420 Throttle Sticking

Hello - the throttle on my '67 420 appears to have mind of its own. When you put the car in Drive and apply pressure to the throttle pedal, the pedal goes full throttle on its own. The pedal moves towards the floor on its own - very strange! Here’s what I have checked so far:

  • Damper oil ckd for proper level.
  • Carpet & padding removed to eliminate binding.
  • Throttle linkage and cable lubed, moved freely, and returns to correct position when engine off.
  • Return spring in place and providing proper tension.
  • Motor mounts tested with no engine lean.
  • No vacuum leak between intake manifold and the distributor (Pertronix distributor, 3 yrs old)
  • Carb Slides cleaned by spraying lots of carb cleaner and wiping.
  • Throttle does not stick when car is moving in Reverse.
    All thoughts will be greatly appreciated. Thank you very much.

Welcome to the forum.
Has this suddenly started happening without anything having been performed in the engine bay by somebody?

I would look at the trans linkage clearance to the throttle linkage when its in drive with the engine off. You didn’t mention if its RHD to LHD.

Bob

thats a good idea,

if its an Auto trans, it will have a BW8, and there is connection between the throttle and a linkage that controls trans fluid pressure…although I would be a bit surprised if this was the issue, as its a sensitive adjustment, and affects trans function

An assistant to operate the controls while you look under the bonnet is helpful

Chock all 4 wheels front & back

Thank you all for replying. Sorry for the delayed response. The weather is now warm enough for me to resume work on my 420’s throttle issue. Let me respond to your suggestions:

  • no one has recently performed any work on the car
  • The car is LHD, and I just checked, there no issue or clearance problem between the trans linkage and the throttle linkage.
  • I checked the the kick-down linkage cable that affects the trans fluid pressure. It does not pull on the throttle linkage.

Any other suggestions?

Thanks again and please keep the ideas coming.

I recently had this problem on the Mk10. If you don’t find a binding linkage, check the accelerator pedal hinge pin. It is a special bolt with a narrow waist and a nut smaller than the head - the same was used on my mk2 and I’m sure on your car. It has a tendency to corrode and stick, especially when you put it to the floor (very inconvenient!). Try not to break it, but you might. If so, you can drill oversize and replace it with a standard bolt.

aye…replace it with an Allen head bolt will save you a great deal of aggravation if it ever needs removing again

have someone operate the accelerator pedal while you look in the engine bay,

the rubber bush on the firewall can degrade

Thank gentlemen. The accelerator hinge pin is not corroded and the accel pedal moves very freely. I lubricated it and it appears to be working beautifully.

How do I check, and replace if needed, the rubber bushing in the firewall? I grabbed the linkage spanning the trans tunnel (firewall) and there doesn’t appear to be much play if any.

Also, the Engine Stabilzer. How do I check to see if there is any play in it? If there is play, how to I adjust it and know the engine is at the proper height at the rear?

Thanks again.