Ignition advance...

It is not silly to strobe your ignition system after you have worked on your motor. A very slight change in the ignition advance can have major effect. The actual value is not important. What is important is that it always stays the same, because you jet to suit the advance curve. If you use methanol fuel, you can get away with a lot more error. That is the reason most race bikes are much faster when methanol is used instead of petrol. I never have to re-jet if the weather changes.
Some of you guys probably need to race a two stroke for a while, they are ten times as bad. - In fact, I race my Seeley 850 as though it is a two stroke. I even feed the throttle on.

For max power a vacuum advance is of little or no use. For part throttle low load cruse fuel economy they work great. A typical wedge type V8 engine may make max power at 35 to 38 degrees (28 to 35 degrees on a Hemi) advance from about 3,000 RPM up. At steady cruse, lite load with a vacuum advance plus mechanical advance and initial static advance may be near 50 degrees. Combined with the carb being leaned out, off the power circuit, fuel mileage is improved. Operating without a vacuum advance we are giving up some fuel economy but not power.

rweb is right, There's a big advantage to vacuum advance when it comes to light loads and gas milage. I tried removing the vac advance on a customers V8 Chevy and the gas milage dropped by about 1/3.

A lot of tuning depends on the intended use of the vehicle. As does the handling and gearing. With road bikes, the manufacturer probably makes decisions on the size and type of the potential market. It is common in most industries to move forward while looking backwards. What has been done already is good to use as a basis for making what will be popular in the future ?

re. ignition timing, the manual says 28 BTDC, but the Boyer instructios say 31 BTDC. What should I set it to, and why is the Boyer different? it has been 31 BTDC for a long time. This is by strobing it using the marks on the alternator rotor and the scale in the primary case, how accurate is that anyway?

daveforty said:

re. ignition timing, the manual says 28 BTDC, but the Boyer instructios say 31 BTDC. What should I set it to, and why is the Boyer different? it has been 31 BTDC for a long time. This is by strobing it using the marks on the alternator rotor and the scale in the primary case, how accurate is that anyway?

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You need to verify the timing scale in the primary cover before strobing, they're often out by a couple degrees. Get yourself a timing disc and piston stop.

I can draw a degree disc on a computer. If I use a degree disc to position the engine to 28BTDC, then replace the primary cover then I will see what the scale indicates and compensate for the error. But is 28 degrees correct? why does Boyer suggest 31 and should I go for that?

Boyer is timed at 31 BTDC, whereas AAU is 28 BTDC, as are TriSpark and other digital systems

As far as "why" boyer is 31? Other more knowledgeable folks will have to chime in on that

The Elektronik Sachse is set up at TDC and when I played about with the settings available the bike was fastest at 28 degrees full advance but was slower when it was set at 30.

The Boyer was 31 degrees because after Dyno testing that was when it worked best. Norman White was the source of this information.

auldblue said:

The Elektronik Sachse is set up at TDC and when I played about with the settings available the bike was fastest at 28 degrees full advance but was slower when it was set at 30.

The Boyer was 31 degrees because after Dyno testing that was when it worked best. Norman White was the source of this information.

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That's interesting. Norman White was probably considering race bikes.

I had a look at the graphs of advance curses dynodave and others have published. My bike has a Pazon and if I look at that graph full advance is reached at about 5000 rpm. At 3000 rpm the advance is about 2.5 degree less.

So if we timed at say 32 degree full advance for 5000 and above we would get about 29.5 at 3000 rpm which is toward the bottom of where I would think most people ride. I find my bike goes smooth at 2400 rpm and 3000 to 4000 is where I ride most of the time. So the advance would be in the range 29.5 to 31 most of the time.

Years ago I did lots of dyno tuning on a 500 Dommie with an SS head and indeed did find within a 28 to 32 advance range horse power was basically unaffected. Although under full race conditions with 10.5 CR more than 32 C advance started to give the tell tail black pepper specks on the plug white electrode. This early warning sign of detonation was highlighted by Gordon Jennings in his papers written in tge 1960s and 70s. I really rate his work. I ended up running that 500 at 28 to 29 full advance on NZ 98 pump petrol and 31.5 full advance on methanol.

Now the 500 has a smaller bore than an 850 and maybe it would like a bit less advance than a bigger bore. CR does change this too. More compression less advance.

In conclusion.

I do think it is worthwhile calibrating the primary cover timing marks and then strobing the bike to the ignition manufacturers recommendation.

Their curves seem to match the solid reliable performance range for ignition that I dyno tested on a 500 Norton SS head. Ie 28 t0 32 degree full advance. These heads are very similar to Commando heads and any difference apart from bore really comes more from piston crown shape.

Yesterday I clocked my crank. It was 2 degrees retarded using the factory mark. Probably better than two degrees advanced.

daveforty said:

I can draw a degree disc on a computer. If I use a degree disc to position the engine to 28BTDC, then replace the primary cover then I will see what the scale indicates and compensate for the error. But is 28 degrees correct? why does Boyer suggest 31 and should I go for that?

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Once you’ve confirmed the degree marker, try both settings and see what the difference is.

You‘re talking about 3 degrees difference... IMHO your Norton won’t even notice the change from one t’ other...

As has already been claimed by some, Boyers actually do keep advancing beyond 5k, but have a relatively slow advance curve, which basically means that whatever you set it at is gonna compromise things at one end of the rev range or the other.

Maybe go for 29.5...

Thanks - splitting the difference sounds feasible... I am more concerned with not misadjusting the timing and thus damaging the engine than tuning it for the most power at max revs. In practical terms, I set it to 31 degrees on the primary case scale lots of years (and miles) ago, so I suspect it isnt badly wrong. I have just started worrying about re-setting it having dismantled the timing case to exchange it for a Mk3 type.

daveforty said:

Thanks - splitting the difference sounds feasible... I am more concerned with not misadjusting the timing and thus damaging the engine than tuning it for the most power at max revs. In practical terms, I set it to 31 degrees on the primary case scale lots of years (and miles) ago, so I suspect it isnt badly wrong. I have just started worrying about re-setting it having dismantled the timing case to exchange it for a Mk3 type.

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In practical terms , advance it until it kicks back then retard it a smidgeon soit doesnt. I think about 1/100th of an inch movement on the boyer/pazonn stator equates to one degree.. Its a scriber andor magnifyingglass job. If you want to set for max performance then you need either a dyno or a suitable stretch of road to see how top speed is affected. by miniscule movements of stator.

oldmikew said:

In practical terms , advance it until it kicks back then retard it a smidgeon soit doesnt. I think about 1/100th of an inch movement on the boyer/pazonn stator equates to one degree.. Its a scriber andor magnifyingglass job. If you want to set for max performance then you need either a dyno or a suitable stretch of road to see how top speed is affected. by miniscule movements of stator.

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I love the words smidgeon....it not used often enough IMHO

Boyer possibly state 31 degrees BTDC static figure as the unit will have a built in safety where it actually reaches max advance around 2000 RPM and retards by 2-3 degrees over 4000 / 4500 rpm
this is very common in ignitions designed for road bikes

Frankie17 said:

Boyer possibly state 31 degrees BTDC static figure as the unit will have a built in safety where it actually reaches max advance around 2000 RPM and retards by 2-3 degrees over 4000 / 4500 rpm
this is very common in ignitions designed for road bikes

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The Boyer analogue ignition doesn't.

if the Pazon graph is correct and the Boyer keeps advancing to 7500 RPM STROBE timing at 5000 RPM at 31 degrees would give a a max advance figure of high 30`s at 7500 RPM

The advance is functionally a capacitor charging curve, the increase deminishes drastically:

as·ymp·tote
a line that continually approaches a given curve but does not meet it at any finite distance.

maybe mid 30's or less @ 100,000 rpm

28 degrees is what you want at full power with 9 to 1 CR or higher. More advance can be tolerated at low compression ratios. More efficient combustion chambers with higher CR want less advance.

28 deg was established by CR Axtell's dyno work many years ago. Give me full advance above 2000RPM for max midrange torque.