Carb issues with bucking

I have a friend with a CJ5 258 with a 2100 MC carb. He was running fine but had a bad shaft bushing in the carb and purchased a new one ( rebuilt ). Now it bucks and jerks and just does some funky hesitations ONLY taking off from idle. If you slam on the gas taking of there is no bucking. but if you take off just touching the gas pedel it with surge pretty bad untill it get up in rpms. We adjusted the float which was way of, and changed the jets to 46's any other ideas it could be?

 

Could this be a choke issue? I know when I get up in to high altitudes (Jeep usually resides in Phoenix at around sea level) I notice the same symptoms.

 

I had a similar condition this past summer but discovered that I had forgotten to replace the vacuum advance line to the carb after checking the timing. Maybe it would be worth looking into a possible vacuum leak at the carb gasket, since it was changed recently. Same reaction when stomping on the gas, since the accelerator pump puts in enough gas to make up for the initial lean condition. Just my 2 cents.

 

This was happening to me also. I bought a junkyard MC2100 from Ebay and rebuilt it. I run 47 jets. I was having problems climbing hills with it as well as bucking at start off. First, I put a manual choke in it, for simplicity purposes. Messed with the accelerator pump linkage, that helped a bit but it still wasn't right. Then I replaced the new power valve from the rebuild kit with a holley. If your ever had backfire through the carb your power valve is probably junk. The holleys are protected from that. That helped a lot too. But the thing that solved all of my problems with it was a fuel pump and line change... Found most of my problems were worse when it got cold. After the new fuel pump, rubber and metal lines it runs like a scared rabbit. Hope this gives you some ideas.

 

If it hesitates on acceleration, that means that the mixture is too lean immediately after opening the throttle. When the throttle opens, the loss of vacuum makes the mixture lean out until the vacuum catches up with the wider throttle opening. There is an accelerator pump that compensates for this condition by squirting raw fuel into the crb bore. These are the jets of fuel you see when you look down the top of the carb and turn the throttle.

You could have a bad accelerator pump diaphragm, accelerator pump check valve, or the accelerator pump is set wrong. The M2100 has a few different settings for the accelerator pump, determined by where you put the linkage in the pump arm.

Another possibility is that the accelerator pump shot would be fine if you did not have a massive vacuum leak that leans out the mixture at all times. If so, then the pump shot may not be sufficient to overcome both the loss of vacuum and the already lean mixture.

I suggest you read the factory manual about the accelerator pump. The manual has some very nice explanations. Reading the entire chapter about the carburetor would help a lot in your understanding of the entire fuel delivery system.

 

Generally a blown power valve diaphragm will cause a 'too rich' condition. You should be able to see it on the plugs and in your fuel consumption.

If you take the power valve cover off and there's gas in there, the valve is blown. Yes, it's a common problem with M2100s.

Going to altitude is also makes the mixture too rich, compared to sea level. Typically you have to block an automatic choke open at high altitude or the car won't start when cold. Too rich otherwise.

 

A whomping big vacuum leak (around the carb base, or something not connected, etc.) is la distinct possibility with a new carb and an amateur installer.

 

Also (4th post to this thread ... !)

Why doesn't your friend post the questions on his own? He's more likely to get a good answer if the facts come to us first person.