Backfire up Carb. Help - Custom 302

[sps_sac]

Finally got my custom build 302 up and running but can't get rid of an idle backfire and it backfires on initial acceleration. Have adjusted Holly accelerator pump, idle jet screws, increased accelerator squitter from 31 to 37, squitter engages immediately. Increased power valve from 6 to 9.5 (vacuum = 11 at idle) Adjusted floats. Moved timing all over,(seems to like it around 3 to 4 degrees, but it still backfires. Runs good at higher rpms, no backfire at all. Compression test shows cylinders from 175 to 183. I am starting to suspect crossfire in the distributor, but it's all new. Need help?

Engine is as follows:

91 Ford Roller cam 302 block.
KB flat top pistons (about 10 to 1 compression).
AFR heads 185's with 58cc. (1388's)
Summit Racing stage II vented intake manifold.
Holly 4150 Street Avenger with vacuum advance secondary’s, electric choke.
Holly Blue fuel pump set at 7 psi.
Comp Cams XE264HR-14, roller cam, 110 degrees separation (not too radical) set at about 3 degree advanced.
Mallory SS42 distributor with vacuum advance.
Mallory Hyfire 6AL
Mallory coil 29440
Taylor 409 - 10.4mm plug wires
Champion RC9YC (one step colder than standard), gap = 0.040".
Moroso electric water pump.
Full length ceramic coated headers, 1 5/8" to 3" H pipe, and dual 2 1/2" Flowmaster Delta 50's.
No smog equipment, egr, or PVC on engine (will be going in a 64 Falcon).

I refitted plug wires at dist. cap. No change. Plugs: # 1,2,& 6, are black with soot. The others are a bit darker than before, but that's likely because of the richer carb settings. Ran compression check: 175,175,178,183,180,180,180,180,180. Could it be that the intake manifold isn't seating well with the heads? But wouldn't that show on the vacuum readings?

[dougbfresh]

Did you check the Power Valve? Try some leaner jets too.

[Saltmine]

Sound like a tight intake valve to me. We used to have the same backfire problem back in the early '80's when GM outsourced a batch of camshafts to a vendor that didn't Parkerize the cam lobes properly. The cam lobes failed in less than 20,000 miles, and one characteristic "tip-off" to the bad cam was the popping or backfiring through the carburator when the engine was revved. Almost always an inspection would reveal an exhaust lobe worn down, and not opening the valve on that cylinder. When the intake valve opened, it would pop back into the intake, just like a leaking intake valve would.

[dougbfresh]

Originally Posted by Saltmine

Sound like a tight intake valve to me. We used to have the same backfire problem back in the early '80's when GM outsourced a batch of camshafts to a vendor that didn't Parkerize the cam lobes properly. The cam lobes failed in less than 20,000 miles, and one characteristic "tip-off" to the bad cam was the popping or backfiring through the carburator when the engine was revved. Almost always an inspection would reveal an exhaust lobe worn down, and not opening the valve on that cylinder. When the intake valve opened, it would pop back into the intake, just like a leaking intake valve would.

My father-in-law has an 82' Berlinetta Camaro that had the cam eaten up by faulty lifter. After fighting w/ GM for months, he won at arbitration and guess what-they already had a standard form for this failure. He got it fixed by GM and it still runs today.

Vacuum at idle still seem a little LOW, do you need ported or manifold vacuum for that distributor???

[Saltmine]

You might be onto something Doug. When emision equipped cars started hitting the streets we quickly discovered that an overly rich idle can cause a huge stumble during "tip-in" off idle. That transition from dripping rich to lean could cause a backfire. Of course, running manifold vacuum to a Ford distributor is also a good way to get a backfire out of one.... The only Ford I know of that used manifold vacuum was the old flatheads with Holley "Load-o-matic" ignition systems, from the early '50's.

[olddog]

It is possible for the distributor to have the rotor not pointing to the right spot - called out of phase. Then as the vacuum and mechanical advance move things, the spark can go to the wrong plug.

See this thread it explains it well.
http://clubcobra.com/forums/showthread.php?t=99922

[olddog]

Originally Posted by Saltmine

Sound like a tight intake valve to me. We used to have the same backfire problem back in the early '80's when GM outsourced a batch of camshafts to a vendor that didn't Parkerize the cam lobes properly. The cam lobes failed in less than 20,000 miles, and one characteristic "tip-off" to the bad cam was the popping or backfiring through the carburator when the engine was revved. Almost always an inspection would reveal an exhaust lobe worn down, and not opening the valve on that cylinder. When the intake valve opened, it would pop back into the intake, just like a leaking intake valve would.

I had a wiped exhaust lobe once. At first it popped back up the carb anytime the engine was under load. It quickly deteriorated to just gunning it in neutral.

A lot of problems wiping cams with flat tappet lifters on start up, however roller cams rarely have any problems. A lot of Chinese junk cam blanks coming into the country, so it is possible.

Pull the valve cover and rotate the engine while watching each exhaust valve cycle. You can see if one is not opening very much. Wouldn't hurt to check the lash while your at it. I doubt the lash tight on the intake, as that should have showed low compression.

[olddog]

The 6AL is known to cause many ignition problems. Many say always always blame it until proven to not be the problem. A lot of guys carry a set of wires made up to allow them to wire past it in a few minutes. It beats getting stranded.

You can do a search on the Cobra Club that I provided the link to and find a lot of information on the 6AL.