Timing a PW3 cam

pommie john · Oct 7, 2011

I've just installed a PW3 cam in my 750 racer and I'm having some difficulty deciding whether I've got it right.
Putting a dial gauge on the pushrod and measuring the lobe centre, I get 107 degrees which is near enough to the 106 specified.
The way I arrived at this figure was to measure the point at which I get 2mm lift ( enough to ensure the lifter is off the opening ramp).
I get 2mm lift at 22 BTDC and also at 56 ABDC. From this I deduce 258 degrees duration at 2mm lift. Half of 258 = 129, minus 22 gives me 107 degrees for the lobe centre.
All good you might think. But to be sure I also measured lift at TDC and came up with 160 thou when it's supposed to be 145. That suggests the cam is too advanced. Another check shows that I get 145 thou lift at 5 degrees BTDC

So which method do I trust? Is the cam spot on or is it 5 degrees advanced, and does that matter?

Hobot · Oct 7, 2011

Best is point of lift vs point of close degrees - but one needs the cam makers data to know which amount of lift or close they used to mark beginning and ending. A silly mm off cam makers opening lift point can confuse the results. The TDC method is less actuate, but mainly because it hard to know if a cam lobe is correctly made to know where your probe actually lands on it. Some just pay for extra dyno runs or top speed in measured distance and tweak cam from that.

Matt Spencer · Oct 7, 2011

Both . Its just the crankcase mouths machined 5 degree off .

:wink: .

Now , all the Valve / Piston clearances are O.K. , is the main thing and off some relevance .

Id wind it up to the OVERLAP , find i.o. & e.c ,THEN of overlap & piston / crank Deg .
and also deg. to centre of overlap @ T.D.C.

Once they start talking to you , the ' intrest ' is that the valves arnt going to tangle with the pistons.
The rest , while not irrellevant is less crucial . A bit early will increase the midrange .
Driveability being more relevant than doing 20.000 with a 1.000 powerband .

Unless your running a C.R. 6 speed box . ?

The ' Winding it Forward to settings allways is the trick .Due to camchain clearance . Theyred be a nudge
variation to readings , running either way .Need to push it through about 10 Deg. before reading . :mrgreen:

pommie john · Oct 8, 2011

ludwig said:
The way I do it , is to find lobe centres about the same way as to find TDC :

first set your pointer at TDC crankshaft .
I suppose you kow how to find TDC ?
Turn the crank till the dial indicator is , say , 0.50 mm before top lift .
The exact value is not important .
Mark that point on the degree wheel (1) .
Keep turning past full lift till the needle from the dial drops to the same point on the dial scale (in this case 0.50 mm )
Mark that on the degree wheel (2) .
Lobe centre is than exactly in the middle of 1 and 2 .
You can do this for in an outlet lobe , or , If you are shure about your lobe split , Inlet is enough .
Then decide where you want your lobe centres relative to TDC .

Hard to see on the pic , but I put a rubber band on my dial gauge to get a little down pressure on the pushrod , to make shure the lifter follows the cam nicely .
Allways turn engine in same direction when taking your readings .
hope this helps ..

I did it that way as well as the 2mm lift way. Both gave me the spot on result, but I'm a bit confused as to why , when I do it by measuring valve lift at TDC I get a different result.

pommie john · Oct 8, 2011

OK, I'll leave it where it is.

Next job is to check valve /piston clearances.

Thanks
John

splatt · Oct 8, 2011

All true only if the cam lobe has a symetrical profile

nortonspeed · Oct 8, 2011

splatt said:
All true only if the cam lobe has a symetrical profile

True, but then again how many non symetrical cams do you know for Norton twins? The only cam I know of (and raced) was the Somerton cam.

Bernhard · Oct 11, 2011

ludwig said:
pommie john said:

.. I'm a bit confused as to why , when I do it by measuring valve lift at TDC I get a different result.

Click to expand...

Like I said ..too many variables .
It could f.ex . be caused by a slight deviation in the position of the barrels vs camshaft ..?

Either you are trying to work with precision, or you are not. Stop and think of what you are attempting to do for one minute, if you do not want to run your engine then leave it. If you are going to just time it, then set tappet clearance cold at Zero, that’s right zero. When you think about it, this is more or less what the engine will be when hot. If you attempt to time it with a dial gauge, when you get .040 thou. lift you will X ammount behind, if the tappets are set as normal!
Zero tappet clearance will cut that dispensary out. There will also be the cam chain lash which you will have to bear in mind.
By only taking a degree disc reading while taking it in one direction only i.e. taking ay backlash out of the chain.
Even then, as the reading may vary, take thee seperate reading turning engine round 360 degrees each time, and accept the matching two.
All in the 'Black Art' Of engine tuning!

lcrken · Oct 11, 2011

nortonspeed said:
splatt said:

All true only if the cam lobe has a symetrical profile

Click to expand...

True, but then again how many non symetrical cams do you know for Norton twins? The only cam I know of (and raced) was the Somerton cam.

Axtell made some cams with non-symmetrical lobes. He called them his "Allegro" grind. He used them in Ron Wood's engines, but I don't know if he ever sold any for other customers.

Ken

Timing a PW3 cam

pommie john

Hobot

Matt Spencer

pommie john

pommie john

splatt

nortonspeed

Guest

Bernhard

lcrken

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