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Old 04-06-2013, 09:54 PM
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Engine backfires under hard acceleration

I am have trouble with my 327 backfiring and popping under acceleration. It usually starts around 3500 to 4500 rpm. I am running a Mallory Unlite with vacuum advance. The vacuum advance is working properly. I put in a points distributor and there is no change. I played around with the timing and it still does the same thing. I have tried different coils, no change. I also sprayed starting fluid around the carb gaskets to check for vacuum leaks and the rpms stayed the same. My plugs all look good. A compression check showed 150 in all cylinders. Could the plug wires not be working right under acceleration. My spark plugs look like they are burning good. I took my Edelbrock 750 carb apart to check for dirt and found none. I also disconnected the power brake vacuum line and plugged the port on the carb because I thought the power brake booster could be loosing vacuum. What could be causing the backfiring?

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Old 04-06-2013, 10:10 PM
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Coil, spark plug wires, engine not grounded to the body.
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Old 04-06-2013, 10:46 PM
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Flat cam!!
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Old 04-07-2013, 02:55 AM
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air anf fuel

judging by your discription in the post you obviously know your way arround a motor and have taken quite a few steps. so im gonna go out on a limb and ask is your air and fuel intake is adjusted properly on your 750 carb. i have one on my 327 and it used to do the same thing and i did a few tests myself i.e comp test\timing\distrbtr cap n wires and nothing changed until i adjusted my carb properly. it seemed to be getting too much fuel and not enough air to compensate so maby try backing the fuel off a little bit and give it some more air. just an idea. same motor same carb same problem maby it will help you as well.. good luck
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Old 04-07-2013, 03:52 AM
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i dont think it would be your plugs because you would notice it when you first start the motor and not just at 3500-4500. plus you had mentioned your plugs looked ok so you know there working on acceleration. Or you could have a flat cam..that when you reach a certian rpm it wont allow the valve in your head to open.. just ideas.. good luck though. sum can be tricky to diagnose and sum can be right in front of our eyes...
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Old 04-07-2013, 08:05 AM
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Have you tried a spray bottle with water and getting the wires damp? Sometimes under a heavy load wires will arc to the covers but work fine other wise. Wire leaks are common close to the headers and V/Cs and on rare occasions x fire on 5&7 if the wires run parallel. Also make sure the plug gap is not too wide. A good ignition will make a spark(somewhere) The wires job is to make sure the spark gets to the plug gap.Check the cap also, for arcing.
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Old 04-07-2013, 08:28 AM
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i had this problem in my pontiac years back. i took all the correct steps as you did.
even bought a bran new carb. points and hei didnt solve it.

turned out that i had a sticky lifter. and would only act up when i hammered down on it.

i came to this conclusion, because i dumped a qt of ATF in the crank case and it didnt solve the situation, but it dramitically decreased the problem...

have you checked your timing chain??
mabe a intake gasket leak? chevys suck for this, because you cant get to the entire runner with spray.
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Old 04-07-2013, 11:16 AM
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Broken valve spring
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Old 04-07-2013, 02:39 PM
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I just bought a new set of plug-wires today so I will get those on after my old back settles down I'm going to take my valve covers off and check for broken valve springs. Would I be able to tell if my cam is going flat by checking how much my rocker arms are moving by eye, or is there another way to check for a flat cam without using a dial indicator?
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Old 04-07-2013, 02:45 PM
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Quote:
Originally Posted by Oregonite View Post
I just bought a new set of plug-wires today so I will get those on after my old back settles down I'm going to take my valve covers off and check for broken valve springs. Would I be able to tell if my cam is going flat by checking how much my rocker arms are moving by eye, or is there another way to check for a flat cam without using a dial indicator?
Yes watch the rockers while spinning the motor over (disabled spark) or at slow idle speed.(messy)
A flat cam lobe will be very obvious.
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Old 04-07-2013, 03:14 PM
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If I had a flat cam lobe or lobes wouldn't it run rough at lower rpms (2000-3000)? It runs good and smooth until I nail it.
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Old 04-07-2013, 03:57 PM
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I had the same problem with a newly built 350, it would only break down under load when i stomped on it ,new everything except carb. Turned out to be some plug wires arching out on the headers. I ran mine under the headers, so twas a bit tricky. Spent the best part of a day routing them away from headers and each other , tieing them up onto whatever I could to get them out the way, put those heat socks on where needed, problem solved.
A crappy set of wires properly routed is better than top shelf wires poorly fitted...
Hope you find it, can be annoying, I know.


Duke
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Old 04-07-2013, 04:27 PM
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Quote:
Originally Posted by thinwhiteduke View Post
I had the same problem with a newly built 350, it would only break down under load when i stomped on it ,new everything except carb. Turned out to be some plug wires arching out on the headers. I ran mine under the headers, so twas a bit tricky. Spent the best part of a day routing them away from headers and each other , tieing them up onto whatever I could to get them out the way, put those heat socks on where needed, problem solved.
A crappy set of wires properly routed is better than top shelf wires poorly fitted...
Hope you find it, can be annoying, I know.


Duke
It is hard to find the wire leak under load. I checked it for arching in the dark and found no sparking but the problem could be inside the wires. Hopefully the new wires will fix it.
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Old 04-18-2013, 12:16 AM
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I found the reason for the backfiring through the carb at WOT. It was the vacuum advance canister on my Mallory Unilite Distributor. I bought a new canister because my original one was bad. For some reason the factory had the Allen adjustment screw turned out 3 1/2 turns. I checked my old one and it was turned all the way in and ran fine. So I guess I was pulling to much vacuum which made it pop and backfire when I stood on it. I'm glad I didn't pull all my hair out! Thanks to everyone that responded to my thread. HAPPY MOTORING!!!
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Old 04-18-2013, 10:57 AM
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Thanks for posting the fix. Always nice to know what solved it.
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