Tech advice on timing ignition (outboard but not Seagull)

[Vic]

I have been asked for advice on setting the ignition timing of a small outboard. We know it should be 2.0 to 2.2 mm BTDC. Thats Ok if you have a dial gauge and the means to mount it in the spark plug hole but I was wondering if it would be easier to calculate the advance in degrees of crankshaft rotation and set it up using that figure.
We know that the stroke is 44mm so according to my maths the cosine of the angle is (22 - 2.1)/22, making the angle 25°.

How's your trigonometry? Do you agree and does 25° BTDC sound like a realistic figure?

The difficulty will be finding TDC accurately as the starting point.

On second thoughts that not right is it? i also need to know the length of the con rod don't I?

Any other suggestions?

[Charles uk]

Vic if you post the bore, stroke & con rod length, I will calculate the angle before TDC for you.
To calculate TDC exactly attach your 360 degree protractor to the flywheel, fasten a bit of coat hanger wire to a suitable crankase bolt & bend it so it makes a accurate pointer on the protractor (it might need a point).

Find something that will go down the sparkplug hole that will hit the piston, I use a sparkplug with a lump of bar welded too it, rotate the flywheel till the piston hits the stop take a reading on the protractor, wind it back all the way till it stops again, take a reading.
TDC is exactly half way between those two readings, set protractor so that TDC = 0 degrees & do it again to confirm your right.

With a 76mm rod like Seaglls use 2.05mm = 22 degrees BTDC & 2.24mm = 23 degrees.

[Vic]

We do not know the conrod length, only the stroke (and bore). I thought I had it figured out, but then realised I needed to include the conrod length in the calcs.

Anyway will be done by measuring with a digital depth gauge.

It is someone rebuilding a Volvo Penta 5hp who I helped out with a copy of the owners manual a while back. Now asking technical questions! At least he has turned a box of bits into a running engine. I suppose I ought to sell mine if people are interested in them. So temperamental I pensioned it off in 1985 after only 7 years light use.

[Charles uk]

Vic if you increase the rod length from 76mm to 86mm at 22 degrees before TDC the piston is displaced by only 0.04mm, I think that will be close enough.

[Collector Inspector]

If the specs call for a mm BTDC, go with that. You do not need a dial gauge. A vernier, pref digital, is all you need. Points open at what you want in mm?

Easy.

Regards

B