258 carb/timing help

@WILLYN your expertise is needed sir.

I can't really give you any info on the MC2100 since I'm still running the stock Carter. Did you do the nutter bypass with this carb? If you haven't, you really should. It takes the ECU out of the equation in order to allow the motor to run like the old CJ setup. The YJ ECU only controls the fuel air mix to the carb and it is very inefficient. Once done you should be able to set the timing to around 12* btdc and it'll run like a champ. There are numerous writeups on Jeep Forum detailing the Nutter Bypass. Once you have it running correctly, look into doing the Team Rush mod and possibly upgrading to a HEI distributor as well. The HEI is on my short list of mods and I should be done with the engine after that.

Quote from Freebird1992

I have MC 2100 1.08 venturis and hei ignition.

Quote from Kuntryboy816

Team Rush mod and possibly upgrading to a HEI distributor as well. The HEI is on my short list of mods and I should be done with the engine after that.

Funny

This sounds like fun to me...I'm kinda weird!
At this point I guess we have to assume that the cam is timed properly. Don't know if you checked your timing marks on the balancer against the actual TDC, but you should always do so because balancers aren't always right. Obviously this would change the timing, or at least what value you set it at.
Assuming the mark is correct and that you are setting timing with the vacuum disconnected from the advance and the hose is plugged, the 8 degrees you are looking for may not be correct for the new setup anyway. Changes in flow and compression usually require changes in the timing. The vacuum characteristics of the motor are different now, so the timing and carburetion should be different also.
As far as the hesitation goes, I hope you are testing that with the vacuum advance connected. You will probably need an adjustable vacuum advance canister to fine tune acceleration. It moves timing ahead when there is high vacuum (idle, part throttle cruise), then when you mash it it retards a certain number of degrees to avoid pinging. The adjustable units, Crane in particular, come with stops for maximum change limit, and an Allen screw in the port changes the vacuum level required to retard it. Watch the timing with a light while you accelerate with the vacuum connected, you should be able to see when it pulls back and get some idea if this is the culprit. You can check this with a vacuum gauge connected as well to get more info. It would have been interesting to note engine vacuum characteristics before and after the rebuild, but oh well. I run a vacuum gauge all the time anyway, it's a great way to monitor performance.
The motorcraft 2100 is one of the simplest carbs there is, and that's good. The jetting should be close to the same, as you are still pulling all the air through the same size opening and it still needs the same amount of fuel added to reach the proper mixture. So don't mess with the jets just yet.
I highly recommend an O2 sensor for carb tuning. I've been using cheap narrow band units on my stuff and they will tell you all you need to know for tuning. They can be had for under $100. At pep boys or wherever. Without one, it can take a long time to get proper tune.
The idle circuit is used by the carb in idle only. When the plates are almost closed the vacuum created under the plate pulls fuel from this circuit only. With the mixture screw all the way seated no fuel should pass and the motor should not run. If it does there are a few possibilities.
The mixture screws may have been cranked in so tight that they damaged the seat, and tuning won't be possible. I have seen damaged needles from this, but usually they will seat fully and stop flow if nothing else.
There is a specification in the carb rebuild manual for how far the plate is open at idle. This is more critical than they tell you. Usually it is given as a distance from the plate edge to the carb bore, and most people set it with a drill bit of the correct size as a gauge, or just eyeball it like I do. It is important because as the plate opens the idle circuit stops working completely, and the carb starts drawing fuel through the transition slot instead. If it is too far open to begin with then it may be running on the transition slot and not the idle circuit, which would render the idle mixture screws inoperative.
With engine off, no foot on the gas, take a look down the carb bore. The plate should be barely open. If not, then adjust the idle stop screw to make it barely open when it is on the stop. Now adjust the mixture screws to 1-1/2 turns out and try to start it. If it runs, try to adjust the mixture in until the engine runs rough, then out until adjusting makes no further difference. The correct idle mixture will be right in the middle of those two, more or less.
If it idles properly and responds to mixture input then you are on the right track. The transition slot and main jet are involved when you step on it, but if the initial setting is correct then they should just work.
When you accelerate, the vacuum becomes less as you open the throttle, and the carb pulls less fuel momentarily. This is overcome by the accelerator pump which gives a squirt of fuel at this critical time, if set correctly. People sometimes think more is better with accelerator pumps, but that's not true. The correct amount is better!
If you start with the accelerator pump adjusted to where there is no pump, adjust it to give a little more each time until there is no hesitation when you accelerate, then stop. More than just a little squirt when you blip the throttle will just be wasted and give you gassy black exhaust smoke. If you get black smoke when you blip it you have too much pump action.
Tuning a new motor setup takes some time in the garage and some time on the road. Without an O2 sensor it's just seat of the pants and read the plugs until you get it driveable. With an O2 sensor you can read what the carb is doing in real time and adjust it accordingly. It's fun to adjust idle mixture and actually watch the reading change. And once you can drive it you can see if your main jetting is correct, which will give maximum and noticeable performance.
With the mods you have done I think I would set the advance about 4 to 8 degrees more than spec and then start to tune the carb for idle as above. Once the carb is adjusted pretty close I would start to drive it to see about the advance and hesitation. The old rule in timing and performance is to advance it till it pings and then back off a hair. Light vehicles can use more advance, and so can performance engines, so you will surely be more advanced than stock when you reach proper tune. The vacuum advance has more to do with retard than advance. If all around the motor runs great until you nail it and then it pings, then you need to adjust the vacuum advance to allow a bit more retard until the ping goes away.
Once this all works and you can do some normal driving you can finally start to read the plugs.
At the moment that's all I've got...hope it helps!

Make sure your vacuum advance is on a ported source under the carb. Too much vacuum will cause this.

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