the motor goes " bang"

[Hugz]

Try opening up the points a bit more. A hacksaw blade is close. 25thou l think. If the banging changed frequency from a points adjustment then a bit of trial and error is in order ie try different gaps. A new plug and lead might help. You could undo the locator stud under the magneto so as to be able to rotate the base plate which will advance or retard the timing and see if that changes or eliminates the banging. Is the magneto base plate tight on the motor? Any movement will change the points gap.

[anne-marie]

the points are 0,5mm at their max, but it is when the piston is on top course, close to the spark plug, right?

[Hugz]

The spark should fire at 23.5 degree before the piston is at the end of its cycle (tdc, closest to plug) If you rotate the flywheel you will see the points open and close once per revolution. When it's fully open is when you adjust the gap of the points. When the points close and make contact with each other is when the spark plug sparks. The magneto is factory set for spark to be in the correct place providing points gap is correct. Have you checked the spark plug gap or put a new plug in?

[anne-marie]

hi Hugz
i have check the spark plug, i undo it so i could see the piston
it seemed a little bit darkened, and the hole seemed greasy too
maybe i'll sand the plug with 600grain
i m gonna take a plug from a spare forty plus and try with it.
are you sure the noise is coming from ignition, that it comes from the timing, not from the rings?
i 'm afraid to damage the block's chamber trying to run it correctly.
anyway, i'll change the rings if i have to

what do you mean by "You could undo the locator stud under the magneto so as to be able to rotate the base plate "? i am gonna have a look at it, but i am not sure to understand...those little screws are tricky! there is the one to regulate the gap, and there is 3 others that are moving the base where the gap one is..is that what you are talking about?

the bad news is that the motor is back to the boat, so i'll have to operate on the water...hope there won't be no "plouf" today!

[anne-marie]

i have try every gaps and screw position for like 3hours and half, but no effects...just at a moment the noise stopped...but the "bumps" restart 5 minutes later
so i am gonna try with a new plug, maybe the plug's gap isn't good anymore.

and the fact is that the first time the "bangs" occured it last for like 3 minutes, then it stopped and the motor was running correctly
and the next day the motor was banging all the time
i guess that was the end of the spark plug

inside the plug hole there was some pieces of calamine, looks like some chips of charcoal, does that have any effect on the ignition?

anyway that was a very shiny day on the water

[Nudge]

Are you using 10:1 fuel to oil?

Try a new HT spark plug wire. I had an engine that had a bad wire and it would bang wvery minute or so. The engine miss fires but still keeps sucking and pumping the fuel through the engine and into the exhaust tube, filling it full of unburnt fuel. Then when it build up enough catches on fire and makes a bang!

Where is Nantes?

[anne-marie]

hello Nudge
i have changed to 4:1 some years ago
effectivly my spark wire doesn't fit it really good; it's moving all the time back and forth
but i don't know how to change it; maybe i'm gonna try to fix it
thanks for your advice
Nantes is in france, north west

[Nudge]

Have a look at this thread, it may help to show how to take the HT apart

viewtopic.php?f=5&t=5597&p=48054&hilit=ht#p48054