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post #1 of (permalink) Old 06-12-2004, 06:31 PM Thread Starter
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FIXED! new engine popping/backfiring under load

Well ive been trying to get this new 401 to run well and just havent had any luck. it idles great and runs smooth until after about 2500 rpms and it gets under load or you stab the throttle then it backfires or pops.

it did it with my edelbrock 1406 and edelbrock q-jet. it now does it with the Howell TBI kit i just installed today.

stock rebuilt 401 performer manifold, stock distributor with a blaster2 coil. I did convert to the TFI cap and wires but that didnt help.

distributor is from my 304. vacuum advance seems ok. who knows about the mechanical or the dist itself. coil going bad? I may just go HEI or MSD.

anything internal in the engine possibly cause this? flattened cam lobe, bad lifter, valve? vacuum leak?

thanks
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post #2 of (permalink) Old 06-13-2004, 12:13 AM
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Re: new engine popping/backfiring under load

</font><blockquote><font class="small">In reply to:</font><hr />
it idles great and runs smooth until after about 2500 rpms and it gets under load or you stab the throttle then it backfires or pops.

[/ QUOTE ]

Sounds like you have tried differnt carbs with the same result. Not likely fuel delivery.

</font><blockquote><font class="small">In reply to:</font><hr />
distributor is from my 304. vacuum advance seems ok. who knows about the mechanical or the dist itself.

[/ QUOTE ]

Vacuum advance is from the correct source?

This is a question, not an answer.
Shouldn't the mechanical advance be very different for a 401 vs. a 304??

If that's not a problem could the distributor just cause a problem like this because it's worn out letting the rotor rattle around?

</font><blockquote><font class="small">In reply to:</font><hr />
anything internal in the engine possibly cause this? flattened cam lobe, bad lifter, valve? vacuum leak?

[/ QUOTE ]

Sounds like the motor is sound and simply has a small issue to deal with and find. Vacuum leak maybe... but at 2500 RPM it would have to be too big to allow for the motor to idle and run well at lower RPMs.

I'm not a great motor head... but will bet you are looking in the right direction with the ignition.

If nothing else I have pushed your post back to the top for some "Motor Head" to see and reply. Add some more detail, someone will jump on this when they read it.

Good Luck

Dale
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post #3 of (permalink) Old 06-13-2004, 09:48 AM
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Re: new engine popping/backfiring under load

A flat cam lobe, bent push rod or a collapsed lifter would do this.(exhaust)
I don't know how the rockers are attached in a 401 but if they are studded it could be a pulled stud.
It was a common problem with the SBC's that were in the larger utility trucks in the 80's.They sat and idled all day and eventually "ate" a cam lobe due to low cam oiling at idle.
also check for a plugged exhaust.
Greg [img]images/graemlins/scs.gif[/img]
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post #4 of (permalink) Old 06-13-2004, 01:19 PM
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Re: new engine popping/backfiring under load

Yes, it could be valve train, flat cam, valve spring, cam in wrong etc. etc --
BUT -
Before assuming the worst I'd start with the simple and cheap - it's far more likely.
Try it without vacuum advance connected - see if it's related.
Check timing advance -
no vacuum, idle, S/B around 8.
No vacuum 2500 - S/B around 20-22,
vacumm connected idle around 8 (same),
2500 around 35.
Use an advance timing light or mark new marks on the damper -- just use the 0-10 degree marks as the width and extrapolate.

Check exhaust for plugged. Vacuum gauge on intake manifold vacuum - idle around 16-18, 2500 about the same as at idle.
Snap throttle hard - should drop to near 0, snap it shut fast - should bounce up higher than before.
If no to the above, suspect exhaust.

Check coil wire - easy way replace with a known good one.

Make sure plug wires are NOT running parallel for much distance, ugly runs nice, pretty doesn't.

Check these and let us know.
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post #5 of (permalink) Old 06-13-2004, 02:22 PM Thread Starter
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Re: new engine popping/backfiring under load

well i pulled the valve cover and checked everything as best i could. everything seems tight, none of the rockers or pushrods wiggle. all the passages are oiling. i ran it with the valve cover off for a while and all seemed well. could still be a valve not sealing. It has the bridge rockers.

It is popping out of the exaust more today, this popping is like a vacuum leak type pop to me. if i rev it up slow it seems fine, i can rev it pretty high and it just slightly stutters, if i rev it fast or put it under load it pops a lot.

Gonna replace the dist, coil and duraspark box next i guess. this things making me nuts. anyone in seattle area have an HEI i could borrow for a day? [img]images/graemlins/smile.gif[/img]
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post #6 of (permalink) Old 06-13-2004, 08:29 PM Thread Starter
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Re: new engine popping/backfiring under load

Its fixed, i think. Its running like a raped ape. Im still not sure what the problem WAS but now ive ruled out the engine having problems.

I did a leak down test and it stopped popping when I unplugged #1. so i pulled the plug and cleaned it then did a compression check which was perfect. I did the same for #3 and it had similar compression. i traded spark plugs between the two and reinstalled. started it up and WTF, its running great?!

I like that its fixed but i hate not knowing why, anyone got a clue what could have been the problem? ive had those plugs out multiple times in the last couple weeks, was it not seating right? compression test knock something back in place?

thanks to those that gave me ideas on all the forums i posted this on.
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post #7 of (permalink) Old 06-13-2004, 08:44 PM
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Re: new engine popping/backfiring under load

</font><blockquote><font class="small">In reply to:</font><hr />
Its fixed, i think. Its running like a raped ape.

[/ QUOTE ]

Great... well it is if it stays that way.

</font><blockquote><font class="small">In reply to:</font><hr />
I like that its fixed but i hate not knowing why...

[/ QUOTE ]

I'm with you there... I work on computers every day and there's alot of that... makes me crazy (well I was anyway).

This comment from RRich wasn't it?
</font><blockquote><font class="small">In reply to:</font><hr />
Make sure plug wires are NOT running parallel for much distance, ugly runs nice, pretty doesn't.

[/ QUOTE ]

I've done that, won't bore you with electromagnetic theory that I don't remember very well anyway (likely didn't even spell it correctly).

Thanks for the follow up.... that's what makes this BBS work. Lots of people with lots of experience in so many different areas with one common goal... make that *&amp;**&amp;&amp;#@ Jeep run the way you want it to.

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Dale [img]images/graemlins/grin.gif[/img]
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