TH400 acting oddly

[DoubleDingo]

A few months ago when I went to the bulk yard, I was backing into the driveway and noticed the truck acting weird. Gave it a little gas, it would go backwards, let off the gas and acted like bearing flat spots and you could hear the driveline chattering as the truck slowly rolled to a stop. That's the best way to explain it. In Drive there was no flat spot feeling. This past weekend I took it out for a drive and it backed up fine for the first day. THEN, it began running like crap, really bad. It felt like the tires were all out of balance. I found three vacuum leaks, each from rotted vacuum port plugs. When I rebuilt the carb a couple years ago I went with electric choke because vortec heads don't have provisions for the warm air choke. Found some fresh plugs that fit, no more vacuum leaks, and also replaced the in-line fuel filter. It still ran like crap. Forgot about the in-carb filter, replaced that and it now runs a little better. However, sometimes when I put it in neutral it wants to creep forward, other times it doesn't. Also, sometimes there is no stumbling from a stop, and other times you can hear it trying to take off, but it stumbles and goes, stumbles and goes. Would a weak spring on the power piston be an issue with partial throttle? I suspect I may have some bad juju in the carb from it being gunked up from not running in a long time, so I've been running Motor-Purr in the tank to clean things up. It fires right up, just a tap of the key like it's always been, it just doesn't run as good as it should and I am running out of ideas.

Dumb question...do you guys think the shifter rod is out of adjustment? Or can a vacuum leak cause some of these weird going forward in neutral things to happen? When everything is running like it's supposed to truck is a blast to drive, it'll take off quickly, shift normal, and run smooth. Then it does its stumble and go thing and loses a lot of power but you can feel it gain it and lose it, gain it and lose it. Any suggestions on what to look for will be greatly appreciated. Thanks.

 

[Snoots]

Vacuum leak?

Pull the dist cap and rotor and see if the advance weights move.

 

[chengny]

However, sometimes when I put it in neutral it wants to creep forward, other times it doesn't. Also, sometimes there is no stumbling from a stop, and other times you can hear it trying to take off, but it stumbles and goes, stumbles and goes.

Dumb question...do you guys think the shifter rod is out of adjustment? Or can a vacuum leak cause some of these weird going forward in neutral things to happen? When everything is running like it's supposed to truck is a blast to drive, it'll take off quickly, shift normal, and run smooth. Then it does its stumble and go thing and loses a lot of power but you can feel it gain it and lose it, gain it and lose it. Any suggestions on what to look for will be greatly appreciated. Thanks.

Improper adjustment of the shifter linkage is first on the list of probable causes - for forward creep in neutral:

Moves Forward In Neutral

1. Manual control linkage improperly adjusted.
2. Forward clutch does not release.
3. Oil pump.
4. Internal linkage.

Adjustment procedure:

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1. Position transmission lever (A) in neutral position by moving lever clockwise to the park detent, then counterclockwise two detents to neutral, Fig. 9.
2. Place column shift lever in neutral gate notch by rotating lever until it drops into neutral gate notch. Do not use indicator pointer as a reference to position shift lever.
3. Attach rod (C) to transmission shaft assembly (B).
4. Slide swivel and clamp onto rod (C), then attach to column shift lever.
5. Hold column lever against neutral stop, park position side, then tighten attaching nuts.

 

[DoubleDingo]

Vacuum leak?

Pull the dist cap and rotor and see if the advance weights move.

I didn't get a chance at this today, but next week for sure. In-laws are in town so time is limited. This truck, I'm finding is not molested very much.

 

[DoubleDingo]

Improper adjustment of the shifter linkage is first on the list of probable causes - for forward creep in neutral:

Moves Forward In Neutral

1. Manual control linkage improperly adjusted.
2. Forward clutch does not release.
3. Oil pump.
4. Internal linkage.

Adjustment procedure:

You must be registered for see images attach

1. Position transmission lever (A) in neutral position by moving lever clockwise to the park detent, then counterclockwise two detents to neutral, Fig. 9.
2. Place column shift lever in neutral gate notch by rotating lever until it drops into neutral gate notch. Do not use indicator pointer as a reference to position shift lever.
3. Attach rod (C) to transmission shaft assembly (B).
4. Slide swivel and clamp onto rod (C), then attach to column shift lever.
5. Hold column lever against neutral stop, park position side, then tighten attaching nuts.

This is great stuff, I got it adjusted and it seems to be running better. Only a very short test drive and smoother than it was. After some longer driving and looking at the dizzy weights, I'll report back.

I wish I had the camera handy when I adjusted the linkage, it had not been touched since it left the factory. The original mark from the adjustment bolt was the one and only mark on the steering column shift rod.

 

[chengny]

This is great stuff, I got it adjusted and it seems to be running better. Only a very short test drive and smoother than it was. After some longer driving and looking at the dizzy weights, I'll report back.

I wish I had the camera handy when I adjusted the linkage, it had not been touched since it left the factory. The original mark from the adjustment bolt was the one and only mark on the steering column shift rod.

I think what happens is that - while the linkage length/alignment stays as original - over the years, slop develops in the nylon bushing (that the outboard end of the shaft assembly rides in). When that happens, the detent points begin to drift out of adjustment.
BTW, that bushing is available if you find that is the case.

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[DoubleDingo]

Yesterday after the second run to the bulk yard, that run was to get a yard of rock dust, the truck running horribly I opened the hood to see or hear anything amiss. Could hear a vacuum leak but I couldn't find it. I got out the idle mixture screw tool to readjust those, but that leak was eluding me. One thing I discovered yesterday is that this running like crap only happens after it gets hot. I drove it yesterday morning and it ran like a top. Then before the first bulk yard run it began the spitting and sputtering BS. In diagnosing the leak, when I sprayed carb cleaner on choke can, the engine really stumbled. This carb is a warm air choke converted to electric. I remember a plastic plug coming with rebuild kit to plug the hole between the can and carburetor body. Today, an experiment, instead of plugging the warm air choke ports that drew air through the tubes, I will run vacuum hose to them so air will still circulate. Easier than pulling carb. If it works, great. If not, I'm pulling it off to pull it apart and investigate how I can plug that hole so no air goes into it.

 

[DoubleDingo]

Curious, would a vacuum leak at the carburetor cause the TH400 to act oddly as described in the original post?

 

[Frankenchevy]

If it’s on the manifold side of the carb, it seems it would effect the modulator. I thought yours was having engagement issues in reverse, but cannot remember...

I think the modulator only changes shift pressure ever so slightly.

 

[DoubleDingo]

It engages all of the gears, but when sitting on level ground and putting it in neutral, sometimes it'll creep forward ever so slightly. Adjusting the rod didn't remedy that. In reverse, when the truck is acting up is when it does the weird thing when backing up, otherwise it just backs up normal. ATF is at the proper level, it shifts through the gears just fine when driving, but they are sometimes unnoticeable, but again, this is when the truck is acting up. I have never had a vehicle run great, then run like crap, then run great, then run like crap all in the same drive. Sometimes when idling at a light you can hear it just purring as it idles smoothly. Then the next light it may idle smoothly or run rough. It's making crazy. It fires right up with a tap of the key no matter how it runs prior to turning it off. It doesn't burn or use oil, has decent oil pressure, doesn't overheat, and when it's running great it really runs great. I guess I need to check what @Snoots mentioned. I haven't done that yet.

 

[DoubleDingo]

Yesterday I did remove the vacuum port plugs for the hot air choke ports, the one on the choke can and the one on the back of the carb, and then ran vacuum hose to link them back up like they were when it had the hot air choke. That way the air can circulate through and not be plugged. It still has the plastic plug in between the choke can and carburetor, but it was worth a try to me, and it did seem to run a little better like that. I didn't get a chance to drive it like that yet, still have half a yard of rock dust in the bed that needs to be hauled to the backyard.

 

[Snoots]

I remember a plastic plug coming with rebuild kit to plug the hole between the can and carburetor body.

PLASTIC? You have got to be kidding me! I never would've thot that would hold up.

 

[DoubleDingo]

These are them.

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[DoubleDingo]

Seems the majority here has nothing to provide on the subject, so I've been trying some research and getting mixed information. I just went to the Cliff Ruggles forums to see if there may be info there because I suspect this is a carburetor issue not a TH400 issue since the TH400 acts just fine when the truck is running properly. While there I saw a couple member's names from here over there. @fussfeld and @da_raabi I thought that was cool. There were a few threads on the hot air choke, and Cliff mentioned melting some lead into the vacuum passage of the hot air choke. I'm wondering if getting a split shot hot enough with a propane torch to make it more malleable and tapping it into that passage will be enough to seal it off?

 

[Snoots]

I'll agree that the problem's not with the tranny.

Don't be so fast to blame the carb. Cliff is excellent when it comes to carbs but you need to check out how well your system is from the gas cap to the carb first.

You could have trash in the tank, some water, a clogged pickup sock, a weak fuel pump, a kink in the line.
You have a lot in front of the carb to eliminate first.

Get a helper. It's certainly worth a 6-pack.