Need some help (motor guys). Long post

[Psycho71]

Let me start by saying I've always felt like a pretty capable guy when it comes to SBC's. But I'm having some trouble isolating a problem, and want to see if anyone here can point me in the right direction.

I have a 350/350th in my '71 truck. I did a fairly extensive partial rebuild on the truck last winter, but kept the same motor in it for now because it seemed very sound, and I think it still is. It never burned a qt of oil, or fouled a plug in the couple thousand miles I put on it before the teardown.

So here's my problem...It's stumbling when I apply up to 1/4 throttle to it. Then it pics up and runs like a top. When I let off the throttle from just about any speed, it pops thru the muffler. It will do all of this when in park/neutral (no load), and when driving (much worse under load). It even idles like crap when I drop it in gear.

Here's what I've looked at so far:
Plugs....Look great (barely tan on the porcelain)

Inspected wires (new accel wires)

Took cap off of dist. (skip white HEI dist) and inspected everything (all looks good, and nothing is bound up on the dist.. It has a clear cap, so I can see it firing, and see the vacuum advance working).

Sprayed carb cleaner all over the vacuum lines, and around the carb itself (Holley 670 avenger, rebuilt 700 miles ago). Also went inside and sprayed carb cleaner on the vacuum actuators for the a/c controls. No change in idle speed.

Took carb off, replaced the base gasket just for grins (all carb functions seem to be right).

Disconnected the vacuum modulator on the trans, and plugged the line to the carb.

Same with vac. advance.

None of this has made any difference with the problem. I was driving the truck last Friday. I went to work, about 12 miles from the house. Then to the bank. Then to the local walmart to get a hunting license. Everything was great. So I get in it at wally word to come home and kiss the wife by (going hunting), and the problem is there. Drove it home, unloaded all my hunting stuff out of it, and had to actually drive a ford to go hunting (which really pissed me off). So it all started at once! Hasn't gotten any worse. But also, nothing I've done so far has helped it one bit. It does this while cold and at operating temperature. But if I stab the throttle, it wakes right up (after a hard stumble) and runs fine.

The way it's popping thru the mufflers on deceleration makes me think it's in the timing/ignition (not burning all of the fuel, or firing late). I know anything can go bad, sometimes no matter how new it is. But everything except the longblock on this truck has less than 1,000 miles on it right now. All new stuff. And the motor is on fine shape internally.


Any advice????? I'm stumped, without just starting to throw money at things. Which is nothing but a shotgun approach. I'd rather be a little more precise than that.


Thanks for any advice y'all can offer. I do not drive the truck daily, so time is not too much of a factor. But I would like to get this thing going so I can drive my baby when I go back hunting this weekend, if possible.

Again, thanks in advance.

Jay