[Austex-Custom-S]

Heyas,

Looking for some advice on diagnosing backfires while driving. Here's the story.

just finished with the top end swap on my 350 block with 670 heads / qjet. I had the engine running pretty well. idles really nice at 650 and a steady temp at 180. I had been babying it around for a week to work and back. This weekend I take her on the highway for about 30 minutes to see how she handles the texas heat. temp is good. I'm cruising at about 55 and I punch it to see what kind of response I get. All four barrels open up, down shifts into 2nd and away I go. Well, after a few seconds of this, the engine sputters and dies. Car starts right back up and I limp it to a friends house who lives near by. Engine is stuttering and backfires a couple times on the way there.

It seems to idle just fine, but when driving it is pretty ****ty and stutters, sputters, backfires, and dies. I have an MSD HEI ready to run distributor. all new plugs. super conductor msd wires. I set the timing to 9 before TDC. I haven't checked it again after this happened though.

From what I know of backfires it could be a timing thing or a valve isn't sealing? is that correct? any other suggestions on what to look at? Could it be the carb or maybe the timing curve in the distributor needs to be adjusted?

Sorry for the wall of text, but I'm hoping y'all can offer some advice.

[prostreet64]

I would start with verifying the timing; check your initial and total, and at what rpm the total timing is reached. If all of that is okay then I would pull the valve covers and make sure the valves are lashed properly. Also sounds like the Q-jet may have issues. Is the carb rebuilt and have you run it before?

[Mudtrux]

How many miles on the timing chain and is it the stock nylon gear?

[Austex-Custom-S]

- yes, the carb is rebuilt. this is the first time running it on this engine. I'll have to look up what initial and total timing is. I'm not sure what that means yet. I set the timing by warming up the engine and then moving the distributor until the marks lined up. I'm not positive what the RPMs were at when I set the timing.

- i'm not sure on the timing chain. i've never replaced it since i've owned the car. and the car has 98k original miles on it. i've also not changed the gear.

[Old Goat 67]

Had that happen twice on my 67 in 45 years. Both times it turned out to be a fuel pump going bad.

[prostreet64]

You need a timing light to set the timing accurately. With the engine at idle and the vacuum advance hose disconnected and plugged, you turn the dist. until have about 8-10 degrees advance btdc. That is to say you are lining up the groove on the balancer with the 8 or 10 degree line on the timing chain cover. You will not be able to verify total timing unless you have a dial-back timing light or an after market balancer with degree marks out to 45 or more degrees. But you can still check to make sure that the advance weights are moving freely so as to provide some mechanical advance. Pontiac motors 'like' somewhere in the 32-36 degree range for total advance.