Use a vacuum gauge.

 

OK, I haven't been following this...
What kind of carb, on what, in what, ect...???

If you have a vacuum gauge, and you can't tune a carb or diagnose an engine without one, Try here, http://www.junkyardgenius.com/tools/vacgauge.html

The only reasons for the idle mixture screws to not be functioning are,

1. Some dummy cranked them in until the nose of the needle twisted off in the adjustment passage.
Remove the screws and check for a point on the screw.

2. The passage is clogged with crap from an imcomplete cleaning before rebuild, or clogged with crap from inadiquate fuel filtering.
Remove screws and try carb cleaner with that little plastic tube attached to the nozzle.
If the passage fills up and the can doesn't have enough pressure to blast the blockage out, leave the passage full, then use a blow gun and shop air!
You aren't going to break anything...
Once the passage is open, rebuild the carb.

3. Some 'Smog Years' had carbs the screws worked backwards.
Lean was out, in was rich.
Since we don't know what carb you have, we can't tell you if this is the case...

4. Float level so high that fuel is flooding the idle passages.
Adjusting float level should have been done immedately after carb install.

 

It is a MC 2100 on a 304 in a 73 cj 5. I had to replace the powervalve due to a really nasty backfire and it hasn't been the same since.

Matt

 

Now we are getting somewhere...
I have a '73 CJ-5 with factory V-8 also.
I'll have a look...

First Picture,
1. This is a clean air vent.
It's intended purpose was to supply clean (and cold) air for the choke regulator.
If it's not hooked up properly with NEW hose, cap it like in the picture.

2. Choke regulator.
Adjust as necessary, instructions on the end of the canister.
Ford had a bad habit of the end of the bi-metal spring slipping off the choke flap arm, so you may want to check that.

3. Ported Vacuum.
This is the vacuum port that's supposed to supply your distributor.
Make sure it has NEW hose or is capped.

4. Idle adjustment screw.
Take the screw out, make sure it has a point on it.
If it doesn't, you are in for a rebuild and new screws, if it does, squirt some carb cleaner in the hole and see if it drains quickly.
Cleaner with the little plastic straw works well here so you can really get down in the hole.

If it drains and the screw has still got a point, replace the screw, GENTLY screw in all the way until it bottoms out, then back out 1-1/2 turns for an adjustment starting point.

5. This will be the large manifold vacuum port.
NEW HOSE to where ever it's going... PCV or brake booster, ect.

6. Factory ID tag.
If you have one of these, the carb probably hasn't been opened, or the guy that did it actually knew what he was doing.
This is the little bugger that will get you the correct parts.
If you ever buy a used carb, look for this tag, and look to see if it looks like it belongs there...
(IE: grease smudged where it mounts, rubs from vibration on the zinc carb bowl parts, ect.)

Picture 2.
4. Idle Adjustment Screw.
This one is out of sight under the float bowl.
Same drill as above for this screw.

6. Factory ID Tag Again.

7. Accelerator Pump Housing.

8. Small Manifold Vacuum Port.
Make sure this is plugged with a NEW hose or a NEW cap.
this can not be left open.

9. Not shown, New base plate gasket...
You must not allow any leaks at the base plate of the carb.
Don't over tighten the bolts or you will warp the baseplate of the carb and leaks will happen!

 

Hey what is your opinion of a holly 500cfm carb on this engine? I had one offered to me for only 100.00. there's nothing wrong with it the guy thats got it took it off to put 4barrell on his truck.
Matt

 

I think most are better off with the Motorcraft carb than the Holley. Comparing both bone stock the motorcraft works much better on inclines. If you are at all mechanically inclined, I would take the carb off, disassemble it looking for problems, clean it thoroughly and adjust the float height. Specs vary depending upon the year, but a good service manual should have the information for your year's float height and teardown procedure.

When its working properly, you should definately have a point at which screwing in the idle mixture screws makes the mixture so lean it stumbles then stalls. You can make it run far richer than it needs and not see that pronounced of an effect unless you are using a vacuum gauge. Keep the two screws in synch, and work slowly and systematically to find the peak lean vacuum. Ie, the leanest point at which it will produce peak vacuum at idle. But don't bother with any adjustments unless you know the carburetor is working properly and the float bowl and fine passages are not full of crud.

 

Thanks guys

It's coming off this week as soon as school gets out.

Matt

 

Holley is a good carb, very tunable.
I run them off road all the time, but you will have to do some mods. Easy enough if you know what to do.


If you take a good look at the MC 2100 you will find out it is a copy of a Holley...

 

Thats my problem I really don't know what to do. I'm ok with everything but major engine stuff. I guess I'm afraid I'll mess it up good. I'm gonna pull the carb off and rebuild it or at least try. I figure if I can put in a whole wiring harness surely I can do this too.

Thanks Matt

 

True the Motorcraft is a stripped down copy of a Holley.
Not quite as efficient as the Holley, much simpler.

The bad thing about Holley is the tiny passages - a piece of something you'd never notice in your eye screws them up.
If you know carbs well, and are prepared to fiddle fiddle fiddle, then Holley's a good choice.

Notice there's a jillion "goodies" you can buy for Holleys - if they were really so good, why are they needed to "improve" them?

Hardly anything available for the Motorcraft MC2100. Makes ya wonder.


JYG - #3 - good hose or "capped" off? You don't run vacuum advance on the distributor?

 

Hey I've got my confidence up and ready to tackle rebuilding this carb and I find out the kit I have has the wrong diaphragm in it. I go the the parts store and tell them what I want. A rebuild kit for a MC 2100 right well now they need numbers off the carb tag but I don't have a tag on the carb. I guess someone before me took it off I don't know. I've called Advanced, Napa, and Autozoo and Oreillys and it's the same story here except for Napa who tryed to tell me there was no mc2100 on this but it was a Chrysler carb. What give's are all the parts stores dummies or is it me. Does anyone know what kit I will need. The kit I got had the diaphragm without a long shaft. My carb has a long shaft attached. If I've missed something here forgive my stupidity and just laugh at me.

Matt

 

A couple links that might be useful:

Exploded View of an American Motors model 2100

Rebuilding the Motorcraft 2150

And rebuild kits:

NAPA

Auto Zone

Hope those help.

/ubbthreads/images/graemlins/40BEER.gif
Dale

 

I have a factory tag on my '73, and I'll get you the numbers.
I should be able to get to the little CJ on Friday... (On the road now)
--------------------------

[ QUOTE ]
RRich Wrote;
JYG - #3 - good hose or "capped" off? You don't run vacuum advance on the distributor?

[/ QUOTE ]

I run a vacuum advance, but you would be surprised at the guys that don't run one, or the guys that run straight manifold vacuum.
Besides, for testing, capping the vacuum ports removes variables, like leaking lines and ruptured vacuum canister diaphragms...

 

I just was wondering if you had some "trick" device instead.

No vacuum is better than manifold vacuum. At least with no vacuum, it's not working against you.
What gets folks confused is often Manifold vacuum is used, but as a power or energy source (just like heater controls, vacuum has nothing to do with generating heat, but it's used to open and close blend doors.) Its switched on and off by solenoids. The actual vacuum curve isn't used.

Manifold vacuum is nearly the opposite from what the engine needs for advance.

I saw a "trick" electronics device that was supposed to be better than vacuum or EST - strange device - it used a strain gauge on an engine mount to determine load. I didn't get a chance to learn more about it - but it sounded interesting. Since you sometimes get involved with stuff like that --

 

Nope no trick devices on my carb or engine. I just took the carb completely apart and I think I found the problem. The little orange diaphragm in the acclerator pup on the front of the carb was all deformed and brittle. I've got to get another kit though as the diaphram in the pump has a long post in it and the kit came with one without a post. Am I correct here? Right now it is soaking in carb cleaner. It was sure nasty inside and all ports were filthy as well. I guess this is a sign I need to go ahead with the new gas tank as well as it is in rough shape. At least I'll know the whole fuel sys is clean.

Thanks for all the help.

Matt

 

He He - Hickory - sounds like you are on the right track - removing the cholestoral from the carb.

My comments on "trick devices" were directed to the genie in the junkyard.

 

Boy the carb now looks like a new one now, however I cannot find the correct kit. We went throough every kit down at the autozoo and none of them were correct. I didn't know there were so mant 2100's. Another store same story ect. ect. I don't think this carb was on it originally. Thye keep asking for numbers but I don't have a tag on it and the only numbers on the flang they tell me are no good. I'm beginning to thin my only option is a new carb. Any suggestions.

 

Tag says,
"AM" (AM General Logo)
"3DM2 AC"
"B 3C 29"

Pictures can be located on an earlier post (this thread) from me.

 

My carb looks a little different than yours. Mine does not have anything behind the throat it's just flat. I'm ggonna try to post a pic tomorrow. On the flange the numbers read 5TZB does these man anything?

Thanks Matt

 

My carb is back together and my jeep now cranks better than ever however I'm getting fuel into the oil again. Is gas supposed to stay in the float bowl when it is not running. If not this is my source of fuel in the oil. I've replaced the fuel pump and now I'm out of ideas. White smoke is a sign of burning oil right well it runs good for about 5min and then starts smoking. Could there be a valve problem here as well.

Matt

 

Oil burns with blue smoke. And yes, the gas is supposed to stay in the float bowl when the engine is shut down. But the volume is pretty small - maybe a quarter cup total or less.

 

Well the gas is going somewhere when it is shut down I'm thinking there is still something wrong with the carb. I was not able to find a new pump diaphragm or power valve for this mc2100, strange I think. My jeep is putting out white smoke what is that then? I did enjoy successfully getting the carb back together after completely taking it apart. Kinda made me feel good!

Matt

 

white smoke combined with it not smoking till the motor warms up a bit makes me think its antifreeze, i'd start looking at either the head gaskets or intake manifold.
try uncapping the radiator (when cold) and run it with no cooling system pressure, if it runs without smoking i'd take that as confirmation of a cooling leak once pressure builds in the system, as for the gas in the oil...are you sure all cylinder are firing? an unburning cylinder or two will gas your oil up real nice like