got my 2 speeder running but…..

[Deleted User]

the bth magneto will only spark on full retard. As i advance the cam ring the spark diminshes as it reaches full advance.I have set the timing with the magneto in the full retard position but with the piston at the correct point before tdc and it starts perfectly and runs well hot or cold…My 2nd enquiry is regarding the de-compressor valves on the barrels.What should they do exactly???They move in by about 6mm when the lever is pulled but make no difference to the running of the bike or make it easier to kick over.I have removed the valves and cleaned them but there doesn’t appear to be too much amiss.Are these valves serviceable.Many Thanks Adrian

[dave bushell]

Hi kas750
It’s possible that you have the wrong cam ring in your mag, or more likely, the wrong contact breaker assembly. The points should open at maximum flux when fully advanced. You can feel this as resistance if you rotate the mag by hand.
The valves are called half compression valves and are supposed to do as they are called! I think most owners leave them disconnected as they seem to be a source of oily smears over the block. I was told many years ago that they help with engine braking but as I have never had mine operational, I can’t confirm if that is true.
Dave

[Deleted User]

One small addition to Dave Bushell’s post, the BTH magneto, like the Lucas, is taper mounted and has a key in the taper to positively locate the breaker assembly on the armature at the correct relative angle.

The point here is that on several occasions I have come across mags where the locating key has either broken off or has been removed. It seemed to mostly happen on machines with an ATC unit because it was it easier to rotate the breaker assembly on the magneto armature than to be on both sides of the bike at once, needful because Lucas in their wisdom were inconsiderate enough to put the ATC on the drive side and the points on the other!

It took me a couple of thousand miles on a £35 Vincent Comet that I bought as a hack, (them were the days!). I got it running with a minimum of attention and only found the missing key when I got around to doing a proper service.

So, just in case, make sure your mags contact assembly has a key, yer never know!

[Deleted User]

I had a cylinder block once that had decompression valves fitted but as it was resleeved they were only there for show…. There were no holes in the cylinder wall!
So I could operate them but they would do nothing.

On my two speeder the valves have always been disconnected and I never missed them.

Cheers,

Erik

[Deleted User]

Thanks for the advice…

[Deleted User]

@efr215 wrote:

So, just in case, make sure your mags contact assembly has a key, yer never know!

Give that man a coconut!!!

Absolutely bang on.The key on the contact breaker assembly seems to of been worn off and the points were in the wrong position.I gently formed the key again with a very small chisel and toffee hammer and the mag now sparks with the cam ring fully advanced or retarded.SO many thanks for that…Adrian

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