TH-400 1st gear dragging

Discussion in 'The "Juice Box"' started by 455Riviera, Jan 21, 2021.

  1. 455Riviera

    455Riviera Active Member

    Ok heres the story.

    71 Rivi with 455 and TH400 Trans. The 455 is mostly stock, and the Trans had a stock rebuild about 5 years ago by a very reputable shop in the area.

    I have been fighting an issue for a while now with enging RPM dropping by a large amount when putting the car in drive. If the car is set to idle in park at 900 RPM, putting it in drive with my foot on the brake drops me down to about 400 RPM. The Trans Shop is telling me its an engine issue, but I'm starting to wonder if that isn't the case. The engine does bog a bit off the line, however after getting off the line it scoots, and the same when stepping on it from a roll. Plus the bog isn't there when the car is in park or neutral. The Trans also has a very sloppy 1-2 shift, but the 2-3 is tight and perfect.

    Am I still chasing an engine issue? or is my Trans the culprit?

    Thanks in Advance!
     
  2. Bens99gtp

    Bens99gtp Well-Known Member

    Only in drive or reverse too

    Sounds like a converter issue to me
     
    john.schaefer77 likes this.
  3. 455Riviera

    455Riviera Active Member

    Reverse does the same. My thought initially went to something being wrong with the converter. I took the car back to the shop that did the work initially a while back, and they swore it was an engine issue. They said its rare for converters to go bad, and since I haven't messed with transmissions much I just assumed they were right.

    Not knowing much about converters, are they able to be rebuilt? Or should I look into purchasing a new one to see if that solves my issue?
     
  4. rkammer

    rkammer Gold Level Contributor

    Even with a stock converter 500 rpm drop is too much. Check your timing to see what is at 900 rpm vs 400 rpm. I’m guessing timing drops when rpm drops which is part of the problem. Also initial timing not advanced enough may be a factor. Check back when you have your timing info and we can go from there.
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    X2. What have you done to check the engine? Have you ever put a vacuum gauge on the engine to see what it is pulling? The vacuum modulator uses engine vacuum to determine engine load. If you have a vacuum leak, that will cause engine and transmission problems.

    I doubt whether there is anything wrong with your converter. It could very well be an ignition timing problem. The more information you provide the more help you can potentially get.

    Detail the engine build. Is it stock, or modified in any way? What carburetor are you using including part number? What distributor are you using including part number? What is you initial, and total timing? Are you using vacuum advance, and does it work?
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  7. BYoung

    BYoung Stage me

    Had a similar issue with my 71 GS. It turned out to be vacuum/timing issues. Mostly carb related.
     
  8. twood

    twood Active Member

    The same thing happened to my 69 with a th400....I changed out the vacuum modulator and problem solved. My brother in-law who is a mechanic couldn't figure it out and it only took me 5 mins to fix it...
     
  9. Matt69olds

    Matt69olds Well-Known Member

    The fact that it happens while the car is t moving rules out the transmission. If the car is sitting still, the input shaft isn’t moving. For the transmission to drag the engine down would suggest that the transmission is putting a excessive load on the engine. If the trans isn’t turning, it isn’t putting any load on the engine.

    The converter could be suspect, but I’d expect noise or lousy performance, along with debris in the pan. If the stator in the converter freewheel (due to sprag failure) the usual complaint is poor low speed performance and normal higher speeds. I said low performance at low speed, not dragging the engine down. A locked stator sprag will accelerate normally, but has limited higher speed performance and usually overheats the fluid. And since 400 transmission don’t have a lockup converter, the converter clutch partially applying isn’t a possibility.

    My bet is something is screwed up in the distributor. Put a timing light on the engine in park, then have a helper put the car in gear. I bet you see a huge drop in timing. Either the advance weights were stuck in an advanced position when the timing was last set (and the weights have no freed up) or the springs on the weights are broken/fatitge, or some other problem causing the timing to change drastically.
     
  10. bobc455

    bobc455 Well-Known Member

    I think "thanks in advance" is a very ironic thing to say (a little distributor timing joke there) because I'd be checking for vacuum leaks and/or connected to the wrong ports on the carb etc.

    -Bob C.
     
    BYoung likes this.
  11. Bigpig455

    Bigpig455 Fastest of the slow....

    Just went through this on a 454 Chevelle - car had 8-10 degrees advance at 800 rpm,, for a total of drop it in gear and idle at 700 and all that advance dropped out and the engine would stumble and die. Easiest thing to do is sub some heavy springs in the distributor (or wire the weigths in closed position), retime and check the idle drop from park to gear. Willing to bet thats your issue, and hope it is cause thats the easiest to fix.
     
  12. 455Riviera

    455Riviera Active Member

    I appreciate the help everyone. Here's what I have at the moment:

    455 is mostly stock, had to have it rebuilt after some oil pressure issues, so it was bored .030 and I added a TA-212 cam. Added the TA Timing Cover and oil pump. stock timing set.

    When I bought the car, it had the wrong Qjet on it, so I purchased the correct one and had it rebuilt locally. I can get the numbers from it the next time I'm out with the car. (sorry it's currently out at my in laws barn for the winter.)

    The Distributor is from a 72, from what Larry told me when I gave the numbers in the previous thread. The vacuum advance is not hooked up, and hasn't worked since I bought the car. as far as I know, the internals on that are all original.

    Transmission was rebuilt 5 years ago. Initially when it was done it drove like a dream. It wasn't until about 6 months later that I started having these problems, and when I took it back to the trans shop they said "that's an engine problem" .

    This car is an absolute mess for both wiring, and vacuum routing. I've replaced most of the vacuum lines, but the wiring was a giant ball of duct tape tucked under the dash for the most part. With the exception of the vacuum line going to the trans, most of it is capped off while I was trying to figure out what, if anything, was leaking. The wiring and vacuum diagrams are a real weakness of mine when it comes to reading and deciphering them, which I know doesn't help, so I apologize for the lack of info there.

    I just went out to the car today, and it quit after running for a few minutes. It appears that my fuel pump has died again, so until I can get that replaced there I'm stuck.

    And I will be picking up a vacuum gauge on my way home tomorrow, so that I have it ready when I get the pump replaced.
     
  13. Bens99gtp

    Bens99gtp Well-Known Member

    As has been stated the two most likely causes are either the timingnis drastically dropping out of it when in gear.......which means its more thsn likely swinging too high at idle.......so stiffer springs

    Or the converter isn't stalling properly inside. Seeing that nothing else is turning the moment you put it in gear.......nothing else can cause this.

    Well on the very very very far off chance tranny pressure could be through the roof and that could cause a drastically drag.....buy i would think seeing pressure is being made at idle not I gear there would be signs there too........but this would be so rare of a problem
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Unless you have a vacuum leak, all of your problems stem from a combination of ignition timing and carburetor calibrations. The TA 212 is a relatively mild cam, but it still may need some modifications to the stock Quadrajet idle system calibrations. Ken Gies (techg8) can help you with that. Fuel pumps are cheap and relatively easy to change.
     

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