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Pinned ZXR250C camshafts

Discussion in 'Kawasaki 250cc In-line 4's' started by DanoHosko, Dec 17, 2023.

  1. gregt

    gregt Well-Known Member Premium Member

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    First comment I'll make is that using the quite common Eutectic overlay to repair the lobes they do not get rehardened.
    The Eutectic stuff work-hardens with use. If you have a cam built up and reground, it's quite possible to mark the surface with a file until it's done enough time running. The cams are cast iron and I suspect the lobe is cast quite close to finished shape
    and chill hardened before being finish ground. The hard layer is very thin.

    Cam lobe centers are simply a convenient way to describe how a particular cam is timed.
    Broadly speaking, convention says the inlet lobe center should be a lower number than the exhaust.
    If you look at online cam catalogues which quote lobe centers the A model is an outlier to convention.
    Convention is accepted practise proven by time.

    My own experience going back yea, these many years - I'm OLD - tells me that downdraft heads like closer lobe centers than
    those with "horizontal" inlet ports. Eg - Suzuki Impulse 400 which is pretty much the last non downdraft Suzuki 400,
    Likes 106/107 lobe centers. The FZR400 which is quite a steep downdraft likes 103/104 lobe centers.
    In big Suzukis, the classic timing for GSX1100 race engines is 110/112 lobe centers. Late GSXR1000's like 107/107.

    Put A cams in on A timing and you'll get A performance.
     
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  2. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    I don't want to hijack Dano's thread, but I guess what I was asking was... if the A inlet cam is the same as the C inlet cam, what difference will having the exhaust open slightly later (the C cam) have on performance? e.g. what advantages/disadvantages are there to changing the duration of overlap and when it occurs? I run A cams in an A engine, but from experience I know that the standard TCI A timing is not for racing or maximum performance at higher revs. I picked up .2 of a second after changing the advance and bike regularly runs 96-98mph at the end of the quarter now (even with 100kg of ballast!). I wanted to investigate race cams, but no-one has any profiles, so the whole idea is a dead end unless I can find someone who knows what they're talking about and can customise a set of standard cams. The fact that the C exhaust cam is different intrigues me as the timing for the C engine is not too different to the A.

    The C engine makes its peak power and torque at different RPM's to the A engine.
     
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  3. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Different bore and stroke on A vs. C models. If using a higher lift exhaust cam, you must carefully check that you have sufficient piston to valve clearance, and that the inlet and exhaust valves don't collide during overlap. At minimum this means carefully turning over the engine with plugs removed and cams/tensioner installed and timed correctly and stopping if it becomes difficult to turn or won't turn. Don't force anything.

    The better way to do it is to remove the cylinder head and use plasticine as a gauge and measure your clearance. I don't know if the service manual will give you piston to valve clearance measurements, that's usually something only considered on custom/built engines, so I can't speak as to how much clearance is needed.

    Regarding ignition timing, adjusting it is kind of difficult unless you can program transistorised ignition units. The cheap way is to slot the mounting holes for the pickup and advance it that way, or find/produce a new ignition timing plate with advance built in. I did this on my R6, there were multiple aftermarket ones available, from -1, +2, +6 and +10 degrees of advance. The -1 degree was for California models because of their ridiculous emissions nonsense, means their model R6 had different cams to the rest of the country and the world.

    Likely there is 'nothing' available for any of the 250 fours unless you search within Japan and know where to look.
     
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  4. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Although there isn't any megacycle cams listed for the ZXR250 or it's bigger sibling the ZXR400 - there may be something to be learnt from comparing cams for 'similar' engines from that time as the lobe centres are comparable.

    Published duration numbers between Kawasaki (ZXR250) numbers and Megacycle numbers cannot be compared it seems - which is a bugger

    I don't know what lift Kawasaki is providing/calculating for their duration for the ZXR250, but it isn't 0.040" by my reckoning.
    Full catalogue is here
    https://www.megacyclecams.com/catalog/catalog_optimized.pdf

    There's some other worthwhile notes in the catalogue with respect to measuring lift as cams with a long duration aren't measured by simply measuring the smallest -v- largest dimensions and subtracting the difference, when the cam is already effecting lift across the smallest dimension, so it must be measured with a dial indicator by rotation, not verniers or a micrometer.

    There's a few conventions - duration and lift increases and the changes to lobe centres which accompany those changes and the cited changes in power characteristics, torque, mid range, top end etc.

    The Yamaha FZR400/600 and YZF600 lobe centre numbers are very close to the ZXR250C and @gregt has already said 103 intake and 104 exhaust works for those engines and the FZR250.

    Screen Shot 2024-05-31 at 9.05.27 am.png Screen Shot 2024-05-31 at 10.03.01 am.png Screen Shot 2024-05-31 at 10.03.17 am.png Screen Shot 2024-05-31 at 9.04.29 am.png
     
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  5. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Yes, the different stroke (48x34.5 Vs 49x33.1) does mean the piston is .7mm closer to the head, but given the valve clearances are the same and the cams have the same profile, I wonder if there will be an issue.
     
  6. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    That's a difference in stroke, the clearance (squish band) between the piston top (flat section) and the cylinder head (flat section) will likely be the same or very similar - and increase/decrease of 0.7mm in squish band between the models would be huge as the squish band is likely to be about 0.8mm - 1mm or thereabouts.

    Consequently piston to valve clearances should be the same or very similar, differences in valve pocket depth notwithstanding.

    There's likely a difference in either, or both, the compression height of the piston, less likely - the conrod length or even less likely - height of the cylinder block (an extreme outside case maybe the engine cases being different) to account for the different stroke.

    I reckon if you put an A and C piston together on a gudgeon pin, there will be height differences between them
     
  7. gregt

    gregt Well-Known Member Premium Member

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    Valve to piston on something as small as the 250's I'd be looking for .050in.
    Bear in mind a man with a mill can easily deepen the valve pockets.

    Big 2 valve engines I use .080in. I have once used .040in on an EX500 racebike - but with a rider who never missed
    upward changes. Next time the head was off the pockets got deepened to give .050in.
     
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  8. DanoHosko

    DanoHosko Well-Known Member Premium Member

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    Just to add, my "good cams" that have been running in the C model have the markings:

    R3 D (intake)
    R4 D (exhaust)

    20240601_151448.jpg
     
  9. jmw76

    jmw76 Well-Known Member

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    The cast numbers usually are not all that helpful on other bike engines I have worked on. The cast in number is just to identify the blank before it is actually ground. Sometimes it also identifies the casting machine/dies for QA purposes. Not unusual to find multiple different grinds from the same blank. There is usually some stamping elsewhere to indicate the actual grind.
     
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  10. DanoHosko

    DanoHosko Well-Known Member Premium Member

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    So do we think the style of the cast numbers has any relation to A or C model? (Rather the the numbering of them)

    Just odd that one pair I have the weird marking with the screw in it?

    I have:
    1 pair from my running C model = No Screw in the Shaft
    1 Pair from spare C model = No Screw in the Shaft
    1 Pair from non-running C-model engine = Screw in the Shaft
    1 Pair from eBay = No Screw in the Shaft

    The non-running C-model engine had the butchered camshafts

    20240516_201803.jpg
     
  11. H05TYL

    H05TYL Active Member

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    To muddy the waters somewhat (sorry) the parts diagrams list two different part numbers for the exhaust cam for the 45hp c/d models, one the same as the a/b models and one the same as the 40hp c models

    1994-1999 zxr250c
    Intake 49118-1069
    Exhaust 49118-1111

    1991-1993 zxr250c/d
    Intake 49118-1069
    Exhaust 49118-1070
    Exhaust 49118-1111

    1989 zxr250a/b
    Intake 49118-1069
    Exhaust 49118-1070
     
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  12. DanoHosko

    DanoHosko Well-Known Member Premium Member

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    That really does muddy the water somewhat :prankster:

    Maybe then the CDI unit needs to be matched with the camshafts?
     
  13. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    upload_2024-6-23_11-50-42.png
     

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