Chainsaw Repair

Flywheel Tape
Will allow you to get port timing and ignition timing numbers without using a degree wheel.

This is nothing new in the car world for finding full advance timing.  Since I have to do this for a car to set the advance curve up where there are not enough lines, thought the process would be adaptable for two-stroke things.

Tools: string, or shoe lace or wire(wire is best does not stretch);  1- foot school type ruler (metric showing mm works best);  stiff wire (used for a pointer);  pen or pencil;  calculator;  painters tape. 

Need to get the circumference of the flywheel first:
Method 1:
1.  Wrap a wire tightly and evenly around the flywheel.  Mark or bend location where the ends touch.
2.  Remove the wire, and measure its length with the metric ruler.  Done.
Method 2:
1.  Measure the diameter of the flywheel using calipers.  (Choose English or metric way below).
If using English unit calipers: 
1.  Multiply this diameter measurement by 3.1415926.
2.  Then multiply by 25.4 to get the measurement into millimeters.  Done.
If using metric unit calipers:
1.  Just multiply the diameter measurement by 3.1415926.  Done.

Using the circumference measurement in millimeters no matter which way you found it:
1.  Divide it by 360.
2.  Answer is just how many degrees equal 1 mm.
3.  Find mm openings as shown below.
4.  Then multiply opening mm by your answer (in #2) to get opening degrees.

Get a length of painters tape that will go around your flywheel ie. Length = circumference length.  Suggest going a bit shorter than this length.

Stick the tape onto something flat.

Lay the ruler on top of the Tape and draw a straight line the length of the tape.

Mark a Zero point on the straight line drawn on the Tape.

Then lay the ruler upside down next to the straight line to draw all the rest of the tick marks (mm) on the tape.  Write in the numbers every 10 mm or so on top of the tick marks.

Take the stiff wire that will be used as a pointer and attach it to a bolt on the motor.

Find TDC.  Mark the flywheel at the pointer.

Affix the Tape all the way around the flywheel evenly, with the Zero lined up to the mark you put on the flywheel with numbers getting larger in the direction of spin.  (Facing flywheel, Zero on top, 90 to left, 270 on right).  Align pointer to the Zero mark at TDC.  Done.  Ready to find openings in mm.


Example 1: 

Flywheel diameter = 4.125 inches.

Circumference = 4.125 x 3.1415926  = 12.99906969 inches

12.99906969 inches x 25.4 = 329.1603702 mm

329.1604 / 360 = 0.91

therefore every 1mm on the tape = 0.91 degrees.

So, if the port starts to open at say 104 mm.......

Multiply 104mm by 0.91 to get degrees.

104 x 0.91 = 95 degrees.


Example 2:
if you want a port to start to open at say 95 degrees......

Divide 95 by 0.91      (0.91 found from above)

95/0.91 = 104 mm mark on the tape.


hope it helps
-joe

Way back before I retired, I used AutoCad on my work computer to generate 'timing tapes'.  Print out full size (1:1).

A BB Chebby likes 34-36 degree advance all in and all done.

You want that to happen by 2400-2600 rpm.

You will have transitional spark knock under heavy load with crap gas.

Downshift!

Quote from: 67L36Driver on July 19, 2016, 09:52:03 am

Way back before I retired, I used AutoCad on my work computer to generate 'timing tapes'.  Print out full size (1:1).

A BB Chebby likes 34-36 degree advance all in and all done.

You want that to happen by 2400-2600 rpm.

You will have transitional spark knock under heavy load with crap gas.

Downshift!

that was a great idea! 

do you need a plotter?... or would a regular printer do real 1:1 ?

think i have a 2D program on a desktop pc.

We had plotters for a long time.  36" wide if I remember correctly but then got a like size printer.  I suppose a desk top printer/scanner/copier should do.

Just check the results with a dial caliper.