67 327/350hp backfire question

[Brad H.]

We have a 67 unmodified 327/350 4spd convertible with an occasional backfire problem when started cold. This engine is a fairly new rebuild with a rebuilt distributor and new Holley carburetor. The engine idles perfectly and runs perfectly in all conditions except that occasionally during cold start it will try to backfire through the carburetor. I have the static timing set up around 10 deg. Do you think that possibly backing the timing off a bit might help this problem? The engine pulls like crazy and has tremendous power with no dead spots or any other complaints. In fact everyone raves about how fantastically it runs. I have never had a 327 pull like this engine does. Anybody experience this behavior and have some possible suggestions? I'm out of ideas.....if the driver semi-floods the engine during start, will that cause a thru the carb backfire of any kind. I've just never seen this before.
Thanks,
Brad

[Timothy B.]

Re: 67 327/350hp backfire question

Brad,

Your engine should have a 3810 Holley carburetor, if you are confident the initial timing is set correctly I would pay attention to the choke adjustment and vacuum choke pull off.

If the choke blade is allowed to open to fast because it's out of adjustment the exact condition you report can occur.

[William C.]

Re: 67 327/350hp backfire question

Agree with Tim, sounds like the choke pull-off is misadjusted and opening the choke too far as soon as the engine starts. There is a sped in the service manual as to how far the choke should open with the vacuum pull-off.

[Brad H.]

Re: 67 327/350hp backfire question

Thanks Tim,
I'll look into that. It has a brand new 3810 just installed.

[Gene M.]

Re: 67 327/350hp backfire question

Brad,
I also find I need more initial timing around 14-16 degrees. But if everything else is good ...........

Make sure the carb is not "dripping" after shut down.

[Brad H.]

Re: 67 327/350hp backfire question

Gene and all, that was one of my questions. I have the initial timing set at 10 deg and it runs perfectly and does not ping, never hesitates, and pulls like a scalded ape. Pardon my stupidity, but would more advance or less advance be more likely to cause a backfire if it were timing related? I think I remember that too much advance is the most likely to cause backfire thru the carb. It does not crank abruptly as is sometimes the case with too much advance. I decided to use 10 deg because I thought it was conservative and not as likely to detonate at acute throttle position, and it runs so nicely there. Am I screwed up in this concept? I assumed this carb backfire was a result of the choke or the timing.
Thanks guys, I really appreciate it!
Brad

[Jim T.]

Re: 67 327/350hp backfire question

With the backfiring, have you replaced a power valve that will rupture in a Holley. Placing a finger over the front vertical vent and the engine dying right away indicates the power valve needs replacement.

[Brad H.]

Re: 67 327/350hp backfire question

I'll check that Jim. This carburetor is brand new manufacture Holley and I assumed their new carbs had anti-rupture power valves but that may be a false assumption.
Thanks,
Brad

[Timothy B.]

Re: 67 327/350hp backfire question

Brad,

You mentioned the distributor has been overhauled, do you know what centrifugal advance curve the distributor has.

If you want to time the engine a different way, remove the springs holding the weights inside the distributor, mark the engine damper to 36* by turning the crank and mark 12* three times using the timing tab and damper marks, make sure you mark the damper on the correct side BTDC. Use your timing light and start the engine and at approx 1300-1500rpm the weights will be fully out and time the engine to your 36* mark. Lock the distributor down and reinstall the springs and recheck the initial timing. Subtract the initial timing figure from 36* and you now know you centrifugal curve in the distributor.

Also, your vacuum advance control should be a #236 and be full in at idle. Make sure you disconnect and plug the V/A line when timing the engine then hook it back up, it should add another 15* to the idle timing.

[Brad H.]

Re: 67 327/350hp backfire question

Thanks Tim, I appreciate it and will check it out.
brad