Cold engine stumble

[Prong]

Let me start off by saying that Quads are not my strong suit.
Specs: 79 Chevy K10 350/SM465/NP205 Edelbrock 1903 Q Jet

The problem which started a couple of weeks ago:
After truck sets overnight and is dead cold, truck starts right up and idles fine. Go a few blocks and hit a stop sign. Take off in 2nd, then hit 3rd gear with moderate throttle (1500-2000 rpm) and it stumbles/loads up bad (like a Holley with a blown power valve or a couple of plug wires disconnected)--but it won't die. Let off gas and cruise for a couple blocks and it will clear up. Once it's completely warmed up it's fine, even if it's been sitting for 8 hours after I get off work. It won't do it again till the next day. Checked choke operation and it seems to be fine (maybe opens a little too fast for my liking). Adjusted choke and no change. Fuel filter is about 4 months old. Tested choke pull off with vacuum pump and that's kosher. Any ideas?

[429bbf]

prong welcome aboard . iv'e never had a 1903 apart but i had 79 quad do this. im going to take a long shot at it.cliff probably knows exactly whats wrong. i think the power piston is sticky.when you step on it and loose vacumn the spring raises the needles. after you cruise for a block or two with high vacumn it pulls the needles back down.once the carb is warmed up theirs enough clearance for things to work.let us know what you find.dean

[Cliff Ruggles]

If adjusting the choke doesn't help, try adjusting the choke pull-off.  It may be opening up the choke flap to an angle that it just doesn't like on a dead cold engine.

If the choke and choke pull-off adjustments don't correct the problem, take the carb apart and replace all the internals with one of our SR kits.  This will get the carb up to par for modern fuel, and give you a chance to check out the power piston, and everything else inside to make sure things are clean, correctly adjusted, and working like they are supposed to.....Cliff

[Prong]

Thanks for the replies. Will adjust the pull off and see what happens. By the way, I called you this afternoon to order a kit. Left a message around 4pm your time. Hope to speak to you on Monday.

[73 Z28]

I found these specs out of a GM service manual that might be helpful. With the front vacuum break fully seated using a vacuum pump, the choke plate should be open betwwen .130 -.200 of an inch. A drill bit makes a nice gauge to use. I believe GM used a range to account for differently
 calibrated carbs. If I remember correctly, I believe cliff had indicated that as the idle circuits are richened up, not as much choke is needed. So if you've richened your idle circuit, the pulloff will need to be adjusted for more opening. By the way, my carb was doing the same thing and this fixed it.

Good Luck!!
Paul

[Prong]

Update:
It turned out to be a bad spark plug of all things!!! They were only a few months old to begin with, but I guess I got a dud. Ended up putting the kit in it anyway because of the age and it runs very well. Thanks again.

[Cliff Ruggles]

Good news, thanks for the update....Cliff