250/350 Desmo Head

[DesmoHead]

I have a Desmo head purchased years ago. It has yellow-Black paint marks. In earlier blogs I cannot find the details on just what the details are for the valve lift and overlap. I used this head for a short time but it showed no improvement over a Mk 3 cam in a std head. I would like to try it again, this time together with a reverse cone megaphone designed for a Mk3 250cc engine. What do I have? perhaps someone knows.

[double diamond]

There was only one production cam for the desmo head, the so called blue/white cam. There was also an optional red/white “corsa” cam. Lift for the blue/white cam is about 9mm intake and 8.35mm exhaust. Timing is 70/82 intake and 80/65 exhaust.

Where is the “yellow-Black” paint Mark you mention? On the head casting? The cam?

Matt

[ducwiz]

Hi,

I presume it's about the color marks on the camshaft.
I know 2 collections of cam data:
- Motoscrubs cam data page
- JimsCamData.pdf

No useful information therein. Best would be to check valve timing and lift with a timing wheel and a dial gauge.

cheers Hans

[graeme]

Most standard Desmo cams I’ve seen aren’t ground properly and need some lapping on the closing lobe to prevent binding. The amount that needs to be removed varies.
All were on 450 engines, same cam as 250 and 350.
Worth checking.

Graeme

[blethermaskite]

Are you fitting the desmo head to a narrow case or wide case mk3 250 and do you know what cam spec is in your valve spring head.
Cheers,
George

[Jordan]

Most standard Desmo cams I’ve seen aren’t ground properly and need some lapping on the closing lobe to prevent binding.

Wouldn't the clearance (zero) be set at the tightest spot?

[Paul W.]

Graeme described the situation perfectly in a post several years ago. Zero closer clearance is what you're aiming for, but the rockers may fight each other at some point mid travel. Resolving such interference merely by shimming would require more opener clearance than specified or desired, so lapping the closing lobe at the just right spot is the answer — and on my 450 the location was exactly where Graeme said it would be, near the end of a flat section of the cam.
I'll take this opportunity to thank him.

[Jordan]

Zero closer clearance is what you're aiming for, but the rockers may fight each other at some point mid travel.

I see, thanks Paul & Graeme.
What was the worst amount of negative clearance that has been discovered?

[Paul W.]

Jordan, In my case it wasn't terribly much. I made some passes on a whetstone and reassembled things to check progress, then repeated until the cam would rotate freely through 360°. Here's a discussion on this topic I found useful:
viewtopic.php?t=1364
D-C-T Bob's bizarre punctuation used to drive me wild, but he really was tremendously knowledgeable. There seems to be a slight lack of consensus among the contributors here; I like Graeme's clearly expressed opinions. Kev's too.

[Harvey]

That was a 'blast from the past'.
I loved talking to Bob, he was just the best person.
There aren't many people on forums that were as good to converse with then Bob, once you worked out his punctuation and who he was quoting.