MC2100 HELP! Google ain't cutting it.

[TrebleHook]

Folks,

I am dying. Literally. Almost daily.....

MC2100/AMC 258 i6 / 4.2l /HEI. 1 ported vacuum to the HEI (not the one under the choke - that one doesn't seem to work). 1 manifold vacuum to the PCV. 12v current to the electric choke (but it's never seemed to actually work).

New manual fuel pump.
New HEI.
New plugs/wires.

Starts like a champ. Once it's warm - idles fine.

I'll be driving down the road and all of a sudden it just starts to stall, like it's out of gas. I'll pull over. Try not to get rear-ended and killed by a speeding mustang (as happened today - seriously scary).

Generally, I can just pump and pump and pump the gas pedal and crank the motor over and it will start and rev just fine...and then it's back to running fine.

I've fixed the vacuum leak at the manifold. No other vacuum leaks I can find.

I've toyed with the 2 idle screws in the front of the carb. Turned both all the way in. Turned out 2 turns. Started motor. Turned them in until it started to effect the idle -- backed them off about 3/4 turn. They are basically matched. Can someone explain to me what exactly each of them do??

Timing was advanced by ear when the idle reduced after fixing the vacuum leak. I just went with what sounded good.

Obviously I need a vacuum gauge and timing light. Once I get them (tomorrow)....then what?

Should I rebuild the carb just to be sure? Wife and kids are out of town the rest of the week, so it's totally possible.

Thanks in advance for the advice. You can check out the entire saga with this pig over on the "build" forum if you want some context - see: TrebleHook's Circus.

Cheers,
TrebleHook

 

[TrebleHook]

Oh - BTW....when it starts to die/stall/sputter I CANNOT feather it back by working the gas pedal.....thought that might be important. Or not.....

 

[elwood blues]

It's okay, the motorcraft/autolite carb doctor is in



Let's start with a couple of vacuum diagrams,


This one I found online a few years back, it's helpful.

This one I made in MSPaint, it's how my lines are ran

 

[elwood blues]

Let's dig right in to this, because at one point I had the exact same symptoms, chronicled here: drifting timing (caution VERY long winded). You have to get a timing light and a vacuum gauge. You can rent them from Advance Auto or AutoZone, either one of them, when you're done and take them back you get your money back. But this won't work without them.

Let's start with; do you know when the last time the carb was rebuilt? Did you rebuild it? When was the last time that you replaced your Ignition Control Module (ICM)?

As for the idle screws, you should not attempt to adjust those (the same for the timing) until the engine is at operating temperature. The idle screws adjust the air to fuel ratio for each venturi at idle speed.

edit: Also, not to be a jerk, unless you've been tuning cars for a long, long, long time, you shouldn't attempt to adjust your timing by ear, use a timing light.

 

[elwood blues]

Alright, so we've checked over the phone:

jets (48F good to go)
float travel (3/4" good to go)
vacuum (steady 20 IN/hg)
PCV valve (not frozen)
EGR valve (removed)
vacuum lines (appear to be routed correctly, but you have an HEI dizzy, so we're not sure)
+12 VDC to the choke

We think that it may be in his HEI unit, but I'm no HEI expert, anyone out there have any other ideas?

 

[TrebleHook]

Mr. Blues -- Really, really appreciate the call and your troubleshooting advice!

Vacuum gauge told me the HEI vacuum line was actually hooked to a port with ZERO advance and the Jeep was running okay (I've been driving it all over town -- but it does lose power at higher RPMs). I thought the ported vacuum below the choke was not working -- turns out it's spot on: Zero at idle, 15ish at throttle.....

So now with the vacuum line attached at the port below the choke and the vacuum actually working, the engine "spark knocks" (sputters?) at higher RPMs. Same symptoms I was having before I moved the vacuum line to a ZERO source.

We also determined that the timing is at approx. 12 (6 little raised dashes from the bottom of the marks -- best I can describe).

So there-in lies the ultimate question: W....T....F?

Any help? Is my Chinese made HEI distributor AFU? For the record, I think it's the same one as Rescue Diver has (as seen in his Definitive HEI MC2100 thread) but I can't seem to adjust the vacuum with an allen wrench like he did....

Hmmmmm.....

Cheers,
TrebleHook

 

[elwood blues]

Mr. Blues -- Really, really appreciate the call and your troubleshooting advice!

Absolutely no issue at all, we'll get you straightend out and running right. BTW, the first round's on you

 

[TrebleHook]

Absolutely no issue at all, we'll get you straightend out and running right. BTW, the first round's on you

You get this one figured out...all the rounds are on me!

 

[IOPort51]

Elwood
are you running a fuel bypass line? If so are you using the ford fuel filter and the jeep filter?
How is this best set up.

 

[priceg]

Just for giggles, run it with the gas cap off.

 

[TrebleHook]

I replaced the OME cap with a vented one about 2 weeks ago....the saga has gotten worse today. I am in the garage pouting...and contemplating actually rebuilding the carb tonight since I found the kit....

This is unbelievable. More to follow.....

 

[priceg]

Do you have a fuel pressure gauge, or can borrow one?

 

[TrebleHook]

Alright - well it's been a long Jeep day.

I left work early with the intent of doing all kinds of things -- ended up working on the Jeep. Can't lie...it's better than the alternative.

So I took the Jeep to my mechanic hoping he could help tune the HEI. Turns out all I really needed was a 2.5mm allen wrench.

He's not a MC2100 guy. He was a bit fumbled by the whole set-up and what I was trying to do...but the shop is a real bunch of good guys and they are honest beyond question. I was there from 1pm-5pm...he easily spent 3 solid hours on the Jeep. Didn't charge me a cent (he still considers this "tune" a part of the HEI install from months ago). Can't beat that.

He got the vacuum advance on the HEI backed off...first test drive it ran like a BAT OUT OF HELL. It friggin' pulls....really hard....all the way through the RPM band. I kid you not, I had it hammered in 4th and was doing 80mph on my speedo (which is probably more like 70mph) and it was still pulling. Not my normal driving habit -- but it was fun.

Somewhere in the tuning process the carb started acting REALLY weird. The idle jumped back up and it was "dieseling" when turned off. It was just all over the place which made it very hard to tune, obviously. We tried all kinds of things...no joy.

So I was right back to where I began...Jeep that runs like . Very disappointing...

I drove it home and it was fine running down highway...the idle was the only real issue. Got it home and decided to pull the carb to inspect. I was prepped for a rebuild (my first) but wasn't really looking forward to it since the wife comes back on Friday and I still have to get the roll bar painted and installed....

Well after much putzing and a beer, I realized that the choke as AFU. The spring was not properly seated and wasn't returning the butterflies. I also found that the choke pull-off diaphragm was totally shot. Replaced it the diaphram-- thank you Autozone.

Reseated the choke spring....reset and backed out the idle mixture screws (2.5 turns). Hooked up the vacuum gauge to manifold vacuum. Set the main idle screw in a couple turns.

Fired it up. Tweaked the mixture screws while watching the vacuum gauge. Hit the timing light - brought the timing to 12 degrees (from 6). Backed off the main idle screw. Vacuum was tuned to a solid 20Hg. Done and done!!

We'll see where this leads -- but I think I found the Gremlin that's been haunting me (choke letting the butterflies slap shut whenever they wanted) and got the HEI/Carb tuned. Today was a good day....

Cheers,
TrebleHook

 

[elwood blues]

Good deal

 

[gert]

Yeah gotta love it when a plan comes together

 

[Rescue Diver]

Congrats!!!!

The choke never crossed my mind. I'm really glad you finally found the gremlin! Sorry it took so long, but look at it on the bright side. You have now fixed several other problems that otherwise would still be around. You no longer have a big vacuum leak at the carb base, and your distributor is tuned correctly!

Not to mention that your skills as a mechanic have improved a whole lot! Think of all the things you learned from all of this!

On a side note, we have yet another member that has disproven that old post saying that the HEI vacuum advance is not adjustable.