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4.2 carb problem

7K views 39 replies 13 participants last post by  pelzworld 
#1 ·
The previous owner did a motor rebuild and swapped carbs to the motor craft 2100/2150
He lost interest in the project so that is how I ended up with the jeep

It runs fine but when you stomp on it it bogs down and dies. If you slowly accelerate it does better but still bogs and if you don't let off the pedal it will die. I'm not sure about the vacuum lines on the carb or to the distributor. It has the nutter bypass done. I just replaced the accelerator pump and that was not it.
Any help would be appreciated
 
#3 ·
I would check the vacuum advance to see if its getting vacuum. Could also be a timing issue, too far advanced will cause it to bog. Vac advance is also adjustable so that might be an issue as well.

Can also be the carb needing a rebuild. If it sits for a long time, it will get tarnished internally and a rebuild/cleaning is the only way to fix it.

Could also be the fuel pump is going bad and cant keep up in the upper RPM's. Might want to check the pump output pressure. Should be between 5-7 PSI.


Its hard to pinpoint just 1 item, since you said it "runs fine", but you said it bogs down. These are the most common problems that would pertain to bogging down.
 
#4 ·
It's a new fuel pump but never got a presure reading

The distributor is moving during acceleration but when it starts to bog you see it slowly moving back to original position. I can't tell if it's losing vacuum and that's what's causing it to stall or it's stalling out so it loses vacuum pressure

You said the vacuum advance is adjustable. How do you adjust???

Also the previous owner did the install for new carb and I have no idea if any of the vacum lines are crossed or missing

Previous owner also did the nutter bypass. If this has been done what vacum lines are still needed???
Carb to distributor
Carb to PVC valve

Am I missing anything. The new carb has several ports not being used. I checked the port for the distributor and it pulls harder as rpm increases so I think the vacuum lines are correct. Thanks again for any input and help
 
#7 ·
So I am in the middle of this issue as well though mine is a minor flat spot at acceleration.

I would first look at the jets see what size you have. If jets are too small it will bog down like you say.

You can also pull a spark plug and look at it.

Black is too much fuel
White is to little
Brown or tan is pretty much dead on.
Can google for pics

Also what size Venturi do you have 1.08 is common should be a raised stamp near accelerator connection I think towards front of carb.
 
#9 ·
The main thing you would have to worry about as far as vacuum lines are concerned is that you don't have any open vacuum in the system. A cracked line or an open port will cause erratic behavior within the carb system. The vacuum system needs to be sealed in order to draw the maximum possible vacuum so that the motor performs properly.

On my 4.2L I have 3 vacuum lines 1 goes to the vacuum advance, 1 goes to the PCV valve, and the 3rd goes to the brake booster. That's it. Plain and simple.

You can try the carb cleaner test. Start the motor, and with it idling, spray the carb cleaner around the carb area and vac. lines. A raise in RPM will tell you that you have a vacuum leak. A vacuum leak around the base of the carb is also a common place to look.
 
#10 ·
I have the same set-up on my 4.2. Make certain you have no vac leaks and your PCV valve is working. I had a huge leak in my PVC vac line that drove me nuts until I found it. Also that carb has a power valve in it that is notorious for going out also. Mine did.

It doesn't sound like a timing issue to me. I suspect your power valve needs replaced. It's a cheap easy fix but rule out a major vacuum leak first.
 
#12 ·
nolayj, what size venturi does the carb. have? The number like (1.08, 1.21 etc..) will be on the side of the carb near the accel pump rod. What size jets are in it?

The PO may have put the wrong carb on the yj, like maybe for a v8 360 or whatever.

It sure sounds like your flooding it when you come off the idle circuit.

Do a google seach for mc2100 or 2150 and you will find tons of info.
 
#15 ·
Could not find any numbers on carb anywhere. Going to do plugs wires cap and coil this eve and set timing. If this don't fix the issue then I'm out of ideas. I need to find a local shop that will work on carbs. Hopefully can show me what the problem is so I can fix it.
 
#17 ·
:banghead: Having same issue with mine. It seems like it's in need of more fuel (going lean) at that throttle point. I've checked all over for possible vacuum leaks, tried different timing adjustments, and carb adjustments, changed jets to what they're supposed to be for my location (sea level) also have nutter bypass and did the team rush upgrade on the dizzy, and STILL having the issue as well as pinging under load/acceleration now... this thing is driving me NUTS!! Good luck on your search for an answer..., I'm still searching. If you come across the resolution, please keep us informed.
 
#18 ·
If he bought it on ebay that may be the problem. awhile back there was a guy selling these carbs rebuilt and supposed to be a big improvment in performance. the problem was he was putting very small jets in and the thing would hardly run. Check the main jet it should be at least a#50 - #52 to run realy well.
 
#20 ·
What a piece of $hit. I had a ASE certified mechanic who worked for dodge jeep Chrysler for 12 years come to the shop and give it a look. Here is what we found
1. There is no markings what so ever on the body of the carb.
What kind is it. It's either a motocraft 2150 or autolite 2150. But there are several different rebuild kits for each. Which one do I use???

2. This was a rebuilt carb, not new. One of the jets was a new 49. The other was never replaced. It was a 51. The 51 was cross threaded when installed. It took the patience of a saint to get it out. Not sure if the female threads are messed up or not.

The Venturi is not marked. The bottom of the Venturi has what looks like a two piece barrel pressed together. One side looks almost seamless while the other side was almost completely separated. It would not stay in place.

After 6 hours o know that I need a new Venturi but don't know the size or model of carb I have

I need new jets but again don't know what size to use or part numbers to order

Need standard rebuild kit but don't know model numbers.

New new gasket for carb to spacer and spacer to intake.

The mechanic recommends getting a holly and be done with it!!! Does anyone have any idea what model holly to use.

I'm at the end of my rope with this thing!!!!

Please help!!!!!
 
#21 ·
What a piece of $hit. I had a ASE certified mechanic who worked for dodge jeep Chrysler for 12 years come to the shop and give it a look. Here is what we found 1. There is no markings what so ever on the body of the carb. What kind is it. It's either a motocraft 2150 or autolite 2150. But there are several different rebuild kits for each. Which one do I use??? 2. This was a rebuilt carb, not new. One of the jets was a new 49. The other was never replaced. It was a 51. The 51 was cross threaded when installed. It took the patience of a saint to get it out. Not sure if the female threads are messed up or not. The Venturi is not marked. The bottom of the Venturi has what looks like a two piece barrel pressed together. One side looks almost seamless while the other side was almost completely separated. It would not stay in place. After 6 hours o know that I need a new Venturi but don't know the size or model of carb I have I need new jets but again don't know what size to use or part numbers to order Need standard rebuild kit but don't know model numbers. New new gasket for carb to spacer and spacer to intake. The mechanic recommends getting a holly and be done with it!!! Does anyone have any idea what model holly to use. I'm at the end of my rope with this thing!!!! Please help!!!!!
If you go with a Holley you'll need a 4bbl intake manifold . Go with a weber 38 Mine runs great
 
#23 ·
Carbsunlimited.com has diagrams andparts for the 2150's. The Autolite was the 2100 and Motorcraft was the 2150. Autolite were the earlier models. What you have is a reproduction with new internals from GRONK in Colorado. There's other guys on here that got these carbs too and found various problems from jets to throttle plate rod slop. I have the same carb and I'm giving it one more shot, then I'm going to get something else or get a 2150 from somewhere else and build it myself. Good luck on yours.
 
#24 ·
Starting to understand the problem. I ordered a $65 rebuild kit that looks like it has several different gaskets to fit several diff carbs. I am not against the idea of going to bone yard and pulling a stock carb to rebuild. Does anybody know what year and make fords I should be looking for
 
#25 ·
In the picture you posted to the right side near the back there should be a number for the size of the venturi. could be in a circle. With the messed up threads for the jet you would be better going the JY for a carb and rebuild it. You are looking for an older pre 90? Two barrel from a small block ford. doesnt matter 351, 302, 289. I know that it looks like a big carb for that motor but it only flows about 300 cfm as i recall ( its been a while i did this ). It sounds like that carb is junk. I had great success with mine. as for the holley an adapter from 2-4 will work. I want to say there should be an adapter to go from four bolts to two bolts. I could be wrong on that?
 
#26 ·
There are no markings at all on this pos. I think it's a cheap knock off. I rebuilt it. Used 50 jets and a new power vale. It's doing a lot better as far as it falling on its face but now it seems like less power than before and very hard start. The new jets did not want to thread in. I forced them. I think the previous owner screwed the threads up. They are seated way better than before but appear to be cross threaded. I worked on it for hours. The carb itself female threads must be screwed up. It's running for now but I will be in the market for a stock ford MasterCraft 2150. Thanks for all the help.
 
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