Weird stuttering and high rpms

[Zach]

Carb # - 17057263
Everything is stock, jets, rods, etc, used Cliffs SR Kit at home and base plate rebuilt by him.

First day I was able to take the trans am out since summer and after a rebuild, 40 degrees and sunny out. Car was running great in the garage, good idle, good cold starts, choke works, etc, etc, no issues. I take it out on the road and immediately notice issues, as I am accelerating it stutters really bad and fails to build RPMs, felt as if I was running out of gas. I notice it slowly runs better and better as I am driving around (20+ minutes and a short 1 mile trip (Choke has been open/off for a long time), still not perfect but it is running ok, still noticeable stutter if I accelerate aggressively. After a while and everything gets hot it starts to idle around 1100, not sure what causes this, and when ever I give it gas it takes a significant amount of time for RPMs to drop back down (to 1100 in this case). Not sure what to check would like some insight, thanks.

I suspect it is running too lean but unsure what to change.

[73ss]

Does car still have the catalytic convertor? Sounds like a classic symptom of a plugged CAT.

[Zach]

Does car still have the catalytic convertor? Sounds like a classic symptom of a plugged CAT.

Cat was removed a long time ago.

[Zach]

Going to summarize my main issue a bit fr after fiddling around with it, idle is great and revving in neutral is great, under load coming off idle, part throttle, and WOT, causes a bog/bucking acceleration, it will accelerate and open the secondaries but it is slow and bucks lightly. Adjusted the APT screw from 3.5 turns out - 4, small change I think, 4 - 4.5, no noticeable change.

[Kenth]

I would use .073" main jets, 4 turns on APT, secondary rods stamped DA or CE with 1/2 turn preload on airvalves spring.

Also, look for binding weights/springs and broken pick-up coil wire insulation in distributor.

[Zach]

I am gonna say it is a timing issue, not sure if my timing marks are correct on the harmonic though. It runs significantly better when there is more timing 14-18 initial (factory) however my starter does not like it. 8 degrees initial is the absolute highest I can go in order for a smoothish crank on the starter. Currently in a decent spot but I know there is more power. I would run a high timing but I don't want to starter to constantly fail on me. The start issue only occurs when the engine is hot however. Maybe I can wrap it and see, don't have materials on hand for that at the moment though. Any ideas?

[Cliff Ruggles]

Stock engine?

Verify that the mechanical spark advance is working correctly.  A stock 1977 400 4 speed engine is really LOW compression and would be fine with the stock timing setting.  "Bucking" the starter with more than 8 degrees initial timing tells me the balancer has slipped or the engine has a LOT more compression than it did stock.....

[Zach]

Stock engine?

Verify that the mechanical spark advance is working correctly.  A stock 1977 400 4 speed engine is really LOW compression and would be fine with the stock timing setting.  "Bucking" the starter with more than 8 degrees initial timing tells me the balancer has slipped or the engine has a LOT more compression than it did stock.....

I think I am fighting heat soak. Starts fine on 18 but isn’t reliable when hot, going to wrap starter and exhaust and see where that gets me.. I quickly checked mech and made sure it moved a few weeks ago but nothing thorough, I’ll do that next. I also am gonna tests for Tdc and see where I am at on the harmonic because likely it has slipped. W72 has slightly more compression 8.2:1 I believe, still incredibly low but more significant than stock. I doubt the engine was ever hopped up. Thanks.