Looking for fresh set of eyes, additional input - Strange starting issue

[marsmann]

Here is the scenario - I am assisting in a complete restoration of a 91 k1500 4wd.

new, and I mean new, everything. All parts/sensors are Delphi or AC Delco. I've also verified every part and sensor to ensure proper voltage and ohm specs and in some cases even swapped with known working parts. Computer has also been verified, wiring, etc.

Just installed the new GM crate. Compression is good, etc. etc. Leakdown tests also good. Here is the problem:

Set motor to TDC, verifying not only that the timing chain was correct but that the marks at 0 were spot on when putting together.
Set the motor on #1 compression stroke, line up oil cam journal, install dizzy, line up the brand new, complete dizzy to #1.
Set motor to run. Set base timing at zero with EST bypassed.
Tightened things up, connected computer and verified timing at roughly 20* with computer advance.

Truck runs beautiful. But, after that first initial firing up, EVERYTIME it is started cold it stumbles for 30 seconds, never shutting off but barely staying on. After 30 seconds, it picks right up. Does this everytime and I am baffled as I have gone through everything a dozen times with all sensors, electronics, timing, and even swapped distributors. It acts like a vacuum or other leak but I have verified it all.

Any ideas? Again, only on cold starts, only for 30 seconds and then it runs beautifully. Actually, flawlessly. One of the smoothest sbc 350s ever.

Grounds are all new and clean on the body everywhere. What would cause something like this to happen for just initial start?

 

[marsmann]

Well, I keep going around in circles for the last couple of days and I got it resolved.

I put in yet another, new distributor. Installed everything the same way as I had done a dozen times before. This morning, with the 3rd distributor it finally started and didn't stumble on cold. I'm still rather perplexed as the 1st distributor, a known good working spare, works fine in my other truck. The second distributor, brand new GM distributor, checks out ok and I installed that in another truck to test with and worked perfectly. But, something about this truck/motor didn't like 'em and so this 3rd one finally resolved it. Very strange. Nothing else changes and nothing else was altered or modified on this truck during that time.

Goes to show you that even when parts are new, or even when things appear to be 'in spec' you always have to try different things and trust your gut.

Thanks to everyone for at least reading and potentially considering offering help or advice.

 

[98_k1500]

I'll bet you have a coil issue that becomes more evident in the future.

 

[marsmann]

Good and very logical theory. I wonder myself if we will see the problem resurface. Good thought to keep an eye out and watch for that. So far she's been running fine, I've been checking it constantly almost paranoid it will return. I did swap coils at one point but it is possible that it couldn't be fully tested in between with all of the swapping, sequence, etc.

I'll be sure to post back with any changes but for now we are still looking good.