Calibration of ignition timing marks (2019)

Status
Not open for further replies.
johnm

johnm

VIP MEMBER
Joined
Feb 26, 2008
Messages
1,605
Country flag
The refurbishment of my 850 is slowly coming to the end and I just did a check which has been on my list for the past 4 decades.

There are several ways to do this butAndover Norton sell a simple disc marked up with TDC and 20 , 30 and 40 degree before TDC. Part no 13.1769.

You set the motor at TDC with a degree disc and then attach the disc to the rotor aligning the TDC marks. Then compare the timing marks on the primary cover to the disc. I have attached a photo. My marks are 2.5 degree out.

I’m pretty happy with this method and recommend it.
 

Attachments

  • Calibration of ignition timing marks (2019)
    2E3B3743-E82C-4038-864D-2507C754DAE1.jpeg
    94.3 KB · Views: 783
Getting close then! Taking it to Raukawa?

You will need a couple shake.down runs... let me know. ;)

Have to get mine to Fullerton for a WOF before xmas.
 
Still slowly working through wiring. Then carbs and exhaust. Then final settings in CNW electric start.

Then check oil etc all the way through. Chain tension and check of all bolts etc for tightness.

Then a test ride and WOF !!

I might be good by March for a ride up the Parapara
 
johnm said:
The refurbishment of my 850 is slowly coming to the end and I just did a check which has been on my list for the past 4 decades.

There are several ways to do this butAndover Norton sell a simple disc marked up with TDC and 20 , 30 and 40 degree before TDC. Part no 13.1769.

You set the motor at TDC with a degree disc and then attach the disc to the rotor aligning the TDC marks. Then compare the timing marks on the primary cover to the disc. I have attached a photo. My marks are 2.5 degree out.

I’m pretty happy with this method and recommend it.
Click to expand...

Sweet. Do you have any pics of setting up the test disc?

And does anybody know if the adjustable MK3 scale will fit a pre-MK3 cover?

Calibration of ignition timing marks (2019)
 
I usually set the ignition timing with a degree disc, then jet to it. I then avoid moving the ignition timing like the plague. Because if it changes, you need to re-jet the carbs, particularly if you usually jet lean enough to get best power.
 
I did a post called "Linear Piston Travel and Advance Degrees." Below is the chart. It is compensated for the spark plug hole angle. I have a dial indicator that screws into the spark plug hole. The tip is ground into a rounded ball which slides across the piston easily. I find TDC by turning the rear wheel in high gear until the dial indicator is at it's peak. This method allows you to find thirty one degrees or twenty and any in between without taking off the rotor or the primary cover. Just set the piston .362 inches (9.205mm) before top dead center and run around the other side to see if your mark is correct on the primary cover degree markings. What to do it is is off is another story and I would try it several times to make sure the dial indicator is repeating accurately.

31°= .320" (8.127mm) BTDC (Before Top Dead Center), 0.362” (9.205mm) (compensated for the 62° spark plug hole angle.)
30°= .300" (7.630mm) BTDC, .340” (8.642mm)
29°= .281" (7.147mm) BTDC, .319” (8.095mm)
28°= .263" (6.678mm) BTDC, .298” (7.564mm)
27°= .245" (6.224mm) BTDC, .278” (7.049mm)
26°= .228" (5.784mm) BTDC, .258” (6.551mm)
25°= .211" (5.359mm) BTDC, .239” (6.070mm)
24°= .195" (4.949mm) BTDC, .221” (5.605mm)
23°= .179" (4.554mm) BTDC, .203” (5.158mm)
22°= .164" (4.174mm) BTDC, .186” (4.728mm)
21°= .150" (3.810mm) BTDC, .170” (4.315mm)
20°= .136" (3.462mm) BTDC, .154” (3.921mm)

The figures in GREEN are corrected for the 62° angle of the spark plug hole. When the gauge measures 1 inch of travel the piston will have moved .8829" down so the gauge needs to read more than the actual piston travel to give you the actual piston travel. According to Jim Comstock the crank offset he has measured was only .5 to 1.0mm on 750 and 850 Commando engines which is not much. And, of course, if the piston is domed or dished the dial indicator will be hitting a different part of that dome or dish at the different distances from TDC as it moves across the piston because of the 62° angled spark plug hole.

(One small disclaimer: If you are running rods of any length other than stock then these numbers will not work for your engine. )
 
Actually my bike does have longer rods but thanks for the info anyway
 
No photos but it is easy.

Set the motor to TDC using a degree disc.

Attach the small disc from Andover to the rotor with the double sided tape supplied aligning the TDC marks.

Put the primary cover back on and compare.


maylar said:
Sweet. Do you have any pics of setting up the test disc?

And does anybody know if the adjustable MK3 scale will fit a pre-MK3 cover?

View attachment 13617
Click to expand...
 
I don't really see where you are coming from with this comment.

One of the simplest things on a Commando is setting ignition timing because you have a built in degree disc in the outer primary case -
provided you know the degree marks are accurate.

This simple method establishes if it is accurate plus any correction that may be necessary.

Jetting is a related issue but has nothing to do with checking the accuracy of the degree marks on the primary cover.


acotrel said:
I usually set the ignition timing with a degree disc, then jet to it. I then avoid moving the ignition timing like the plague. Because if it changes, you need to re-jet the carbs, particularly if you usually jet lean enough to get best power.
Click to expand...
 
motorson said:
I did a post called "Linear Piston Travel and Advance Degrees." Below is the chart. It is compensated for the spark plug hole angle. I have a dial indicator that screws into the spark plug hole. The tip is ground into a rounded ball which slides across the piston easily. I find TDC by turning the rear wheel in high gear until the dial indicator is at it's peak. This method allows you to find thirty one degrees or twenty and any in between without taking off the rotor or the primary cover. Just set the piston .362 inches (9.205mm) before top dead center and run around the other side to see if your mark is correct on the primary cover degree markings. What to do it is is off is another story and I would try it several times to make sure the dial indicator is repeating accurately.

31°= .320" (8.127mm) BTDC (Before Top Dead Center), 0.362” (9.205mm) (compensated for the 62° spark plug hole angle.)
30°= .300" (7.630mm) BTDC, .340” (8.642mm)
29°= .281" (7.147mm) BTDC, .319” (8.095mm)
28°= .263" (6.678mm) BTDC, .298” (7.564mm)
27°= .245" (6.224mm) BTDC, .278” (7.049mm)
26°= .228" (5.784mm) BTDC, .258” (6.551mm)
25°= .211" (5.359mm) BTDC, .239” (6.070mm)
24°= .195" (4.949mm) BTDC, .221” (5.605mm)
23°= .179" (4.554mm) BTDC, .203” (5.158mm)
22°= .164" (4.174mm) BTDC, .186” (4.728mm)
21°= .150" (3.810mm) BTDC, .170” (4.315mm)
20°= .136" (3.462mm) BTDC, .154” (3.921mm)

The figures in GREEN are corrected for the 62° angle of the spark plug hole. When the gauge measures 1 inch of travel the piston will have moved .8829" down so the gauge needs to read more than the actual piston travel to give you the actual piston travel. According to Jim Comstock the crank offset he has measured was only .5 to 1.0mm on 750 and 850 Commando engines which is not much. And, of course, if the piston is domed or dished the dial indicator will be hitting a different part of that dome or dish at the different distances from TDC as it moves across the piston because of the 62° angled spark plug hole.

(One small disclaimer: If you are running rods of any length other than stock then these numbers will not work for your engine. )
Click to expand...
Interesting. Is there any concern about differences in head gasket thicknesses being used, different states of head bolt torque, or even carbon build up on pistons?
 
Tornado said:
Interesting. Is there any concern about differences in head gasket thicknesses being used, different states of head bolt torque, or even carbon build up on pistons?
Click to expand...

Measure the thickness of a line on the scale and then look at the differences between adjacent degrees in piston heights, even at their greatest 30 to 31 degrees is 20 thou or 0.5mm.
 
Interesting. Is there any concern about differences in head gasket thicknesses being used, different states of head bolt torque, or even carbon build up on pistons?
Click to expand...
You are measuring piston travel from TDC. The thickness of your gasket, torque of the head and even carbon on the piston will make no difference. (except to make the surface of the piston rough) The formula for piston travel in relation to crankshaft degrees is available on line. The length of the rod and the stroke of the crank are the main variables. For a different length rod a new list of distances from TDC would have to be generated. My compensation for the spark plug angle was simple trigonometry. Here is a link to one free degree calculator: http://dansmc.com/mc_software2.htm
 
It does not matter if the ignition timing is slightly wrong, as long as it never changes - because you jet to it. The two are interconnected - within limits, advancing the timing is similar to leaning off the jetting. Normal practice is to set the timing, then tune the carbs.
Because you set your engine up according to the book, that does not necessarily mean it is right for the fuel you are using. With fuels, sometimes the manufacturer specifies the amount of advance which can be used, but usually we don't bother to find out what difference is recommended. Getting the carburation right is more important than a minor difference in ignition advance.
 
Last edited:
One of the first bikes I ever owned was an Indian Scout. It had advance and retard on the right hand twist grip, and an adjustable jet on the Schebler carb. You could change them both as you rode along.
 
I hope this is relevant. I use a piston stop to find TDC and then roll the crank back 30 degrees. Unfortunately there are no marks on the rotor anywhere near the 30 degree mark on the timing plate mounted on the timing cover. The nearest one shows an estimated 50-60 degree advance. This is a reassembly of a abused and battered machine, however I am sure the key is in the rotor. What I do is mark the rotor at the 30 degree mark on the timing plate when I degree wheel the crank to 30 degrees advance. However when I start the machine the strobe shows 40+ degrees advance and I have to use the full adjustment on the pickup plate to get the advance to +- 30 degrees. Boyer Brandson ignition, set up as per mfg instructions. (probably 25 years old). Engines runs but I am wondering if I have an error in thinking regarding my method.
 
Method sounds fine as long as the rotor is marked wrongly. I find Boyers are normally too advanced when set statically and then strobe checked, my worst was 8 degrees too advanced. You will need to break the boyer rotor off its taper in the camshaft and rotate it so the stator plate has more adjustment.
 
David Schandall said:
Unfortunately there are no marks on the rotor anywhere near the 30 degree mark on the timing plate mounted on the timing cover.
Click to expand...
1973 750 I believe?

With the crank correctly set then under normal circumstances the scribed line on the raised pad furthest from the key slot is the correct timing mark, therefore, should be somewhere close to the 30 degree mark with a second (unused) raised pad and mark 180 degrees from that.
Calibration of ignition timing marks (2019)


David Schandall said:
The nearest one shows an estimated 50-60 degree advance.
Click to expand...

The rotor doesn't happen to have a machined face with 3 timing marks 120 degrees apart (or 3 marked 'A' and 3 marked 'B') as in the picture below by any chance? If so, then the rotor has the wrong side facing out?

Calibration of ignition timing marks (2019)
 
Status
Not open for further replies.
Back
Top