Petrol engines aircooled Ignition 01

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mudroch: Its probably a simple fix, but i cant see it.. warmed engine ( 2ltr CU ) ,been running ten minutes or so,checked the auto chokes are off, but idle speed is way too fast. pulling the vacuum pipe off the advance unit on the distributor the idle drops to normal, put it back on and it speeds right up again! checked for leaks, fouling of carb linkage etc, all is good, anyone got any ideas as to whats causing this, or what to check?

HarryMann: Distributor vac adv/ret hoses connected properly? Blocked at either end, carb orifice for hoses blocked? Hoses leaking?

cumbriankeith: Here's a pic of the vac/adv hoses (Mine's a CU):

CU EngineBay 01.jpg

The pipe to the cone side of the diz vac unit should come from the left carb and the pipe to the other side of the vac unit, next to the diz body, comes from the balance pipe at the back of the airbox.

I've just extracted a speil I did a while back for someone that might explain stuff to you a bit more: 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.

And if you want more about these carbs and the ignition system have a look at this:

murdoch: Thanks for that Keith, as suspected pipes are in the right place (good to be able to check though!) What I did notice today though, was the throttle wasn't shutting off completly, and as i've been swapping bits of carb to make two complete ones, I hadn't thought about checking all the adjustments, just swapped like for like, had a little adjust of the connecting rods and the problem has gone - runs good now (Embarassed) I'm hoping its solved it, but a good old play at the weekend will find out!!