Pinned ZXR250C camshafts

Good job Dano. It might actually be a different exhaust cam. Same amount of duration (and overlap), but just a wee bit earlier on the ZXR250C.

 

Yes between the A model and C model they've got the same duration for the exhausts but they're timed ever so slightly differently

It could be the bolt holes located differently on the cam gears OR the holes for the bolts on the cams themselves drilled differently - same net outcome, the sprockets in different 'positions'.

I'd be looking at the sprocket bolts holes being different between them

 

A little arithmetic says the A model cams are on 104 inlet lobe center and 102 exhaust lobe center
The C model is on 104/105 lobe centers - which is a much better setting.
I've no idea why the A is arse backwards to convention - emissions ?
I suspect you'll find the A and C model cam sprockets are different part numbers if they are bolt ons.

I've only ever been into a couple of ZXR 250's and seem to remember the sprockets were pressed on.
Which meant a bit of time spent marking a datum line, pressing off and pressing back on.
Then dialling them up to check I'd gone the right way. Lacking a manual, but knowing what bigger Kawa heads liked
I put them in on 103/104 lobe centers. Never had any complaints about power.

 

I wasn't thinking straight when I thought of bolts, rather I was thinking of GSXR250's with the cylinder/cam layout in the middle between cylinder pairs that are likely bolted on...anyway...

I'd go with the suggestion of @gregt on timing - now would the cam wheel, because it's moving at 1/2 the speed of the crank need to move 6 degrees to give a 3 degree difference in crank rotation - I think it would, (happy to be corrected on that if I'm wrong) which means it's a larger amount to measure, which is a good thing, makes it easier

 

I like this degree wheel calculator - this is the input numbers from the actual cam duration specs to get 103/104 lobe centres
https://blocklayer.com/us?i=2804

 

Hey Dano (& @gregt), can you document the process of removing the cam wheel and re-pressing it onto the cam when you do it? As far as I am aware, there's no way to advance (or retard) the timing on these engines using the timing wheel, so I'd like to understand the process/procedure for changing the timing and overlap periods/degrees by manually altering the cam position(s).

 

CNC could be useful for rotational positioning, otherwise a rotary positioning table will work, less complex.

Even less complex two discs, can be pretty much anything, plastic even, make them say 20cm diameter - one affixes to the cam in a manner which can be repeated, the other affixes to the sprocket, in a way that can be repeated.

At that diameter of 20cm, each degree is very close to 1.75mm movement at the circumference.

Position the discs, mark them to cam and sprocket and to eachother - press the cam and sprocket apart, reposition the cam/sprocket to the new timing using the positioning discs, press back together...

You'll just have to think carefully as to which way an intake cam would rotate -v- an exhaust cam because their lobe centres are given differently;

Advancing an intake cam (opening earlier) make it's lobe centre number decrease because it's given as ATDC
whereas
Advancing an exhaust cam (opening earlier) makes it's lobe centre number increase because it's given as BTDC

Use the degree wheel page to change the numbers and see the changes in the lobe centres -
https://www.blocklayer.com/degree-wheel

I added (advanced - opening earlier) each cam by 10 degrees in this example (what was added to the opening time - earlier, was removed from the closing time to keep the duration the same, each time you change the numbers uncheck/recheck the lobe centres check box and it will update the numbers for intake lobe centre, exhaust lobe centre and lobe separation.
Note that the intake lobe centre has gone from 104 -> 94 and the exhaust lobe centre has gone from 105 -> 115.

 

If I remember right - it was years back - i used a meaty 3 jaw gear puller and a little heat to remove the sprockets.
At the time I had access to a local engineering firm's press - a very large pumped hydraulic unit.
So this was used to replace them.

You could shrink fit them without too much trouble.

But do not forget to mark them before removal......

 

I'd be considering getting a set reconditioned - the pit(s) welded and the surfaces ground to restore OEM lift specs then re-hardened.

Pick a set with the best cam journals because that's the important bearing surface for the cylinder head cam cap to cam journal clearance

 

So Greg, what would the outcome/result be if I stuck a set of ZXR250C cams in a ZXR250A engine without making any changes to the cams (position etc)? My novice brain says the inlet will open at the same point/position as ZXR250A cams, but the exhaust will open a few degrees later for less overlap? Does this do anything for power? I don't understand your comment about the 103/104 lobes in the ZXR250C exhaust cam making it better? Happy to be pointed to something to read or watch.

I have a programmable TCI unit on the bike, so I can change the advance if needed.