304 engine miss following tune up & exhaust re-route - JeepForum.com
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post #1 of 4 Old 10-21-2013, 09:40 PM Thread Starter
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1979 CJ5 
Join Date: Feb 2013
Location: Mason
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

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post #2 of 4 Old 10-22-2013, 03:46 AM
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1982 CJ7 
Join Date: Feb 2010
Location: FL.
Posts: 431
recheck firing order, its way easy to cross a plug wire....ocg
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post #3 of 4 Old 10-22-2013, 06:25 AM
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1982 CJ7 
Join Date: Jun 2010
Location: Marysville, WA
Posts: 5,508
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.
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post #4 of 4 Old 10-22-2013, 06:50 AM
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1981 CJ7 
Join Date: May 2011
Location: Elbert
Posts: 19,012
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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