Find and down load the original article on using a vacuum gauge I wrote if you don't already have a copy.
Hook a vacuum gauge to the cotton picking thing, and I think you might find the problem...
You said it did it with a different carb, and I know you upgraded the ignition recently, did you change the distributor during the ignition upgrade?
If so, did it do it with both distributors?
Have you looked into the possibility of the module being bad?
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Here is what I'm thinking,... so keep in mind this is just thinking out loud....
If it's popping through the intake, the only way it could be the ignition is...
1. If the module malfunctioning around 1,300 rpm and firing the cylinders too soon...
That would still require an extraordinary set of circumstances to occur.
You might trade modules with someone and see if their jeep does it and yours stops doing it...
2. Check to make sure the centrifugal advance and vacuum advance are working as advertised. (Since it only happens at a specific rpm, I don't see how this could be affecting it.)
3. Verify the timing mark on the balancer. That is a long process, but worth the trouble to make sure your timing is correct. (again, this is a long shot)
4. Check the resistance on the distributor pick up coil. A break in the winding can cause some strange things... (another long shot)
5. Do you have a factory tach? I have seen the factory tachs do some strange stuff when they are going bad... (another long shot)
6. If all else fails, swap distributor, module and coil with someone, and see if it still does it.
A lot of work, but that would rule out the ignition system entirely...
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The only way to make ignition 'Pop' back through the intake manifold is to fire one or more cylinders so early the intake valve is still open.
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Bad cam timing, bad lifter or lifters, sticking valves, worn out valve guides, pitted or grooved valve seats, weak valve springs, bent or broken rocker arms and bent push rods can all make the symptoms you describe.
So can too lean of fuel mixture, vacuum leaks, leaking intake manifold, or raw fuel leaking into the intake tract.
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Hang a vacuum gauge on that sucker, and verify your idle mixture.
Do a compression test with all the plugs out.
I'm betting if it's not idle mixture or bad accelerator pump, it's one or more cylinders is below 90 lbs, and you have very weak valve springs...
(Bad accelerator pump will cause exactly the symptoms you describe. But if you changed carbs, what are the chances of two carbs having the same problem....)
If that is the case, have a head job done, and get some new valve springs. Check the cam for lift, and check the lifters for wear and function.
Pay real close attention to the head gasket for anything that looks like a blow through between cylinders.
If you reinstall and still don't have any compression, time to go through the short block.
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I'm also betting you have a cam timing problem. The AMC engines had a real problem with cam chains stretching so much the valve timing is screwed up.
They stretch so much they can actually jump a tooth on the crank gear...
Again, check the cam for lift. AMC cams like to round down the lump on the cam, and that WILL cause the problem you describe.
(can't call it a 'cam' anymore, you have to call it a 'round'..../wwwthreads_images/icons/wink.gif)
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I tore down about 30, mostly factory assembled, AMC engines in the last six months, and they all had one thing in common, worn out cam Bering, flat cam lobes, and timing chains that could be worn by fat guys as belts.
Lots of front covers damaged by timing chains.
Lots of lifter faces worn through, especially on the even fire bank.
Lots of factory retarded cam timing for some reason.
I assume emissions control, but that is a guess.
Most all of them showed oil loss to the #7 & #8 rod bearings in a big way.
Most showed signs of pitiful valve spring pressure... Lots of chatter, slop, and bounced valve locks.
All had factory rear main seals leaking like a sieve...
Most had fuel pump drives, cam gears, and distributor gears worn out for lack of lubrication.
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Just thinking out loud...
Let me know what you find please...
If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?
