Bucking and jerking

[teammo3]

Thanks for your time, My 1970 Spider always had a jerking motion ( fore and aft) when in first and second gear. I decided to replace the motor mounts, suspension bushings, tierods and alignment, driveline bushings and u-joints. With all that work complete, the car still jerks up and down like it's not getting fuel when you take off normally. If you take off with plenty of power it does not jerk, but taking off like a bat out of hell every stop is a pain as well. I'm thinking the clutch is bad, or throttle linkage is somehow not correctly opening the carb right?? Anyone else ever battle this condition? Thanks, Mike

[azruss]

The 70s came in the country with a very leaned out carb (emissions issue). Many of them, including mine, jerked like yours on acceleration. At the suggestion of a Fiat mechanic, I drilled out the main jets to the next drill bit size up. Did not solved the problem completely and had to go up one more size. This did solve the problem but dropped my fuel economy by 8 mpg.

[teammo3]

Interesting. So that would explain why when I give it plenty of throttle it runs good. Did you have to change spark plugs or timing to account for the richness in fuel? Thanks for your reply!

[azruss]

Didn't have to make any other adjustment. It is interesting the jerking occurs early in the carb cycle. This tells me you may just need to increase the size of the primary main and leave the secondary alone. I did mine so long ago, i don't remember if i just did the one jet. You aren't actually creating a rich condition, you are just fixing the lean condition. As I recall, the scoop was if Fiat could keep emissions under a certain level, they didn't need to add a smog pump. They found by leaning out the carb, they could meet the requirement. It was interesting to see that some 70s ran just fine and others, like mine, bucked like a Brahma bull.

[RoyBatty]

I'd be willing to be that your problem is not within the engine, but rather on it's exterior.
If I am guessing right, your Spider has a solid linkage between your foot and the throttle shaft.
In other words there is no cable in your throttle linkage.
Instead you have an arm in the engine compartment on the throttle pedal pivot opposite your foot.
If you look down at the linkage you will see a ball link connecting the pedal assembly to a bellcrank that has more arms and another ball link tying the bellcrank to the throttle shaft.
If this is the setup in your car, you are experiencing a phenomenon that the magazine editors all complained about when our cars were new.
Basically, there is no "give" in this style linkage.
As you start rolling and then say then let off the throttle a bit then try to step into it again, the engine will rock a bit in the motor mounts, this movement is transmitted back into the throttle linkage, causing an osscilation, what I call the hippity hop action, that will only get worse unless you depress the clutch and start all over again with a higher RPM and no deceleration/acceleration in the lower gears.
Best way to cure this, install a cable type throttle linkage.

[Redline]

This was exactly what caused kangarooing in many of the magazine road tests from the late 60s, early 70s. My Coupe does that as well (rarely) with its solid linkage to the twin IDFs. I had a hilarious bouncy moment the other day when I rolled out of the underground garage on part throttle in first...

[TStark]

Just what Roy sez

Switch to a cable throttle linkage.

[azruss]

Personally, i think the solid linkage gets a bad rap. I have put well over 300k miles on 124s with solid linkage and the lurching effect was minimal. The biggest issue is the bushing in the firewall gets worn and makes the matter worse. Also, motor mounts need to be in good shape.

[vandor]

Roy is right. Get stiffer motor mounts or convert to the later cable throttle linkage and the problem will go away.

[sptcoupe]

Never have had a solid linkage that didn't buck like crazy. Do a cable set up and enjoy the car.

[bradartigue]

Absolutely its the throttle linkage. For whatever reason the tiny DHSA1 carburetor adds to the misery. I'd get a DHSA2 or DFEV on that car and a cable linkage from a later model. Your problems will go away.

[teammo3]

Thanks for all the input! I do have the solid linkage and the bushing through the firewall is worn out. I'll look around for a cable hookup and try that. Can I buy the cable hookup from IAP? Thanks, Mike.

[vandor]

No, most parts are not available new. Here's what you need from a 1978-80 carbureted Spider: gas pedal, bushings that hold gas pedal, throttle cable, throttle bellcrank, throttle rod, and intake cam cover (throttle linkage bolts to it).
The part of the pedal that pulls on the cable has to be bent toward the passenger side, as it wants to be right where a bundle of cables come through the firewall. Just bend it, then drill a hole for the cable in the appropriate place.
BTW, getting the gas pedal in place is difficult because of it's shape, you may need to remove the radio console.

[ConcordNH]

I had a bit of bucking and jerking, then the car died while running. I narrowed it down to a bad coil, replaced the coil and it solved the jerking as well.