Rough Idle | Puffs from Exhaust - JeepForum.com
 
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post #1 of 8 Old 04-02-2020, 09:54 PM Thread Starter
Brrang
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Rough Idle | Puffs from Exhaust

I was wondering if anyone here has ever had their jeep act like this (see video). I am thinking it has a sticking valve. I rejetted the carb to make it more lean (better for my altitude) and the puffing got a little worse. I am thinking about putting a new head on it, but before I do that I want to make sure I have checked every possible cause for this. If I put the vacuum advance on the motor it dies instantly on manifold vaccum. On ported vacuum it doesn't change the idle, but it causes back fires. I have always ran ported, but on the wrangler forums they were split about 50/50 on whether it should be one or the other, so i tried both. Adjusting the vacuum advance does not seem to affect it much. If I hold a piece of paper to the exhaust when it puffs it sucks the paper up to the tailpipe.

Little background.
The jeep sat for 5 years before I got it. Once I got it I put an HEI Distributor on it and a MC2150 carb (now with 46 + 47 jets). I have looked all over and cannot find any vacuum leaks. It has a new carb gasket and new exhaust and intake manifold gaskets. I have tested the intake manifold for leaks and i cant find any(carb cleaner hose down).

I think I have included all relevant information. If I forgot something I will add it later. If you have any questions please ask. Any information would be helpful. I am hoping to work on it this weekend.

I cant post links yet, so just add this link to the end of youtubes url

/watch?v=-kLCzv9A_xw

Thanks in advance.

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post #2 of 8 Old 04-03-2020, 04:30 PM
Wabatuckian
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It sounds lean.

Quote:
If I hold a piece of paper to the exhaust when it puffs it sucks the paper up to the tailpipe.
That's a sticky exhaust valve, but isn't necessarily the reason it dies.

At what RPM does it die when you slowly give it gas?

What year is that YJ? I6?

Josh
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post #3 of 8 Old 04-04-2020, 08:33 PM Thread Starter
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Quote:
Originally Posted by Wabatuckian View Post
It sounds lean.

Quote:
If I hold a piece of paper to the exhaust when it puffs it sucks the paper up to the tailpipe.
That's a sticky exhaust valve, but isn't necessarily the reason it dies.

At what RPM does it die when you slowly give it gas?

What year is that YJ? I6?

Josh

It was rich, but I rejected it. It only dies when I connect the vacuum advance. It actually runs reasonably well considering. Once the vacuum advance is connected to port vacuum it causes it to backfire once giving it gas. I assume its advancing too much for the sticky valve.

It's a 1990 YJ 4.2L. Sorry I left out the most important detail.
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post #4 of 8 Old 04-05-2020, 04:36 PM
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What is your initial timing setting? It should be around 8.

Do you have a vacuum gauge? If not, go get one. They're not all that much.

Josh
Attached Thumbnails
vacuum readings.jpg  
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post #5 of 8 Old 04-06-2020, 04:26 PM Thread Starter
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Quote:
Originally Posted by Wabatuckian View Post
What is your initial timing setting? It should be around 8.

Do you have a vacuum gauge? If not, go get one. They're not all that much.

Josh
Thanks for this chart i have not seen it before. I would say the vacuum gauge acts like a sticky or burnt valve. Timing is around 10*, HEI instructions said it should be advanced a little over stock and it seems to run better around 10*. When I compression tested the motor it was around 120 psi per cylinder. Do you guys think a new head would cause it to start sucking oil passed the rings? I can't find a definitive on what the stock compression was, but i see between 150 and 140.

thanks
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post #6 of 8 Old 04-07-2020, 06:49 AM
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Hello,

Richer mixtures burn faster. I'd set initial timing to around 5, and adjust it from there.

FSM is 120 to 180 or so, IIRC, and you want the cylinders to be close to each other.

Did it do any of this before you re-jetted and installed the HEI?

Josh
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post #7 of 8 Old 04-07-2020, 12:50 PM Thread Starter
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Quote:
Originally Posted by Wabatuckian View Post
Hello,

Richer mixtures burn faster. I'd set initial timing to around 5, and adjust it from there.

FSM is 120 to 180 or so, IIRC, and you want the cylinders to be close to each other.

Did it do any of this before you re-jetted and installed the HEI?

Josh
Yea, but it seemed more pronounced once I installed the HEI. After the carb re-jet it doesn't seem to really be any different. Maybe slightly louder, but I couldn't say that for a fact. All of my cylinders were within 5 psi. I am thinking in a month or so I can put a new head on.

I am Honestly considering a 4.0 swap. I am just looking into it. If they bolt right in and right up to the transmission I would do that before I spend the nearly $400 on the head. Then I get fuel injection, newer motor, and all the amenities that come with it.
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post #8 of 8 Old 04-08-2020, 05:43 AM
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Quote:
Originally Posted by Brrang View Post
Yea, but it seemed more pronounced once I installed the HEI. After the carb re-jet it doesn't seem to really be any different. Maybe slightly louder, but I couldn't say that for a fact. All of my cylinders were within 5 psi. I am thinking in a month or so I can put a new head on.

I am Honestly considering a 4.0 swap. I am just looking into it. If they bolt right in and right up to the transmission I would do that before I spend the nearly $400 on the head. Then I get fuel injection, newer motor, and all the amenities that come with it.
I would like another carbureted Jeep. I've not owned one since I sold my CJ5 in the early 2000s.

Fuel injection is nice, but sometimes I just want the older stuff.

If I recall, you can put a 4.0 head on the 4.2 engine. That might be fun

Josh
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