A.T.C. and timing magneto
- Thread starter Michel
- Start date
I would always retime if swapping out the ATD. Not a big job, and you won't have to worry about movement when the old is removed. With the ears of the ATD wearing, it may affect the retard position of the new unit. What is the reason for exchanging? if the springs are worn new ones can be placed with just removing ATD cover. The fibre gear seem to last for miles, but have heard they seperate in time. I have blamed the ATD for many a bad thing, only to find it was my poor carburator adjusting, dying magneto condenser, bad fuel, burning exhaust valve, etc. The only thing I notice is a bit of run up when idling at the stoplight. I tried one of the electronic mags, but have gone back to the original unit, no problems this month. Good Luck with the tune up. Cheers, Dan
It sounds as though you have not done this before. If that is true then expect to have to be patient and do the job several times. You will have to check the timing as once the ATD is removed there is not a good way to ensure that the magneto does not rotate a little. Even if you clamp the magneto you will still find that there is a problem. What happens is that when you have everything in place and the timing carefully set (with the ATD temporarily set in the fully advanced position) and you then start to tighten the nut in the middle of the ATD you will get some rotation. There is a taper in the middle of the device which has to fit onto the tapered shaft of the magneto and in my experience even if you start to tighten by hand and then try to hit the end of the nut with a hammer to drive it onto the taper without rotation there will still be some movement. I expect to have to do it once or twice and find out how much the system is rotating and then do it again with the timing offset a little so that with the movement it ends up in the correct place.
One other thing to consider is that modern fuels burn quicker than fuels did 60 years ago and most of us would now like to time the bikes with less advance than originally. If the new ATD has 19 or 20 degrees of movement, which gives 38 to 40 degrees on the engine then you will find that if you time it at, say, 36 degrees BTDC then it is slightly retarded for starting. Ideally we should now have ATDs with slightly less movement but most of us are compromising and using nearly the original figure, perhaps retarded to 36-37 degrees, which is a compromise between easy starting and the engine pinking when the throttle is opened quickly. Good luck.
One other thing to consider is that modern fuels burn quicker than fuels did 60 years ago and most of us would now like to time the bikes with less advance than originally. If the new ATD has 19 or 20 degrees of movement, which gives 38 to 40 degrees on the engine then you will find that if you time it at, say, 36 degrees BTDC then it is slightly retarded for starting. Ideally we should now have ATDs with slightly less movement but most of us are compromising and using nearly the original figure, perhaps retarded to 36-37 degrees, which is a compromise between easy starting and the engine pinking when the throttle is opened quickly. Good luck.
I have found that if I finger-tighten the nut to take up the clearance and then fit a socket over the nut and rap on that, that the timing doesn't slip. I have lapped my armature taper to the ATD female taper which helped with this. I haven't lapped my Roy Price ATD to my armature yet, but will see how it goes before going that far.
The taper is in the atd body not the nut,do as Bruce suggests otherwise the magneto shaft will not grip the atd. I also use a pair of long nose vise-grips to hold the ears of the atd in the full advance position and can easily do the job on my own, you could also make up a wedge to hold the atd in the full advance position..John