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Unread 04-28-2013, 07:17 PM   #1
bigb229
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1998 ZJ 
 
Join Date: Feb 2010
Location: ga
Posts: 679
Fresh build backfiring

Alright fellas. We just fresh built the 4.0l back to standard. Everything is new. Initially thought it was the timing but its correct. She idles smoothly but when you get on the skinny peddle it backfires through the exhaust and intake occasionally. Thinking fuel pump but haven't checked the pressure on the rail yet. Opinions?

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Unread 04-28-2013, 07:20 PM   #2
jnowak
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Did you lap the valves? If not the have to seat in.
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Unread 04-28-2013, 07:27 PM   #3
bigb229
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Took them out one at a time replaced the seals and lapped them
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Unread 04-29-2013, 06:58 AM   #4
bigb229
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bump
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Unread 04-29-2013, 07:16 AM   #5
speedbucket
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Any codes?
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http://www.jeepforum.com/forum/f22/speedbuckets-zj-build-1361322/
V8 zj at 6" . Double tri rear and true 3 link front. Stretched to 108"
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Unread 04-29-2013, 08:43 AM   #6
bigb229
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not throwing any codes...was hoping that it would throw an o2 code and it be an easy fix....NOT!
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Unread 04-29-2013, 10:00 AM   #7
jnowak
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1993 ZJ 
 
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New rocker arms? Curious if you got the right ratio.(shipping error) usually if it back fires through intake ,valve is still open on combustion.
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Unread 04-29-2013, 05:21 PM   #8
bigb229
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Location: ga
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Original rocker arms and valves... Just had the seats and seals done checked the fuel pressure today and everything is fine
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Unread 04-29-2013, 05:29 PM   #9
jnowak
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How old is the gas? I know my small engines start the crap when I got bad gas.
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Unread 04-29-2013, 05:31 PM   #10
bigb229
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Also fresh....I really don't want to admit defeat and take this to the shop
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Unread 04-29-2013, 05:38 PM   #11
jnowak
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Know your pain. Is there a flame coming out with the tail pipe back fire. Might be a vacuum leak? Try.a good can of carb cleaner on all gasket seams.
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Unread 04-29-2013, 06:00 PM   #12
bigb229
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No flames. She idles soooo smooth and is solo quiet....no vacuum leaks...I think it's possessed
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Unread 04-29-2013, 06:27 PM   #13
denn34
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Sounds like cam timing to me. Timing marks aligned right?
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Unread 04-29-2013, 07:56 PM   #14
bigb229
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Yeah. Even pulled the cover and reset to tdc and recheck distributor timing....it's perplexing
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Unread 04-30-2013, 01:44 AM   #15
denn34
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1995 ZJ 
 
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Is the chain tight... Maybe you got a bad one
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