Author Topic: Weird stuttering and high rpms  (Read 1539 times)

Offline Zach

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Weird stuttering and high rpms
« on: April 06, 2023, 05:58:55 PM »
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.
« Last Edit: April 06, 2023, 06:18:29 PM by Zach »

Offline 73ss

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Re: Weird stuttering and high rpms
« Reply #1 on: April 07, 2023, 05:46:15 AM »
Does car still have the catalytic convertor? Sounds like a classic symptom of a plugged CAT.

Offline Zach

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Re: Weird stuttering and high rpms
« Reply #2 on: April 07, 2023, 10:24:33 AM »
Does car still have the catalytic convertor? Sounds like a classic symptom of a plugged CAT.

Cat was removed a long time ago.

Offline Zach

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Re: Weird stuttering and high rpms
« Reply #3 on: April 07, 2023, 02:05:48 PM »
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.

Offline Kenth

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Re: Weird stuttering and high rpms
« Reply #4 on: April 08, 2023, 12:05:52 AM »
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.

Offline Zach

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Re: Weird stuttering and high rpms
« Reply #5 on: April 08, 2023, 01:58:21 PM »
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?

Offline Cliff Ruggles

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Re: Weird stuttering and high rpms
« Reply #6 on: April 09, 2023, 01:37:55 AM »
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.....

Offline Zach

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Re: Weird stuttering and high rpms
« Reply #7 on: April 09, 2023, 06:22:58 AM »
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.