Revs drop when i dip the clutch?!

As above really, doesn't happen all the time, and does it usually once car is warm and been driven, my college teacher says it can be the thrust bearings,  he said to have a look at the crank pulley while someone dips the clutch to see if the pulley pushes it self out at all, i have done this and from what i can see nothing happens (it does not move) then he said if it's not that then its possibly my clutch and or release bearing.
Anyone have this problem or know what it is?
Cheers.

?? :dontknow:

Sounds like a daft condition, but is the car moving and what gear is it in at the time?

Happens when in any gear and neutral, stand still and when moving.
But this doesnt happen all the time.

I don't think its a transmission issue, more like some kind of ignition/mixture running fault. If it were a transmission issue, it would do it consistently. Obviously if you're moving and in gear, then drop the clutch, the engine speed will decrease to idle speed. If its not dropping to idle properly, ie when you do this it drops below idle, then its a fuelling issue (too rich), if that helps.

But how is this possible? I have a weber 32/34 carb and electronic ignition, how is it that when i dip the clutch the fuel mixture should enrich?
Seems near enough impossible

Because the load on the engine changes.

Right right, so what can be causing the extra load? I still don't understand how it enriches the fuel??

With the clutch engaged, the engine will be driving the clutch mechanism, the driven plate, plus at least half the gearbox internals. If the car is cold, the gearbox oil will be thicker and it will cause drag.

The extra load will cause a slight increase in vacuum in the inlet manifold, which will deliver more fuel to the engine and thus the revs will increase slightly.

As soon as you remove the load (by pressing the clutch pedal down), the vacuum will decrease and the amount of fuel puled into the carb venturi will decrease, dropping the revs slightly.

that's my theory, anyway

I understand what your saying, but it still doesn't make any sense. How can i go about fixing this?

I would have the fuelling checked - its possible that the idle adjustment is out - incorrectly adjusted idle screw could cause the tickover to require fuel from the main jet, which would be affected by inlet vacuum

Its been many, many years since I've run a carb'd car, so I wouldn't know where to start to check it - however, I seem to recall the Gunsons do a "colour-tune" device that you use in place of a spark plug which monitors the mixture and gives a reading

Note that it could also be ignition timing - a bit of vacuum could also be advancing the ignition a bit, which would cause a rev change at tickover

My idle screw is done by the butterfly opening and does not work in conjucntion with the CO level screw, well thats what i learnt about weber 32/34s, i set my idle screw so the the engine idles at around 950 rpm, and i also set my CO screw right down and until the engine cut off then went back 1 half, althought i must admit my carb does need setting up properly, i jetted it using vauxhall nova 1.3 jets but ran it more lean.

your C0 screw is way to far in you turn it till the car start to hunt then knock it out half a turn.

I was once told winding co screw in then back 2 turns is a good starting point, not sure how accurate that is. I would take it somewhere to get the carb set up proparly with the emission testing equipment etc

sounds like a blocked idle jet to me.  Check your brake servo pipe for vacuumm leaks too, the air flap for cold/warm running and the vacuumm advance pipe on the dizzy.

just brought a 91 clipper and it cuts out when driving like the idle is messed up on the carb then wont start unless i play with the butterfly