304 engine miss following tune up & exhaust re-route - JeepForum.com
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Unread 10-21-2013, 09:40 PM   #1
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1979 CJ5 
Join Date: Feb 2013
Location: Mason, Ohio
Posts: 2
304 engine miss following tune up & exhaust re-route

I have a 79 CJ5 with the 304. I had the vehicle running very well following a rebuilt MC2100 Carb install and a new coil.

It had side pipes that were loud and I didn't like the constant exhaust smell.
I had a mechanic run the pipe out the back via a custom exhaust using a “y” pipe run into a single muffler, very much like the stock configuration.

It sounds perfect. However, the vehicle has developed a pretty significant miss. I had the mechanic then replace both fuel filters, put new plugs in, spark plug wires, distributor cap and cap button. The engine feels really crisp now but has a miss. It idles as if it has an oversized cam shaft.

I have re-routed the vacuum lines to the distributor from the carb vs the intake manifold and this did nothing.

I am still getting pretty strong fumes and the inside of the carb is very wet around the barrels when I look inside. The choke is fully opened and no issue with it being stuck closed … it is actually stuck in the wide open (no choke) position.

What could have happened with the new exhaust and potential added back pressure?

I used to be able to run along at really low RPMs and it was smooth. Now, it just chugs. It actually runs best a full throttle.

The baffling part is why it ran so well previously and now with exhaust reconfiguration, it is running rough.

Another mechanic suggested possibly a stuck valve. I have not performed a compression check but am still questioning why it would have gone from running well to a miss like it has.

Thanks in advance for any tips! It feels like I am on my own to figure this out.

79 304

Apollo66 is offline   Reply With Quote
Unread 10-22-2013, 03:46 AM   #2
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1982 CJ7 
Join Date: Feb 2010
Location: FL.
Posts: 358
recheck firing order, its way easy to cross a plug wire....ocg
Oldcraneguy is offline   Reply With Quote
Unread 10-22-2013, 06:25 AM   #3
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1982 CJ7 
Join Date: Jun 2010
Location: Marysville, WA
Posts: 4,660
Sounds like a vacuum leak. Take a look at all the connections on the intake.

Another reason why I never trust a "mechanic" and do all the work myself....
Beat it to fit, paint it to match!

Wheel, Wrench, Repeat.
82JeepCJ7 is offline   Reply With Quote
Unread 10-22-2013, 06:50 AM   #4
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Matt1981CJ7's Avatar
1981 CJ7 
Join Date: May 2011
Location: Elbert, CO
Posts: 9,452
The "miss" is probably not related to your new exhaust system, unless there's a significant leak.

What do you mean by "both" fuel filters? A stock 304 only has one. How is the fuel delivery configured? Post up some photos.

What were the spark plugs gapped at...it's worth a check. Check the firing order, as well.

Good luck,

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