So I went HEI, but MC2100 still stumbles - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 04-26-2009, 08:14 AM Thread Starter
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So I went HEI, but MC2100 still stumbles

So I installed this new HEI dist. that I got off Ebay. ($80...drop in, one wire hook up...not a bad deal...)
I notice a great improvement in acceleration. It accelerates very smooth, and more powerfully...like a new car. A slow new car, but still...

But still the main annoyance is still there: a stumble just off idle. If I punch the gas, the engine will die.

Anyway, it is a recently rebuilt 1.08 MC2100, and I think #48 jets(but its been a while since i looked at 'em...) Could this be too much carb for my tired old 258?
I'm gonna install new plugs and see where that takes me today...
Before I ditch this carb, you guys think #47 jets would make a difference?

Thanks
Tim
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post #2 of (permalink) Old 04-26-2009, 12:13 PM
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My initial thought... have you confirmed you ARE getting a shot from the accelerator pump? It's used to transition from the idle circuit to the run circuit (where the jetting takes over).

If not, you can confirm the accel. pump is working by looking down the venturis (prop the choke open so you can see), thumb the butterflies (snap the throttle open), and look for a squirt of fuel coming from the carb body into the venturis...

If so, did you adjust the accel. pump linkage by moving the pump linkage to another hole (don't recall which, but have seen it mentioned in NUMEROUS 2100/258 write-ups). If not, you may be getting TOO much fuel from the accel pump and causing a bog (short-term flooding)

LMK

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'72 "Mossy Edition" Commando
the Trail Forge

Last edited by Caver Dave; 04-26-2009 at 12:17 PM.
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post #3 of (permalink) Old 04-26-2009, 03:24 PM
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OR -
Is the HEI stock original?

Most, if not all HEI's were used with carbs that had a very weak ported vacuum signal. The MC2100 has a fairly strong one. What can happen is at "tip-in" the advance comes in way too much and too fast, slamming the advance all the way in momentarily, giving a stumble. (Yours is on ported vacuum already I hope. Manifold vac will give the stumble all by itself.)

Try it without the vacuum connected. If it goes away - bingo!

Then you'll need to get an adjustable vacuum advance diaphragm - good parts houses have them, cheapies won't.
I prefer Crane, about $25.

If that's it, let us know and we'll guide you through the installation and how to set the curve.
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post #4 of (permalink) Old 04-27-2009, 12:56 PM
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Warnings first.
The 'Ebay' and '$100' HEI clones have been killing engines right and left, so I suggest you do a couple of tests to find out if you are becoming a victim.

First off, you need to remove the distributor,
Take an 1/8" drill bit, and rotate the distributor gear 90 to the roll pin, and try to drill the apron of the gear.

If you can't drill directly into the gear, it's HARDENED STEEL instead of ductile cast iron, and it WILL eat up the camshaft gear!

Link here,
GM HEI vs. Motorcraft/DuraSpark Ignitions

..................................

Secondly,
There is a HUGE batch of housings and shafts that are TOO LONG!

This loads the oil pump too much, and wipes out the oil pump and the drive gears.

For this one, Install the distributor WITHOUT the gasket in place,
Make sure the distributor flange SEATS ON THE TIMING HOUSING... WITHOUT THE GASKET.
IF so, then the housing isn't too long.

Once the distributor is seated WITHOUT the gasket,
Hold the distributor hosing in place with one hand,
And lift up/down on the distributor shaft with the other.
See if the shaft has some play in it...
(Should be about 0.035" or more, so it's noticeable with just your bare hands)

If the shaft moves Up/Down, then the shaft isn't too long and you are ready for reinstall WITH the gasket.
------------------------------------

Now,
Your stumble off idle,

There are TWO things you should do to cut down on that 'Bog'...

The first is to drive 55 MPH or so, on flat, level ground, without moving the throttle pedal...

This is called 'Part Throttle Cruise'.
You should take a vacuum reading from the carb base plate or intake manifold at Part Throttle Cruise to determine what POWER VALVE you need.

Most times, with Street/Mild Trail Vehicles,
you will want 2 In.Hg. difference between Part Throttle Cruise vacuum and the power valve opening point.

Example,
If your Part Throttle Cruise vacuum is 8.5 In.Hg.,
Then you want about 2 In.Hg. lower power valve, around 6.5 In.Hg. so the Power Valve stays closed during cruise and saves gas.

If you run trails heavily, you might want to increase that gap to 2.5 or 3.0 In.Hg. so the power valve stays closed when you are trying to 'Finesse' obstacles...

If you rock crawl or blast wide open in mud pits,
You might want to block off the power valve all together, and use larger main jets to compensate.

If you use your vehicle as a 'Daily Driver', you might want to 'Lessen' the difference, 1.5 In.Hg. difference so you get fuel faster when you are jockeying in traffic.
---------------------------------

Your bog should be worst from standing start if it's an accelerator pump.
It shouldn't be so pronounced when coming off idle from rolling start,
If so, it's from fuel starvation when you open the throttle blades.

There is a transition period, when the blades first open, where the idle fuel isn't sufficient for the air flow,
And before the main jets supply enough fuel for the larger air flow...

This is where the ACCELERATOR PUMP comes in...
It delivers raw fuel to the intake to soften the transition period.
You probably aren't getting enough accelerator pump shot for your engine.

You will find a rod that connects the External Accelerator pump arm to the throttle blade linkage.

There will be adjustments at both ends of that rod.
One set of adjustments determines how MUCH fuel is injected into the intake,
The other adjustment set determines how LONG the accelerator pump shot lasts.
I suggest you try a different adjustment, play with it to determine if you need more fuel, or fuel longer to elminate the 'Bog' or 'Stall'...
------------------------------

Once you have taken care of that 'Bog', and you can highway drive your vehicle...

And see if you have a 'bog' in driveability that HEI distributors often cause in AMC engines...

When you are at 'Part Throttle Cruise', try accelerating a little, but just a little...
Like the vehicle in front of you speeds up, and you are trying to keep up with it in traffic,
Or you were to go up a hill...

You will PROBABLY (most likely) have a 'Bog' when you try and accelerate lightly at 'Part Throttle Cruise'.

Some people have 'Spark Knock', 'Ping', 'Valve Clatter' or what ever you want to call DETONATION, and no matter what you call it, or if you can hear it, or just feel it as a power loss, it's slowing destroying your engine!


This is because the HEI was never designed to work with an AMC engine, and it's laying in WAY too much advance at once!

AND!
If you take a look at the link I posted above,
Take a good look at the broken valve and ruined piston

The fix for this is an 'Advance Limiter' for the vacuum advance. You will need to take some of the total vacuum advance out of the system,
And it's EASY, but you will need to fabricate a limiter,
OR buy a specialized vacuum advance canister for about $30 or so...

Let me know directly if you have this part throttle 'Lag' or 'Bog' or 'Knock' and I'll show you how to fabricate and install a limiter for a dime...
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post #5 of (permalink) Old 04-27-2009, 03:37 PM Thread Starter
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Now I'm kinda concerned about this. The Ebay ad confirmed that this distributor has "a hardened steel gear for use with cast and steel billet cams". .I just got done reading some posts on this issue and its sounds pretty scary. I've already put 50-60 miles on this thing...sutmbling aside, it seems i've got a new bigger problem. What should I do about this dist gear?
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post #6 of (permalink) Old 04-27-2009, 04:12 PM
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Yes, get the softer gear for sure.

TR - it's not that it's an AMC engine, the Ford MC2100 carb has too strong a ported vacuum signal. It "slams" the advance in full too soon. The stock can is already limited to about 15 degrees. Most of the HEI's had Q-Jets - very weak ported signal.

The best cure is the Crane adjustable advance unit - last time I bought one it was about $22 at Carquest. Then it should be adjusted to have the limiter cam (comes with it) installed to limit it to about 15 degrees. Without it the adjustable will give it 40+ degrees by itself -- way too much!
Set it to about 1/3 of it's travel.

AND the adjustment screw needs to be backed out all the way - CCW (through the nipple) till it's at the "least sensitive" setting. That compensates for the "too strong" ported vacuum. CCW seems backwards but it's not.

And - best to use the original STOCK springs and weights. Toss out the springs and weights the Crane kit comes with. They are good for drag racing, but for the street and off road they too bring advance in too fast.

Last time I set one up was last week! The guy had a stumble so bad it almost would quit. All smiles now!
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post #7 of (permalink) Old 04-28-2009, 03:43 PM Thread Starter
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so I replaced the HEI gear with the gear off my OEM dist. It looked nearly identical...the HEI one was maybe 1/32" longer...that should be fine, right?

that was a boat load of info on TR, thanks for that thoroughness.
About the accelerator pump, and the linkage:
I'm getting a good squirt of fuel with engine off
How do I know which holes to use?
Currently, i'm using the TOP hole on the throttle arm, and the furthest out hole on the accel. pump linkage.

As for the advance, I couldn't tell any difference in the stumble with the advance port on the side of the carb un-hooked and plugged...I'm gonna do more investigating this evening.
Tim
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post #8 of (permalink) Old 04-28-2009, 04:14 PM
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That setting on the accelerator pump will give the least amount of additional fuel on tip in. You might try moving the link closer to the pivot and see if that helps. If it doesn't, change it back.

EVERYTHING's easy for the guy who doesn't have to do it.
B. Dash Fabrication
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post #9 of (permalink) Old 04-28-2009, 04:49 PM
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Jim's on it...

From a Southern Jeeps article on 2100/258 (a lot more info in the full article)...
Quote:
I have found that for my application the accelerator pump linkage works best set to the third hole down from the top (of four holes) on the arm at the throttle shaft.

Caver Dave
'72 "Mossy Edition" Commando
the Trail Forge
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post #10 of (permalink) Old 04-30-2009, 05:29 PM
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Quote:
Originally Posted by Tim-E View Post
Now I'm kinda concerned about this. The Ebay ad confirmed that this distributor has "a hardened steel gear for use with cast and steel billet cams". .I just got done reading some posts on this issue and its sounds pretty scary. I've already put 50-60 miles on this thing...sutmbling aside, it seems i've got a new bigger problem. What should I do about this dist gear?
Remove the distributor, drill the gear to see if it has a hardened gear or not.
It's really the ONLY way to test for sure at home.

I can guarantee you DO NOT have a STEEL BILLET CAMSHAFT, so you camshaft gear is at real risk!
-----------------------------------

Quote:
Originally Posted by Tim-E View Post
so I replaced the HEI gear with the gear off my OEM dist. It looked nearly identical...the HEI one was maybe 1/32" longer...that should be fine, right?
That's one problem you shouldn't have to worry about now...
The factory gear is EXACTLY what you needed!
Should have looked in the hole and had a look at the camshaft gear to see if it was damaged while the distributor was out.
Might save you some problems later on!...
------------------------------------------------------------

Quote:
That was a boat load of info on TR, thanks for that thoroughness.
That's one of my pet peeves,
People trickling out information like it was costing them something!

Be complete, be through, be precise or don't post up to confuse things!
Some times I over simplify things, and people get offended, but you never know what the experiences and skill level of the people you are talking to, so you DO have to include the basics... Or you are just giving HALF the story...
-------------------------------------------------------------

Quote:
About the accelerator pump, and the linkage:
I'm getting a good squirt of fuel with engine off
How do I know which holes to use?
Ones on the throttle shaft linkage determine how long the total accelerator pump shot lasts,
The ones on the accelerator pump arm determine how much fuel gets dumped with each activation. (more fuel sooner or later)

Quote:
Currently, i'm using the TOP hole on the throttle arm, and the furthest out hole on the accel. pump linkage.
Move things around and see if that helps or hurts your 'Issue'...
-----------------------------------

Quote:
As for the advance, I couldn't tell any difference in the stumble with the advance port on the side of the carb un-hooked and plugged...I'm gonna do more investigating this evening.
Tim
That rules out the distributor being the problem with the 'Off The Line' stumble...
Off the line stumble is almost always a problem with accelerator pump shot volume...
Not enough fuel initially, not fuel long enough for main jets to take over, Shot not happening quick enough, ect.

If it tries to take off, then stumbles or bogs, that's a DURATION, or length of shot problem.
This can be wrong linkage settings,
Or it can be an accelerator pump that is worn and doesn't like to deliver.
Might also be clogged lines to the discharge nozzles...

If the stumble/bog happens just as soon as you touch the throttle,
Then that is a deliver TIMING problem.
The pump shot isn't starting SOON enough.
The pump shot should be starting as soon, and I mean the INSTANT the throttle blades start to move!

If it's a general bog that last over the entire range of the accelerator pump shot,
Then you aren't getting enough VOLUME to the throttle bores.
That can be wrong adjustments, plugged lines or weak pump (broken spring, leaking check valve, crud/corrosion in the ports) ect.
------------------------------------------------

Think long and hard before you scrap this carb!
It's an early Holley design,
It's VERY simple,
And it's easy to adjust/rebuild/maintain!

It also works pretty good off road for a stock carb!

Last edited by TeamRush; 04-30-2009 at 05:50 PM.
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