Weber IDFs and Sticky Throttle

[Ian Bissett]

On a 302 block, I have Weber IDF’s. Unfortunately, the vacuum created by the Webers is leading to a sticky throttle. The foot pedal was modified to bring the cable connection as close the pivot point as practicable, and the bellcrank was extended slightly with a bracket to increase the leverage. Both were sufficient to get the car through its recent IVA, but the throttle is still resistant especially at the beginning of the travel. As the resistance is worse at the start of pedal travel, looking at the attached photo, my belief is that this is due to the initial angle of pull on the bellcrank (I have checked the foot pedal movement and the inner cable and there are no restrictions there).

I was thinking of replacing the bellcrank with a pulley so that the cable pull is always at 90deg to the fulcrum, however, I am concerned that if I unbolt the bellcrank, it may release the captive nut below it into the manifold. Since I did not fit the Webers myself, I do not know how the bellcrank centre bolt is secured into the manifold. Can it simply be unbolted in place without risk of losing the lower captive nut? Any advise from other IDF owners appreciated.

Also, I am sure that my problem is not unusual and I would imagine others out there have experienced the similar problems with a sticky throttle. Any suggestions or ideas from other owners that have had these same problems would be greatly appreciated.

Thanks in advance
Ian

 

[Chuck]

The pedestal the bell crank sets on is threaded. Removing the screw will NOT cause a nut to fall into the intake.

If the throttle is sticking in the idle position, could the stop screws need adjustment? When in the idle position there should be slight clearance between the throttle plate and the side of the carb for that reason.

Typically only one idle stop screw is used for all four carbs.

There should be no need for the pulley arrangement you describe. Bell cranks have been used in that location by many before.

Linkage is ofter the weak link in Weber set ups. Make sure your bell crank and related hardware is beefy enough and that there is no flexing. When I had IDFs they came with off shore hardware which was of inferior quality. One had to literally 'tune' out the slight flexing that would change the rate the throttles opened from idle to slightly off idle. That is one area where spending a bit more will pay dividends.

 

[Bob Putnam]

If you want a more progressive linkage, consider rotating the cable connection (90 degrees to the initial pull) to be more in-line with the pull levers to the carb. Right now you're defeating any progressivity by its forward angle. Using my geometry will make the system more progressive than using a circular pulley. You may have to move the links to the carbs to the outer holes though.

 

[Rick]

You should move the links to the outer holes on the bellcrank. They must be parallel through their travel. You may lengthen or shorten them to move the starting point of the cable connection. The additional piece of metal you have added will not be necessary. The problem you are experiencing is caused by incorrect ratio. If necessary get more pedal travel. Adjust the lengths of the links so that the remaining arm on the bellcrank is perpendicular to the throttle shafts at 1/2 throttle IE: near 40 degrees to the left at full throtttle, near 40 degrees to the right at idle. A little trial an error will be necessary. The links must ultimately be the same length. Connect the cable to the bare aluminum arm.
Lastly raise or lower the pivot mounting point so the arms are as close to horizontal through their entire travel, and install cable to pull parallel to the throttle shafts.

Like below

 

[Rick]

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[Ian Bissett]

Thanks for the suggestions, much appreciated.
Ian

 

[Robert May Eng.]

Its hard to say from the plan view but is the bell crank sat too low and causing the pull rods stress in trying to get the butterfly spindles over center? It may be worth a look.

Bob