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Posted by: Tdskip Mar 16 2022, 07:05 PM

Hi - the '75 DJet car has a stumble that happens at a couple points in the rev range and at certain RPMs when the throttle is held constant. It rarely happens when the throttle is being exercised up/down the RPM range (like accelerating through turns etc).

I'm thinking worn throttle body "fingers" or the trace pad they travel against (yeah, wrong terms I know) but I believe this could also (maybe) be a sticky advance weight in the distributor.

Any why to test the spare throttle body so I don't swap out the wrong thing?

OR

Should I just pull and open the plastic cover on the one I have to see what is up? Doesn't look that hard to remove it.

What say you gentlemen, and please do liberally apply terminology dope slaps.

Thanks!

Posted by: emerygt350 Mar 16 2022, 08:34 PM

Go ahead and pull the tps cover and check it out. It is an easy and cheap rebuild. However, that kind of stumble can also be caused be a lean mixture.... How's your engine temp/head temp? How do your plugs look?

Posted by: Dustin Mar 17 2022, 05:11 AM

This page has links to two articles about testing and calibrating the TPS. One article for 2.0 and one for 1.7 1.8.

https://www.pelicanparts.com/techarticles/914_cooley/914_cooley_throttle_switch.htm

Posted by: Tdskip Mar 17 2022, 08:07 AM

Thanks for the respsones.

Found this earlier which is what I'm wondering about;

Track Wear: Over time, the wiper track for the accelerator function will wear. Wear will be especially high at moderate to light throttle angles, corresponding to part-load cruising. Click here for a link to a 60X photo of accelerator track wear. While this TPS track is still good, the re-deposition of gold worn from the contact fingers by the wiper can be seen, and eventually will become sufficient to bridge the traces. This wear causes arcing and poor contact, resulting in the car "bucking" at a constant throttle angle. "Bucking" is a fairly common complaint and is almost always due to track wear. Check by disconnecting the harness plug to the throttle switch and driving at a constant throttle angle under part-load. If the bucking is gone, it's due to the switch. As I mentioned earlier, if you go to http://www.914world.com/ and search for user "davesprinkle", he's made a kit to replace the worn circuit board that restores your TPS to like-new condition.

https://members.rennlist.com/pbanders/DJetParts.htm

Posted by: Lockwodo Mar 17 2022, 08:08 AM

QUOTE(Tdskip @ Mar 16 2022, 06:05 PM)

Posted by: DRPHIL914 Mar 17 2022, 08:08 AM

QUOTE(Tdskip @ Mar 16 2022, 09:05 PM)

Posted by: Jett Mar 17 2022, 08:58 AM

We had the same symptoms with our 73 2.0 stumbling at 2700 - 2900 RPM when not accelerating, and it was a worn distributor shaft, that was rebuilt using two other units.

On our 75 2.0 we had the TPS rebuilt with new board but failed within a month, and with the ignition on (motor not started) we could count the TPS clicks and was missing four in the middle, which also caused the car to miss and buck. Found an NOS unit and sent my right arm for payment

Posted by: Tdskip Mar 17 2022, 11:32 AM

Took the cover off the spare one that I had and there are clear lines that I’ve been worn here, whether or not that’s problematic I don’t know yet. But you can see a consistent arc from where it’s travel highlighted by red arrow.

Posted by: GregAmy Mar 17 2022, 12:06 PM

The board is, what, $50?

Just replace it.

Posted by: rjames Mar 17 2022, 12:53 PM

The board definitely looks worn, but it's also easy to disconnect the TPS see if the stumbling is still there. I'd do that first before anything else as it may be a air/fuel mixture issue as emerygt350 suggested.

If it's the board, you also may be able to bend the TPS contacts out just a bit so they aren't going over the same worn area on the board. Again, easier to do than replacing the board and worth a try before tossing it- especially if the reproductions aren't reliable (I' not saying they are, just going off of the comment by Jett).

As far as inspecting the throttle body, a smoke test (with it installed in the car, or off the car if you can seal up the bottom and top ends) will tell you if it's worn. Often they leak from the idle adjustment screw and the cure is to install a bushing. However, a leak would more likely result in a high idle, not a stumble in the middle of the RPM range.

Posted by: Tdskip Mar 17 2022, 05:40 PM

Thanks for all of the response and information.

Fortunately that unit pictured is a spare which is off the car, will try disconnecting it on the actual car and see what happens.

Kind of neat to be diving into this.