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Mixing trigonometry with "timing stick"

Trying to obtain optimum running of my '49 ES2 and not have to strip the drive side covers/fix timing disc every time I tweak it.

I think for my purposes reasonable re-peatable accuracy can be obtained using the "stick down the plughole" method.

In fact I'm using a "sliding rod in a dummy plug" type of tool

As we know the plug is set at an angle however, and this skews correct measurement of the required distance before TDC.

History, English and Geography got me reasonable marks [and remarks] at school, however Mathematics.......

Apparently all I need to do is:

Establish plug angle in relation to axis of piston, multiply the cosine of that angle and the distance the timing rod travels to give me the actual distance the piston has moved in the bore.

An alternative seems to multiply a factor based on the plug angle to the desired distance before TDC, giving the increased dimension to be read on the timing stick - which seems much easier if only I knew the factor!

I include the bits of information gleaned from a trawl of the interweb in case my interpretation lacks somewhat!

How do I go about this please?

Rob.

".....if the plug thread were horizontal, an indicator in the plug hole wouldn't move no matter what the piston did - its movement could be infinite and would still show nothing. If the plug thread were vertical (parallel to the bore axis) the indicator movement would be exactly the same as piston movement.

In between (which is almost everything except some flatheads) the indicator distance is always larger since it's a vector: one component is horizontal (based on the angle) and the other is vertical (based on piston movement).To find the actual piston movement, multiply the indicator reading by the cosine of the plug angle. If 0? is a vertical plug, a 30? plug indicator reading will be higher than the actual movement; multiplied by the cosine of 30? (.866), each .100" of indicator travel = .0866" of piston movement.45? plug: .7071, 15? plug: .0966."Attachments capture-jpg
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Hi,

If x = distance before TDC

A = Angle to vertical

y = distance prodder needs to move.

Then

y=x/Cos A

So if distance before TDC is 5mm

Angle to vertical is 20 degrees.

y=5/Cos 20

y = 5 / 0.93969

y = 5.321mm

Regards

Tony

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Thanks Tony,

I reckon my plug to be at 30 degrees [Cos 0.866], and the distance BTDC to be 13.5mm. [which gives an ign advance of 38deg]

So:

13.5/0.866=15.58mm on "stick"

I've also worked out that mutiplying the BTDC figure by the 30 deg angle factor of 1.155 gives an acceptable:

13.5 X 1.155 = 15.592 on "prodder"

Which should work out easier to try other BTDC figures with greasy mitts on the garage calculator!

Rob.

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Or simply draw a right-angled triangle to scale, with the hypotenuse at, in this instance, 30 degrees, then measure said hypotenuse.

Ianwink

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Or, with no maths required:

Setpiston to the correct positionas you have always done, then with rod in tool loose and 'fully up', fit tool into head, drop probe onto piston, tighten retainer and job done.

To confirm position is correct, wind engine back a little and slowly return until stopped by the tool, and check the reading on the degree disc.

You said that maths wasn't your strongest area, so you were going to check your calculations by comparing to the method you have always used weren't you?

George.

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Previously robyn_howard-evans wrote:

Thanks Tony,

I reckon my plug to be at 30 degrees [Cos 0.866], and the distance BTDC to be 13.5mm. [which gives an ign advance of 38deg]

So:

13.5/0.866=15.58mm on "stick"

I've also worked out that mutiplying the BTDC figure by the 30 deg angle factor of 1.155 gives an acceptable:

13.5 X 1.155 = 15.592 on "prodder"

Which should work out easier to try other BTDC figures with greasy mitts on the garage calculator!

Rob.

yes 38% fully advanced this mean advance the Handel bar leaver as well and set it at full advance position , so when you retard it to start the machine it goes back to retard position . this is sometimes over looked yours anna j

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Well, I have never been very strong in the mathematic department, and after reading all the above, I don't think Iam missing out on anything, In fact,I'm ratherglad I have a thin piece of wooddowel in my kitthat I have used for years to do my ignition timing on my bikes (and others) with success. With sparkplug removed, it stands almost vertical and I'm sure that a few degrees of the perpindicular is going to make stuff all difference to a 65 year old engine, considering I find I get the best performance from my long stroke engine by manually adjusting the advance/retard lever according to the load and revs. Lugging up hill, a bit of retard makes the engine more smoother and tractable, while on the flat, at speed, up tofull advance is used. They are not over sensative to spot on timing,..... on paper, no doubt the figures show, but in practice, I find there is a bit of leeway. Having the ignition slightly over advanced will let the rider 'tune' the timing manually while on the road in actual riding conditions.

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Whilst I completely agree on keeping things simple, once this set of figures are sorted [as it is now]and the angle factor for my "TDC Tool" is now known it becomes a straightforward matter of multiplication on the calculator to obtain my distance BTDC if I would like to vary it.

As far as I was always given to understand the main point of ignition timing accuracy with a magneto however was to utilise the optimum point of sparking from that device itself.

Proper utilisation of the flux of the magnetic field means full power of spark is given at maximum advance setting of the Mag.

So it seems to me that running with some advance "in hand" so that more can be dialled at will would not be using the optimum of the magneto for all the rest of the time.

Rob.

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I'm with Paul on this - my 16H is much better set 1/8" further advanced than the book so I have some advance "in hand" . I cannot use full retard for any purpose whatosever apart from demonstrating how slowly it can tick-over. It cannot pull anything in such a position.When the engine is running there's plenty of spark even at the non-optimal field position. Surely the best position for the point of optimum spark at collapse of field should be at the usual kick start position - i.e. at maybe halfway to full advance?
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Previously robyn_howard-evans wrote:

Trying to obtain optimum running of my '49 ES2 and not have to strip the drive side covers/fix timing disc every time I tweak it.

I think for my purposes reasonable re-peatable accuracy can be obtained using the "stick down the plughole" method.

In fact I'm using a "sliding rod in a dummy plug" type of tool

As we know the plug is set at an angle however, and this skews correct measurement of the required distance before TDC.

History, English and Geography got me reasonable marks [and remarks] at school, however Mathematics.......

Apparently all I need to do is:

Establish plug angle in relation to axis of piston, multiply the cosine of that angle and the distance the timing rod travels to give me the actual distance the piston has moved in the bore.

An alternative seems to multiply a factor based on the plug angle to the desired distance before TDC, giving the increased dimension to be read on the timing stick - which seems much easier if only I knew the factor!

I include the bits of information gleaned from a trawl of the interweb in case my interpretation lacks somewhat!

How do I go about this please?

Rob.

".....if the plug thread were horizontal, an indicator in the plug hole wouldn't move no matter what the piston did - its movement could be infinite and would still show nothing. If the plug thread were vertical (parallel to the bore axis) the indicator movement would be exactly the same as piston movement.

In between (which is almost everything except some flatheads) the indicator distance is always larger since it's a vector: one component is horizontal (based on the angle) and the other is vertical (based on piston movement).To find the actual piston movement, multiply the indicator reading by the cosine of the plug angle. If 0? is a vertical plug, a 30? plug indicator reading will be higher than the actual movement; multiplied by the cosine of 30? (.866), each .100" of indicator travel = .0866" of piston movement.45? plug: .7071, 15? plug: .0966."

My 2P Above words have an anomoly, Para 2 starts 'In between.....' Indiactor distance is always 'larger'.... Not so-think about it, it actualy is smaller.

Now further along with this diatribe we are told that there is a 'best' ignition point on a magneto. This may very well be, but there is little joy in getting the best from your mag and running the engine at less than optimum ignition advance. The later is the crucial point here if it is far enough out you get rough, hot running, wasted petrol, and if you are lucky holy pistons-your choice. The correct engine ignition point is first consideration, every thing else is secondary.

Saying that as is also said here, an old nail like one of these at the end of the day very little is gained or lost by any small errors here. BY the way if any one out there was in the club about 30 years ago they would have seen this very item with the maths and the lolly stick in Roadholder. and the bottom line thne was 'nothing to worry about, two halves of not a lot in technical terms'

Cheer Al Oz-back to my electrics.

 

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