjustments I have is idle mixture and idle throttle position right? There isn't any way to adjust mid-range mixture is there? Or my accelerator pump? The spark plug blew out when it leaned out at WOT for an extended run, how do I ensure that I have enough fuel flow at WOT? Does that just come down to float level and overall fuel flow?

 

[alldodge]

If the carb jets are sized for the motor (no one has changed them) then after cleaning the carb it should pass sufficient fuel to run correctly. To check, run at mid throttle for a bit, then slow and stop without much idling. Remove some plugs and check color. Do same at other rpms up to WOT. You don't want to idle long because this will skew the readings

Best way is with air fuel ratio meter and O2 sensors but that cost

 

[Rick Stephens]

As AD said, as long as the carb was calibrated to that motor, then getting the carburetor clean is most of the process. I would do the on water tests to make sure you got it.

 

[Tanker_01]

Ok so another update to this. Have not gotten a chance to take it out on the water yet but I did change the fuel pump out because I wanted to eliminate that being an issue in the future and it looked very old. Good thing I changed it because it had a ton of gunk down inside of it and the rubber was all dry rotted. With the new fuel pump in the boat runs great on the trailer and doesnt bog when I rapidly increase throttle anymore. I wouldn't think with a carb setup that would matter much so it might just seem better because I changed other stuff out too. I did notice however that my in-line fuel filter now stays completely filled with fuel at all times now instead of only when running at mid-range rpm. Is it possible my fuel flow at low rpm was insufficient to keep the bowl filled but enough to push fuel into the intake and caused my engine to lean out?

 

[alldodge]

Is it possible my fuel flow at low rpm was insufficient to keep the bowl filled but enough to push fuel into the intake and caused my engine to lean out?

Yes, very much so
With finding a bunch of gunk in the pump, I would not be surprised that a bunch made it into the carb

 

[rad1026]

Rochester carb may have a filter on the inlet housing. Take a look at that and clean it. You really should just rebuild the carb. Sounds like you have enough ability. Kits are inexpensive, doesn't take long and some of the orings, accelerator pump rubber, etc. is much more tolerant of today's fuels than the older stuff.

 

[Rick Stephens]

I've never seen a glass inline fuel filter that was legal in a boat.

 

[nola mike]

I've never seen a glass inline fuel filter that was legal in a boat.

I think he's talking about the mesh or ceramic filter right at the carb inlet. You don't have one on yer fancy 4bbl.

 

[Tanker_01]

I was able to get the boat out for testing it out. It starts and idles perfectly but now it falls on its face above 80% throttle and wants to die. I can get it up on plane just barely but if I try to accelerate to full power it sputters and tries to die. I was able to go all day staying under 80% throttle and started and stopped it multiple times without any issues starting or driving around. What would prevent me from being able to go full power?

 

[Bondo]

What would prevent me from being able to go full power?

Ayuh,..... Crap in the carb,...... rebuild it,.....

 

[Rick Stephens]

Running lean will eat pistons.

 

[Tanker_01]

Ayuh,..... Crap in the carb,...... rebuild it,.....

I just rebuilt it. It was a little dirty before but its 100% fresh now and still not liking full throttle. Low compression maybe? It acts like it is too rich now instead of too lean like it was before I rebuilt the carb. If I smash the throttle to full it gives me about 5-10 seconds of power then tapers off and tries to die. I can smash the throttle to shoot the accelerator pump to get myself up on plane then come back to 75% throttle and it acts like nothing is wrong. It only has an issue if I leave it at 100%

 

[alldodge]

Either weak fuel pump or a restriction (antisiphon valve, fuel line collapse, tank pickup tube clogged)
Check pressure 3-7 psi

 

[Rick Stephens]

High range on the Roc is metered out of the piston and needles in the center of the carb. All it takes is a little dirt on the piston or built up corrosion deposits to stop it from richening. In other carburetors there's a power valve, the Roc runs off that piston and needles.

 

[Tanker_01]

High range on the Roc is metered out of the piston and needles in the center of the carb. All it takes is a little dirt on the piston or built up corrosion deposits to stop it from richening. In other carburetors there's a power valve, the Roc runs off that piston and needles.

My carb most definately has a power valve. See #34 in the picture I attached. Could I have assembled the power valve incorrectly when I rebuilt it? It has a spring loaded shaft that goes on top of it. Could it also be possible my float is set incorrectly or would that not have anything to do with WOT?