2100 venturi 1.08 or 1.14

[viking4949]

1979 CJ7 AMC 258 i6 / 4.2l AMC 258 i6 / 4.2l . Bought with a cobbled together 1.08 venturi autolite carb. Last year, Couldn't get it to idle, did my first ever rebuild, got closer, but still have a slight miss at idle. Had the head redone, fixed the common lifter tick, now have an issue with having to set the idle high to smooth out the stumble. Thinking about rebuilding again, simply because now I understand a little more, and will have some time this winter. Today, I was given another 2100 that has been rebuild long ago. Here is a link to the exact model, but not my carb. C7AF-AS . So now I have 2 questions. First off, should I stick to 1.08 or switch to 1.14 venturi, all other things being equal. I will be rebuilding one or both anyway. After reading some of Elwoods posts, the CFM rating of the 1.14 seems closer. Question 2. I am using an electric choke. I have read other places that the small hole, on the bottom of the carb on the back, choke housing side should be epoxied shut, and could be to blame for the stumble at idle. Is this correct? What should I do?C7AF-AS

 

[CJ]

Cant help but...

Paging Elwood, Mr Elwood!!!

 

[elwood blues]

I'm getting ready to walk out the door, it's date night with the wife

I'll get back to you later tonight or tomorrow with all of the details, I hope you can wait.

 

[elwood blues]

I have some questions for you before I try to explain anything.

1. How is the MC2100 attached to your intake manifold? A good small 2 barrel to large 2 barrel adapter and gaskets (fel-pro).
2. Have you ever measured your manifold vacuum?
3. I need you to describe in more detail what you mean by you can't get it to idle. Will the engine die when you let off the excellerator? Will the engine not sit at a constant idle speed?


Here's what I think. I think that you need to double check and probably replace both your manifold adapter and gaskets, as well as most of your vacuum lines. That's most likely what's up with your idle issue, at lower RPMs a vacuum leak (no matter how small) will cause the engine to stumble and not sit at 750-800 RPMs like it should. This may be caused by the carburetor if it is very old, but most of the time the vacuum leak isn't in the carb.

You also need to check the size of the power valve, no matter what carb you choose to go with. The power valve should be 1/2 the reading at idle on a vacuum gauge. So, if your manifold vacuum reads 17 in Hg at idle, you need a 8.5 in Hg power valve. Holley power valves work in an MC2100.

As for venturi size, I ran a 1.14 on a stock '78 AMC 258 i6 / 4.2l that originally came with a single barrel carb. I thought the throttle response was a lot better with the 1.14 than with the 1.08, but the gas milage went down a probably 2MPG.

I personally would go with the 1.14. Here's a diagram for the vacuum lines.

 

[viking4949]

Thanks Elwood,
I only have two vac lines hooked up, the distributor advance and the PVC. Using an aluminum adapter with gaskets on both sides. The mating surface seems to pass the carb cleaner rmp test. The only change when spraying carb cleaner around seems to come from the choke housing. The two vacuum lines are good. I will be getting a vac gauge tomorrow to get a good reading. At 600 rmp, it fights hard to stay running, 700-750 gets a descent idle, but a noticeable miss. Have the team rush rotor, cap adapter and cap, new plugs, wires, blaster coil. Still have wells ignition module however.

Hopefully it is as simple as an external leak. Either way, what about the choke housing passage into the air chamber, epoxy or not.

 

[mtnwhlr]

You should have it set at 750 RPM at idle.

 

[elwood blues]

Either way, what about the choke housing passage into the air chamber, epoxy or not.

You should have it set at 750 RPM at idle.

I've read about the epoxy in the air passage, I honestly have never done it. If you have the link saved, would you post it please? I want to read it again.

I'm betting you're on the correct track. Get the vacuum lines straightened out and that will help the engine run better. mtnwhlr is correct, idle speed for a manual Transmission should be around 750 RPMs, you will never get a AMC 258 i6 / 4.2l with a carburetor on it to idle at 600 RPMs, that's just too low.

 

[alpha6]

Forgive my ignorance, but looking at the vacuum diagram, I can see everything but not sure what the CTO is where all those lines come together. I am finishing up my mc2150 install tomorrow and would like to get the vacuum hook ups right the first time. Thanks for all the great info you guys!

 

[alpha6]

Never mind...I figured it out.

 

[diddy2003]

I have some questions for you before I try to explain anything.

1. How is the MC2100 attached to your intake manifold? A good small 2 barrel to large 2 barrel adapter and gaskets (fel-pro).
2. Have you ever measured your manifold vacuum?
3. I need you to describe in more detail what you mean by you can't get it to idle. Will the engine die when you let off the excellerator? Will the engine not sit at a constant idle speed?


Here's what I think. I think that you need to double check and probably replace both your manifold adapter and gaskets, as well as most of your vacuum lines. That's most likely what's up with your idle issue, at lower RPMs a vacuum leak (no matter how small) will cause the engine to stumble and not sit at 750-800 RPMs like it should. This may be caused by the carburetor if it is very old, but most of the time the vacuum leak isn't in the carb.

You also need to check the size of the power valve, no matter what carb you choose to go with. The power valve should be 1/2 the reading at idle on a vacuum gauge. So, if your manifold vacuum reads 17 in Hg at idle, you need a 8.5 in Hg power valve. Holley power valves work in an MC2100.

As for venturi size, I ran a 1.14 on a stock '78 258 that originally came with a single barrel carb. I thought the throttle response was a lot better with the 1.14 than with the 1.08, but the gas milage went down a probably 2MPG.

I personally would go with the 1.14. Here's a diagram for the vacuum lines.

Thanks for the picture. Very helpful.

 

[gert]

As a side note, sometimes weak spark plug wires and a crappy ECU can cause you to have a miss just to further complicate things a little

 

[wm69]

I was wondering what idle speed should be. I've got mine at @500 RPM and thought about idleing it up a little bit (just swapped a new to me 2150 on it after some issues with my old 2150 and it turned out I had a bad fuel pump).

 

[JR74CJ5]

I was wondering what idle speed should be. I've got mine at @500 RPM and thought about idleing it up a little bit (just swapped a new to me 2150 on it after some issues with my old 2150 and it turned out I had a bad fuel pump).

You should have it set at 750 RPM at idle.

mtnwhlr is correct, idle speed for a manual trans should be around 750 RPMs, you will never get a 258 with a carburetor on it to idle at 600 RPMs, that's just too low.

As stated, for Manual Transmission around 750rpm. I have my AMC 304 idling around 550-600 rpm, but thats just for around town.