Advance/Return curves... diagram please!

[redstar]

anyone help with the above please... im at me wits end withe the 1981 cu! time it to the right degree, then bout 450 revs after im near on max advance! do i just need a new dissy? or springs? or new bus?!

[cumbriankeith]

I take it that you're using all standard - carbs with dual vac/retard diz?
Are you leaving the vac pipes connected and timing it to about 4 or 5 ATDC? which is the stock method... but this does rely on the throttles being correctly adjusted to develop the right amount of vacuum in the right place at the right time.

If you want to check just the mechanical side of the diz then disconnect the vac pipes and plug the one going to the idling supply pipes (at the back of the airbox near the bulkhead) The timing should be about 7 or 8 BTDC at 850ish rpm and should advance to no more than about 28 or 30 at higher revs. If you replace the retard pipe (port on diz next to main body) the idling timing should move to 4 or 5 ATDC. Then try revving - should cause the vacuum to collapse in the induction pipes and the timing to move back to 7ish BTDC before advancing on the mechanical system. Connecting just the advance pipe (the one on the cone shape part of the vac unit) and plugging the retard should not change the timing at idle - as this is connected to a ported vac takeoff point, just above the throttle. As soon as you open the throttle, vacuum develops and the timing should advance. This advance factor is added to the mechanical (centrifugal weights/springs gubbins) at higher revs to give a total advance of 35 to 40. Clear as mud?

Are the rubber pipes cracked/perished? Are your carbs old and leaking air round the throttle spindles? Is the vac unit good (suck on it - arm/plate should move. Then stick your tongue in the hole at end of a good suck (!) It should stay put and shouldn't hiss)
Centrifugal system can get sticky but can be overhauled with care and patience. Springs can get weak over time - I suspected this, but on closer and patient inspection found the vac system to be faulty.

[redstar]

thanks keith.. i have done the pipes 'round the back' i am pretty sure its gonna be weak springs... dont know if i should just buy a new one? i have timed the beast with all connected, with no difference in about 10 degrees either way!!! really dont know what to do now....

[cumbriankeith]

Before you write off your diz just hook the timing light up and go thro those tests I've suggested and report back. Especially the first one where all you're looking at is the mechanical springs/weights function.

I thought the springs in my diz were past it but eventually learnt how all these systems interact and found that the diz was ok apart from a bit of a clean and service.
If it is dodgy you should know that new replacements are no longer available so you will have to hope that one can be had thro our noble breaker brethren!