camshaft timing

Discussion in 'Classic Motorbikes' started by PePe, Oct 21, 2004.

  1. PePe

    PePe Guest

    I'm thinking of upgrading a CB250RS-engine with a big-bore and a different
    camshaft. The big-bore was no problem, but the camshaft turns out to be a
    real pain. There are several different parts readily available, all with
    different timings and promises on torque, top-end, mid-end etcetera. For
    now, I've been comparing the timing-numbers, but I can't find any logic in
    that. Since I have the choice in having the stock one reground or a new one
    delivered for about the same price, I would like to do some 'calculating'
    myself if that's possible. Is there anyone who can help me in the search for
    a clear understanding of, or maybe a calculation applet or formula?

    For the record, the standard profile reads 10/40 and 40/10. I've found
    similar timing on a XL350, and the most stated are:
    22/48 and 50/22 for a better mid- and top-end.
    33/63 and 60/33 for maximum top-end.

    I want to gain mid- and top-end since it's going to be a street and racebike
    (50-50), but I do not want to suffer too much reliability.

    Any pointers please? Many thanks!


    '00 Ducati M600 - '82 Kawasaki KDX 175 - '81 Honda CB250RS
    PePe, Oct 21, 2004
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  2. PePe

    TOG Guest

    To be honest, I'd junk the idea of boring out the 250 lump and simply
    go out and buy a 500 engine from an XL500, FT500 and the like.

    They're damn nearly a drop-in fitment, and will give more power than a
    tweaked 250 ever will. I've seen a 500-ised CB250RS: you really had to
    look twice to appreciate it for what it was.
    TOG, Oct 21, 2004
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  3. PePe

    Pip Luscher Guest

    Well as you've probably figured out:

    10/40 = 10deg BTDC inlet opens / 40deg ABDC inlet closes


    40/10 = 40deg BBDC exhaust opens / 10deg ATDC exhaust closes

    thus giving 20deg of overlap at the top of the stroke.

    The 22/48 and 50/22 simply means that the valves open earlier and
    close later giving more overlap (44deg), and so on.

    33/63 and 60/33 are even more extreme.

    In each succeeding case, the valves are open for longer and overlap
    more. This will *tend* to improve breathing at higher RPM and hence
    more power because:
    a) the valves are open wider for longer and
    b) the increased overlap will allow any exhaust resonance to pull
    through more mixture

    The down side: Wider cam timings will probably stuff lower & midrange
    torque. Jetting will probably have to be changed, possibly across the
    full range, and the exhaust & inlet may need modifying to make full
    use of the altered timing.

    As TOG says, better to just get a bigger bike.
    Pip Luscher, Oct 21, 2004
  4. It was somewhere around Barstow, on the edge of the desert, when the
    drugs began to take hold. I remember [email protected],
    AARRRRGGGGHHHH!!!!!!!!!! Not the dreaded FT! Itym XBR500.



    GS 850 x2 / SE 6a
    FUB KotL OSOS#12? UKRMMA#19 COSOC#10
    Grimly Curmudgeon, Oct 21, 2004
  5. PePe

    platypus Guest

    Exactly my reaction.

    When the XBR500 was launched, testers of the time praised it as being "like
    a CB500RS".
    platypus, Oct 21, 2004
  6. Well, the FT is OK as long as you convert it to kickstart, throw away
    the exhaust (which you'll have to anyway) and fit a less restrictive air
    The Older Gentleman, Oct 21, 2004
  7. PePe

    platypus Guest

    Can you get replacement exhausts, or do you just use the Big Collector Box?
    platypus, Oct 22, 2004
  8. PePe

    Pip Guest

    There ain't no substitute for cc's.
    Pip, Oct 22, 2004
  9. Someone must do them. If not, it's not hard to source a custom-made
    The Older Gentleman, Oct 22, 2004
  10. PePe

    platypus Guest

    Not that I'm planning to get one. Actually, I was having a close look at a
    v. clean VN1500 nomad/Classic Tourer thing in the local emporium yesterday:
    99T reg, spotless, green, added sissy bar and spotlights, 9K miles (on a
    5-year-old bike ffs), £5895. Which I think may be on the high side, but
    these are pretty rare, so it's hard to establish a frame of reference.
    platypus, Oct 22, 2004
  11. AOL to that
    oldbloke at work, Oct 22, 2004

  12. My dealer's got one which sounds very like that, i his floor, too. Can't
    recall the price. It's the only thing to do wiuth a big Jap cruiser,
    IMHO - give it a screen, panniers, and maybe a decent seat, and turn it
    into a sort of budget tourer.

    But 6k sounds like silly money, really, for a tart's boudoir with
    panniers. 5k, better still 4.5k, and it makes sense.
    The Older Gentleman, Oct 22, 2004
  13. PePe

    platypus Guest

    That's sort of what I was thinking. I was mesmerised by the bigness and
    shiny-glitteriness of it, but in reality a Cali would be 1-1.5K cheaper, be
    more powerful, economical, and have seriously better tank range.
    platypus, Oct 22, 2004
  14. Nail - head - BANG!

    Plus it would handle and stop much better. Only the seat would be the
    issue - the Cali has a seat that is utter total crap.

    Nothing that can't be (relatively) cheaply rectified, mind.

    I really, really liked the injected Cali I tested in 1997 (shit! - seven
    years ago).
    The Older Gentleman, Oct 22, 2004
  15. PePe

    platypus Guest

    A Cali would cope with luggage and a pillion, and do the v-twin, cruiser
    thang, so replace both the RT and the Drifter. One thing the Trophy did was
    focus my mind on what I want and need from a bike.

    What I'd save on the purchase would leave me plenty for something from
    Corbin or the like.

    I'm torn between the looks of the T3-style Cali and the later Cali
    III/1100/LAPD styles. I suppose the injected motor would be the clincher.

    platypus, Oct 22, 2004
  16. PePe

    Pip Guest

    Yeah, yeah Max - the blower ... I know.

    You take a 750 motor and turbo it - then hog it out to 1100 and turbo
    that. See my point?
    Pip, Oct 23, 2004
  17. The original Cali, the T3 850cc engined one, is the best looker.

    The injected one is the best for riding.

    The carbed 1000s are almost all shite.
    The Older Gentleman, Oct 23, 2004
  18. PePe

    platypus Guest

    That was my perception. Roll on the NEC, then.
    platypus, Oct 23, 2004

  19. Remember that Yamaha was developing an XS1100 Turbo before sanity
    prevailed (or possibly, after they'd killed all their development
    riders) and they decided to turbo the XJ650 instead.
    The Older Gentleman, Oct 23, 2004
  20. PePe

    Pip Guest

    (The Older Gentleman) struggled
    to ejaculate:
    Pah. Wusses.

    Supercharged V-Max, that's what they should have been developing. Now
    *that* would be worthy of Champ's plate logo.
    Pip, Oct 23, 2004
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