My Fiat "jumps" at low speed... ?

[strive]

I recently got my hands on a 1968 spider that needs a bit attention. Not really a lot of rust nor any major mechanical issuses.
What really bothers me though is when I drive along at low speed (1st or 2nd gear) and hill starts, the car "jumps" ahead.
Kinda feels like the clutch engages and disengages without me touching the clutch pedal.

Anyone with the same problem or any ideas what it could be?

[sptcoupe]

This is the famous "rubber band" starts the early cars were known for. I remember trying to chase this problem for months in the early 70's on my '72 coupe. Even dealers back then diagnosed it as a timing/carb issue. The problem is the throttle linkage. It is hard mounted to the firewall attaches directly to the carb. So when you start in 1st or 2nd, the engine moves with the torque, which opens the throttle plates. The car surges, you momentarily lift off as the pedal depresses, and then it surges again, etc.

If your car still has the original linkage, you can swap it fairly easily with a later model linkage that attaches to the intake cam box. This linkage moves with the engine and eliminates the rubber band effect. I think the later linkages are pretty readily available, but I don't know what year FIAT made the change.

[BEEK]

replacing the motor mounts will also help, the early type were hollow with springs inside them, the later style solid rubber fair much better. but +1 for the linkage change too

[strive]

Thank you BEEK and Sptcoupe for your replies.
What you said about the throttle linkage sounds fairly easy to change. And I'll definitely replace those engine mounts when the engine comes out.
Currently it's fitted with, as far as I can tell, a DHSA cerb but I'm planning to replace it with dual carbs in the future. No idea which one yet, just want to put dual in for the looks and sound.
Is it likely that a new carb will have this new better throttle linkage so my problem disappears?

[juvius]

The older throttle linkage comes through the firewall right at the pedal. You probably can see exactly what I mean on your car. The later ones use a throttle cable. You'd need a newer accelerator pedal because of the way the cable attaches to it, a cable, the linkage that mounts on the cam cover and the rod from the linkage to the carburetor. You'll more than likely need a cam cover from a later car that will allow you to bolt the linkage to. If my car was that far together, I'd take pictures for you.

The good news is that all that should be easy to find right here on the forum without breaking your wallet

[majicwrench]

That "rrubber banding" shouldn't be a problem unless something is wrong, motor mounts, tune/carb etc. My 72 will do it sometimes cold, but really not a big issue. So make sure everything right, then change the linkage if you wish.

[RoyBatty]

My 71 coupe has the mechanical linkage and would do the same oscillating hop off you didn't feather the clutch just right at low speed in 1st gear.
In the last month I installed new, more rigid, engine mounts from Auto Ricambi and the problem is almost non existent.

My 75 spider on the other hand uses a cable only with no mechanical bellcrank and has never had this problem.

Go with the mounts first. Test drive and move on from there.

[strive]

RoyBatty. Those new engine mounts you installed, won't they affect the ride quality in a poor manner?

[70spider]

Thanks for the post, I just thought I hadn't got the carb tuned right.

[GeorgeT]

RoyBatty. Those new engine mounts you installed, won't they affect the ride quality in a poor manner?

Motor mounts are not part of the suspension system and have no effect on ride quality. Suspension bushings on the other hand...

[RoyBatty]

RoyBatty. Those new engine mounts you installed, won't they affect the ride quality in a poor manner?

Nope.
The new stiffer motor mounts just keep the engine from rocking back and forth as much, which helps to minimize unwanted input to the throttle linkage.

[micbrody]

Can this happen on a 1981 fuel injected car?
Yesterday, car drove fine. Made a quick 10 minute trip. Then about 20 minutes later took car for very long ride. The initial 5 minutes (only in 1st gear) seemed to be doing the rubber band stuff. After 5 minutes I could not re-create. Would go away when shifting into 2nd gear. Drove for about an hour , 85 degrees outside; some highway driving; some local. Entire route was uneventful (25 miles).
After about 6 hours made the return trip. No problems except when I got off highway and had to start from a stop. And even then it seemed hard to consistently re-produce. Always from stop in 1st gear.
Any ideas ?

[azruss]

Your 81 has a cable actuated throttle, so no direct linkage. May want to check your motor mounts. Also look at all the air hoses from the air filter to the plenum. Look for hose cracks and the connection points.

[micbrody]

Very intermittent.....I drove to work today (4 miles); no issues.
I doubt air leak; I have been over the system many times; and problem was so rare yesterday.

The motor mounts: how does one check them?