Checking timing marks

I have a ‘73 850 Commando with Boyer ignition. The bike is running rough and backfiring when hot. I want to make sure the ignition timing is right. Before I check with timing light I want to check that the timing marks are correct.
So if I use a CD size degree wheel attached on the ignition rotor in conjunction with a piston stop will this give an accurate enough reading to check my timing marks?
I believe that if using a degree wheel on this side the readings are half that of the crank side. Is this correct?

I would use a crank mounted degree wheel to eliminate any error from the timing chain.

And the bigger the timing disk the more accurate the readings will be.

kommando said:

I would use a crank mounted degree wheel to eliminate any error from the timing chain.

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Strictly speaking I'd agree with kommando, but the reality is that a cam mounted degree wheel is quite adequate for the purpose. I have a "kit" that I purchased years ago that was designed to screw into the cam; is available, I believe with 26 TPI and 24 TPI shafts, and it is calibrated in engine degrees, not cam degrees You can enhance the "accuracy" of this tool by turning the engine in the direction of normal rotation only, or if you go past your mark back the engine up substantially and approach the mark going in the normal direction of rotation.

Knowing where your timing marks are is a smart move.

I would never rely on timing marks. The timing of both cam and ignition are set in relation to crankshaft rotation. Setting ignition timing in relation to camshaft rotation is not smart. If the degree disc is mounted on the end of the crank, you are working with certainty. A couple of degrees error in setting the ignition timing on a petrol engine, can substantially change the jetting required. With my bike, after I have used the degree disc, I use a strobe light. To find TDC, I use the soap bubble method.

soap bubble method???

You put a soap bubble on an empty spark plug hole just before TDC, as you ten slowly move the piston higher the bubble will rise until TDC and then lower after TDC, adjust until the bubble is at its highest.

I & Wes had water in bad fuel cause roughness and miss fires like skewed ign. timing till cleaned out. Otherwise practical down/dirty obnoxious Boyer method is diddle time plate till sweetness then enjoy riding till weather interferes to put a time light &/or wheel on to see where it lands for your particular steed factory skewed marker or your own correct marks. If Boyer trigger wires not replaced/upgraded/investigated could be wasting our/your timing efforts till then. A loose/worn cam chain will also stymie accuracy protocols into the weeds. If more lazy than a proper procedure mechanic as hobot - way to find TDC > apply air in plug hole to force to BDC. Also can't forget main bane of Commandos, a bad power supply conduction path after heat and road vibes add up. Oh yeah too often after some poor tune, bad fuel, electrical/mechanical ign gremlins miss firing intervals the plugs get kind of fouled to further stymie until replaced to carry on. I've found out that float level setting or fuel flow through Amal tiny needle seat going lean to normal with splashing to act like over advanced spark.

No doubt the soap bubble method works fine,
But for me a positive stop (extended spark plug) you can not go wrong.
Rotate engine forward until it stops rotate backwards until it stops then split the difference set the degree disc to zero ,
Then start from there you may need to scribe a new line for strobe timing.
But as above all wiring and connections must be perfect with Boyer ignition, check pickup wires /connections/earth and battery.
Silly things can catch you out like dodgy spark plugs,dodgy coil or tank vent etc

Agreed, any system that uses TDC as the aiming point is suspect as at TDC the piston is hardly moving at all, a positive stop forward and back at say 30 will give you a more accurate TDC.

acotrel said:

I would never rely on timing marks. The timing of both cam and ignition are set in relation to crankshaft rotation. Setting ignition timing in relation to camshaft rotation is not smart. If the degree disc is mounted on the end of the crank, you are working with certainty. A couple of degrees error in setting the ignition timing on a petrol engine, can substantially change the jetting required. With my bike, after I have used the degree disc, I use a strobe light. To find TDC, I use the soap bubble method.

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Using a cam driven degree disk may not be prudent for someone that is racing their Norton and wants to be sure that their ignition timing is holding up its end to provide every available erg to get down those straights first or out of the corners first; after all, second place is first looser, right?. Using a cam driven degree disk in conjunction with a piston stop or the bubble method should provide a level of accuracy that is more than adequate for the street rider and is considerably less involved then opening up the primary. Consider that attaching a degree wheel to the crankshaft involves removing the vary scale you are trying to calibrate, which means that once you have established the crankshaft position for the advance you are seeking, you must then remove the degree wheel, which may turn the crank, re-install the cover and see what is indicated.

If you are not fortunate enough to have a specialized adapter that replaces the alternator rotor nut, then you are faced with removing the stator and rotor, degreeing the crank then replacing the rotor to check the timing marks calibration, high chance that the crank will move somewhere in that process. For those individuals that have the expertise to sufficiently attach a degree wheel to the crank, that will withstand not moving during engine acceleration and RPM to set timing, more power to you, that's why you race, I think we all get that. The average Norton owner isn't going to do this, can't do this.

The cam mounted degree disk is good step in the right direction and will give the owner some good data. I suggest that anyone doing this average the data from 3 or 4 trials, set their timing and then go on to worry about something else.

Hm baz et al, timing Norton is like controlling a tempest in a tea cup as the mechanics of the cam and drive on trigger mechanism is not nearly as stable as the Nth degree accuracy being taught. Real world is get close enough to start then diddle best ya can by what ever means to compromise/compensate for jiggle jossles and state of tune, then somehow mark were that lands and at your leasure rig up time wheel to get rotor mark indexed against stator marks or the always suspect factory gauge. Just be gentle sticking anything inside jug as piston hits it at good angle to mostly bend or break it in half rather than pushing out hole. I snapped eraser end of pencil off on seeming effortless turning doing this, had to fill with diesel to float and lift out, then suck out most the diesel. ugh.

The soap bubble method is so tricky I doubt anyone here tried it twice but gave me idea on a simple cheap TDC device. Could just fill jug with diesel and diddle to see level rise to max in plug threads, mark and suck out. What little diesel remains will not dilute oil and a few smoky firings clears jug film.

I agree the cam-mounted disk is a step in the right direction. But I'm nothing like a racer, did my calibration off the crank, and didn't have to remove the scale or the rotor or the stator. And I have no specialized adapter. Not bragging, just sayin it's not all that difficult.
I did have to replace my old rotor nut with https://andover-norton.co.uk/en/shop-details/16346/rotor-nut-06-0387- a fully-threaded version, thread a pc of 5/8 aluminum round stock with a 20 tpi die, print and mount a degree wheel (found at blocklayer.com), and make a piston stop from an old spark plug. All fairly straightforward, and kinda fun, too!

Your symptoms - running rough/backfiring when hot are not typical symptoms of incorrect basic timing. I'd look at the ignition secondary - plugs/wires first, then the Boyer itself, specifically the voltage it receives under load. A poor spark (Boyer, bad wires/plugs) will cause the problem you are experiencing. This normally occurs under load. Does the rough running/backfiring occur at a steady speed running down the road at, say 70 MPH or when accelerating , especially in the higher gears (worse in 4th at WOT than in 3rd)

As far as checking the timing marks, I agree that the proper way is with a degree wheel on the crank and piston stop. No other way is as accurate. OTOH, as has been noted, these are not Formula 1 motors. Frankly, I can't imagine that the OEM timing marks could ever be off enough to really matter as far as general operation is concerned.

What I'm saying is that IMO the problem is NOT in the timing marks though, of course, there is nothing wrong with checking them at some point. FWIW, the marks in my '73 850 were "off" by less than the width of the mark on the rotor.

kommando said:

You put a soap bubble on an empty spark plug hole just before TDC, as you ten slowly move the piston higher the bubble will rise until TDC and then lower after TDC, adjust until the bubble is at its highest.

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I learn something every time I get on here.

To be honest the only time you would really need to fit a timing disc is to set or adjust the cam timing.
It's nice to know if your rotor timing marks are accurate but depending on the fuel that is available to you you may want to back the the ignition timing a little after strobing any way,
As stated by others your problem lies elsewhere I reckon,cheers

baz said:

To be honest the only time you would really need to fit a timing disc is to set or adjust the cam timing.
It's nice to know if your rotor timing marks are accurate but depending on the fuel that is available to you you may want to back the the ignition timing a little after strobing any way,
As stated by others your problem lies elsewhere I reckon,cheers

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There are a few things going on with this bike. The carbs were out of sink and one cylinder was running really rich. I checked valve clearances. I wanted to check the timing to make sure that was OK. With the Boyer I refitted the rotor as per instructions and had to advance the stator to its full adjustment just so it would run. That’s why I want to check the timing marks before I time with strobe light.
I think the mech who did the work on it just had it running so it started OK and idled because he knew the owner wasn’t going to ride it. Makes me wonder about other work he did on the bike

Stuffa said:

Makes me wonder about other work he did on the bike

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And so it should.

There's a few about like that, I bought a t160 that supposedly was A1 running etc but I had to work my way around all the usual things to make it run right and just how I wanted it

Took me three days to get my Commando to start, purchased from a well-known US dealer in old Brittbikes who assured me the bike was "ready to ride cross country" when I bought it via phone/internet. When delivered, aong with being unable to start, it was unsafe to ride around the block for a variety of other reasons. So, though I believe your main problem is not timing mark error, I TOTALLY AGREE with thoroughly going through the bike. You might find things like:

Rocker arm spindles installed facing the wrong way (BIG oil burner)

Non functioning rear shock (ruptured seal so no resistance on one shock)

NO oil in front forks - a bit of tar in one and just rust in the other

Iso clearance nearly 3x the stock spec.

Tire sizes totally mismatched so handling was, well...SCARY!

Incorrect clutch pressure plate (750 plate in an 850)

and the list goes on, and on, and on...