timing - degrees to mm

[ajh]

Hi, anybody have a conversion of degrees to mm BTDC for ignition timing? I'd liek to check my timing using a dial gauage and measure BTDC

My gs550e has been retrofitted to points and so Im not sure what timing marks are on the advance governor

I think i should be using the later settings of 15degrees and 40 advanced but I suspect my marks may be for the points bikes of 17 degrees 37 advanced.
I recall when I got the bike I had to modify the points plate so I could get the timing marks to even line up and the bike to run well. (It still feels like it needs a touch more advance at higher revs so the wrong advance governor might explain this as given the specs, it seems to have 5degrees more advance)

Thanks

 

[zed1015]

You just need to divide the stroke by 180 and that will give you AVERAGE piston travel per degree BUT because the crank and rod travels in an arc you get less movement per degree toward the top and bottom of the stroke so it's not a practical way to calculate your ignition timing.
You are better off using a degree wheel on the crank to confirm what you have and then mark your timing plate with the ones you want to run at if different..

 

[Brendan W]

This is a bit messy but should work. You need to know the con rod length. The throw is just half the stroke.
d - crank throw mm
l - con rod length mm
a - angle from TDC
h - distance below TDC mm

h = l - sqrt(l^2 - (d sin a)^2) + d(1 - cos a )

example
d - 28mm
l - 100mm
a - 10 degrees

h = 100 - sqrt( 100^2 - ( 28 sin 10 )^2) +28(1 - cos 10)
h = 0.543 mm

 

[bitzz]

https://lambretta-images.com/tuningh/port-timing-calculators/degrees-to-mm-timing-calculator/

 

[Brendan W]

https://lambretta-images.com/tuningh/port-timing-calculators/degrees-to-mm-timing-calculator/

That's cheating

 

[ajh]

thanks all for the replies, I knew all the theory but was being a little lazy and hoped someone had crunched the numbers as I dont know the the conrod length, I dont think its in the manual? I think as Rob says I might invest in a degree wheel

Is my logic right in thinking the later advance unit gives 5 degrees more travel? If so I think I'll get a later unit and go to a Dyno S system

 

[Brendan W]

thanks all for the replies, I knew all the theory but was being a little lazy and hoped someone had crunched the numbers as I dont know the the conrod length, I dont think its in the manual? I think as Rob says I might invest in a degree wheel

Is my logic right in thinking the later advance unit gives 5 degrees more travel? If so I think I'll get a later unit and go to a Dyno S system

A google search will throw up degree wheels but not con rod verniers
Never seen it in a manual. I checked the 850 and couldn't see it anywhere.

 

[ajh]

degree whelel from a quick look online.

degree whelel from a quick look online.

degree whelel from a quick look online.