I found this site and have been using it for the past few weeks to diagnose my own problems, but i decided to make my first post about my recent delima.

First off my background. I have a 1980 with a frankenstien engine. The block, pan, crank, and rods are from a 73, 10.25:1 flat top pistons, 2.02 double humps from a 65. edelbrock performer intake, 272 adv. dur. .454 lift crane cam, crane 1.5 full roller rockers, holley 650 street avenger carb. procomp distributer w/ lighter springs (literally just put them on) AC and a.i.r. pump removed.

Now my delima is that when I put the Holley onto my car it ran much better (coming from a worn out Qjet) but I had a major problem w off-idle stumble, so tried adjusting the carb, replaced the factory dist. with the procomp, without any luck. today i discovered that i made the mistake of using sparkplugs for 1980 heads instead of for the 65 heads that are on it, so i replaced them and that made a big difference in performance but still had a bad stumble. I then redid the timing by Lars(taking of the spring, setting it to 36 degrees, then putting the springs back on) well it ran great, only a slight stumble and great acceleration, but the initial timing w/ the spring on is now only about 2-4 and that sound very low to me (although the idle was a little low)
I will next try to adjust the carb again w/ some instruction I bookmarked from a post.

So my question is, duz that initial timing sound correct, and would the carb be the definite cause of the stumble? Thanls for your help ahead of time!!!

 

I would try to get your initial to at least 8°.
Finish your timing before the carb.

On holley's you may have to crack open the secondaries a bit to help in the transition.

 

your off-idle stumble is probably a carb issue rather than an ignition issue.

if your total timing is at 36º and your initial timing is down at 2-4º than your mechanical advance is too much. I know nothing at all about the Procomp distributors but you want to have the initial around 12º and your total at about 36º. Try to get the timing all in at 36º somewhere between 2500-3000rpm. You can always fine-tune it from there if you want but this is an excellent starting point anyway.
The stock GM distributor is a very good unit so unless there is something that is a major problem with it I'd pull the new "hot rod" distributor and reinstall the stock unit if it was my car.

I'd recommend carefully reading two excellent papers on my website's tech articles page that is here:
http://69.253.166.197/page1/page65/page65.html

for the carb, find the article called:
Holley Carb paper.pdf by: Lars Grimsrud

for setting up the distributor find the article called:
MAPPINGADVANCE.pdf by: John Hinckley

 

I moved the initial timing up to about 12 and now im getting 36 degrees at about 2000rpm. It runs great now, but still have a slight stumble.. not near as bad now, it is actually tollerable. I also put a lighter spring in the secondary diaphram(not to fix the problem but jsut to see how it did) I feel it open up at about 1600 or so w out a stumble or jump.. now I belive I just need to fine tune the carb to get rid of the slight off-idle stumble and im all set.. but itz started raining around noon and has been the rest of the day so i guess that will come another day...

Also the reason I put the new dist. on was because I figured with a 25 year old dist.(minus new cap and rotor of course) it would not hurt to replace it with a fresh one, and with the procomp only costing me $90 as opposed to a little over $100 for a new factory distributer, I figured why not.