Engine Revs/Rpm Increase when Clutch Disengaged while driving

[Jrex19]

Hi All
Anyone had the lovely experience while driving where you disengage the clutch (press the clutch pedal) when shifting up/down and the rpms/revs on the engine increase? This only happens when driving the vehicle as if I depress the clutch when the car is stationary in idle, there is no rpm increase.
I have already cleaned and examined the idle control valve. I have also checked the bowden tube on the clutch cable and it only slightly rubs the throttle linkage on the bellhousing. I dont think it is enough to create an issue.
Could it be a vacuum leak?
Faulty deceleration valve?
Possibly the throttle and clutch cable are twisted in the tunnel?
I am running a 993 Varioram with a 915 Euro transmission in a 1970 chassis. Lightened flywheel and clutch. Do you think I need to tune the ECU for this?
Any thoughts, feed back, or advice would be greatly appreciated as I am stumped.
Thanks
Jason

[blpetry]

Edit: read your post fully and you already know about the binding issues.

But, its easy to tell if this is the culprit because when it happens you can feel the binding in the accelerator pedal if you leave your foot on it gently when engaging the clutch.

With the engine off, you can also run the clutch and accelerator through their ranges of motion and feel for any binding.

Original post:

If you've replaced your clutch cable or pedal cluster recently, you could be experiencing binding between the two. It happens most often in front up near the pedal cluster in the tunnel or near the front of the transmission where the clutch cable gets close to the throttle linkage at that location.

[Jrex19]

I will check the binding issue tonight when I get home.

Any thoughts on it being a vacuum leak?

[Jrex19]

Issue resolved!

Issue close to pedal box with rubbing cables and Bowden tube resting on the throttle linkage at the bell housing.

[Trailbraker]

I have taken video of my Bowden tube while depressing the clutch pedal - not surprisingly, the Bowden tub deflects quite a bit (recalling maybe 1/4 to 5/8 of an inch) at full clutch pedal depression. The Bowden tube is flexible, it is NOT a relatively stiff pipe, but an articulating tube, quite a compromise design (to achieve adjustability). As the tension in the cable increases to full pedal depression. an increasing bending force is imposed on the arch of the Bowden tube, and it deflects. So if your cable were resting on the accelerator bell housing (mounted to the transmission), and the bell housing is inside the arch of the Bowden tube, it will bind up the bell housing pivot. That binding actually supports the Bowden tube, making it more 'efficient' (less bending deflection) - so if this is happening, after the Bowden tube is moved so binding does not occur, the clutch cable needs to be readjusted to get the right throw.

[Jrex19]

Thanks Trailbraker. Clutch cable was adjusted accordingly after the modification.