(repost by) Steve Ohara Mc timing method

[David Luciani]

Here is how you set up the ignition without a degree wheel, dial indicator, protractor etc. Start with a five degree flywheel and an un-modified coil. Install the flywheel and then install the coil using a .012 spacer between the coil legs and the section of the flywheel with the magnets. With the coil leg screws slightly loose, rotate the flywheel/coil (they are stuck together by the magnets) in the clockwise direction to take up all the slop in the coil mounting slots and tighten the screws. Remove the shim and hook up your simple continuity tester for a timing light and check the timing by comparing the position of the raised line on the flywheel between the two magnets with the center leg of the coil. Adjust the points to cause the timing light to go off/on when the line on the flywheel lines up with the left edge of the center leg of the coil
For 99% of all Mac engines starting with the Mac 90/100 models and later, that setting is going to be 24 - 25 degrees BTDC if checked with any of the more precise tools. It is possible to get a goofy part from time to time... I have about 60 Mac flywheels here and I've probably checked another 40 over the last ten years and I have only found one factory screw up... I have a 9 degree flywheel and the key slot has not been messed with.. It is just a bad part.
I've played with static settings from 20 to 30 degrees btdc and have never found anything that works better than the 24/25 setting I described above. I have gone to great lengths to find ways to get a few ounces more spring tension but it was a waste of time too.
I usually get around 16 ounces out of the 91 style points and up to 20 from the 101 type. Both of those settings are plenty. Make sure to sand the point contacts to have a good clean contact across at least two thirds of the surface and you are good to go.
Use an Autolite AE901 or equivalent spark plug if you can find one... I think the current number is 411.
If you stick with the system I just described you will never hurt your motor with an ignition based tuning problem nor will you ever give up any meaningful performance gain.
Now... go work on the carb... that is where all the magic is anyway
Steve O'Hara

[David Luciani]

    Your timing light should turn off/on when the piston is in the position desired.
At that point the contacts will still appear to be touching.
Using a feeler gauge is the least accurate method as it does not account for the relative position of the side plate to the block.       You can push the side cover all the way clockwise and set the points with a feeler gauge and then check the timing, then loosen the screws and move the side plate all the way in the opposite direction and tighten it down and re-check the timing without touching the points and the timing will be different.
  I stopped using degree wheels, dial indicators, feeler gauges etc decades ago after discovering that the static timing didn't have any effect on performance unless you get way off in one direction or the other. Here is how I set mine.... I assemble the motor and take up the slop in the side plate, if any, by pushing it in the clockwise direction before tightening the screws. Next, I install the flywheel with the key and install the coil with a gap of .012 -.015. The slop in the coil should be taken out in the clockwise direction before tightening the screws.
     Once the coil and side cover are in the proper position I set the points to break the circuit at the point where the long raised line on the flywheel lines up with the left edge of the center leg of the coil. The alignment is not real critical... the full thickness of the line can be all the way on or off the left edge and you'll be in good shape. If you end up at the center of the middle leg you'll be at or close to 30 degrees advance and get into trouble with detonation at low engine speeds.
  If you set it say .100 to the left of the left edge of the center leg you'll be down around 23 degrees and you may feel a little less power if you have a low engagement speed clutch or run direct drive. I ran my 101 on alky at Adams in February with the line about .050 to the left of the left edge. Why did I set it there..... I got tired of chasing the damn thing too far left, too far right, too far left..... you get the idea When I got it within .050 I decided odds were the next change was going to be for the worse!.
   In conclusion, if you have a lot of free time to kill go ahead and set up the instruments and get fussy.... if you want to move on just use the timing marks and stay conservative ( stay away from the middle of the center post) and your motor will run great.         One last thing.... always clean the points before you set the timing. I use a piece of 600 or higher grit sand paper and fold up a piece about 2" long and 1/4" wide with grit on both sides. I turn the crank to open the points and insert the sand paper tool and then turn the crank to close the points and pull the paper out using the point tension as the pressure source. Repeat several times and then blow all the dust out.
    When you check the timing with a continuity light you want to see the light turn on and off decisively at the point of contact. If your light starts coming on at less than full intensity it is a sign you points are dirty or don't touch evenly and you should keep sanding until they give a nice bright, sharp on/off signal.
      Last but not least.... keep the point tension up around 20lbs. if possible. Some of the points floating around for the 91 series were hard to get more than 12 to 16 and if you have them it would be a good idea to junk them. Also, I have seen some pretty butchered up point springs over the years..... if you have to mangle the point spring to get the tension you'll never get the points to meet square and consistent.
That's about all there is on the Mac ignition.... simple stuff unless you want to get into the real trick stuff????
Regards,
Steve O'Hara

[David Luciani]

It's a good idea to use a thin film of sealer on the intake manifold gasket, stuffer gasket and the surface between the side cover and the block where the large o-ring groove is located. I use RTV as it is easy to remove when doing rebuilds. You can also use the spray on copper coat sealer on the head gasket. I don't recommend that you use any sealers on the gaskets above or below the reed cage as there is too much chance for blockage of the pulse holes to the carb.
Regarding setting the timing.... you can try the method I use.... here is how I do it.
Install the points and run the screw down till it is snug, then back it off just enough to allow the metal plate with the slot to be movable with light force and move the plate to a position that creates a large gap in the points. Rotate the crank around until the plastic arm of the points is sitting on the highest point of the cam on the crank then insert a .016" feeler gauge in between the points. With the feeler gauge held in place with the left hand, use your right hand to push the contact on the metal plate over to contact the feeler gauge and continue pushing enough to see the cam follower lift off the cam. Once you see that you have made the cam follower lift off the cam just a bit, gently release the assembly with your right hand and let the point spring tension return the cam follower back to contact with the cam and then use your right hand to tighten the screw. The process is easier if you have a small feeler gauge that allows better "feel" in the left hand while the right hand does all the work.
I like to use a 1/4" wide by .016" feeler gauge since it is pretty flexible and you can't influence the process much with sloppy work by the left hand. Once you have them set you can check to see where you are with a degree wheel or by measuring piston position relative to TDC.
On your Mac 91 25 degrees BTDC is equal to .100" from the top. Each degree plus or minus from that position is approximately .008" so .116" will give you 27 degrees and .084 would be 23 degrees.