Help BOYER!!!

Help! Spent all weekend getting 850 MK2 back together. Bought bike recently and heard it running well. Took boyer unit off to change points oil seal and reassembled today. Bad news was there was no timing plate behind the inspection window in the primary chaincase. Good news is the bike has a belt drive so that £400 saved. The belts is 30mm wide so it looks like the timing plate has been ditched for clearance as the rotor is further out. Anyway there was a mark on the resin alternator ring that looked like TDC. With a screwdriver carefully down the cylinder we established it was TDC or near enough and I then measured the rotor and worked out that 20.5mm equates to approx 31degress BTDC and marked another mark on the resin alternator ring for the correct timing. Bike won't fire despite playing around with all the adjustment.

The question is whether the boyer fires when the magnets are in line with the coils (?) because it tells you to line up the centre of the magnet with the timing hole in the plate which is clearly well before the magnet aligns with the coil. ?????

Don't waste your time boys. Swapped boyer trigger unit wires and bike started first kick.....

Doh

I used to know all that Boyer detail stuff until I got to know all the Boyer detail stuff.
Your TDC is too crude to do anything but get in ball park to try to start. Make note of where the trigger plate is starting at, then advance in small steps till fires and runs then back off until easy starts w/o back fire then mark trigger plate and alternator. If that don't work then from initial trigger plate setting begin retarding in small steps till - same as above, then mark for future reference and someday can even put a time light on it and make more marks so never need timing light again but for fun. A degree wheel helps a lot in this. The piston stop method with hand held stop is fine for initial starting but might be a degree or 3 off from a more definite repeatable stop is all. BE Aware that any probe in hole is not being pressed on by piston at 90's but mostly from the side and can pinch or bend trapped in hole rather than pushed out as expected.
Much easier if you go to trouble of removing primary but I wouldn't either if not extra reason too.
Not the higher state of tune and set up the lower the full advance needed.
31' may not fit your case, but real road use will help diddle final timing.

hobot

Glad you got it running. However, if it were me, I would mount a degree wheel on the crank and using a positive piston stop determine the exact TDC. Then back the engine up about 40 degrees and rotate it forward to 31 degrees (or wherever you want max advance). Pull off the degree wheel and scribe a timing mark on the stator.

That way you can strobe time it accurately.