Conversion help: mm BTDC to degrees BTDC

Anybody want to take a crack at it? (If you don't know for what BTDC is
an acronym, don't bother trying to answer.)

My bike's points open at 0.8 mm BTDC, and I want to measure that with a
degree wheel and strobe light. I trust dymanic timing before static,
because the points are at the end of a four-gear drive train. Lash, you
know.

Ted

 

not positive, but pretty sure the differentiation of mark vs mark (with the
strobe) exceeds by far the 0.80 mm accuracy of the dial indicator. it has
been my experience that whatever gear lash is involved is taken up by only
checking/setting the distance by turning the engine in the normal rotation

 

Here's the formula for x, piston position relative to TDC:

r: stroke / 2
l: length of connecting rod
theta: angular position of crankshaft from TDC

x = l + r - r * cos(theta) - sqrt(l^2 - (r * sin(theta))^2)

I'm not bright enough to simplify that, let alone to solve for theta, so
I wrote an HP48 program that calculates x from l, r, and theta. I would
then try solutions using a binary search type algorithm (manually) to
home in on a value that yields .8mm.

For info I just ran a test case with stroke of 60mm and rod length of
120mm and came up with 11.869° for 8mm BTDC in about a minute.

If you post stroke and connecting rod length, I can give you an answer.

 

Turn the crank steadily in the normal direction of rotation,
and gear lash, etc. is not a factor.
Measurement of piston travel is a more accurate way of setting
timing than measuring crank rotation in degrees, provided that the
spark plug hole is parallel to the cylinder centerline.
It takes a hell of a trigonometry exercise to make a table
equating piston travel with degrees of crank rotation. The point of
diminishing returns is rapidly surpassed.

 

try this:http://www.scooterhelp.com/tips/timing/timing.calc.html

 

Thanks, but I already tried it. Their calculator won't let me input the
values for my bike, only for various scooters.

 

Can't turn it steadily because of resistance from compression and
springs.

Right, that's the problem. The spark plug hole is off-center in the
roof of a hemispherical chamber.

I want to be able to use a degree wheel and strobe so I can experiment
with various ignition timings. Anyway, I found a formula (yes, it's
complex, but that doesn't scare me) and now all I need is the length of
my connecting rod. Not easy to find for some reason, and I don't feel
like taking the motor apart to find out by direct measurement.

 

Uhhh.. had you ever considered removing the spark plugs ?
Rumor has it that this reduces compression greatly.