Desmo Valve Timing

[jp1]

Can anyone clarify this for me, please?
I am currently restoring a 900SS and can't get it to start. The engine was refurbished many years ago and I have recently rebuilt the bike. I don't know what the problem is (carburation, electrics or whatever) but I decided to make sure that the machanicals were in order, in particular the valve timing of the front cylinder in relation to the rear. It may be correct, but I really don't know. I remember that both cylinders were removed to adjust the shims and that tthe heads should have been replaced with the timing correct, but maybe the front cylinder could be out of phase with the rear. I do know that all of the bevel dots are aligned (the front camshaft bevel dots as well) but it is possible that when the lower dots and rear camshaft dots are all aligned, after the front camshaft has been turned a few times with the head off it could have been put back in the wrong position. I can't find any definite information about this particular step in rebuilding the engine in any of the manuals, only to make sure that the bevel dots are aligned. (lower bevels & rear cyl timing is fairly easy).
The engine turns over freely - no valves touching the piston, but could the timing still be out by a 1/4 turn or so?
It would be great if I could actually see a working engine with the bevel covers off to check the dot positions of the front bevels when the rear ones are aligned.
Any help with this greatly appreciated.
jp1

[jp1]

Great to see the forum back - could have been gone for good!
Added my rough diagram of what I have and hope that it is correct.

[Gardner]

I was working on posting this when the forum went missing.
Not written by me but I saved it from a forum and have been using it.


BEVEL DRIVE CAMSHAFT TIMING GEARS AND ALIGNMENT MARKS
August 1980
By Peter Shearman

The timing marks on a bevel drive engine, are only all lined up once every six engine revolutions (Three cycles).
This is because of the ratio variations between the gears.
For each turn of the crankshaft, the bevel drive shafts turn 2/3 of a turn, and the camshaft turns 1/2 of a turn.
The point's drive cam also turns at half engine speed on the round case models.

HEAD BEVELS (All Models)

Each turn of the drive shaft turns the camshaft through 3/4 of a turn. The head timing marks are only
lined up once every 4 turns of the drive shaft (Three turns of the camshaft).

It can be seen from this that the drive shaft will line up with its mating tongue with the camshaft in four
different positions. Only one of these is correct and that is when the timing marks are lined up on the
head bevel gears. This puts the drive shaft and the camshaft in the correct position to fit onto a correctly
timed lower end.

LOWER END SQUARE CASE 750/860/900)

With the head drive shafts in the right position, it is possible to have the engine in two different
positions where the tongues will be in line.

The only way to decide which is correct, without going to the trouble of removing the timing case side
cover, is to set the rear piston at TDC with the front piston moving down. This should line up both drive
tongues with their flats facing away from the engine at 180 degrees. The flats described are the ones on
the shafts coming up from the engine that are inside the tubes next to the cylinders.
These flats mate up with the flats on the shafts in the head.
The correctly timed heads should now slip straight on.

860 MODELS TIMING CHART.

Crankshaft Revolutions -- 0*____1_____2______3*_____4________5__________6*
Lower Bevel Drive Revs -0*_____2/3___1-1/3___2*____2-2/3_____ 3-1/3_____4*
Camshaft Revolutions --- 0*____1/2____1_____1-1/2___2_______2-1/2_______3*
* = Timing Dots

[jp1]

Thanks, Gardner,
I just need somethimg clarified. I have studied the details of the cam gear ratios but it just leaves me confused. These details are all fixed and the only problem I have is knowing whether I have put the front cylinder head on correctly. (could be any of four camshaft positions where the half-shafts match to allow the head to go on)
The engine is in the bike with both heads on and hopefully I don't need to pull the front head off again.
As can be seen in the rough diagram I posted, the rear cylinder is at TDC with all the lower and rear-upper bevel dots aligned . The front cylinder upper bevel dots are also aligned exactly the same as the rear ones- i.e. both sets of upper bevel dots position is exactly the same relative to one other when the crank is turned. They all coincide as per the diagram every 6 crank turns.
I am pretty sure that this is correct, but I would appreciate it if someone more qualified than me could confirm if I have got it right (or not).
Greatly appreciate any help,
jp1

[Gardner]

That all sounds right to me.
Rear piston at TDC, front piston on its way down and all the timing marks lined up, turn the crankshaft 6 revolutions and all timing marks lined up again.

[jp1]

Thanks again, Gardner.
When turning the engine, I notice that the front inlet valve begins to open appr. 90º before the rear cylinder is at its firing point. Would this be correct?
(I struggle to understand the firing order between the two cylinders).
jp1

[Gardner]

Afraid I never paid attention to details like valves opening when rotating from all timing marks lined up.
All the gears are keyed to their shafts so if the timing marks are all lined up, that's it.

[Gardner]

If you really want to understand cam timing it would be hard to beat this.
https://www.ducati.community/threads/cam-timing.10995/

[Gardner]

Found a link to a pdf of the same article but this has the diagrams that make it a bit easier to understand.
https://www.ducati.community/attachment ... pdf.72624/

[BevHevSteve]

^ thats a great read ^

[jp1]

That's a great article for those who want the best power and smoothness from their engine. I just need mine to run when I crank it over, otherwise the bike is just an expensive ornament. I am pretty sure that the timing is corrrect but I would like to be 100% confident before I pull the carbs off again.
I wish I could look at a working motor with the bevel covers off to see if the dots are aligned the same as in my diagram. Unfortunately I don't have access to another engine.
I should have been more careful when I took it apart all those years ago, but I am where I am now. The rear cylinder timing is pretty straightforward, but the front cam could easily be out of synch. with the rear one and my limited brain calls can't work out the correct relationship between front and rear. I just assume that both camshaft bevel dots move round more or less in parallel to one another (as in the diagram I posted earlier).

Anyway, Steve, I,m glad that the forum is up and running again and here's hoping it stays.
jp1

[Gardner]

Do you have the original Bosch ignition?
If you do you have some wire replacing to do or maybe go with a new Sachse ignition.
https://forum.bevelheaven.com/viewtopic.php?f=19&t=771

[Gardner]

These details are all fixed and the only problem I have is knowing whether I have put the front cylinder head on correctly. (could be any of four camshaft positions where the half-shafts match to allow the head to go on)

It can be seen from this that the drive shaft will line up with its mating tongue with the camshaft in four
different positions. Only one of these is correct and that is when the timing marks are lined up on the
head bevel gears.

So 3 of the 4 ways, the timing marks will not be lined up when you put the head on.
And all the timing marks will never line up every 6 turns if its wrong.
You have all the marks lined up with pistons in the right positions and you can rotate 6 turns of the crank and they line up again.
I think you have it right.

[BevHevSteve]

and always remember that the crankshaft turns in the opposite direction that your wheels do when driving down the road...

[jp1]

I had another look at this and get the bevel dot positions (see attachment) in relation to one another when turning the crank ( crank anticlockwise - cams clockwise). All upper bevel dots moving in parallel.
Just another clarification of what I have and I have to assume you are correct (unless anyone disagrees).
Thanks again for your help.
jp1