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AMC Jeep 360 V8 w/ motorcraft 2150, trouble with initial stumble off idle under moder

JeepScrambler

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City
Claflin
State
KS
I'm needing some direction with an issue I'm having on my 1979 Jeep Cherokee S 4 door. The Jeep has what appears to be a stock rebuilt 360 with the 2 barrel Motorcraft 2150 carburetor. The Jeep had sat for several years before I purchased it.

The issue I'm having is, right off idle when you hit the throttle hard, the Jeep seems to stumble for just a second or two then catches up with the throttle and revs up fine after that point. If I ease into the throttle there is no issue. It only occurred under moderate to hard acceleration.

When I was first getting the jeep running before the carburetor was rebuilt, I had several backfires through the carburetor due to a few vacuum leaks on some old lines. I wanted to replace the power valve after the backfires and that is what lead to the rebuild. I have checked the float and the fuel bowl level with the engine running and they are spot on. When I replaced the vacuum lines I eliminated most of the CTOs, Air pump, and other various emissions equipment, except for the EGR. The distributor advance is now run off of manifold vacuum (I have tried ported vacuum too and that did not chance the off idle stumble).

Distributor is currently sent to 8 degrees BTDC. I have tried advancing and retarding the timing; I even timed it with a vacuum gauge, none of which got ride of the dead spot under hard throttle. I did check the distributor advance and and it is working and will hold vacuum.

My best idle mixture screw adjustments when tuning with a vacuum gauge came in right at 1-1/2 turns out (will double check this) for each at 18 in Hg (one side note to this is with the mixture screws in so far for the best vacuum reading the jeep would not start after a day without dumping gas down the carburetor, I backed the screws out to 2-1/2 turns and this starting problem went away). I have since tried several different idle mixture screw settings and nothing has eliminated the hesitation off idle.

The accelerator pump has been replaced twice, thinking maybe the first one was bad; no luck. I have also tried moving the accelerator pump rod to the other positions, still nothing.

The only other issue I can think of is the throttle shaft on the driver's side appears to have a very small vacuum leak around it. If I spray carburetor cleaner on it the RPMs will vary slightly. Don't think this could be enough of an issue to cause my problem?

I'm about to pull the Motorcraft off and build a junkyard TBI from parts I gathered from a salvage yard while I lived in Houston. I've never had so much trouble with a carburetor in my life!

Short summary: Stumble off idle under hard acceleration.
-rebuilt carb
-manifold distributor advance set at 8 degrees BTDC
-idle mixture screws set to the rich side at 2-1/2 turns out.
-replaced power valve twice
-replaced accelerator pump twice.
-very slight vacuum leak on driver side of throttle shaft.

Anyone have any input on what to try next? I've spent the last several nights trying to get this working correctly but no matter what combination of settings/adjustments I try, the off idle stumble is still there under moderate/hard acceleration. Will post pictures of the Jeep as soon as my frustration level comes down!



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mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
You need to have the distributor set at 10 degrees without ant vacuum and use a ported vacuum source. What's happening is you have 8 degrees of timing at idle that is all from the vacuum advance. If you unplug the vacuum source you will be ATDC instead of before. When you go to accelerate, you lose the vacuum and the little advance you had already and a stumble happens. You probably have heating issues too from the retarded ignition advance.
 

JeepScrambler

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Lifetime Member
City
Claflin
State
KS
AMC Jeep 360 V8 w/ motorcraft 2150, trouble with initial stumble off idle und...

Set timing this morning to 10 degrees and hooked the advance back up to the ported source, no change. It still has the hesitation right off idle. From what I have been reading the manifold vs ported distributor advance argument goes on forever, but from what I gather most go with manifold. My sunshine, what's your take on this? Also, any other recommendations on what to try? I'm at a loss on this thing...


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mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
I'd choose ported vacuum if I was setting it up. Is the mechanical advance even working? What is the total mechanical advance?
 

JeepScrambler

CJ-oholic
Lifetime Member
City
Claflin
State
KS
Dist. advance is working, I can pull a vacuum on it with the engine running and it does increase the engine RPMs. Not sure on my total advance, I just have a basic timing light and no timing tape on the harmonic balancer. I played with the accelerator pump rod placement with the ported vac. source and the timing, still no change. No matter what I try, the hesitation is still there...
 

mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
That shows the vacuum advance is working but not necessarily the mechanical advance (internal weights and springs). You should be able to disconnect the vacuum hose and watch the timing with your timing light and verify that it works.
 

JeepScrambler

CJ-oholic
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Claflin
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KS
With the Jeep warmed up idling, and the vacuum advanced plugged, it is still at 10 degrees BTDC. When I rev the engine the advance increases.
 

mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
You might spray down your plug wires to look for an ignition related stumble. I'll assume it's hitting on all eight cylinders right?
 

JeepScrambler

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City
Claflin
State
KS
Changed all the plugs and plug wires from another 360 I have that I know runs good, still didn't make a difference..


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mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
Running on all 8 cylinders? Have you done a compression test to make sure the engine is any good without making assumptions?
 

JeepScrambler

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City
Claflin
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KS
Haven't done a compression check on the engine yet. It's idles great and runs very smooth at all RPMs, except for right off idle. I will do a compression check this weekend on it. I would think weak compression in a cylinder would cause more issues than just the off idle stumble?


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mysunnshine

Legacy Registered User
City
Phoenix
State
AZ
If you had a cylinder that was low on compression, then the other cylinders will have to carry it upon acceleration. They mostly show up as a studder at tip-in.
 

jammer1

Scrambler Junkie
Lifetime Member
City
Maple Hts.
State
Oh
Do you have another carb (like from your other 360) that you can put on just to give a drive. It's possible that your carb has some of the inter passages partly restricted (example would be the transition circuit. Might not be giving enough fuel when it goes from the idle circuit to the main circuit. Another carb would tell you right away. It's easy to clean and rebuild the carb and still have dirt (or old fuel varnish) in the carb.
 

JeepScrambler

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Lifetime Member
City
Claflin
State
KS
AMC Jeep 360 V8 w/ motorcraft 2150, trouble with initial stumble off idle und...

Before I swapped the carburetor out for another 2150, I spent a few more late evenings of work on it and finally got the problem straightened out. There was a very small vacuum leak at the firewall where the vacuum line for the emergency 4wd switch had a small split. The split was very small and right at the grommet where the lines pass through the firewall. I still had good steady vacuum before (18 in Hg), but now I get just under 20 in Hg with this fixed. This also fixed the need to dump fuel down the carb to restart the Jeep after it has been sitting a day or longer with my idle mixture screws set at 1 1/2 turns out. Prior to this, I had to set the carb rich (2 1/2 turns out) for the Jeep to easily restart the next day.

The next major issue I found was when I replaced the accelerator pump the second time, I somehow put the spring in backwards! This allowed the accelerator pump to still work, but since the small end of the spring was holding the red rubber umbrella valve mostly closed, it couldn't keep up after a couple hard accelerations; causing the miss and lean backfire through the carburetor. Not sure how I managed that; I'm blaming it on lack of sleep from my teething 8 month old! Thanks for the help along the way with this!

And just because I like pictures, here is the Jeep in question.
ee30f6ecf29b81fee63f24cf2af65abf-1.jpg

da9cabb3ba7d33f76872e85a4316f822-1.jpg
 
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