Advance and retard

[Sailsy]

I have an engine flat spot which I am investigating. I have changed the obvious and am now looking at the advance retard.

Just testing the action of the advance retard on the dissy. When I suck on the pipe the "arm" in the dissy barely moves. I am guessing the diaphragm has a hole in it. How far should it move? It barely seems to twitch when I am sucking for dear life.

I am right in assuming this is goosed?

Cheers

Andy

[itchyfeet]

No probably you just can't suck hard enough, I remember once watchjng what2do trying to test his and he just could barely move it whereas I could move it lots.

if it has a hole you would be able to suck easily.

worth oiling the mechanism under the dust cover and the felt pad, I wonder if anybody does that?

[ajsimmo]

In your signature you have a DG, so you don't have a retard function on the dizzy can, it's advance only. If you suck on the pipe and it moves at all - it's working. (I'm assuming it holds vacuum from your description.)
The DG flat spot is often caused by the Pierburg carb, and more specifically the accelerator pump timing and quantity. If you take off the pancake on top of the carb and look down the nearest venturi, then work the throttle, you should see a fine squirt of fuel straight down (about 7/8 o'clock as you look at it). The arm that works the pump for this is bottom left side of the carb, riding on a cam. It should be touching the cam at idle, not floating free. The preload of the cam itself is adjustable. If you mark it's current position, then experiment with a little more or less preload, you may find it is the cause. If you find there's not enough fuel delivered you might need a new accelerator pump diaphragm, which is in the rebuild kit from Brickwerks.

[Sailsy]

Cheers for the replies. I will try lubricating the dissy and checking the carb.

I had the carb rebuilt only a couple of months ago by Roger at Bromyard so I didn't expect that to be the problem.

I have also read a faulty coil can also cause flat spots. I checked the resistance of the primary and secondary circuits yesterday and they were 1.17 (primary) and 2.87 K (secondary).

It is the late green label coil (211 905 115D) so the primary circuit is a tad high, expected is 0.5 to 0.8. Is 1.17 too high and do I need to get a replacement coil?

Any help much appreciated.

Cheers

Andy