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Old 04-23-2010, 12:57 PM
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backfire through carb

Hello All, I just got back into hotrods again kids grown and left nest. I just bought a nova with a 327 yr 62-67 date code. HEI . I'm getting backfire thought carb on fast kick down. I have checked comp 135-150 hole, found bad plug wires, replaced cap rotor plugs wires. also checked timing?? I do know rotor is on one and timing marks line-up on balancer. it head a 650 DP on it, I have buddy that loaned me a 600 vac sec carb, seams to run good but on kick down stiil backfires through carb. Please Help not sure what to do next. I do know last owner put a mild cam in could timing gears be off ??? or am I reaching. Thanks for all your Time.

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Old 04-23-2010, 03:32 PM
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Several things can cause a backfire, by kick down are you meaning
opening the throttle from idle?
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Old 04-23-2010, 03:43 PM
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Hello, yes when I open thottle rapidly, is time of backfire, have any idea's? thanks for your time. Kelly
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Old 04-23-2010, 03:47 PM
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Kelly, try a larger shooter size on the primary side of the carb.; also check to
make sure there isn't any play in the pump shot arm.

Also check ignition advance at idle, are you using a vacuum advance?
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Old 04-23-2010, 05:06 PM
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i would think any backfire through carb is ignition problem. its fireing before the valve is closed. adv. and try again.
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Old 04-23-2010, 05:22 PM
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Quote:
Originally Posted by kentuckytruck
i would think any backfire through carb is ignition problem. its fireing before
the valve is closed. adv. and try again.
Slow burn from a lean condition, fuels still burning during overlap. This is
normally the cause. Solutions, richen the mixture or add ignition advance.
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Old 04-23-2010, 06:41 PM
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I agree with Automotive Breath, put a bigger accelerator pump shooter on the primary side of the carb. May need to do this also on the secondary side of the Double Pumper carb, but do primary side first.. Very common problem and easy to fix on the Holley carb due to its modular design.

Carbs are virtually never right the way they come from Holley( or Edelbrock, etc), to many variables involved. They are set from the factory to basically get you running and it is your responsibility to tune it to your engine. This does not mean just turning the screws, it often requires you to change individual parts of the carb.
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Old 04-23-2010, 07:06 PM
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Quote:
Originally Posted by kellynova
I have checked comp 135-150 hole, found bad plug wires, replaced cap rotor plugs wires. also checked timing??
Check to see that there are no vacuum leaks and that the vacuum advance works, and is connected to manifold vacuum.

If more initial timing and/or more accelerator pump shot doesn't cure it, there may be other problems. The compression should be higher than what it was, unless the cam is quite large, or the engine quite worn, like rings, valve seal and timing retarded due to a worn/stretched timing set.

The compression reading should be done w/a fully warmed up engine w/the throttle blades wide open and ignition disabled. If a squirt or two of oil in the plug hole doesn't improve the readings quite a bit, you could be looking at worn valves or badly retarded cam timing.
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Old 04-26-2010, 06:28 PM
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hello all, Thanks for the reply's I've been at work last couple days, I did have a DP 650 holley, now I have a 600 vac sec holley on it, still backfires on rapid acc, also I"m not the best on the timing? with the vac advance plugged the timing marks balancer and zero on timing cover line up. with it plugged to carb base it moves to very top of timing cover marks?? I think this would be advanced?? if i go the other way you can here the cam better but car runs bad and will backfire a lot more, I'm a little lost. thanks again for all the great replies let me no on any more ideas. Thanks Kelly
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Old 04-26-2010, 06:35 PM
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Set the timing to 8-10 advanced with the vacuum advance unhooked from the distributor and plugged. After you have set it hook the vacuum advance back to the distributor and don't worry about how much it raises the timing, it is supposed to do that. Take it for a test drive and see if it is better. Right now you have the timing set way too low.
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Old 04-26-2010, 07:22 PM
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Thank you, I will start there after work tommorrow, Thanks for the reply I let you know Thanks again everyone
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Old 04-27-2010, 07:09 AM
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Is it back firing 1 time are a rapid succession of pops? This could mean a bad camshaft,IE a rounded lobe. Hard to tell without actually hearing it.Hope this helps and hope your problem is something simpler.
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Old 04-27-2010, 07:27 AM
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With the cam changed you may want to run a little more then the 8-10 at idle. Typically our base recommendations are 10 to 12 for cams less then .220 @ .050 , 14 to 16 for .220 to .240 and 18-20 for.240 to .260.
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Old 04-28-2010, 12:03 PM
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Thanks everyone for the great advice!! last night I did ajust timing to between 8-10 degrees, I toke it out and drove it, Seams to run better I just have not got on it hard yet? someone also told me about a rounded lobe I hope not, My plan was to Drive it this summer and put my other motor in the car over the winter [468BB project] this of course depends on money. well I"ll see what happens and leave a post. Thanks again!! nice to get advice from true hotrodders!!!
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