From the factory these engines are, typically, set-up with the timing 2-5 degrees advanced...which is about 15 degrees retarded from ideal.
Interestingly enough the Chinese do understand that 20 degrees advanced is a good starting point...The crankshaft key slot is cut at 20 degrees from T.D.C...Unfortunately the magnet guy ran his broach through at, (you guessed it), approximately 20 degrees...which completely nullifies the 20 degrees of advance that the crank maker built in.
Jim
I have not pulled off the magneto armature ??? - but I had a thought, and this one is a good one.
A simple solution -
Make an OFF-SET Key.
How to - this is a little bit fiddly - going by the HUGE shaft and all...
Someone will have to fill in the dimensional omissions on this for me.
OK the KEY is a simple cheap and mechanically bad way to align two mating parts on a shaft - with repeatable accuracy.
It's BAD because it's a stress raiser - which causes shafts to break, and so it's only used on LOW power low stress, transmissions - i.e. engine shaft to magneto.
IF the magneto must be advanced by 12* - this will either be an easy or nearly / good enough solution.
I wish I had the dimensions... anda stripped magneto and key and it's shaft in front of me.
OK a key in terms of radians or degrees, occupies SOME space - and the magneto is held on by the lock of the taper.
One can forgo the use of the key, IF the shaft and the magneto are lapped to a dead accurate fit, washed with fuel, wiped with rags, washed with acetone, wiped with rag, primed with a chemical primer for the compound i.e. Loctite, and then wiped over with Loctite and promptly done up, keeping the magneto and the crank shaft IN the premarked and scribed alignment - probably by setting the piston at TDC and aligning the magnetio - with it's tiny center punched dimple (from you) with a tiny center punched dimple on a thick part of the engine casing.
But use a LOW strenght shaft / bearing locking compound.... and use a LOW heat to soften the bond, to remove the magneto.... and a mechanical puller...
This is ONE way.
The other way, which I cannot tell you the exact advance or the range of advance, because I do not have the dimensions or an accurate scaled picture in front of me to work from.
But off the top of my head, this will work...
Get 3 sets of keys - grind or file all three of them into half thickness, the hardest bit of this is holding them while you do it...
Get one key and grind the top of it off (the flat part), so it sits flush to slightly under the profile of the shaft, and then get the second key, and grind the bottom of it off, so it sits flush inside the magneto..
Now by using the third "half thickness" key - you can assemble the magneto to the shaft by using a half width half height key as a packet in the shaft, and a half width half height key as a packer in the magneto, and a half width and full height key in the remaining half keyways.
Again - hand lap the magneto onto the shaft and IF the advance is pretty right, use a thorough cleansing with clean rag, fuel, a wipe over, then a wipe with acetone, and then another wipe over, and then the primer and a low strength shaft locking compound...
(The magnetism may be an issue here - probably use an adhesive to hold the keys in the slots prior to assemmbly
Now since 12* advance is not a HUGE amount... in terms of offsetting the rotation of the magneto to the shaft, without the dimensions in front of me, I am making an estimated assumption.
It would be appreciated if SOMEONE could provide the measurements and draw up a simple diagram.
Half a key thickness advance MAY be spot on... I think it is, but it may be not enough or doubtfully a bit too much....
Best to check.
I actually use Libre Office DRAW for all of my engineering drawings...
It's very good.
So this is two more of several options to select from.
IF I had a gear puller though and I could do do up the magneto tightly into the shaft, I'd be selecting the keyless option, and aligning the magneto by TDC and punch marks, and then using the loctite option to cement the parts together on the tapered lock, between the lapped together parts.
