strange miss fire issue, help please!!!

well had a cabby a year now and the MOT tester let me off last year with to much hydro carbons

anyway… its not a miss fire but the engine drops revs when idling, soo goes from 800rpm to 600rpm then up to 1000rpm

now this isn't at regular intervals, it random so not on the beat of the engine. so some would say it runs lumpy :dontknow:

now here the list i have changed

new weber carb and the emissions is set spot on
new lead
dizzy cab
rotor arm
fuel pump
swirl pot
compression is spot on
hydro tappets are correct

now with all this done and checked i'm running out of idea's

the things to do if i can't figure this out is the stem seal and reseat the vavles but i don't think i need to do this

has anyone got any ideas what it might be please help as i promised the mot man that i'd sort it for this year and don't wanna let him down

have you tried it without the vacum advance plugged in.

if you have another I would change the distributor it sound like something to do with the advance mech or springs.

good thinking

i have checked the timing and if i do pull the pipe off then it does chance

i will double check tho

thanks, any more thoughts

anyone esle with any ideas

I would still thing it has something to do with the distributor.
 there is a centrafugal advance under the metal plate. it could be sticking.
what happens when you take the vacume hose off the advance?

when i take the advance of the engine drops and almost dies

Get a strobe timing light on her, preferably one where you can dial some degrees up on it, useful when you want to see what the advance and retard curve is like. Good old Haynes tells you what it should be and how much advance the vacuum should dial in.

Has yours got a box that sits on top of the TCI unit in the scuttle ? It has two round plugs and was used to manage the idle by advancing and retarding the timing when the engine experiences extra load, like lights on. You can unplug the connectors and plug them together.

i know the timing is good as i check this a few times

never checked the curve tho, is this in the haynes manual??

i'll have a look for this thing in the scuttle as i've not notice it before

cheers

Not really in the Haynes, but it has some rpm points and what the advance should reach. When the dizzy springs are worn or broke strange things happen to the timing, with a strobe that you can vary the timing point you are looking at you can see how smooth the advance is higher up the rev range. The vacuum advance allows more advance because of engine load.