Atlas 66 Magneto setting

Setted magneto 35 degree.
I know, is a little bit advanced but i thought that with the stock concave pistons the bike has only 7.5:1 compression ratio and the fuel used here (named green because without lead) is very poor of octanes.
Ciao
Piero

Listen for pinging (pinking) when you open the throttle and make a load on the engine.

Even with 7.5 CR, 35 degree magneto setting may be too much with poor octane fuel.

Slick

35 is too much, 7'5 : 1 requires 32.
If it pinks on your poor octane fuel, then use a octane booster, and avoid hard acceleration if it pinks or back off if you detect engine knocking sound.

What is the theory behind advancing timing for low compression? 32 degrees was the stock advance for the low compression pistons and magneto when the bike was new. While the twin engine was designed to run on really low octane "pool" petrol, by the 1960's in the US we had easily available high octane fuel.

Bodger said:

What is the theory behind advancing timing for low compression? 32 degrees was the stock advance for the low compression pistons and magneto when the bike was new. While the twin engine was designed to run on really low octane "pool" petrol, by the 1960's in the US we had easily available high octane fuel.

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There was/is a manual advance/retard available for both the Lucas K1F & K2F magnetos, as soon as you hit the "sweet spot" for the engine you are testing your ing timing on cut the engine, leaving the A/R lever alone, then you can set up a large diameter timing disc , the bigger the better, and take your reading from there. Once set up , then you know you are in the zone.
Of course, it can change if you go to a lower/ higher octane rating gas/fuel.

Resetted to 32 degree but this time i have had some problem to tight the AUU wthout move the set.

And make sure you get to point looking the right way around as the plug lead next to battery box is the one for the drive side Or Left-hand side, The points will look upwards and the heel will be on the bottom of the cam ring to set point you must remove rocker covers to find the stroke your on when you have found TDC on the left side with all valves closed on that side then fit your timing disc at TDC and next fit a bit of stiff wire for a pointer I use the back stud on the cylinder base nut to fix a pointer too then with a spanner on the crank nut turn your engine back to 32% before TDC full advanced and with the ATD witch should be opened and a small bolt to keep it open you then find were the points just open and close with your timing light and hold it there and do up the AutoTiming Device bolt it has a double tread when you have got this done then you can check it against the timing light you should have the firing point at 6% after TDC then fit things back as they where

With a mag always check the timing is the same on both cylinders. Not uncommon for them to be 10 plus degrees difference.

And I agree I wouldn't go more than 32 degree.

I experimented a lot racing 500 Dommies with 10.5 to 1 and 97 octane fuel and ended up at 28 degree advance.

Observing other guys I decided most people seem to use too much advance and compensate by going very rich. My 500 did 51 bhp on the dyno and completed over 120 races with no DNFs so I'm pretty sure I was pretty right on the advance.