Sticky throttle butterfly

[Middle Laner]

I've just changed the throttle cable, and I noticed that with the cable off, if I released the throttle and let it "snap" back, the butterfly got stuck. It then needed a fairly hefty pull with an 11mm spanner next to the return spring to open it. I cleaned/lubed everything in sight but that did no good. With the new cable back on, it seemed fine, but if I abruptly released the throttle, I found a slight but annoying stiction before I could apply any more throttle. It's almost as though the butterfly is going over-centre in some way and getting itself stuck. There seems to be a small thread on the plenum that acts as a stop to the throttle (next to the return spring) but I can't see any way of adjusting that without damaging the thread.

Hope that made some sense and any advice appreciated!!

[GreenV8S]

The throttle butterfly/spindle will spin through 360 degrees, the throttle linkage is the only thing that limits it. If yours is actually jamming inside the housing it may be that you have a build up of gunk that needs to be cleaned off.

[eliot]

You may find the throttle plate is binding inside the throttle bore - take a look down the bore and look for any witness marks.

If so, best bet is to remove the throttle plate, pull the shaft out and give it all a good clean. (the screws are split at the back of the plate, remember to squeeze them back together).
When reassembling - get the plate on the shaft with the screws loose and allow the plate to float around to the most central position and nip them up - then split the screws apart again (i would locktite them too)

www.mez.co.uk/ms12.html#71mm

Edited by eliot on Sunday 25th March 09:45

[GreenV8S]

eliot said:

If so, best bet is to remove the throttle plate, pull the shaft out and give it all a good clean. (the screws are split at the back of the plate, remember to squeeze them back together).

If you're contemplating doing this then I'd suggest you take the plenum top off the engine first. It's a little fiddly and you *really* don't want to lose anything down the intake.

[Middle Laner]

I cleaned and cleaned the butterfly and surroundings and extracted loads of gunk, then lubed with silicone. This helped a bit, but in the end I did adjust the throttle stop a little and that's cured it. The idle's now about 900-1000rpm, will check the bible sometime to see if that's in tolerance - the car drives fine though. Looked at the split screws in the butterfly, but didn't have the bottle to touch these, let alone take the plenum off as I image the reassembly is quite tricky! Maybe when I have more time...

Thanks again for the help and suggestions

[GreenV8S]

Middle Laner said:

in the end I did adjust the throttle stop a little and that's cured it.

In order for the semi-random idle speed control to do its thing you need to get the throttle pot set to give you the correct reading at idle, moving the throttle stop could have thrown that out.

[ACT]

Middle laner, is yours a large throttle ? these are prone to snapping shut tight as they are oval and a fine tolerance fit.
when assembling i normally set them with a feeler gauge so that with the throttle on the stop you have a 3 thou gap top & bottom, then as GreenV8S said you'll need to set up the throttle pot with a digital multi meter, .32V throttle closed ignition on.

[spend]

ACT said:

when assembling i normally set them with a feeler gauge so that with the throttle on the stop you have a 3 thou gap top & bottom,

I have .002" for normal throttles in my notes, just in case...

Dave

[Middle Laner]

Hi
I think it must be "normal" size as it's round. Following eliot's suggestion, I can indeed see some witness marks on the bottom of the the inlet bore. Will get a mirror and check the top too sometime but presume the butterfly must be off centre and needs moving upwards. What's the best way of doing this? Do I just pinch the split screws, loosen them a bit and adjust with the feeler gauge top and bottom and retighten? I think this problem has only come about since changing the cable because all the frayed bits were snagging enough to prevent a fully closed throttle anyway...

Once all's done, do I just follow the instructions in the bible (p105 "adjusting the idle speed" with the grub screw adjustment and then set the throttle pot.

Thanks for everyone's help btw!