Forum

[Westyboy]

Hello forum,
Is there a method to calculate the ignition timing from fractions of an inch (btdc) to degrees?

I have recently replaced the contact points in an 18D2 dizzie and fitted an electronic trigger unit, this signals a twin output coil (doing away with the rotor arm in the dizzie).

I noticed other makes of twin (Triumph/Norton etc) give timing in degrees but Enfields say 1/32" btdc @ retarded or 3/8" btdc advanced etc.
Bike by the way is a 1960 Super Meteor.

Thanks Westy

[Norm]

Hi Westy, 28 degrees will be very close, mark the rotor and stator with white paint, timing light and bring it up to revs and you can set the timing accurately

[Colin F]

You can do it if you know the length of the conrod.Draw a circle 90mm in diameter
(stroke)then add a straight line the length of the conrod up from there.Now come down 3/8"
and draw a line that intersects the circle dia line.And that's the degrees you measure off
with a protractor.
cheers
colin

[Norm]

Hi Colin, Twin rod lengths are all the same 6.125 inches

[Colin F]

Hopefully Westyboy will share with us the figures he comes up with.I've allways just
used a stick down the plughole.
cheers
colin

[Scalyback]

Somebody already wrote a nice application to do this, just convert 3/8 inches and 1/32 inches to mm, and your away!




timing mm to degree converter


(Thanks EXILE! for your teachings!)

[Scalyback]

OOOPS,



timing mm to degree converter



or...in old fashioned copy and paste...

[Les H]

Hi. Here is a nice calculator in graphic form: (note the other calculators in the main menu list on the left)
http://www.torqsoft.net/piston-position.html

You will also benefit from the Imperial to Metric converter:http://www.euronet.nl/users/grantm/frans/length.html
(The Imperial units go in the boxes near the centre....click on the black background for the calculation)

Also note that quoted timing figures are just a good safe setting. Better results can be found by small adjustments from this, (set advance just below engine pinking on high load with large throttle opening) but on a low tuned engine the gain is fairly minimal but worth doing if you can be bothered.

[Howzie]

[Howzie]

Ignition Timing Reference Pointer 1959 250cc Royal Enfield Clipper

The information below is only intended to be of help to anyone

Make a reference pointer (plastic lid) & attach to the stator wiring cable clamp as shown in photo.
At T.D.C. on the compression stroke make the first mark on the rotor lining up with the reference pointer.
The second mark (to the right of T.D.C.) will be for 1/64” or 5 degrees for static timing.
The third mark (to the right of T.D.C.) will be for 7/32” or 30 degrees for advanced timing.

To work out the ignition timing try this, multiply the rotor diameter by 3.142 (Ï€)
My rotor diameter was 70mm x 3.142 = 220mm (circumference of the rotor)
Next divide 220mm by 360 (degrees) = 0.61mm.
Therefore one degree of the rotor circumference = 0.61mm.
Static timing needs 5 degrees so multiply this by 0.61 = 3.00mm.
Transfer this 3.00mm to the right of T.D.C. and that’s your static timing mark.
Fully advanced needs 30 degrees, so multiply this by 0.61 = 18.3mm.
So your last mark wants to be 18.3mm to the right from T.D.C.

When using the manual they ask you to turn the piston back 1/64” from T.D.C. this is ok if you have the cylinder head off, not so easy when the engine is fully built & you do not any experience or have dial clocks / callipers etc.
This is not as difficult as it looks, read it over a few times till it sinks in, the main thing is to make sure your T.D.C. is spot on! Do not go over T.D.C. (Try Hitchcock’s T.D.C. tool)

This works on my Clipper no problems at all.
Alter timing measurements to suit different machines.