Strange running condition....
 

Hi,

I have a 1971 Cutlass with 350 and 4 barrel q-jet. Work done recently as follows:
-Points upgraded to HEI with new 8mm wires. Starter wire from R terminal tied in with new 12 gauge power wire run to new distributor. 12 gauge wire connected to IGN on fuse box.
-Stock air cleaner changed to 14" after market type to clear new HEI. Breather installed on passenger valve cover now that the stock air cleaner connection is gone.
-All rubber vacuum lines replaced with new and open/unused ports capped.
-New professionally rebuilt q-jet carb with new gasket.
-New intake manifold gasket to rule out internal vacuum leak.
-Stock manifolds and single exhaust changed to headers and dual exhaust system. Remflex header gaskets used.
-New fuel pump.
-New trans cooler lines.
-Rebuilt TH350 transmission.

I think that is all the major stuff. So, I have been chasing what I thought was timing and fuel mixture issues but it always seemed to idle rough no matter what and once in gear, even slight pressure on the gas resulted in severe hesitation and bogging. It will idle at a vacuum pressure of 16" to 17.5" depending on how aggressive the timing is - between 16* and 22*. I have tried ported and manifold for vacuum advance, not really any difference. I have also tried adjusting that secondary clock spring with little to no difference, but from what I understand at idle and light acceleration I would see a difference. It also seems to have a flat spot around 1000-1100 RPM even after warmed up. From a cold start it also usually takes a few starts, first time it fires up, runs for a few seconds then dies, then gets going on the next attempt or two. I have checked the choke and it operates as expected.

But, to my surprise yesterday, it started better, only requiring one restart, went into a nice high idle. I kicked it out of high idle at about 130F and it seemed smooth, so I took it down the lane. There was no hesitation and it drove and sounded really good. So I hit the street for a few laps. Drove great, sounded great, I was finally able to test the kick down on the trans which worked great. But after about 5 minutes or so all of a sudden it fell on its face and went back to bogging really bad. I limped it home and put it away. I was reluctant to mess with the timing and fuel any more because it was proven that those settings can run very nicely when the mystery intermittent problem is not happening.

Today was similar, started first shot, no restart needed. I kicked it out of high idle around 120F and hit the road. It drove OK for about the same amount of time, then instantly back to bog city. This time I noted that it was at operating temp for a good 5 minutes before it went bad, so it isn't like it hit a certain temp and start struggling.

But, it got me wondering is it maybe a wire or wires getting heat soaked that once they are heated to a certain temp start to cause issues. Perhaps some or all the plug wires reach a temp and can no longer deliver a strong enough spark, or maybe the 12 gauge wire to the HEI gets too hot and reduces its power? I did try pulling each plug wire off the spark plug to see if it changed the way it ran looking for a dead cylinder. They all were getting a spark, but some were so hot I could hardly touch them, which got me wondering about heat. Maybe the header just make that much more heat than the manifolds and it is causing issues?

Any other ideas? As far as I know vacuum leaks are not intermittent, the timing hasn't slipped, the secondary clock spring hasn't slipped, I don't know what else could just change on its own that would explain this issue.

Any other ideas?

Thanks to anyone who made it though my rambling post, lol.