Valves timing

S

sergio750sf

New member
Location
France
A problem with my 750 engine.

Valves clearance are ok.
I have checked the real TDC ( the mark on the case was not good: 6/7 ? out) it's the same thor the two sides.
When the Crankshaft is at the REAL TDC and the marks on the head and the camshaft are perfectly aligned,
I notice that the valves (on both cylinders) open 7? in advance (inlet 67 ? and Exhaust : 85 ?).

Don't know what to do.




 
I just assembled an SF and it was just like that, a bit of pull against two valves to get the marks into place but they lined up perfectly between head and cam sprocket at true TDC. A worn camchain can have them significantly retarded. Good that you used true TDC, I always use a degree wheel and wire pointer.
 
Sergio, measure the difference in inlet valve lift when turning the engine by 7? around TDC. When the valve moves only a few 0.1mm down, there shouldn't be an issue with valve to piston clearance. And forget the bullshit that I have written her before, got my numbers wrong. :D With the timing 7? off the engine might run not as good as it should. Did you measure cam timing at max. valve lift? How did you measure true TDC, 6 to 7? off looks a bit steep to me? Have you double-checked this, just in case the factory managed to make proper TDC markings on your engine?



 
the question is how did you read the degrees in the first place, are you quoting crankshaft degrees ?
if so half the figure for camshaft degrees

seen Laverda engines far worse than that on cam timing, I check cam timing on every engine I build and it is very rare that I don't have to adjust cam timing in some way

 
How did you assess opening degrees? From what I've learnt, cam timing should be checked at maximum lift and the standard in cam design should give you max lift exactly halfway between opening and closing numbers. No way can you get an accurate figure by looking at a degree wheel and matching that with when the valve opens. Your valve clearance would only have to be .001mm out and you'd get a different figure.

All the Yoshimura and Moriwaki builds we did specified that technique. Apparently it's very rare to not have lobe centre = max lift. Red's point is very important as well.
 
"Can I run the engine without problems ?"
I woudn?t hesitate. If the marks line up it is safe. If you are concerned about piston to valve clearance, just use a lever to compress down the rocker arm as you rotate the crank the degrees as the pistons are closest to TDC (I can send you some how-to photos if you want) , but I?m sure that on a standard motor there will be tons of clearance.
Certainly the motor will be sharper if you degree in the cam timing (a fair bit of mucking around), and set compression to spec but it isn?t necessary for the safety of the motor.
 
Tippie, I know how it can be done on the triples, but how is cam timing adjusted on the twins? Would have thought with that slotted keyway there is not much left to fiddle with?

Regards,
Stefan
 
You have to either widen the keyway slot in the chainwheel or dispense with the bit of the key that passes through it, and slot the holes in the flanges for each cam half. On my 270 motor I have no keyway through the chainwheel, and slotted holes with allen screws that secure each cam flange to the chainwheel, such that each cam half can be adjusted independently, the same could be done with a 360 motor using some new holes in the flanges and chainwheel. My 270 motor has done a lot of racing and road miles without issue.
 
So what's the correct way to measure?
If we take e.g. 4S inlet with 60? opens, 68? closes:
does "opens" mean the moment the rocker begins to
move or maximum lift?
"Closes" no more movement on rocker?

Ernesto
 
Ernesto, for the 4/S inlet (opens 60? before TDC, closes 68? after BDC)  it calculates for the inlet 60?+180?+68?=308?, then 308?/2 -60? = 94?, thus you should measure max. lift on the inlet valve at a crank angle of 94? after TDC when timing is in tune.
 
It does have to be said though,

that however you "adjust" the timing on the SOHC twins, it will be at the centre driven portion of the cams, i.e. the keyway alignment (not drive) slot or the flange bolts, but whatever you do you are adjusting inlet and exhaust simultaneously, the two cannot be separated individually, just the left or right "halves", so you get a situation where the inlets on both sides are spot on, but the exhaust are out, so a happy medium (not a drunk spiritualist) has to be found, or concentrate on the inlets and sod the exhausts.
CLEM
 
Ernesto said:
So what's the correct way to measure?
If we take e.g. 4S inlet with 60? opens, 68? closes:
does "opens" mean the moment the rocker begins to
move or maximum lift?
"Closes" no more movement on rocker?

Ernesto
Click to expand...

Numbers such as these are just about meaningless unless it is known at what valve clearance they were attained.  Opening/closing figures change drastically with different valve clearances.  They can however, be used as a base line.  Clocking the cams at what ever chosen clearance shoud yield figures evenly spaced from such specs (if they are correct!).  These figures enable one to somewhat assess the characteristics of a cam.  But, opening/closing are only part of the equation, valve acceleration, deceleration and max. lift are also factors that decide what a cam can or cannot do.

Only figure that doesn't change with clearance is the point of max. lift, so this figure is usually used to dial in cams.  Sometimes not so easy to find due to cam shape and ensuing dwell time, but usually easy enough to get within a couple of degrees.  Unless you're chasing the very last pony, these couple of degrees won't hurt.  Far more important is that both cylinders are supplied evenly.

piet
 
As Piet said, it's not easy to pick the exact point of maximum lift. I usually verify it by looking at points of equal valve lift just before and after maximum, and split the difference on the degree wheel. A bit like finding TDC with a piston stop.
 
That's easy enough with the triples, using flat tappets, Cam.

Radiussed rockers/followers, as with the OHC 750's, can be a bit troublesome if not not perfectly symetrical (what they ideally should be), as the radius has a direct effect on the lift of the valve in relation to cam position.  Not too important for general road use, as differing/changing valve clearances during service can cause just as much deviation from "spec".

There are lots of differing rockers out there by now.  Few are as they left the factory, some have been re-worked, some modded with stellite rub plates.  To achieve a sweet runner, it should be checked that the rocker radii are the same all round.  Even the radius on the tip of the adjuster screw should be considered!

piet
 
You must log in or register to reply here.
Back
Top