Sticky Throttle

[240Z240Z240Z]

My 72 has a problem with a sticky throttle pedal, I searched the forms and coundn't find any threads concerning this issue, so I'm asking the group.

Problem :

If I press down on the throttle with the engine off, no problem, it's smooth as silk.

If I press down on the throttle with the engine running, it take a bunch of effort and then it lets go. This makes it hard to take off from a start without feathering the clutch a bunch.

It almost seems as if the vacuum on the back side of the throttle plates is holding them closed. I tried some weaker throttle return springs and they work better, but then it doesn't always return to idle without blipping it.

I've checked the linkage, and lubed everything and no change. It's been this way since I've owned it (25 years) and I'm not sure if it's just mine, or if everyone has this problem to some degree.

Thoughts ?

[EScanlon]

Throttle plates? Are you running Webbers or Holley's?

The S.U.'s use a piston that rides up and down and wouldn't be affected by vacuum in the manner you cite.

I'm sure the mechanical gurus will speak up and give you more enlightenment but the S.U. carbs wouldn't be doing what you are describing.

Now, it could be the infamous Sticky Throttle linkage that's causing your problem, and it's due to the geometry of the transfer link that's mounted right on the firewall. The link I'm referring to is the barrel with two arms at approximately 90° from each other that transfers the forward/back movement of the accelerator link to side to side movement required to rotate the carburator link connecting both carburators.

The characteristics of this "stickiness" is that the throttle will push in OK for a short distance up to a point then you need to push harder to overcome the resistance you meet and it releases suddenly causing you to "floor" it.

There have been a couple of fixes suggested, from cutting and re-welding the arms of that link, to changing the link arms, to tightening the play in the links in order to eliminate this "kink" in the travel of the accelerator.

I'll let the mechanical guys kick in their thoughts as to the best way to overcome this.

HTH

E

[Arne]

SUs do have throttle butterflies in addition to the pistons.

This theory on the vacuum holding the throttle closed is interesting. I'll have to compare the feel of mine when the engine is not running. I drove it to work today, so might remember to test this on my way home this evening.

[240Z240Z240Z]

SU's have throttle plates too. If the problem is associated with the linkage at the firewall, then why does it make a difference if the engine is running or not? I suspect that independent of the linkage, the carbs are harder to open when running and that's the crux of the problem.

[240ZX]

The problem you are experiencing is with the adjustment of the said linkage (for the SU's)! I wish I could remember where I saw the article but, I went through the same exercise of trying to figure it out. Basically, it has to do with the positioning of the bell cranks! As a bell crank travels through it's travel path, there are variations in linkage speed. Sorry I can't provide details on the desired adjustment but, it's pretty easy to make changes. Other items that tend to effect the linkage while accellorating are....worn out engine mounts (rubber is seperating and the engine lifts, which = pulling the throttle open)...also worn out bushing where the throttle shaft pivots at the firewall. Anyway, I hope this helps you out to some degree!

Tom

[1 Bravo 6]

There was a thread with a similar problem some time ago.

You say that you've had both the car AND the problem for 25 years is that correct ??

Can you recall whether you replaced the linkage that runs across the firewall or that which runs from the firewall to the carbs. I'm not absolutely certain, but I think there's a difference in length and angle between the 240Z and 260Z and not compatable.

Could be wrong.

Rick.

:devious::devious:

[240Z240Z240Z]

There was a thread with a similar problem some time ago.

You say that you've had both the car AND the problem for 25 years is that correct ??

Can you recall whether you replaced the linkage that runs across the firewall or that which runs from the firewall to the carbs. I'm not absolutely certain, but I think there's a difference in length and angle between the 240Z and 260Z and not compatable.

Could be wrong.

Rick.

:devious::devious:

Thanks for the response.

The linkage is all original and has never been apart.

[hd240z]

I think this is the old thread discussing the issue and several possible solutions.

http://www.classiczcars.com/forums/showthread.php?t=5545&highlight=throttle+surge

[DaveN]

Re sticky linkage -- Steel on nylon ball & socket joints will stick and slip when dry. The best lube I've found is a homeade paste of graphite powder (lock lube) in silicone grease. A graphite coating on the nylon cures the stickiness. The grease is mostly a binder. Silicone grease catches less dirt than petroleum based greases. The same paste or moly assembly lube also work well in the metal to metal joints & bushings.

Re aggressive linkage geometry -- 73 and later linkages have a slower opening geometry. I've heard 73-74 bell cranks can be used on 70-72 cars (with some adjustment of push rod length), but haven't tried it or seen an example.