Final Update 5-16-20

Solved. When I had disassembled the TB initially, the return spring assembly had come apart. I did not know the proper rewinding of either the outer or inner coils nor did I know if the cartridge itself needed to be prewound. All it needed was two pre-load turns to the inner spring to the single preload turn of the outer spring as it sits on the closed throttle arm. After that, perfect. Appreciate all the help here! The problem was that there was not enough return spring force.

Original Post:

Ok, I'm stumped and before I press the issue, I thought I would tap the wisdom and experience here.

As many of you know, stock, rebuilt L98. Just slightly warm cam.

On many drives, often the idle would need a little throttle kick to come down when coming to a stop.

The stock TB was the original and it had some play in the shaft so I rebuilt it. I got the bushings and reamer from Carb Junkys in Georgia. Nice guys. It was a really interesting job. I will doing a write up on it a little later.

Got the TB all setup, TPS .54, IAC at 34, just fine, no hanging, snaps back, smooth, plates are closed.

So here's what it does. ONLY when the engine is running (and not in gear), I can raise the throttle a little (say to 3,000 RPM and below) and it will not fully return to the stop. If the engine is off, and I raise the the throttle it just snaps all the way back like no big deal.

The engine is literally holding high idle, say 2000 RPM, I shut it off, TB snaps closed the rest of the way and it idles correctly when I restart it right after that.

So I figure, it has to be the vacuum keeping it open. Has to be. That's the only difference and the only that changes.

So, I thought maybe my return spring was tired, so I reindexed the inner one about half a turn for more return force. It helped but not fully. It's not a huge increase but I don't know how much is too much force.

So before I raise the return force another notch, I wanted to see if anyone had any similar weirdness.

 

The throttle blades should be offset a little so that manifold vacuum pulls them shut. It may be that there isn't enough offset due to play in the screw holes.

Or that the blades were installed in the wrong direction.

RBob.

 

Thanks Bob. I will look into that. There didn't seem to be any other way to assemble the TB. I took pics from before and after. I agree it has to be throttle plate related. That's where the action is.

I did have to grind off the back of the throttle plate screws as they were tack welded and that took a little meat off the shaft but that's so close to the axis, it would have very little torque effect on the rotating assembly. It's gotta be the orientation of the outer edges of the plates.

Now that my coffee is kicking in, It could be that the base idle set screw is also used to keep the plates from rotating too far back so that they CAN have offset. Hmmmm. I had back my screw out to get the plates centered in the bores. Maybe they are too centered!?

 

Well, messed with it today. Took it apart, set the plates up as high as they would go and it still does it. Weird. Its the same TB. Even adjusted the base idle screw out to allow for more angle of the plates to get them higher up above the shaft. Got the IAC count to 16. Vacuum still yanks it open. Ugh. I’m not seeing what has changed to start causing this.

 

It may be the throttle cable or cruise cable hanging it open a tad. With the engine running see if you can replicate the high idle by manually moving the throttle. If so, then remove the throttle and cruise cables and try again.

RBob.

 

Thank you Bob.

I am very happy to report the problem is solved.

I disassembled the TB again- I literally can remove it, and completely disassemble it in five minutes. Reassembly and installation is not much longer. What a talent I have acquired! Lol.

So I recalled the shaft being a small amount out of straight but there was very little drag on the assembly. Very light- but not that loose, "clanky" zero. So I thought what the heck. Took it apart and ran over to my Dad's house. He loves this kinda stuff. He has a small machine shop in his garage and is an old school machinist hobbyist.

Here he is working on it.



He got it to within half a thousandth and it became that proper, loose "clanky" when opening and closing (bushings and side clearances were all tight though).

I then reset my return spring to stock index setting, stop screw to stock and simply installed the plates. I had measured them and the holes, bores, and shaft bore are all on center (no bias). Threw it on and bam- 100% perfectly behaved TPI engine. Holy moly.

 

Ok. Not entirely fixed. It's MUCH better but still holds 1500 or so once in a while. Additional spring force does not help, even a little. Weird weird weird. There is a tiny vent hole on the top of the driver's side bore at the top right about where that throttle plate closes. It vents to the chamber under the "Tuned Port Injection" plate. I wonder if that has anything to do with it. Thinking hat on again.

 

I fixed a similar problem on a FIRST® intake throttle body. A slight play in the throttle plate shaft was creating the problem. When releasing the gas pedal slowly to idle, the TPS lever would be raised a few thousands. That was increasing the TPS voltage, thus increasing RPM at idle. The ECM thought I was feathering the gas pedal, thus opening up the IAC a bit. The problem would not happened if the gas pedal was released quickly or a gas pedal jerk was given. I found out by datalogging and observing the TPS data. Solution was to custom make a TPS stopper and adjust it so the TPS would rest a few thousands higher to compensate for the shaft play.

 

Son of a Biatch! Never thought of that. I'll look into that ASAP.

 

Nope. All good there. I just got a BBK 58MM today. The spring is waaay stronger than my stock TB. I don't know what stock fresh should be so I'm going to crank it up.

 

Adjusted the throttle return springs for more force and that solved it.