Is there a way to dampen throttle response

[richardsonta]

I just bought a '71 a couple of months ago for my wife (I had one in college and when I showed her a pic she fell in love with it). It's a great car but I've had to iron out a couple of issues. I don't have a good idea how to fix this one. The pedal is so sensitive that any actuation at all will make the car jump. I can drive it pretty smoothly but it's a challenge to keep it from surging and my wife can't drive it at all. Any suggestions on how to dampen the throttle response? I considered adjusting the linkages so that it has less travel but I was hoping either someone with more knowledge would sign off on that or suggest something better.

Thanks,

[geospider]

Well, I think all of us may struggle with this one. Normally we have something screwed up to dampen response and we are trying to fix that. I would say: A good problem to have.

What carb is on this? I'm guessing a synchro carb vs progressive would be pretty responsive; but I don;t have one of those May just be a learning thing.

[richardsonta]

I thinks it's an EMPI 32/36 carb. I'd post a pic but I haven't figured out how so here's a pic I found on line that's identical except my choke cover is yellow instead of black.



It's tuned pretty well. Starts up easily and runs nicely with some punch but any oscillation on the gas pedal makes the car start lurching pretty violently. I've been driving stick for decades including racing but it's even touchy for me.

[18Fiatsandcounting]

I also have a '71 spider, and the stock carb for this is a 28/36 DHSA-2. Since you have something different, it could be set up to be "especially responsive." My guess is that playing around with the carb jettings and linkages will get you something you can live with. If not, just retard your ignition timing by 10 degrees and that will solve your problem (but also make the car a dud in terms of performance).

-Bryan

[davidbruce]

The throttle linkage on the early cars was a poor design. Being tied to the stationary firewall on one end and the rocking engine on the other was the cause of bucking or jerking upon throttle input. Depressing the throttle causes the engine to rise which reduces throttle action which cause the engine to drop and repeat the cycle. The problem is exacerbated by worn linkage bushings and soft engine mounts. The fact that the carb has been changed makes me wonder if the throttle linkage the PO used is the cause of your problem. Perhaps when your engine rocks it is advancing the throttle? The stock linkage has to be modified to work with a DFEV. A picture of the linkage may provide a clue.

[richardsonta]

davidbruce - that's a very interesting response. I'll post a couple of pictures of the linkage tomorrow and I'll check the play in the linkages.

[18Fiatsandcounting]

Now that I think about it, I think davidbruce is exactly right. In fact, this is how my '69 and '71 spiders used to behave, but I switched to a cable setup back in the 80s and had forgotten how the mechanical linkage used to set up a positive feedback loop where the whole system would start oscillating, with the result being a car that bucked around like it was practicing to be a mechanical bull when it grew up.

I'm not aware of a solution for the original flawed design, other than replacing with a throttle cable.

Kudos to Dave for thinking of this.

-Bryan

[SteinOnkel]

Oh, ha...what a coincidence.

I've been battling this exact issue on my other car for the last 6 weeks. That one is all custom, see pic. Finally have a linkage that works great.

I think in your stock configuration you will have a much easier time. First and foremost - check the springs on the carb. They are spiral springs and can be installed in a number of ways, not all of them correct. You can also "tighten" them by installing them one full rotation more. You will need a set of hooked metal picks to achieve this and some patience. Wear gloves and eye protection, those little springs can pack a punch. If it's an aftermarket carb, check the diagram and make sure all the return springs are there. Aftermarket carbs are easily the automotive part with the poorest quality control and I don't care if it's from Spain, Brazil, China or whatever; they slap those things together willy nilly.

Next up is the linkage on the car. Not sure what a '71 has, but on my '78 it's a rod with two heim joints and a pivot point on the cylinder head. There is a spring there, too. If that is missing, damaged, worn it would cause your issue. Make sure you get full travel out of the connecting rod. To determine this, have a helper press the gas pedal fully and see if the carb opens up all the way. If it does - and I cannot stress this enough - leave the connecting rod alone! Changing it is a surefire way to mess up the entire system.

Lastly, you can add a spring in multiple locations. The easiest is probably an extension spring on the pedal. Autozone sells a kit of various springs to try out. Some of them come doubled up. If two are too stiff, just take one out by rotating it. Bear in mind that rotating springs almost always trump extension springs, because they usually don't bind anywhere.

[davidbruce]

The problem Bryan and I are talking about is the linkage from the pedal to the engine. Pre 75 did not use a cable but a solid rod that went from a solid mount to a moving one. Your's should be a cable from the pedal to the linkage mounted to the engine. If my memory serves me. When I first installed my single plane and DFEV the only modification I made was to shorten the adjustment rod from the carb to the pedal rod. With only about 1/8" between the lock nuts on the rod. This was simple and gave me full throttle range. However, with the engine running and blipping the throttle the rod from the pedal would actually fall out of its mount on the engine. My solution was to file a groove in the rod and install a flat washer and circlip so the rod was captive at the engine mount and put a few washers either side of the coil spring that was supposed to keep it in the engine mount. It was very driveable and like that when I sold it.

[18Fiatsandcounting]

Yes, Dave and I are on the same page here: It's not an issue with the carb or the gas pedal in the early spiders; it's the bad engineering design for the linkage between the two that was changed in the later model years. If you have not experienced the feedback loop, picture this in a Fiat spider:

https://www.youtube.com/watch?v=My3hX0_dnPQ



-Bryan

[redcars]

I have a 70 coupe with the same setup with IDF's new motor mounts and good bushings will minimize the problem. I feel the best solution is to go to a cable for the throttle. Mine only really bothers taking off in 1st.

[richardsonta]

I checked my linkages and everything seemed pretty tight. I took a couple of pictures but didn't think to take a pic of the way the pedal is attached to the other linkages. I'll check that tomorrow but I'm pretty sure there's a solid rod that goes through the fire wall. I'll post the pics tomorrow too. I forgot I'd have to host them somewhere first.

[richardsonta]

Here are a couple of pics I shot. I think my motor mounts are pretty loose so I'll start there unless anybody sees an issue with my linkage setup...


https://photos.app.goo.gl/5z57bqad8FbPxBid8

https://photos.app.goo.gl/wjnwiSZJWTePC39S6

[18Fiatsandcounting]

Richard, yes, that's the infamous goofy throttle linkage in the early 124s. You press on the gas, the engine shifts slightly which has the effect of closing the throttle ever so slightly, and so the engine "relaxes" a bit and closes the throttle, and since your foot is in the same place, the whole thing repeats and you get into a "waa-waa-waa" cycle of bucking and snorting that is difficult to get out of. Picture the mechanical bull. It's just a bad design, and there's only so much you can do with engine mounts and throttle linkage grommets.

There are only 2 solutions to this poor throttle design: 1) live with it and learn to start the car in 2nd gear, 2) convert to a later throttle cable linkage.

-Bryan

[richardsonta]

Throttle linkage it is then. I’ll start researching how to do that. Thanks for everybody’s input. I’ll post the results once I’m finished.