Camshaft Timing Figures

[Duccout]

I am checking my cam timing on my 250 Wide case which has an unknown cam in it and I am getting figures of 50/82 In and 62/45 ex, so what cam have I got?

According to 'Jim's Cam Data' white cams have figures of 32/75, 35/75 and 27/75 Inlet and 55/44, 55/35 and 60/32 Ex. There is also a grey cam with figures of 62/76 and 70/48 but my figures are nothing like these. If I advance or regard the large bevel gear by one tooth I still get nothing like the book figures - 35/95 and 50/58 or 65/62 and 82/23.

Any ideas?


Cheers,

Colin

[LaceyDucati]

Colin

How did you obtain the figures? The factory figures are very subject to using the stated clearance for that particular cam. If you use the wrong clearances you can be a long way out. All in all the factory way of checking cam timing is very "airy fairy" as the rate of acceleration at that point is very slow, Hence the +/- 5 degree tolerance. If you let me know a bit more of your method I maybe able to help, maybe give me a call. Did you take any measurements from the cam before you installed it? The base circles and lifts are very specific to each original cam and that will give you a better guess as to what it is and what to set the clearance to, to check the figures.

Regards Nigel

[Duccout]

Hi Nigel,

Thanks again. I bought the cam used and was told that it was a white cam, but had no way of verifying that. I checked the timing figures using the clearances in the handbook for the white cam, but if it is not a white cam then they will be wrong. I set the valve clearances to 2 thou and with the feeler gauge inserted turned the engine and felt for movement on the feeler gauge to show when the valves opened and closed.

I didn't take any camshaft measurements before I fitted it, but I suppose base circle diameter and lift would have confirmed what cam I have? I guess that reprofiled rocker lobes and wear in the shafts and bushes will affect the timing too. I was mainly trying to make sure that I had the timing right, as I have an Italian cam with Mototrans bevel gears.

Cheers,

Colin

[LaceyDucati]

Colin

There are two sets of white timing figures quoted by Ducati

N/C 250 SCR 32/75 55/44 with 6 thou inlet and 8 thou exhaust clearances

W/C SCR 27/75 60/32 with 2 to 4 thou clearances

The reason for some difference in timing is possibly the difference in clearance specified as the timing is taken using the running clearance. However the inlet closing is quoted as the same....maybe they are different cams....as I said a bit "airy fairy". certainly there are different base circles on the W/C and N/C versions.

The method of checking as per Ducati is to open up the clearances by 4 thou (0.10mm) then insert a 4 thou feeler gauge in the gap. You do the "vague" feeling via the feeler gauge but the actual running clearance is still present. Hence if you are running 6 and 8 thou clearances you would open the gaps to 10 and 12 thou and then insert the 4 thou gauge to do the checking. The manual is about as clear as mud and this is often overlooked by many and worse still interpreted as using 10 and 12 thou as running clearances!! Once the checking is complete the clearances should be returned to the running clearance. If you didn't check your timing in this method, then your figures will be out.

Regarding your timing figures, the lobe centre figures are not wildly different to the white cams as quoted, certainly within the "vagueness" of these checking methods and maybe you not using the exact clearances or checking methods. Add to this any "slop" in the bevels components including the joint and figures can vary by a large amount. It is always possible the cam may not be a white cam. However without further measurements from the cam, it's a bit like stabbing around in the dark working out what it may be. What can be said is your figures are not wildly out lobe centre wise to imagine that you are out a tooth out on the bevels. If you were a tooth out, then the centres would be wildly out as one tooth is 17 or 25 (and a bit) degrees out, which you are not. As to whether the timing is "optimum", that's a different matter, but without more info difficult to identify. Even if it was a few degrees out, frankly it won't make a startling difference.....maybe 6 degrees or so it may.

Hope that helps Nigel

[Duccout]

Hi Nigel,

Thank you, I really appreciate that. As I am mixing Mototrans bevels with Bologna cam I wanted to be sure that the timing was at least 'in the right place'. I did check it with one tooth advanced and one retarded and guessed that I did have it right, even if the timing figures were a bit off the mark.

The engine runs very well and is much smoother than before the rebuild, so I think that the timing is probably correct, but I will check it again using the information that you have supplied and see what I come up with, just for interest.

Cheers,

Colin

[Duccout]

Had another go using Nigel's advice:

I believe that I have a wide case white cam, due to the smaller base circle diameter. The figures, obtained using a 4 thou feeler gauge in clearances of 6 thou inlet and 8 thou ex were inlet opens 52 BTDC closes 72 ABDC exhaust opens 70 BBDC closes 33 ATDC.

White cam figures are inlet 35/75 and exhaust 55/35 (approximately, as there is discrepancy in the figures). So my inlet is opening 17 deg early, but is almost correct in closing, and the exhaust is opening 15 deg early and is almost correct in closing......

Cheers,

Colin

[Duccout]

Just to add some more confusion, what about Mototrans cams, have I one of them? The figures for the Road model are:

Inlet opens 55 closes 90, exhaust opens 85 closes 90.

It is a pity that Ducati did not use a stamped number to identify their cams.


Cheers,

Colin