67 300 HP Powerglide throttle return spring

[Joe R.]

The throttle return spring shown in the photo below does not conform to what is shown in the AIM, but I have a few questions that I hope someone can answer for me.

The 1967 AIM shows two identical springs, both coming from the rear of the carb. One spring ties to the kickdown linkage as shown in the photo. The second spring connects between the bottom of the carb throttle lever and the tab on the crankcase vent tube support.

Using two identical springs obtained from Dr. rebuild I can duplicate the arrangement shown in the AIM, but the throttle return spring is a bit loose, and the point where it ties to the carb linkage results in an undesirable over-center position at wide open throttle. It just doesn't work very well.

The forward-mounted black spring in the photo is what the car had when I got it, and it works very well. The spring is attached to a mounting plate that would otherwise be unused if I followed the AIM. I also note that the diagram in the AIM shows an AFB carb and is clearly a carryover from an earlier year.

So, it the arrangement in the photo just plain wrong, and should I try to duplicate what the AIM shows, despite the functional shortcomings? Or is it possible that there was a running change some time during 1967? My car is an April 1967 car.

[Geoff C.]

Re: 67 300 HP Powerglide throttle return spring

Joe

I really don’t know what faith we can put in every item illustrated in the AIM. According to it and the part books 2 identical green springs are used on powerglide, but as you see it’s weak that way. I know the 427 powerglide is different also and it that is not shown in the AIM. It also uses two springs also, but both are different: one long and one short. And neither is like the 327 powerglide. With only a few hundred made, I wonder how well they documented some features in the Corvette AIM.

The AIM often can show the previous years arrangement on occasion if it was not a major change, or a generic carb or a 68-72 radiator support with brackets for both Copper and Aluminum radiators simultaneously, i.e. a design never built. They aren’t perfect but they are good.

My basic undergraduate degree in Corvettes was obtained by hands on disassembly. Then years later, the AIM’s were reprinted. I’m glad I learned most from the actual cars, than trying to get the car to match the AIM. I will admit I don't know it all and I'm forgetting what I did know as a faster rate but I'm learning more about other things like tomatoes, lettuce, peppers.

Geoffrey Coenen

[John H.]

Re: 67 300 HP Powerglide throttle return spring

One of the photos in Noland's book and two in the Dobbins book show '67 M35 300hp cars with what appear to be two identical light-colored small-diameter springs, with the front one anchored at the bracket at the front corner of the intake.

[Joe R.]

Re: 67 300 HP Powerglide throttle return spring

Hi John:

Thanks for the suggestion to look at those old photos. It looks to me that the photos show quite a range of implementations.

The photo on page 391 of Noland's book (second edition, in case the page numbers changed) shows both springs coming from the rear, as shown in the AIM. The photo on page 392 shows a large diameter throttle spring coming from the front.

In the Dobbins book, the photo on page 244 shows a small diameter spring coming from the front, although it is somewhat shorter than the kickdown spring. Is there another Dobbins photo that I missed?

I checked my 1972 parts book, and at that time Chevrolet was still calling out two identical springs for the 67 PG, although the part number had changed from what is shown in the AIM.

I'm starting to suspect that perhaps the two identical rear-mounted springs were in fact installed at the factory per the AIM, but it was a poorly engineered solution that may have generated customer complaints due to inadequate throttle return pressure. A simple dealer fix would have been to install the forward spring used on non-PG cars.

I note that the 4th edition of the JG calls for "two lime green return springs running to the vent tube," so I think I will install the throttle return spring that way only when the car is being judged (by the way, the kickdown spring does NOT connect to the vent tube, it connects to the ignition wire harness support). When I'm driving my car, I will use the forward mounted spring as shown in the photo I posted.

Lastly, is it true that the forward spring mount that is bolted to the front two intake bolts would still be installed, even if it wasn't used per the AIM?

[Joe R.]

Re: 67 300 HP Powerglide throttle return spring

Hi Geoffrey:

Thanks for your comments. I guess that the small block PG was a sufficiently rare option that we may never be certain what the configuration looked like when the cars were delivered. As noted in my response to John Hinckley, there were three different implementations shown in the three photos I could find in the Noland Adams and M. F. Dobbins books.

Do you know for certain that the 5.7" free length of the spring you sell matches the 3904245 spring called out in the AIM? If so, I think that GM had a poorly designed system. While the installed length of the kickdown spring is about 7 inches, the installed throttle return spring length is only 6 inches. Stretching a 5.7 inch spring to 6 inches provides almost no return pressure on the throttle at low throttle positions. In fact, I think the spring could almost fall off from engine vibration.

It seems odd to me that GM would specify the same spring for both locations when the working range is so different.

[Geoff C.]

I'll look for the blueprint *NM*

[John H.]

Re: 67 300 HP Powerglide throttle return spring

Joe -

Looking at the page 244 photo again, more closely, it appears that the front spring is smaller in diameter than the rear spring; based on all the conflicting photos, it appears most folks "fixed" the spring setup.

[Geoff C.]

throttle return spring search

Earlier this year I brought home hundred of blueprints from our warehouse. I thought it was among these, however it is not. Many are still located inside individual files at our warehouse. I’ll look there also.

Geoffrey Coenen