Cams and Ignition Timing

[frankfast]

Since my 350 Widecase Scrambler cam was trashed, I replaced it with a 250 Scrambler cam. Since the profiles are very different, where is a good place to start for static ignition timing?

[Jordan]

What are the full advance ignition timings for both bikes?

[frankfast]

Ignition timing for the 350 Scrambler is 5 - 8 degrees static and 33 - 36 at full advanced. The only info I a can get for the 250 Scrambler is for the narrowcast edition and that is 38 - 41 degrees fixed. I'm not quite sure what that means. Does that mean that it does not have an automatic advance mechanism and static timing figures are the same as the full advance figures? I've noticed that there is a difference in cam profiles between the 250 N/C and 250W/C. Not sure if ignition timing specs are different and I'm also not sure which 250 my cam comes from. Are cams interchangeable between N/C and W/C machines? Thanks for any help you can lend.

[Jordan]

Why would a bike have fixed ignition? It used to be standard for 2 strokes, but some retard action is desirable on 4 strokes to ease starting and a nice idle.
Bikes without batteries don't have the luxury of being able to provide a wide range of ignition advance.
Unless it has some extra technology like a battery eliminator (regulator and big capacitor).
Some off-road Ducatis did not have a battery or eliminator, instead having an advance-retard device with a much restricted range - better than nothing.
That was so the instantaneous voltage being produced in the alternator at ignition time was high enough to get a usable spark. As the voltage is not steady, as it would be from a battery or big capacitor, but a sinusoidal wave, too much adv-ret range would fall outside of acceptable level of the variable voltage.
I didn't know that some Ducatis did not have any advance mechanism at all? It would be harder to start, but when the engine is running it would not be a big problem with fixed ignition, especially on a scrambler bike. A 250 would be more manageable than a larger engine too.

On the bike in question, the full advance spec is about 5 degrees different for the two camshafts.
Assuming correct timing is somewhere between the specified range extremes, to be conservative, you could try 33 degrees, and see if it gives good power, if not try a bit more at a time, listening out for unhappy noises like pinging (in which case, go back immediately to timing that doesn't produce pinging). Or you could take the bull by the horns and go for 41 degrees, and drop back until any manifest problems disappear. You evidently have a battery and wide range adv-ret mechanism, so starting should be acceptably easy.

As the heads are interchangeable between widecase and narrowcase, and fit both 250 and 350, the camshafts would be interchangeable too, in terms of being able to fit.

[frankfast]

I believe that the 250N/C Scrambler and the Diana came with a magneto type alternator supplying AC to the coil. That is probably the reason for a fixed advance. Thanks for the help.

[amartina75]

There are two timing specs for NC scramblers with and without advance mech, look again it's in the manual and probably in the owners manual. Don't have time now but can post that later tonight if needed

[frankfast]

The gear that drives the points cam on my 350 Scrambler has two marks. One is a dot and the other is a slash one tooth before the dot. I'm not sure which is supposed to line up with the dot on the gear that drives the oil pump since when I removed the cover the ignition timing gear came out with the cover. I didn't release the screw that holds it to the points cam. By eye, it seems that using the slash would be the better option for the points opening at the correct advance of TDC statically. Is anyone familiar with these marks? I can't find info in any of the manuals that I have.

[graeme]

The dots are for timing all but 450's, the 450 uses the slash.
(as far as I'm aware)

Graeme

[frankfast]

By changing cams from a 350 Scrambler to a 250 Scrambler and therefore a different location of the cam woodruff key, is it possible that the gear driving the points cam would be in a different location and not line up with the dot on the gear that drives the oil pump? I'm out of adjustment on the points plate and therefore can not time the ignition properly, Trying to achieve static timing of 5 to 8 degrees BTDC is impossible. The points are open with the points plate adjusted clockwise as far as it will go.

[frankfast]

Never mind. Got it. Silly mistake.