78 CJ7 Carb, 2100-2

[Air Force]

I believe this is a carb question, so I'll lead the witness. I just finished a complete frame off restoration on a 78 CJ7 , AMC 304 with the 2100-2 carb. It ran OK before the restoration, but it was torn apart for about a year. What I have now is that it runs smooth until it warms up. After that it is OK at low RPMS, but around 45-55 mph (Higher RPM's) it will spit and sputter and feel like it is going to shut down. If you let off the gas it will settle out, but as you get going again it starts again. I thought it was the accelerator pump, so a carb rebuilt was done. That did not seem to fix the issue. Anyone have any ideas on what to look into?

 

[mtnwhlr]

I would say the carb isn't getting enough fuel could be a weak fuel pump or it's having trouble getting fuel from the tank. It could also be you don't have enough timming.

 

[Air Force]

Interesting on the timing idea. I'll double check that. It was set when the rebuild was done. The strange part about this is it does not do any of this until the engine warms up really good.

 

[elwood blues]

IMHO, its not your timing. If your timing was off, it would spit and sputter at idle the same as it does at high RPMs. In an MC2100 series carb, the idle circuit is completely separate from the acceleration circuit. I sounds like your not getting enough fuel. I would check your fuel filter, and then move to the carb. Check your needle and seat, make sure that the needle nicest freely, and then check your float. Ensure that its moving freely and is adjusted correctly. If those two don't help it may be in your venturi cluster, you may have to remove the whole venturi assembly and let it soak in carb cleaner or gasoline over night and then spray it out with LP air. If none of those fix your issue let us know and we'll figure it out.

 

[Air Force]

Timing was OK. I took the carb off again and cleaned it again and re checked everything. I had put a new fuel filter on it during the rebuild, but checked it anyway. It looks clean. I have only run two tank fulls of gas through it since the retoration. Could this be an electrical issue? I have a new rotor, cap, sparkplugs, and wires on it, but it has the original coil and ignition box.

 

[TroyaCantrell]

I believe that the 2100 has multiple fuel metering systems two of which are responsible for delivering more fuel under a load. Both are related to vacuum. If it sat too long the rubber could dry out and have fine cracks so check the diaphragm. There is also an air bleed in the main metering system (used under normal acceleration ) and if that is clogged it could cause back pressure against the diaphragm.

One other thought is if it is under even heavier load it could be a secondary enrichment system where increased file is needed for the load. Vacuum is supplied from the manifold and is negative pressure from the intake manifold. It puts vacuum on the power valve diaphragm to overcome the spring. If the passage way is plugged it can't overcome the spring and no extra fuel will be supplied.

Just a couple thoughts.

 

[DHugg]

I would say the carb isn't getting enough fuel could be a weak fuel pump or it's having trouble getting fuel from the tank. It could also be you don't have enough timming.

My Dad used to disconnect the fuel line to carburetor inlet, direct the line into a pint fruit jar, and have me crank for about 15-20 seconds with the distributor wire disconnected. I seem to have a picture of about 1/4 to 1/3 of the fruit jar filling as we did this. This was a fuel line-filters-fuel pump check all in one.

You may be better off, and safer, putting a fuel pressure gauge on the line instead of a fruit jar.

This is in the same category as setting timing and idle needles at idle by watching a glass of water sitting on the engine valve cover, or cast-iron head in the case of an older flathead.

 

[tarry99]

I believe this is a carb question, so I'll lead the witness. I just finished a complete frame off restoration on a 78 CJ7 , AMC 304 with the 2100-2 carb. It ran OK before the restoration, but it was torn apart for about a year. What I have now is that it runs smooth until it warms up. After that it is OK at low RPMS, but around 45-55 mph (Higher RPM's) it will spit and sputter and feel like it is going to shut down. If you let off the gas it will settle out, but as you get going again it starts again. I thought it was the accelerator pump, so a carb rebuilt was done. That did not seem to fix the issue. Anyone have any ideas on what to look into?

Vacuum advance on the distributor?
How about a vent problem at the tank? Take the cap loose and try driving it.
Sounds like fuel delivery..............but?