I have a worked a12, at low revs it runs on 2 cylinders, but when i increase the revs it suddenly jumps into life and runs on all 4.

I have just fitted new leads, new dizzy cap and a new rotor, and the spark plugs seem good

I have checked tdc against the dizzy, and the firing order is 1-3-4-2 (right?)

still don't understand what it is.

Please help.

Thanks,
Simeon.

Pull the leads at idle one at a time and see if all 4 are sparking (most likely are) so it could be a fuel air issue?!

Possibly a vacuum leak so at high inlet vac. situations it sucks too much air to burn (ie when the throttle is shut)? Just a thought.

Hi, thanks, 1 and 2 aren't firing at idle, the dizzy is locked and has no vacuum advance,

Thanks for the quick reply.
Simeon.

try a compression test, maybe a sligthly blow head gasket. cheers

try a compression test and check tappets, maybe a sligthly blow head gasket or tight tappets. cheers

Manifold leak cyl 1/2?

Spray some WD40/CRC around the manifold joins to see if it changes anything.

Does the engine have twin carbs, DCOE`s etc? Could be linkage or idle circuit issue. But more likely to be tappets as mentioned above(valve recession).

That being said, It wouldnt "jump into life as revs increase" if it`s tight valve clearances. It would progressivly run smoother as the revs increase.

You can check for a vacuum leak with aerostart.

Let us know what you find.

I wont be able to do a compression test till the weekend,

Tappets were reacently done, Sprayed wd-40 on manifold,

Yes, it has twin dhla 40's on it, I have cleaned the back one, but i might have to do the front as well.

But i didn't think it would need it because it was going great at higher revs

Thanks for all of the help,
Simeon

ahh.. twin carbs!

Check idle and progression circuits on cyl 1/2, they are seperate from the main circuit. Actually it will run on the progression circuit alot until probably over half throttle so focus on the idle circuit.

Remove and clean all jets, check the number of turns out for the idle screws..

Actually, thinking about it some more, if it is both 1 and 2 with the issue it is more likely to be a balance problem than a blocked idle circuit as there is a seperate idle jet for each cylinder and the odds of both being blocked is slim.