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Unread 08-21-2013, 02:56 PM   #1
Koko4
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Sputtering Upon Acceleration - *Only While Warm*?

I have been battling a huge problem with sputtering / bucking upon acceleration for awhile now. I narrowed it down to a MAJOR vacuum leak between the carb gasket and the base of the carb - I replaced the gaskets and put a paper gasket between the manifold and insulator, and between the carb base and the spacer underneath it. Seemed to fix the problem immediately. It accelerates like a dream now - the difference is night and day.

However, I took it for a 30 minute drive today and after about 20 minutes, once the engine was nice and warm, it started stumbling again. It started by doing it in 3rd gear, when I was at a bit low of RPM's (probably could have been in 2nd gear at that time), and then it started doing it while accelerating anytime. I don't know what to make of it. I had set the distributor by using a vacuum gauge and getting the strongest vacuum that I could from the manifold, which was about 18. It seemed to be running perfect until this small issue happened. Any thoughts? I am baffled because it only seems to happen (or is way more pronounced, at least) while the engine is nice and warm. I don't want to go back and mess with the timing / idle mixture / anything because I thought I had it all dialed in pretty well, considering how it was performing.

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Unread 08-21-2013, 03:10 PM   #2
LumpyGrits
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Make sure your intake manifold and carb base plate bolts are tight.
LG
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Unread 08-21-2013, 05:11 PM   #3
JWMII
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It sounds as if after warmup the vacuum leak is back due to the warmup and possible warping of various surfaces that are supposed to be sealed.
Such as the areas you corrected with your paper gaskets. May need more and thicker gaskets to compensate for things heating up. Also look for vacuum leaks where the intake manifold runners attach to the head(s).
Spray some penetrating oil around all of those areas that should be sealed.
Do this with the engine at idle. Any vacuum leak the oil reaches will cause the engine idle to momentarily increase. There are usually vacuum access ports that may or may not be plugged Check those for leakage using the penetrating oil. It's messy but will find leaks. Jack.
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Unread 08-22-2013, 02:32 PM   #4
Koko4
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So here is what happened. I tuned the Jeep without the timing light, because when I tried to set it to 12 Advanced it ran poorly and just wanted to die. Instead, I had hooked up a vacuum gauge to the intake port and got the reading as high as I could, which was about 25psi if I remember correctly. I turned the distributor until it got to that reading..any more either way and it would start to go down. It sounded GREAT at idle, and even drove really well for the first 20 minutes or so, until it started stumbling like I was talking about earlier.

I got home and decided to try advancing/retarding the timing just a bit, to see if that would make a difference in this acceleration problem. I turned it clockwise just a HAIR until I heard the idle change, and then tried to drive it. Way worse. Then I turned it a HAIR counterclockwise and it started sounding like it was idling poorly - going up and down at idle, BUT when I drove it and accelerated it felt MUCH MUCH better. So I think I may have found the sweet spot, but it still is idling like crap. Any ideas as to what I'm onto now? Pretty sure it's not vacuum related at this point. I turned up the idle speed a bit, but I think that's just masking the problem - when I turn it down to around 600 (by ear) this "up and down" is more pronounced. I feel like I'm close, but yet so far.

I also checked the carb base nuts before I left home today, and tightened them down a bit more (evenly, of course). Thanks for any suggestions. I know it's hard when you can't be there to actually see/experience it yourself.
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Unread 08-22-2013, 03:04 PM   #5
Matt1981CJ7
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A vacuum gauge works well for setting air/fuel mixture, but I don't recommend it for setting timing. Most likely you are too advanced, if you are setting it to the highest vacuum reading at idle.

You really should get a timing light and set the initial timing somewhere between 5-10 degrees advanced at around 550 RPM, with the vac advance line disconnected and plugged. Then hook the distributer to manifold vacuum, which will add advance and should give you a smooth idle.

Matt
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Unread 08-22-2013, 03:23 PM   #6
Koko4
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So I just turned both the idle mixture screws all the way in, and then backed them out 2 full rotations. Jeep wouldn't start - backed it out to 4 full rotations, each, and it still idles up and down in the same manner. Thought the mixture might have been too rich, but I guess not.

I can try setting it to 12 Advanced (which was what was originally recommended, but would 5-10 be better?) and I could work on getting it going from there. It is pretty damn advanced, BUT when I try retarding it to 12A, it dies. I hooked up a vacuum gauge to the carb port as well and the only time it provides vacuum is upon acceleration, so I'm guessing at idle it wouldn't do anything?
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Unread 08-22-2013, 04:06 PM   #7
Matt1981CJ7
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I think 12 degrees initial is a little too aggressive for your elevation. Guys at much higher elevations can probably get away with that much.

Generally speaking, the factory vacuum layout used full manifold vacuum to the distributer when the engine is cold, then a CTO valve switched it over to ported vacuum when the engine warmed up. That said, many of us have found our engines run much better on full manifold all the time. Try it, I think you'll agree.

Matt
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Unread 08-22-2013, 04:30 PM   #8
Koko4
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Thanks Matt. So I should start over, and try to set it around 5-10? I don't know what to do because I can't even keep it running at that setting. It just stumbles and dies. I will try using the constant manifold vacuum instead of the ported and see what happens. Thanks for helping me through all of this - I am still realizing there is SO much to learn.
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Unread 08-22-2013, 04:34 PM   #9
Matt1981CJ7
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How tired is the engine? Timing chains can stretch, even jump a tooth, if they get bad enough. The result will be inaccurate timing readings.

What carb do you have, and when was the last time it was rebuilt?

Matt
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Unread 08-22-2013, 04:37 PM   #10
Koko4
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Its a freshly rebuilt engine with less that 100 miles on it, and new timing chain. And I have a motorcraft 2100 carb that I bought about two weeks ago, it was rebuilt from AutoZone.
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Unread 08-22-2013, 05:14 PM   #11
Matt1981CJ7
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Quote:
Originally Posted by Koko4 View Post
Its a freshly rebuilt engine with less that 100 miles on it, and new timing chain. And I have a motorcraft 2100 carb that I bought about two weeks ago, it was rebuilt from AutoZone.
Hmm...It should run fine at normal timing settings, then.

What's the venturi size on the 2100? How about the jets?

What ignition are you running?

Matt
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Unread 08-22-2013, 05:14 PM   #12
81azcj7
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Quote:
Originally Posted by Koko4
Its a freshly rebuilt engine with less that 100 miles on it, and new timing chain. And I have a motorcraft 2100 carb that I bought about two weeks ago, it was rebuilt from AutoZone.
it kinda sounds like you need to re jet the carb, when mine was doing that and I needed bigger jets.
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Unread 08-22-2013, 05:16 PM   #13
Koko4
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The carb is 1.08 - the one recommended for the AMC 304. I'm not sure what the jet size is.. But its only doing tthis when its warmed up. When its cold it seems to be a much less prominent issue
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Unread 08-22-2013, 05:21 PM   #14
81azcj7
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Have you had the intake manifold off before? Sometimes they warp or if you re install it, the gasket can tear and cause a leak. Happened to me once.
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Unread 08-22-2013, 05:27 PM   #15
Koko4
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We installed it with a new gasket, and used a TON of RTV. I hate the intake manifold!
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