Hope someone can help me with this. I've just bought an early twin cylinder bike which has no timing marks on the flywheel. However the manual says "Max timing 35 degree BTDC,Initial Fixed 5 degrees BTDC. It has a bore 84mm and stroke 58mm. It has twin points so I want to set it up manually. Just wondered if there is a formular/calc I can use to get the mm stroke travel BTDC when the points should start to open! I'm thinking (35/360) * (58*2) = 11.3mm BTDC.Does that sound right! Cheers Glyn
crzyhrse,
May 23, 12:43am
No, that formula doesn't work not just because you're doubling the strokeinstead of halving it but because the position of the piston at any crank angleis a SINE wave. So the distance from TDC that the piston has travelled (or has yet to travel) is ((STROKE/2) - (STROKE/2 x COS A)) where A is the crank angle relative to TDC. In your case that's 28.89mm. As ½ stroke is 58mm/2 = 29mm that's 0.11mm from TDC.
Put a DTI down the bore and mark TDC on the flywheel and engine casing. Then get a piece of paper, put it between the contacts and advance the timing until it just pulls out. Start the engine and check the base timing.
Is the ignition advance centrifugal only!
crzyhrse,
May 23, 12:46am
Oh, and that makes 35ºBTDC 5.24mm.
SOHCAHTOA.
(58mm/2 - (29x COS 35)) = 5.24mm.
crzyhrse,
May 23, 1:02am
Actually, ignore that calculation. Because of the position of the gudgeon relative to the crown it's not quite that straight forward. If you know that measurement and the conrod centres though, you can easily work it out.
skin1235,
May 23, 8:59am
create a protractor, mark in your 5 degrees etc, find tdc and some recognisable method to mark that position ( split in case, slight mark - even a scratch with a srewdriver will suffice) set tdc to that mark, move almost 2 revs on crank, set to btdc mark ( 5! degrees) time points accordingly
what bike!, they all had timing marks on them somewhere, either a cap or plug or even on the cam end
glyn4,
Mar 25, 7:54am
Many thanks for that info. It's a Benelli Tornado. There may be marks somewhere but I can't find them. It's very Triumph like and reading on the net static timing was fine for those (apparently). I don't want to take the motor apart at this stage but just want to get it running to see it everything sounds OK. I'll go with the that advice.Many thanks.The Triumphs were 38 degrees BTDC with a stroke of 82mm. They used points starting to open at 6.4mm before TDC. Cheers everyone.
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