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Anybody recognize this cam?

Discussion in 'The Garage' started by jay760, Oct 31, 2014.

  1. jay760

    jay760 XS650 Junkie

    as the title says, does anybody recognize this cam, I thought it was a megacycle 250 40 until I bought one, it's nothing like it the lobes look to be much larger, only identification is an 8 78 which I assume is the date it was made and a c on the outer lobe.

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  2. jay760

    jay760 XS650 Junkie

    Just for comparison a picture of the lobe on the 250 40 cam I received this one has 250 40 stamped on the cam.

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  3. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Yes, that's an interesting lobe profile, weld-up/regrind. Long duration, looks to have wider lobe centers. Why the woodruff key notch?

    In the '70s, you could get cams built-up/reground to custom profiles. Mine was done by a local hobbyist type, using a profile-follower cam grinder, and I selected the profiles. But, mine's unmarked. Yours, being marked, might signify a more prominent grinding business, like Crane, or something else identifiable as 'C'.

    I'd do a test install, minus pistons, use just inner springs, degree wheel and dialgauge to get valve timing specs. Might find something interesting there. Could require special setups to run it, or it could be just a failed experiment. Since you're UK, could check suppliers to Tony Hall...
  4. jay760

    jay760 XS650 Junkie

    I was running it in the bike last year, bought it from the US, it was on ebay they had the hole engine on there, I bought the complete top end, didn't fancy the postage for the whole engine, head was ported with 750 barrels and pistons, when you looked at the pictures of the bottom end the studs were notched so you could remove the top end, the other interesting thing about the engine was it had 256 rods on a 447 crank.

    It ran well, best time of 12.2, highest speed recorded was 115 in 4th, that was without being set up properly, just put in to where the marks were before, now I'm rebuilding the engine would have liked to get it set up properly.
    Last edited: Nov 1, 2014
  5. jay760

    jay760 XS650 Junkie

    The only thing I can find with reference to C is either as twomany says Crane, or Andrews made a C grind camshaft but can't find either with xs650's
  6. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    256 rods on a 447 crank, might have been a 306 or 366 crank, since they have the starter gearteeth, unless the sprocket has the 447 toothcount.

    Have you thought about the degreewheel/dialgauge reverse-engineer thing to get its timing specs?
    gggGary likes this.
  7. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Comp Cams dates back to around that timeframe. They have a tech forum that may help...
    gggGary likes this.
  8. jay760

    jay760 XS650 Junkie

    Just measured this cam inlet is approx .435 exhaust .395 this is using the rocker ratio given on xs650.org.au
    gggGary likes this.
  9. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Jay, all I have is this Megacycle cam chart.

    You will probably need to reverse-engineer your cam, make a chart like this Shell #1 cam profile.
    Paul Sutton likes this.
  10. jay760

    jay760 XS650 Junkie

    Just managed to measure the cam

    Inlet 22 btdc 64 abdc 0.430 lift
    Exhaust 60 Bbdc 45 atdc 0.400 lift

    Any clues on this cam?
    Last edited: Dec 2, 2019
  11. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Strange timings on that cam, offset lobe centers.
    Dunno what lift you're measuring at, 0.000", 0.012", 0.040", or 0.050"?
    Intake duration 266°, lobe center at 111° atdc.
    Exhaust duration 285°, lobe center at 98° btdc.
    Intake opening sure looks late.
    Would need to shorten the intake guide.

    Suppose you were to advance its timing by 11°, even up the overlap.
    That would give:

    Inlet 33° btdc, 53° abdc, 0.430" lift.
    Exhaust 71° bbdc, 34° atdc, 0.400" lift.

    Suppose you were to advance its timing by 6°, equalize the lobe centers to about 105°.
    That would give:

    Inlet 28° btdc, 58° abdc, 0.430" lift.
    Exhaust 66° bbdc, 39° atdc, 0.400" lift.

    Strange grind, indeed.

    You could try to get a copy of Signal's "cam lobe centers" spreadsheet.

    Grind thru this.

    Play around with various advance/retard timings until you get something sensical...
    jay760 and gggGary like this.
  12. jay760

    jay760 XS650 Junkie

    Measured at .040 lift with 0 clearance for the opening
    Lift I measured from 0
    Thought I'd measure it the same as with the megacycle cam, when I hot it there were no valve seals as when they went on it hit them so changed the guides, the cam does work quite well.
    gggGary and TwoManyXS1Bs like this.
  13. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    That long duration exhaust is somewhat backwards from traditional cam timings. May have been trying to solve the exhausting problems on the 750cc engine.

    For reference, the 256 cam and 447 cam.
    Note the 256 longer duration intake.
    Note the 447 symmetry...


    jay760 and gggGary like this.
  14. billy icon

    billy icon test pilot,gunnie pig, and all around experiment

  15. jay760

    jay760 XS650 Junkie

    Lobe centres are

    Inlet 72 Bbdc
    Ex 84 abdc
  16. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Ok, can also look at it from TDC.

    Inlet 72 Bbdc = 108° ATDC
    Ex 84 abdc = 96° BTDC

    The thing with cam timing is that it's a part of the total package, the goal being to maximize induction and scavenge super-charging. Cam profiles, port design, valve design, inlet and exhaust geometries all have to work together to get this. There's some general principles to apply, and established setups for a particular engine, but without exotic test equipment and experimentation, it's like throwing darts in a darkened room...

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