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valve timing on a 1928 short frame 600cc scout

  • 13 Apr 2019 9:49 PM
    Message # 7281276

    hi everyone 

    could anyone tell me the exact time inlet and exhaust valves should open and shut please? when I install all the cam and intermediate gears according to the marking (as per book) the exhaust valve opens 13mm before the piston reach bottom dead centre at explosion stroke, the intake valve start to lift 5mm before top dead centre at exhaust stroke (the exhaust valve is still open and shuts at 3mm past top dead centre

    I have a reproduction pinion shaft gear, so I thought the marking could be incorrect, so I've forwarded the cam gear one tooth from the marking on the cam gear and now, I have the intake valve opening 6mm before top dead centre, the exhaust valve is still slightly opened (shuts at top dead centre) and the intake valve fully shuts when the piston rise 6mm from bottom dead centre on compression stroke and the exhaust valve start to open 20mm before the piston reach the bottom at explosion stroke  

    now I've retarded the cam gear one tooth from the pinion gear mark and here is the result

    the exhaust valve opens at bottom dead centre and shuts 6mm past top dead centre on intake stroke, the intake valve fully shuts 20mm past bottom dead centre on compression stroke!

    I love a challenge but I thought that valves should opens or shuts at top dead centre or bottom dead centre (I am not a mechanic so i'll accept any suggestions form the 101ers, I am sure they'll know right from wrong!)

    cheers 

    Daniel Guillon 


  • 14 Apr 2019 6:53 AM
    Reply # 7281462 on 7281276

    Hello Daniel ,   your first measures sound about right . there must be what is called valve overlap which allows one action to assist the other i.e. when the inlet opens with the exhaust exhausting it helps pull in the inlet charge and same for the late closing of the exhaust , incoming charge helps the exhaust . If you have the old pinion gear , check the position of the timing mark against the key slot . Hope this helps and best of luck with your bike , Ken

  • 14 Apr 2019 7:09 AM
    Reply # 7281465 on 7281276

    thanks Ken lee, I've actually got some information from a friend, he lent me his book and I copied a page so here it comes, could help someone else in the future!

     

     

    Valve timing of the Indian scout published by J B Nicholson in the MODERN MOTORCYCLE MECHANICS book (1953 fourth edition)

     

     

    Checking valve and ignition timing

      It is good policy to make a quick check on valve and ignition timing to piston movement before fitting cylinder heads. Revolve motor slowly via rear wheel with high gear engaged. On standard twin models, there is a slight amount of valve overlap, the inlet valve opening when piston is about 1/16” before top of exhaust stroke and exhaust valve closing when piston is about 1/16” down on the inlet stroke. Special speed models using special cams have considerably more overlap. On the Bonneville motor models, inlet valve commences opening with piston approximately 3/8” before top of exhaust stroke and exhaust valve closing 3/8” down on piston on inlet stroke.

       Valve timing will be correct unless timing gears have been removed and refitted out of time.

      With ignition control advanced and breaker points set .020” fully open (coil ignition), or .015” (magneto ignition), revolve motor slowly in running direction and note that the points just commence to separate on the narrow cam when front piston is 3/8” before top of compression stroke (both valves closed) and that points just separate on the wide breaker cam when the rear piston is 3/8” before top of compression stroke, 3/8” advance on piston is used on standard 45” models.

      If timing is checked on both pistons, the possibility of having one cylinder out of time due to accidentally timing on the wrong cam will be eliminated.


    cheers

    Daniel 

  • 15 Apr 2019 12:02 PM
    Reply # 7283241 on 7281276

    If you are running standard parts in your engine it is better to rely on Indian factory numbers from the time. Here is data from 1929 Indian note book.

    Last modified: 15 Apr 2019 12:06 PM | Carl-Erik Renquist
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