Hi all...I have been having trouble with my just finished rebuild of a M21. Its been running really hot and having trouble starting. As well I had to replace the piston as I damaged it. thought it was because I had the carry jetted wrong with a 150 main jet instead of a 160 main jet (concentric 626 carby). However, I just pulled out the Maggy thinking that the condenser may be causing the starting issues and I found that due to cable breakage it is stuck on retard, Pesto I have found the heating problem. However before I put the maggy back into the bike I wanted to test the condenser just incase its done. I tested it by heating at 150 degrees celsius in the kitchen oven for 25 mins, then I hooked a drill upto it and ran it anticlockwise, it had plenty of spark. My question to the forum is "is it a good enough test? I have already ordered a Brightspark condenser product incase and I do not want to put it back into the bike until I am sure.

Hello Bryce,

If the mag was fully rebuilt lately and the condenser replaced, It is probably good.
There is no easy way to bench test these condensers, as the capacitance of it is very low.

(nominal capacitance of 150 nF [0.15 µF])

Some multi-meters do have a capacitance testing ability.

https://www.youtube.com/watch?v=DRD94arK964

Or there is another way, which I less trust:

https://www.youtube.com/watch?v=FxD1OpPQaS0

On the other hand,
I would not cook the mag at 150° Celsius and for so long,
because it never gets this hot, (I would say about half of it)
And this cooking may damage the isolation
Cast over the internal coil wrapping, and melt the bearing grease,
If no "Hi temp" grease was used.
The Capacitor are also not rated to work in such an intense heat.
For example, "easy cap" say that their products heat specs are
"Over the temperature range 0°C to 60°C"
So the heat probably does not come near that figure, at least at the area of the points.

The problem you experienced was probably the advance cable, as you already found.
I do not know how good are your skills with the magneto,
But if it is still out, I would pull the armature for a quick inspection
And re-grease the bearings, just to be on the safe side.

Good luck,
Noam.

150C in an oven ........... bloody hell, its wonder its works!

The grease (even hi temp) will have melted out of the bearings, the Paxolin insulation degrades around 120C, magnets certainly deteriorate with heat, as for the insulation ......... the potting may have survived but the copper wire insulation is only a type of lacquer with what would i presume is a lowish melting point?

Still, it does work ....

As for starting - Concentric's are notorious for the idle/start jets blocking with crud/corrosion - the first thing that suffers is starting.

Why on earth would you cook your magneto in the oven?

To prepare a Swedish speciality perhaps?

email (option): wd.register@gmail.com

you sure those main jets are the correct size :

the M21 had an amal 1 1/16" 376 monoblock and 250 jets.

(M20 had 1" 376 and 240 main)

from here the advice for a concentric was to start with the monoblock jet sizes.

my M20 with 626 runs ok with a 240 jet

jet references here:

http://archives.jampot.dk/technical/carburation/amal_monobloc__numbers_type_and_use.pdf

Well spotted that man...The 160 main jet is the standard jet for a 276 separate float chamber carb...

Concentrics use a substantially larger jet for the same application...

Monobloc jet sizes and set up should indeed be used when a Concentric carb is fitted...Ian

email (option): ian@wright52.plus.com

Amal changed their method of measuring flow through a jet - between the wars, so the jets sizes pre and post war, are not directly equivalent.

Impressed it even ran with such a small main-jet