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Cam timing made simple

Discussion in 'The Garage' started by jay760, Oct 28, 2019.

  1. jay760

    jay760 XS650 Junkie

    Can anybody explain in simple terms how to time the cam, I'm going to have a go at it with the 250-40 I have but not sure how to do it, do I set the tappets .040 and wait for the valve to start moving or set the tappets to 0

    Plus there is lots of information on regarding the cam for the different types of racing but not coming from the US I'm not sure which needs the most power I'm assuming the mile track does so set the engine to that?
    What is tt?
     
    gggGary likes this.
  2. grizld1

    grizld1 Grumpy old man Top Contributor

    Hi Jay,
    Re. tappet setting, it depends on the manufacturer's specs. All aftermarket performance cams that I know of are dialed in with tappets at 0 clearance. Here's the drill.
    1. Remove alternator (left side of crank) or primary gear (right side of crank) and install a degree wheel on the crankshaft and a pointer on the case.
    2. Accurately locate TDC and zero the degree wheel and pointer.
    3. Set up a dial indicator on a valve retainer with shaft in line with the valve stem.
    4. Turn crank in the direction of running rotation (CCW as seen from left, CW as seen from right) until valve opens to the manufacturer's specified lift (usually .040" or . 050"). Read the degree wheel and record the number. IIRC Megacycle specifies .040" lift.
    5. Turn the motor until the valve closes to the specified lift. Record the number.
    6. Calculate the lobe center and compare to manufacturer's specs. Procede to next valve.

    If we're talking dirt track racing, the mile requires the most power. If we're talking road racing, it depends on the track. American TT bears no resemblance to a British Tourist Trophy event. It's essentially a dirt flat track event with a jump and at least one right hand turn thrown in, with tractability favored over raw horsepower.

    Come on back if you need more, things can get tricky first time around!
     
    Moabite, TwoManyXS1Bs and gggGary like this.
  3. grizld1

    grizld1 Grumpy old man Top Contributor

    Confirmed Megacycle inspection lift; it's .040". Lobe centers are calculated as follows: Valve opening event + valve closing event +180 = total valve duration. Lobe center = (Total duration/2) - smaller valve event.

    As example, here's the calculation for the Shell #1 exhaust lobe, which should open at 45* BBDC and close at 25* ATDC. Total Duration = 45*BBDC + 25*ATDC + 180* = 250*. Lobe Center = (250*/2) - 25* = 100*.

    Megacycle's catalogue, which contains specs, procedures, and necessary mods (shortened guides, performance springs, high compression pistons, etc.) is available at https://www.megacyclecams.com.
     
    TwoManyXS1Bs and gggGary like this.
  4. Jack

    Jack XS650 Junkie

    You'll achieve the most power with that cam retarded 4 to 6 degrees, short duration high lift. That cam has very aggressive cam lobes make sure the valve springs are in top condition and I would us R&D springs for taming the opening and closing valve events, that's a radical cam so spend the money on quality parts for lasting durability where its needed. To get the most gains from that cam,spend some time on the head addressing the flow curve to cam lift.
     
  5. jay760

    jay760 XS650 Junkie

    Thanks for the reply I'll have a go when the engine goes back together, it's apart for inspection at the moment, do it every winter just to check the crank, I have had a welded crank spread before so just a habit I've got into .
     
    peanut and grizld1 like this.
  6. Signal

    Signal XS650 Junkie Top Contributor

    Jay I made an bracket to fix the DTI to the rockerbox this speeds up the process a lot, as the first time I did it I did a lot of measuring and calculating. and it took a long time.
    Set up 3.JPG Set up 2.JPG

    The next time was faster and I wrote a spread sheet to do the calculation shown in Grizld's post # 3.
    I can send you it via a pm if you want a copy.
     
  7. jay760

    jay760 XS650 Junkie

    Good idea with the bracket, I had a short day yesterday so had a quick look, in the end I fixed one of the back engine mounts to the head but was still struggling to get the dial around to the valve,
    If you could pm me the spread sheet please, anything that's going to help, the readings I seemed to be getting seem to be way off,
    Last night the closest I was getting was opening at 40 btdc closing at 60 btdc missing the abdc all together.
     
  8. jay760

    jay760 XS650 Junkie

    Thanks for the help, didn't have to adjust the timing in the end, it was spot on just one tooth out, so with the dot level with the head surface it was 20 degrees advanced, took a while to get my head around it but seem to have got there in the end, I'll mark the cam with a line so I remember in future.
     
    grizld1 likes this.
  9. Jack

    Jack XS650 Junkie

    You have that engine ready for next season already?
     
  10. jay760

    jay760 XS650 Junkie

    Not yet, still a bit more work to do.
     
  11. jay760

    jay760 XS650 Junkie

    Just going to do a bit of work on the exhaust port, it won't be as pretty as yours but will flow more, not going to touch the inlets as I'm worried I'll do more harm than good
     
  12. Jack

    Jack XS650 Junkie

    Flow in the intake would be the guide boss ,tapering guide end and shorting it and roof. Alot of flow improvement will be in the valve head tulip shape degrees and seat work, which will be dependent on throat %,sometimes a oversize valve is needed to correct this% and for a good three angle valve for increased flow.
    Hope it all works out improving your times.
     
    grizld1 likes this.
  13. grizld1

    grizld1 Grumpy old man Top Contributor

    Re. guides, Megacycle's instructions for the 250-40 specify that guides must be shortened by .080". The complete catalogue with specs can be downloaded from www.megacyclecams.com. I didn't see the XS650 listed in the table of contents, but if you go to the Yamaha section you'll find it there.
     
  14. Jack

    Jack XS650 Junkie

    Don't know what guides Jays using but the Kibblewhite guides are clones of the Lillie guide I believe.
     
    grizld1 and gggGary like this.
  15. jay760

    jay760 XS650 Junkie

    I'm using kibblewhite valve guides, my old cam needed the guides shortened as well, the previous owner got over this by not running with guide seals on.
    I've also fitted xv750 valves as I was told they were a better shape and work much better, I was a bit worried when I saw the inlet valves a the back of them are almost flat.
     
    grizld1 likes this.
  16. Jack

    Jack XS650 Junkie

    You won't lose flow with the virago valves, they're very good valves.
     
    grizld1 likes this.
  17. jay760

    jay760 XS650 Junkie

    Just thought I'd check it may seem like a stupid question but when you retard the cam by a few degrees it opens closer to tdc rather than if you move the cam forward it opens sooner, it may seem like a stupid question but just want to make sure.
     
  18. grizld1

    grizld1 Grumpy old man Top Contributor

    Right, retarding the cam means moving it so that opening events occur later. To accomplish that, the cam would be moved opposite to the direction of engine rotation.
     
    TwoManyXS1Bs and jay760 like this.
  19. jay760

    jay760 XS650 Junkie

    Thought that's what it was thought I'd better check though just in case, as my cam was set by the previous owner to 25 degrees just had a senior moment thinking regarding it it would be in the 30s
     
    grizld1 likes this.
  20. grizld1

    grizld1 Grumpy old man Top Contributor

    IO at 25* BTDC would be 2* retarded, but inspect the exhaust events as well. It might not follow as a general rule, but I've found myself doing a lot of short-shifting (and retiming) if the EO was too far advanced.
     

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