4.3LX & Weber Carb Service

Swedefj40

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Jun 9, 2018
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182
So I'm trying to dial in some issues on my 1996 Mercruiser 4.3LX 4bbl Weber carburetor. The motor has about 800hrs on it but not sure about the carb. I have a new Fuel/water filter, and recently a new fuel pump installed.

So what I'm experiencing is that it runs a little rough at 1st start up. Almost like it's missing a cylinder and I have to wait a good 5 minutes of about 2000 to 2500 rpm before I can punch the throttle to get up on plane. If it's a little cold it'll bog down slightly however, if I gradually push the throttle, she'll go. Not a big deal, just annoying. When it's warm, it's fine, no problems.

Now that I have the boat home, I made a VIDEO up of a cold start. If you have any expertise on a Weber/Edelbrock 4bbl Carb and 6 minutes to kill, let me know your opinions on how it looks/sounds etc. I'm curious to know about the choke position as it gets warm and what (or why) the little fuel squirting nozzle thing on top of the carb is doing?

I plan on adding a 1" phenolic spacer and I'll also do a carb rebuild at some point this winter.

Thanks in advance peeps.
 
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alldodge

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The fuel squirt is the bowl is to full, float needs adjusting or needle valve is weak

The divorce choke don't work very well and do take more time because the intake has to heat up. Just nature of the beast

Probably after and good cleaning and rebuild it will be better.

Don't think the spacer will help but don't see it hurting
 

harringtondav

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May 26, 2018
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2,438
I rebuilt my 1996 4.3LXI Weber and my pal's 1994 350 Mag
Weber. The kit isn't very expensive. If your carb has never been opened, a good cleaning is probably overdue.
It's fairly easy if you follow the included instructions step by step. Set up a clean work surface and lay the parts out in order of disassembly including removable jets. Remember to remove the spark arrestor stud from the air horn. It clamps the horn down.
Very important is keeping track of the little check ball, and spring in some models, under the accelerator pump nozzle. It falls out when you turn the carb body over. You'll be dead in the water if you loose it. Don't mix up the primary and secondary jets.
Soak the body in immersion cleaner. Wash it in very hot water with TSP. Blow it dry, shoot all passages with aerosol carb cleaner, both directions if possible. Reassemble parts in reverse order. Be sure to verify/adjust the float distance.
 

Scott06

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Apr 20, 2014
Messages
5,737
The fuel coming up and dribbling off of boosters is evidence of float bowl overflowing bad or misadjusted needle and seat/float, or too much fuel pressure. Until engine is running fuel pump should only be running when starter is cranking. Get a good carb kit from a place like mikes carb and rebuild it. then you can adjust choke for best cold start. I don’t think the choke was too far off as is but dont think it’s your problem.
are your ignition components in good shape ? Timing set properly
 

garbageguy

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May 8, 2012
Messages
1,543
agreed on the spacer, below is also another opinion on that

"for every 5 degree reduction in air temperature, increases power by 1%"
 

ROY WILLIAMS

Petty Officer 1st Class
Joined
Aug 8, 2022
Messages
367
So I'm trying to dial in some issues on my 1996 Mercruiser 4.3LX 4bbl Weber carburetor. The motor has about 800hrs on it but not sure about the carb. I have a new Fuel/water filter, and recently a new fuel pump installed.

So what I'm experiencing is that it runs a little rough at 1st start up. Almost like it's missing a cylinder and I have to wait a good 5 minutes of about 2000 to 2500 rpm before I can punch the throttle to get up on plane. If it's a little cold it'll bog down slightly however, if I gradually push the throttle, she'll go. Not a big deal, just annoying. When it's warm, it's fine, no problems.

Now that I have the boat home, I made a VIDEO up of a cold start. If you have any expertise on a Weber/Edelbrock 4bbl Carb and 6 minutes to kill, let me know your opinions on how it looks/sounds etc. I'm curious to know about the choke position as it gets warm and what (or why) the little fuel squirting nozzle thing on top of the carb is doing?

I plan on adding a 1" phenolic spacer and I'll also do a carb rebuild at some point this winter.

Thanks in advance peeps.
the carb of the Throttle body cleaner inside .... the take the top off the carb and rebuild it .....did the cloth / brush ... marvel mystery oil did the cloth ...
I was NOT OF THE CARB CLEANER !!!! 50 years ago ...
 

Swedefj40

Petty Officer 2nd Class
Joined
Jun 9, 2018
Messages
182
The fuel squirt is the bowl is to full, float needs adjusting or needle valve is weak

The divorce choke don't work very well and do take more time because the intake has to heat up. Just nature of the beast

Probably after and good cleaning and rebuild it will be better.

Don't think the spacer will help but don't see it hurting

The fuel squirt is the bowl is to full, float needs adjusting or needle valve is weak

The divorce choke don't work very well and do take more time because the intake has to heat up. Just nature of the beast

Probably after and good cleaning and rebuild it will be better.

Don't think the spacer will help but don't see it hurting
Good to know that's what the squirting is from. I'll check the floats during the rebuild.

The spacer is going to be added to help with lessening the heat at the float bowls and evaporating the fuel. My only problem with it will be the linkage to the temp coil as it stays with the manifold. I have thought about switching to an electric choke though, any thoughts on this?
 

alldodge

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Mar 8, 2009
Messages
40,880
Those carbs leak out over time. A rebuild should stop the leaking. That said given enough time and fuel will evaporate. Had same carb on my other boat , same problem

Electric choke will fix the long time for choke to open
 

Swedefj40

Petty Officer 2nd Class
Joined
Jun 9, 2018
Messages
182
I rebuilt my 1996 4.3LXI Weber and my pal's 1994 350 Mag
Weber. The kit isn't very expensive. If your carb has never been opened, a good cleaning is probably overdue.
It's fairly easy if you follow the included instructions step by step. Set up a clean work surface and lay the parts out in order of disassembly including removable jets. Remember to remove the spark arrestor stud from the air horn. It clamps the horn down.
Very important is keeping track of the little check ball, and spring in some models, under the accelerator pump nozzle. It falls out when you turn the carb body over. You'll be dead in the water if you loose it. Don't mix up the primary and secondary jets.
Soak the body in immersion cleaner. Wash it in very hot water with TSP. Blow it dry, shoot all passages with aerosol carb cleaner, both directions if possible. Reassemble parts in reverse order. Be sure to verify/adjust the float distance.
Good info, thanks.
 

Swedefj40

Petty Officer 2nd Class
Joined
Jun 9, 2018
Messages
182
The fuel coming up and dribbling off of boosters is evidence of float bowl overflowing bad or misadjusted needle and seat/float, or too much fuel pressure. Until engine is running fuel pump should only be running when starter is cranking. Get a good carb kit from a place like mikes carb and rebuild it. then you can adjust choke for best cold start. I don’t think the choke was too far off as is but dont think it’s your problem.
are your ignition components in good shape ? Timing set properly
Replaced the coil, plugs and wires, and the distributer cap a few seasons ago. The timing and idle air/fuel screws done at the same time and also a compression check.
 

tango13

Seaman
Joined
Mar 10, 2015
Messages
71
Do you find that it floods after shutdown when warm? I suspect I'm having similar issues on my Weber (7.4), not so much the choke problem but def the dribble. The warm shut-down flooding being the biggest issue, was told floats could be out of spec or a needle issue. Starts great cold but is an absolute bear to start warm...
 

Swedefj40

Petty Officer 2nd Class
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Jun 9, 2018
Messages
182
Do you find that it floods after shutdown when warm? I suspect I'm having similar issues on my Weber (7.4), not so much the choke problem but def the dribble. The warm shut-down flooding being the biggest issue, was told floats could be out of spec or a needle issue. Starts great cold but is an absolute bear to start warm...
No, not so much... if I try and start it within a few minutes after it's been up to temp she'll start right away. Throttle in neutral and just bump the key and it goes. If it sits for a half hour or so then it'll have to turn over for a few seconds to fire.
 

Swedefj40

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Messages
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So here's an update... stupid me tried to pry off the top plate with the air cleaner stud still screwed on and warped it and so I ordered a new carb. Btw, the old carb's floats were way out of spec as some of you had suggested.

I set up the new carb's floats to spec and installed it with a 1/2" phenolic spacer and tried to start it up and nothing, no fire. This was the 1st attempted start since October (6 months ago). In between start attempts I can hear the fuel pump running and when I look into the carb, I can see down the secondaries that there is fuel flooding into them.

I pulled the new fuel pump off, thinking it's giving too much pressure, and connected a pressure guage to it and it was showing 10 psi. I checked the old original pump, and it was at 9 psi. A bit high from my research for these carbs but the old pump worked fine for years on this engine. (I tested it by connecting the fuel pressure guage direcrly to the pump outlet and putting the inlet tube in a bowl of gas.)

I'm kind of stumped now as to where to go from here. Thoughts?
 

Scott06

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Joined
Apr 20, 2014
Messages
5,737
So here's an update... stupid me tried to pry off the top plate with the air cleaner stud still screwed on and warped it and so I ordered a new carb. Btw, the old carb's floats were way out of spec as some of you had suggested.

I set up the new carb's floats to spec and installed it with a 1/2" phenolic spacer and tried to start it up and nothing, no fire. This was the 1st attempted start since October (6 months ago). In between start attempts I can hear the fuel pump running and when I look into the carb, I can see down the secondaries that there is fuel flooding into them.

I pulled the new fuel pump off, thinking it's giving too much pressure, and connected a pressure guage to it and it was showing 10 psi. I checked the old original pump, and it was at 9 psi. A bit high from my research for these carbs but the old pump worked fine for years on this engine. (I tested it by connecting the fuel pressure guage direcrly to the pump outlet and putting the inlet tube in a bowl of gas.)

I'm kind of stumped now as to where to go from here. Thoughts?
9 or 10 psi is too high fuel pressure and will cause flooding.

When you say you bought a new carb was it new or new to you (aka reman). There are some cheap knock off carbs out there that are 50/50 if you get one that works
 

Swedefj40

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Messages
182
9 or 10 psi is too high fuel pressure and will cause flooding.

When you say you bought a new carb was it new or new to you (aka reman). There are some cheap knock off carbs out there that are 50/50 if you get one that works
Yeah that's what I was thinking about the fuel pressure, but it's a merc low pressure pump and has worked fine and never had any issues with it before. I dunno, maybe my new fuel pressure guage is wrong? The carb is a new knock-off one from eBay. Maybe I'll strip everything off of the new carbs top plate and use it on the old carb and try that?
 

Scott06

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Messages
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Yeah that's what I was thinking about the fuel pressure, but it's a merc low pressure pump and has worked fine and never had any issues with it before. I dunno, maybe my new fuel pressure guage is wrong? The carb is a new knock-off one from eBay. Maybe I'll strip everything off of the new carbs top plate and use it on the old carb and try that?
Muscle car solutions you tube channel does a pretty good review of the Edelbrock/webber knock offs.

My advice would be to send the knock off back, tell them it didnt work correctly, and buy a new Edelbrock 1409. Take the jets out of your original carb put them in the new carb
 

ROY WILLIAMS

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Aug 8, 2022
Messages
367
Yeah that's what I was thinking about the fuel pressure, but it's a merc low pressure pump and has worked fine and never had any issues with it before. I dunno, maybe my new fuel pressure gauge is wrong? The carb is a new knock-off one from eBay. Maybe I'll strip everything off of the new carbs top plate and use it on the old carb and try that?
the carbs 6-8 psi gasoline ..
 

Swedefj40

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Jun 9, 2018
Messages
182
Another update... I bumped the float gap from 7/16" to 1/2" and mounted the new carbs top plate to the old carb and tried it. That seemed to stop the flooding but still not starting. I do get the odd backfire though which it hasn't done before.

I did a compression check and found the the #3 cylinder down to 95 psi. The other 4 were around 140 to 120 psi with #6 at 160 psi. (This was with a cold engine if that makes and difference). 4 years ago I did a comp check and all the cylinders were between 140 psi and 160 psi. (#3 was at 167 psi). I'm wondering if a stuck valve in #3 would cause this low compression and backfire?

I did have the distributor cap off to have a look inside and give it a clean as well but I double checked that it was back on right.

Thoughts???
 

alldodge

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Compression is way off
Seeing 95 to 167 is to much of a span. How your doing the test and accuracy of gauge can have a bearing but my also be engine problems

It use to fire up and run rough but not hardly fires and when it does there is backfire. So check spark and timing
 
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