Newby help for Commando Points Cam

Hi all - newbie here requiring some assist:
got a 1975 850cc electric, and had to remove timing cover etc - removed points cam as per manual - but before I re-assemble - how the dickens? does the cam relocate on the taper?? does it go on in ANY position or what?? I'm more used to Jap bikes with proper fixings/marks. The points/cam plate was not disturbed, and I only fitted a new rear seal.
BTW when opened up there was no spring or cup for the new oil pump fixing - and looks as if the casing was never opened since it left the factory! so how did it manage 16,000 miles without major sumping - at least until I got it??????
Thanks folks.

The mechanical advance unit is the Main Primary Reason people will-must put up with the sluggish Boyer advance curve, because no one make the mechanical version new and they wear out - essentially un-fixable.

Best to have a degree wheel as cover dial can be off up to 12 degrees reported.
Find TDC via the back and forth turn to a stopper and split the difference, then put engine at max adv. timing, 850's @ 32' BTDC, then the mech-adv is locked by bolt/washer or just held in full adv. and placed on cam as close to right as ya can.

Set point gap on cam peak to .015". Then wire up trigger wires to correct coil-piston side and try to start and then diddle adv/retard till starts w/o back fire, ie; adv till is does then retard till just don't, then once warmed up, put time light on and then mark where ya like the results and then can record in numbers as an after thought.

I paint the correct alternator rotor line and the Alternator stator @ TDC and 30' BTDC as future reference. Its kinda hard to judge by time light as cam chain flap makes it a judgment call on center estimating the moving time line.

Hi Auto,
Sorry to say, but yes the AAU ( automatic advance unit) can be installed in any random position. Unfortunately only one position (maybe two) is going to work correctly. By trial and error (hopefully less error) you will have to work out your crank, AAU, and point plate postion in order to re-establish the correct engine timing.

GB

autoxtraz said:

removed points cam as per manual - but before I re-assemble - how the dickens? does the cam relocate on the taper??

Click to expand...

...Re-install "as per manual" (850 Mk3 manual, Section C39).

I'm sure the 850 workshop manual will have a picture of the advance unit installed showing the correct "approximate" position. The 750 manual does. Then it's a matter of adjusting the gap and setting the opening of the point with the correct side, with the AAU completely advanced, there is a special washer to keep it advanced, or you can make one out of a few washers and glue or tack weld them together. Like hobot says, it may be wise to check the TDC and know where your timing mark is for real, some are off 4 degrees.

Dave
69S

Factory dial if still present, has been reported up to 12 degrees off, so beware. In avatar photo that left R knee shattered and dangling by skin-hide only, that is when leg lifted only part above knee lifted while lower leg and boot stayed put on ground til skin stretched buggee'd it up to smack the living daylights out of me 1st time,

Got shattered-smacked half way to smithereens again by 1st kick Back*Fire - because during initial 3 wk recovery had time to put time light on Boyer to "correct my - gosh & by golly timing happy way - to set by instructions and dial indicator to 32' - Seven months gimping like Festus to ponder following instructions blindly and expecting it's correct in all cases.

Its also good lesion to reverse the trigger leads in elctro ign system - once.

Grease everything. Then grease again after 1000 miles. Bobweights ,cam,anything in the cavity that moves. Set gap on filed flat points, re-time. Points work fine, just regrease often ,re-time ,gap,lightly grease. Then 1000 miles later you can open up ,inspect and do it all again. Fun ,kinda zen like and very involving ,therapudic and time consuming. Ahaaaa....

A rather less complicated way of refitting the points cam, is to measure piston position on the compression stroke, setting this to the firing point. Then refit the points cam, with the points for that side cylinder just opening, with back plate in the centre of adjustment slots.

At the same time you can check whether the factory timing marks are accurate or not, and if so after setting points dwell angle, you can finely adjust the timing using a strobe light.

Its possible to measure piston position with something like a thin wooden dowel if you are very careful, but a piston position indicator of the type used on dirt bikes, which screws into the plug hole makes things much easier, and these dont cost much money.

Always did it with rod down plughole and get T.D.C. posn , then the right distance back .

Takeing time and a few coffes . Spot on timings first priority for well mannered performance.

As stated there , the points faces should mate perfectly flat .
If it erodes one side , the condensor rateing is out of kilter with coil.

Fitted a washer on end to lock unit to eliminate slop , fixed advance .
Half hearted swings on starter are unacceptable , particularly with
pull=back bars. The knee gets thrown into the handlebar end.

Good practise for appearing not to limp, and grinning and bearing it
for the rest of the day on restarts !

Was given a BSA 650 unit as he'd fitted Boyer ign. Luckilly the worn
bits were differant on both 6CAs , Gave 36 deg. retard from the 31 deg. fully advanced
and a 500 rpm idle , with Mk II Amal concentrics. Had heavier springs fitted . Which
held it of till maybe 3.500 rpm . So under 3.000 one could be careless with throttle without being spat off.

If you put a rod down the spark plug hole to find TDC, be aware how rapidly the piston will also trap, bend or break said rod when it comes to the top of the stroke! Somebody here had a story about breaking off a pencil and having to remove the head to clean it out.

With that warning firmly in place, you can use a stop tool and a degree wheel to find your proper advance. It is a good exercise.

Russ

For those who might have problems using a simple rod down the plug hole to measure piston position in the cylinder, its possible to make a simple piston position tool out of an old spark plug, using a length of rod fitted into this after the insulator has been removed.

Alternatively the TDC tool pictured here https://www.masclassics.com/vtwin/1226.php is pretty much the same thing, and only costs $26. Certainly accurate enough to check the factory timing marks, and means there is no need to mess around dismantling the motor to fit a degree disc to the end of the crank.