The "advanced" piston position is given as 7/16" BTDC for both engines IIRC.
However, due to the stroke length difference, the actual crankshaft position will differ. The following figures assume centered wrist pins.
M20 (94mm stroke) 36.5°
M21 (early, 105mm stroke) 34.1°
M21 (late, 112mm stroke) 32.8°
Any more information on this? Since rebuilding my engine to M21 spec and setting the timing at 7/16", I have found that the engine pinks under load, necessitating constant retarding at the lever. Ron
The M21 has several things going on at once:
1. smaller rod angle may (not always) need more advance due to faster piston motion ATDC
2. higher compression ratio usually needs less advance
Obviously, they're not going to cancel out except by coincidence.
What's your piston to head clearance @ TDC?
The next thoughts are the obvious (and not specific to the M20/21).
1. obstructed exhaust system (damage or carbon build up)
2. accessories in front of the engine (scabbard, tool kit)
3. reduced oil delivery
4. heavy paint on barrel and/or cylinder
5. high driveline load (tight chain, low tire pressure, wheel alignment, tight hub bearings)
6. plug is too hot for the conditions
7. weak mixture (adjustment or manifold leak)
The obvious test here is to lower the needle clip 1 notch (raising it in the needle jet), but even if it helps it may ruin the mileage.
Which carby?
