1973 Rebuilt 460 with a one cylinder backfire

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frankpalumbo - 22 Aug 2005 17:07 GMT

Thanks in advance for any help. I built a 460 with 10:1 compression, MSD
box, mild cam, headers, magnetic dist., and a 750 carb. I have a series of
small backfires confirmed from cylinder 4 (passenger's rear-most). I
swapped out the plug, changed the wire and cap, readjusted the valves,
triple checked the firing order, it runs great accept for the backfire. Do
I have a valve problem? Thanks

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BOB URZ - 22 Aug 2005 18:39 GMT

> Thanks in advance for any help. I built a 460 with 10:1 compression, MSD
> box, mild cam, headers, magnetic dist., and a 750 carb. I have a series of
> small backfires confirmed from cylinder 4 (passenger's rear-most). I
> swapped out the plug, changed the wire and cap, readjusted the valves,
> triple checked the firing order, it runs great accept for the backfire. Do
> I have a valve problem? Thanks

What does a compression and leak down test show on that hole?
No leaks on the intake?

Bob

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lugnut - 22 Aug 2005 18:40 GMT

>Thanks in advance for any help. I built a 460 with 10:1 compression, MSD
>box, mild cam, headers, magnetic dist., and a 750 carb. I have a series of
>small backfires confirmed from cylinder 4 (passenger's rear-most). I
>swapped out the plug, changed the wire and cap, readjusted the valves,
>triple checked the firing order, it runs great accept for the backfire. Do
>I have a valve problem? Thanks

What you describe is classic of a valve train problem like a
bent pushrod, a weak valve spring, a sticky valve or a flat
cam lobe - well, it isn't really "flat". It may have gotten
the nose knocked down a bit if the breakin wasn't done the
way it should have been. It only takes a few thousandths to
really screw it up. Did you follow the cam grinders
instructions on cam lube and breakin procedure? Also, if
you are using points for a trigger, a bad or shorted
condenser will do that.

Lugnut

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frankpalumbo - 23 Aug 2005 11:55 GMT

Thanks Lugnut. I definitely had the cam well lubed and did not drop or
force it into place. I pre-lubed the lifters before installation and the
engine before starting, and ran it up real easy. Not to say I still dont
have a jocked cam, but your note confirms my feelings that I have to chase
the valve train, starting with the push rod.
Thanks again - FP

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dmac - 24 Aug 2005 18:50 GMT

don't suppose you are getting crossfire from another wire?

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Dave MacLeod
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dmacleo@midmaine1.com

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Tom Adkins - 25 Aug 2005 05:33 GMT

> don't suppose you are getting crossfire from another wire?

Aaah, you might be on to something there. I recall being told on most older Ford
V-8s to always have the wires to cylinders 7 and 8 cross each other because they were
inline in the firing order and the long run to the cylinders could cause inductive
coupling between the 2 if they were side by side for the entire run. That is a long
forgotten tech tip from my youth.
The problem does sound like a worn lobe on the cam though. For breakin, I was taught
to initially start the motor and run it at ~1800 rpm for 20 minutes to break in the
cam. The big thing was to get it above idle for full oiling and to maintain steady
speed for this time to minimize wear.

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frankpalumbo - 08 Sep 2005 15:51 GMT

Thanks Tom

I'll check it out. I currently have the head off and will be looking into
the condition of the CAM.

FP

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