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post #1 of (permalink) Old 03-05-2012, 07:39 PM Thread Starter
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MC 2100 carb help

Ok, guys, got a new issue for you. I haven't seen, let alone tuned a carb in probably 10 years, so needless to say I'm a little out of practice. I have a stock 304 with a stock 2bbl, it does have headers, but no crazy cam that I know of. What I have going on is, everytime I would accelerate from a stop, (hard or slow) it would hesitate real bad and sometimes die. I pulled the carb off cleaned it up and rebuilt it. Reinstalled the carb and reconnected all the vacuum lines. Runs much better, however my problem has now reversed itself. When I come down from a high idle (to a stop) or when it sits for too long at an idle it sputters and SOMETIMES dies. Runs very well at take off and through the range now. Thanks again for the advice.
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post #2 of (permalink) Old 03-06-2012, 05:16 AM
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Sounds like an ignition timing problem to me. You may need a little more initial advance. Just a guess.

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post #3 of (permalink) Old 03-06-2012, 07:36 AM
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Probable vacuum leak. Could be as simple as loosening the base nuts and retightening in an X pattern.

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post #4 of (permalink) Old 03-06-2012, 09:03 AM
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Quote:
Originally Posted by jeepno4 View Post
Ok, guys, got a new issue for you.
I haven't seen, let alone tuned a carb in probably 10 years, so needless to say I'm a little out of practice.

I have a stock 304 with a stock 2bbl, it does have headers, but no crazy cam that I know of.

What I have going on is, everytime I would accelerate from a stop, (hard or slow) it would hesitate real bad and sometimes die.
Bad accelerator pump, drivers side front corner of the carb.
When you accelerate, this is the ONLY fuel the carb is getting for that acceleration.
You aren't 'Idling' anymore, so the idle mixture screws can't supply enough fuel,
And you don't have enough air flow through the Venturis to pull main jets in yet,
So the Accelerator Pump supplies a big shot of raw fuel to get you moving under load and low RPM.

Quote:
I pulled the carb off cleaned it up and rebuilt it.
Reinstalled the carb and reconnected all the vacuum lines.

Runs much better, however my problem has now reversed itself.
When I come down from a high idle (to a stop) or when it sits for too long at an idle it sputters and SOMETIMES dies. Runs very well at take off and through the range now.
Thanks again for the advice.
Vacuum leak, idle mixture or float level.
Probably float level.

--------------------------

Do you go back through tuning when you rebuilt?

Set the idle mixture with a vacuum gauge, or at least do the 50 RPM drop the manual recommends?

Make sure your timing is correct at idle, remove and plug the vacuum advance line to the distributor, then check base Initial timing...
Then tune with idle mixture with the vacuum like still plugged.
(Jim_Lou's suggestion)

The idle mixture needs EVEN screw adjustment,
All the way in, back them about 4 turns out EVENLY,
Then adjust in until the RPM drops by about 50 RPM,

Or use a vacuum gauge and screw them in EVENLY until you get the highest STEADY vacuum reading (Engine RPM will drop when you get close).

----------------------------

Once you get the idle mixture set, make a big effort to find vacuum leaks.
They are hard to find until you get the idle mixture set so the RPM will drop or increase when you find a vacuum leak...

Propane is one way to do it, turn a propane torch way up, with the engine running, and look for the idle to go up when you find an intake leak.
Be aware this isn't real effective since the fan is going to be blowing the propane away at a pretty good clip.

Another way to find vacuum leaks is a squirt bottle full of water,
Hose down all the gasket surfaces, plugs, vacuum connectors, where manifold meets heads, ect.
Idle will change when you find a vacuum leak since the water will temporary plug that leak.

You might also check the brake booster if you have power brakes.
A bad booster is a HUGE vacuum leak when the brakes are applied,
So if it seems like the brakes are doing their jobs, taking more pedal effort to get the vehicle stopped, it's probably the diaphragm in the brake booster...

Once you are SURE you don't have vacuum leaks,
Then you are left with float level.

MC 2100 is pretty sensitive about float level,
(Just a question, but did you weigh the float or shake the float when you had it out? That's usually a good way to see if the float is leaking)

When the float rides too low (Low fuel level) when you brake, the fuel sloshes forward in the float bowl,
If you see black smoke when you stop, then it's flooding, since the fuel moving forward out from under the float and the valve is opening,
You usually get a 'Chugging' before the engine dies.

If there is enough fuel in the bowl to keep the float/inlet valve closed,
Then you uncover the Jets and fuel to the idle passages, and you starve for fuel, this often results in a lean misfire condition, and eventually the engine dying if fuel isn't recovered in time.

Too high of float level will flood out the engine when you stop from higher RPMs.

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post #5 of (permalink) Old 03-06-2012, 08:59 PM Thread Starter
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Again thanks for the input guys. The timing is good. Did all that when I did the tune up a couple of weeks ago. I believe it is the float level issue. The float checked ok as far as leaks go, I think I just mis-adjusted when I was installing. No black smoke, so I think its set a little low. Will be pulling the top off tomorrow to double check. New vacuum lines on it. pretty sure I don't have a leak, but need to go buy a vacuum gauge and do some investigating anyway. On the subject of brake boosters, does anyone know what make or model will fit on my 79. No one seems to offer a new one for that year.
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post #6 of (permalink) Old 03-07-2012, 11:12 AM
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Quote:
Originally Posted by jeepno4 View Post
Again thanks for the input guys. The timing is good. Did all that when I did the tune up a couple of weeks ago. I believe it is the float level issue. The float checked ok as far as leaks go, I think I just mis-adjusted when I was installing. No black smoke, so I think its set a little low. Will be pulling the top off tomorrow to double check. New vacuum lines on it. pretty sure I don't have a leak, but need to go buy a vacuum gauge and do some investigating anyway. On the subject of brake boosters, does anyone know what make or model will fit on my 79. No one seems to offer a new one for that year.
I can't remember for sure, just changed the '79's brake booster a year ago,
I'm thinking it was the same as the FSJ from the same year, the two door wagon...

I had the same issue, dying when I slowed down, but the brake booster crapped out so slowly over time I really didn't notice the extra pedal pressure.
Until I could actually HEAR the sucking sound when I applied brakes did I think about the booster...

And I rebuilt the carb also thinking the issue was with the carb...
Automatic made things worse, since I wasn't pushing in a clutch to slow down, it would idle really low and rough when I went to stop...
So I went to do brakes, couldn't really find anything wrong on the bottom end, everything working and adjusted,
And that's when I took notice of the 'Sucking' sound when I applied brakes,
Put that and engine wanting to die together...

New booster fixed both issues,
But I nearly threw myself out the windshield!
The brake pedal issues had come on so gradually I really didn't think about it...
So new booster, hammered the brakes like I got used to when driving that pig, and liked to slam my face on the windshield!
That bucket nearly stood on it's nose! Pedal went down with so very little effort it wasn't funny!
Had to learn how to drive that thing all over again!

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post #7 of (permalink) Old 03-08-2012, 06:30 PM
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On the subject of brake boosters, does anyone know what make or model will fit on my 79. No one seems to offer a new one for that year.
Rock Auto carries a remanufactured booster.

Quote:
I'm thinking it was the same as the FSJ from the same year, the two door wagon...
I know the J-20 booster is not compatible with the CJ. Rock Auto lists the same booster for the Cherokee and Wagoneer as the J-20.

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post #8 of (permalink) Old 03-09-2012, 10:56 AM Thread Starter
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I found a local guy that has disassembled his cj and is parting it out. he has a brand new bosster on his rig. It is the upgraded one for the 4 wheel disc. He is only asking 120 for the complete assembly. My question is, will this be compatible with my rear drums? Will it blow out my wheel cylinders? I wouldn't think there would be a lot of difference.
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post #9 of (permalink) Old 03-09-2012, 02:06 PM
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It should be fine as long as you have the stock multi-function (AKA proportioning) valve in the system. One of it's jobs is to limit pressure to the rear brakes. Anyway, what the booster really does is to add to the muscle power of your leg, so it has the same effect as your working out on the Stairmaster six hours a day. It only applies as much pressure as you want applied to make the stop.

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post #10 of (permalink) Old 03-11-2012, 03:12 AM Thread Starter
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Thanks Jim. I don't think my legs are getting weaker so my booster must be leaking. I can't hear it when I press the pedal, but the engine is a little loud. Haven't tried adjusting the carb anymore, until I get the booster installed. Wll be picking it up frideay. In the mean time, guess I won't need to hit the gym.
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