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Mid range Engine miss w/ MC2100

2K views 15 replies 6 participants last post by  DREDnot 
#1 ·
Recently installed HEI with new wires and plugs along with a MC2100 non smog carb. Carb was rebuilt and appears in really good shape. Set timing, checked for vacuum leaks and adjusted carb. Engine idles a little high (900-1000 rpm) and there is a frustrating miss in mid range. The miss occurs primarily between 1800 & 2300 Roma when you are just riding along. Acceleration is excellent. My next steps are to replace power valve with 6.5 Hg unit and check compression. Any suggestions on what else could be the issue? 1989 4.2 manual with simplified vacuum hose routing and small air cleaner.
 
#2 ·
First things first: Timing at 8* at 750rpms? Vacuum advance hooked up to ported vacuum port? Achieved peak lean vacuum with vacuum gauge? Was it a cheap distributor that could eat up your cam gear like a $50 ebay one? Get the idle down to 750 and try to keep it there during proper tuning.
 
#4 ·
I agree with what c5wagner has posted 100%. You have to get that idle and timing in 1st. and then go for an adjustment on the mixture screws for max vacuum. All the while keeping an eye on the timing and idle speed.

Idling at 1000rpms is way too high. the working range of a 4.2L starts at 1200 and goes to 2500rpm.

A little tid bit of info...the 2100 is an Autolite carb the 2150 is a MotorCraft. Both still a Ford product, 2100 was just designed for a smaller motor will still work the same, just takes a different power valve.

And, just because it "looks" good, doesn't mean that it is. 1 backfire up through the carb is all that it takes to ruin the power valve. But it will still "look good" from the outside.
 
#5 ·
Appreciate the input. Think the timing is correct, but will work on getting the idle down and rechecking the timing. The throttle arm tab is bottomed out not allowing the idle screw to be adjusted any lower. Need to fix this some how. In regards to HEI gear I used the old gear since typically gears wear in together and I did not want to take any chance. Waiting on new power valve to see what difference it makes.
 
#7 ·
I'm running the #47 1.08 jets. Carb originally had the 1.21 jets. If anything my float is set too high at 3/8" vs the 1/2" recommended. I've attached pictures. Please let me know if there are other more specific pictures that would be helpful. As noted above the throttle response is excellent. The miss occurs when you are just driving along with a light throttle. I think all vacuum hoses are correctly connected and have triple checked for leaks. Have a new power valve on the way. Also will replace the catalytic converter as it does not appear to be functioning properly.
 

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#8 ·
"1.08" jets? "1.21" jets?

That's the primary venturi diameter

Does your float bowl have 1.21 cast in the side? If so, that carb is wrong(too big) for your motor.



Why did you set the float too high? You cant adjust a carb with the first and most fundamental setting off like that. Reset all settings to spec. And don't skip any.
 
#9 ·
I installed the 1.08" jets even though the carb has 1.21 stamped on the side. Not sure how to measure the Venturi diameter, but will give it a try and post a picture. The float was set correctly originally, but was apparently bent during disassembly and reassembly as I did this several times. Never messed with auto carbs before, so gaining experience as I go.
 
#11 ·
I installed the 1.08" jets even though the carb has 1.21 stamped on the side.
Thats the bore diameter. Its not something you can replace. The concensus is that the 1.21 flows way to much for your engine and it will be next to impossible to tune to run well
 
#10 ·
Like drednot said. The 1.08 and 1.21, which will be stamped on the side of the carb is the diameter of the carb barrels in inches. These are not interchangeable. The jets read in 0.mm so depending on your elevation you will probably be running 47-50 size jets.
 
#12 ·
There are other jets, tubes, orifices and needles that are sized for the CFM rate of the 1.21. They are for 318 and 360 V8s. You need to find a 1.08 bore carb and use that.
 
#14 ·
Wanted to provide an update. As recommended I obtained a 1.08 carb. rebuilt it and installed it. set the timing to 8 degrees before TDC. Set idle to 700 rpm. Jeep would not run well. After both timing (14 degrees BTDC) and mixture screws at 3.5 turns its running great. At 16 degrees it pings a bit. Not sure why my timing is different. May be wear, jumped tooth, etc. but good news is it's running well.

As a side note found out another reason why the 1.21 would not run besides it being oversized. When I mounted the carb the electric choke wire was under the carb. This allowed extra air to enter and I ultimately cracked and broke off the foot. Not very smart in my part, but learned a valuable lesson. A zip tie solved this for mounting if the 1.08 carb. Hopefully, my stupidity will be a good reminder for others.

Again, thanks for your help and Merry Christmas.
 
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