Frustrated noobie (MC2100 conversion)

Noobie here from Pleasant Grove, Utah.

Hello all, I've been reading your forum for a while now and am finally frustrated enough that I need some advice.

The history:

Several months ago, a crack in my exhuast headers expanded itself to a 1" hole, right underneath the carburetor. A few weeks later, my leaking Carter YF splashed a little fuel on that exhaust hole, resulting in the necessity to remodel my engine compartment. Thanks to my fire extinguisher, all that was damaged was all of my vacuum hoses and the wiring for the headlights (the engine wiring and distributor are all on the other side of the engine).

What I did:

First, I replaced the melted wires and plugged off all the vacuum ports. Then I started it long enough to verify that all else was in working order. I decided to replace my entire exhaust system and upgrade to a MC2100. So I tore out the entire exhaust and removed the intake manifold and headers. I purchased a professionally rebuilt MC2100 (1.08 venturi w/ #46 jets) and a new header.

I put everything on and started her up. She would only idle if I kept pumping the gas and then only very roughly. After some time messing with it, I decided I had an extreme intake/vacuum leak, but couldn't pinpoint it because I couldn't get the thing to idle. So I took everthing off, cleaned up all the mating surfaces again, and reassembled, torquing everthing carefully. In the process, I stripped out one of the bolt holes that holds the intake/exhaust to the head (I decided to ignore that for the time). I started it up and got it to run well enough to take it to the exhaust shop to have the exhaust system built and installed.

Then I started trying to tune it. I replaced the points, condensor, plugs, wire, cap and rotor. Hooked up vacuum lines and adjusted the idle speed, mixture, and timing. It still wouldn't run quite right. As long as it was idling it would run excellent, but if I opened the throttle a little it would stumble. If I would accelerate a little more aggressively, it would be fine until I let off the pedal a little, then it would stumble again.

Knowing that the one intake/exhaust bolt hole was stripped out, I decided there was still an intake/vacuum leak. So I pulled everything off again, drilled out the hole, heli-coiled it, and put it all back together, torquing everything carefully, again.

The problem:

I started it up again and noticed it was idling even better than before, but I didn't have any of the vacuum lines attached. So I attached everything and took it for a drive. Same thing as before, it would idle well, accelerate well under a heavy foot, but stumbles during light acceleration or coasting. My first thought was that maybe my carburetor jets were a little too lean; then, because it was idling so great before I hooked up the vacuum lines, I disconnected the distributor's vacuum advance hose and plugged off the port on the carburetor. It runs perfectly, except for a little pinging under heavy load.

Why would it run perfectly with no vacuum advance, but stumble with it? I am completely stumped. BTW - I'm running an MSD 6 ignition box (so timing is the only ignition adjustment that is necessary, the points are only a trigger mechanism for the ignition box, they do not determine the dwell angle).

Thanks in advance,

Ron

 

I don't think so. The carburetor has been carefully wrapped in bubble wrap and put inside a plastic bag everytime I've had it out of the vehicle. When I depress the throttle, I get a nice stream of fuel, that's the accelerator pump doing it's job isn't it?

 

you're letting the motor get up to temperature, right?

i ask because i was having carb icing problems, and the symptoms were similar. it would idle fine. with a light/easy pedal, it would stumble/buck. put the pedal to the metal (err, fiberglas), and it would go fine. once the carb warmed up suitably to melt the ice, and prevent further buildup, it worked properly. just a thought.

 

Vacuum is really hateful when you have a leak, but, if you don't, it's pretty simple.

Check out the picture below. This is the 2100 on my 304. I took the picture from the front driver side fender (to give you the angle).

Use that vacuum port to connect to your distributor. This will give you direct vacuum, which provides your distributor extra vacuum advance through your range.

It sounds like you are running ported vacuum now. Of course, some engines actually run worse on direct vacuum, so if you are already using that, try ported.



Edit: just to be clear, the vacuum line is in the bottom right corner of the picture, left of the choke.

 

Actually, I have had that same problem myself in the past, but the temp here has been between 70 and 80 the last few days I was messing with it, so it's not icing up.

And yes, I waited for the engine to come all the way up to temperature before I adjusted everything. I plan on adjusting everything again tonight unless someone here comes up with a smoking gun for me.

 

I thought that direct vacuum pulls full on at idle, then drops off as you open the throttle, while ported vacuum pulls more and more as you open the throttle, which is what's needed for the vacuum advance.

Or do I have my terminology wrong/backward?

In any case, that's a great idea. I will try the port in your picture tonight. Anything's worth a try.

 

Just trying to gather more data here; When it stumbles on slow acceleration, have you tried to slightly cover the top of the carb with your hand or a rag to richen the mixture. If so, how does the motor respond to the richer mixture?

 

so I have a clear picture.... 74 258
you've gone from the YF 1bbl to the MC2100 2bbl. mounted on the factory 1bbl intake, correct ?
your intake does not have an EGR valve, correct ?

 

I haven't tried that. It's a good idea. I had assumed the mixture was a little lean since it's fine when you get on the gas a little and it's dumping extra fuel down the carb. But when it ran good with the vacuum advance disconnected, I gave up on that theory. I'll try the hand over the carb thing and report back tomorrow.

 

Correct on everything except the EGR valve. There is one there, but since the original exhaust manifold has been replaced with headers, the EGR has been blocked off and is not connected to any vacuum source.

 

http://www.jeepsunlimited.com/forums/showthread.php?t=727061

Here is a thread I started, but quickly turned into a great diagnostic tool...

http://www.jeepsunlimited.com/forums/showthread.php?t=735139

 

A few things occured to me: What is your elevation? Sinc ethe O2 is a little thinner up there, the 46 jets should be fine.

Is the BASE of your 2100 completely sealed by your adapter plate? The autozone "Spectre" brand adapter plate, which I thought I had to use, only seals the whole way around an older model of 2100. The newer 2100s end up having a HUGE gap on the back side of the carb. This can be solved with a really thick gasket off of an F-150. (Can be bought new, instead of scrounging the boneyards...) Here is a thread that documents another fix: a Wagoneer gasket:

http://www.jeepsunlimited.com/forums/showthread.php?t=652949


What do your RPMs do when you pull a vacuum line? Stay the same or go up? Find vacuum leaks with a can of GOOD starter fluid. (Cheap stuff won't give you a good indication...) Tuning the carb before having vacuum issues fixed is akin to peeing into the wind.

The 2nd thread I posted above was gold for me: W/O COAS' advice, I'd still be trying to plug vacuum leaks on the outside of the carb, instead of plugging them with that port.

 

I don't know specifically about the 1V manifold - mine has an '85 2V manifold since it was easy and cheap to find and the post-80 manifolds are supposed to flow much better than the siamesed manifolds.

46F should be fine - the two carbs I've found recently had 48F and 44F (CA E100 and CA 360 Cherokee resp).

If it runs at a constant speed and idle well, but does not run under constant low vacuum acceleration, I'd suspect the power valve. Usually these blow out and stay open, but they are intended to richen the mixture under low vacuum conditions. I presume you have a single-stage valve? They are cheap and tuneable (by selecting the right one).

Most of this advice applies to the Motorcraft carbs too, since they are actually cheapo OEM Holleys.
http://www.holley.com/data/TechService/Technical/power_valves.pdf
http://www.lovefords.org/tech/holley.htm

 

I too am suspecting the power valve but wanted to hear the test results of the manual enrichment technique.

 

Mongo-

-It ends up I was already using the vacuum port you suggested from your picture. You're carburetor is oriented 180* different from mine, so that port is on the passenger side of my Jeep. For fun though, I tried the other port, it ran worse connected there.

xlr8n-

-I did the rag over the carb test. When I hold the throttle open at the point where it stumbles worst, then put a rag over the carburetor opening, it doesn't seem to react at all until the point where the carburetor is completely covered; then it just dies.

Mugzilla-

-My elevation is about 4500 ft.
-There is no gap on the base of the carburetor (I know what you're talking about, I was worried about that when I first saw the carb and the adaptor, but the gasket/spacer that came with it seals the base completely).
-When I pull the smaller vacuum lines there doesn't seem to be any noticeable rpm change at idle, but there is one big fat vacuum port to the right as I look directly at the linkage. The engine coughed and then died when I pulled the plug over that port.
-I haven't tried the "GOOD" starting fluid technique of finding vacuum leaks. I've been using a spray bottle with water; I've heard both ways work (stumbles w/ water, speeds up w/ starting fluid). I'll try the starting fluid tonight.
-As far as the port on the inside that is used for the choke, my carb was setup to use my stock thermal choke, which I assume means that the port you are speaking of is NOT plugged? However, I do not have the choke connected; the "heat tube" that attaches to the headers has different threads than the carb so I have not connected it. Instead I adjusted the choke so that it is wide open even at colder temperatures, which shouldn't matter anyway since I'm doing all of my testing at full temperature, right?

xlr8n/mugzilla/timgr-

-Sounds like some of you were leaning toward a bad power valve? In one of mugzilla's links, it suggested testing the power valve by turning the mixture screw all the way in. If it keeps running, the power valve is bad, if it dies, the power valve is good? Does that sound right? Mine dies when I turn the idle mixture screws all the way in. Any other suggestions on testing the power valve.

What about the " automatic choke port" on the inside of the carburetor, should I have that plugged up since the choke isn't actually operational?

 

The idle-screws test will only work if the PV is ruptured and pouring gas into the intake. You might have the opposite condition, where the PV won't open - then you would have a lean condition under load (low vacuum).

The choke port is a vacuum leak, albeit a very small one. If it's not used, it should be plugged.

 

how is your fuel pressure?