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bigb229 04-28-2013 07:17 PM

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?

jnowak 04-28-2013 07:20 PM

Did you lap the valves? If not the have to seat in.

bigb229 04-28-2013 07:27 PM

Took them out one at a time replaced the seals and lapped them

bigb229 04-29-2013 06:58 AM


speedbucket 04-29-2013 07:16 AM

Any codes?

bigb229 04-29-2013 08:43 AM

not throwing any codes...was hoping that it would throw an o2 code and it be an easy fix....NOT!

jnowak 04-29-2013 10:00 AM

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.

bigb229 04-29-2013 05:21 PM

Original rocker arms and valves... Just had the seats and seals done checked the fuel pressure today and everything is fine

jnowak 04-29-2013 05:29 PM

How old is the gas? I know my small engines start the crap when I got bad gas.

bigb229 04-29-2013 05:31 PM

Also fresh....I really don't want to admit defeat and take this to the shop

jnowak 04-29-2013 05:38 PM

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.

bigb229 04-29-2013 06:00 PM

No flames. She idles soooo smooth and is solo vacuum leaks...I think it's possessed

denn34 04-29-2013 06:27 PM

Sounds like cam timing to me. Timing marks aligned right?

bigb229 04-29-2013 07:56 PM

Yeah. Even pulled the cover and reset to tdc and recheck distributor's perplexing

denn34 04-30-2013 01:44 AM

Is the chain tight... Maybe you got a bad one

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