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Timing for alternator engine with magneto

Submitted by Tim Dekkers on Wednesday, 22 October, 2025 - 19:05
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Weird one:

On my racing project, I'm using a Model 50 alternator engine (as that was an engine I could get my hands on). But, I'm going to use an SR1 magneto, as ignition should be period for that racing club.

Magneto engines should be timed at 11/16 degrees BTDC, but alternator ones at 15/32 degrees BTDC. I was inclined to use 11/16 as I'm using a magneto now and thought the different timing might have something to do with differences between magneto and alternator and somesort of delay in one or the other.

However, I vaguely remember earlier engines have 7" conrods, while the later ones have 6,5"? Mine's definitely the latter. So could that be the reason and should I use 15/32 degrees BTDC timing?

Submitted by gordon_johnston on Thursday, 23 October, 2025 - 06:30

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There would be no difference in ignition timing between magneto or coil ignition all other things being equal. A different con rod length would have minimal effect on the geometry either.
A quoted difference in ignition timing is most likely due to other factors - cams and compression ratio being the more likely causes. Or a misprint. 

Submitted by Tim Dekkers on Thursday, 23 October, 2025 - 22:06

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Cheers Gordon,

The cams on the late engines are supposed to have quieting ramps, though I don't know about timing or lift. Nor about compression.

Although I try not to bother the club experts too much, today I had to contact Mike Pemberton on another matter and threw the timing question in there too. He told me 38 degrees would be a good starting point.

Submitted by Don Anson on Monday, 27 October, 2025 - 22:50

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I'm with Dan.
I highly recommend the vernier sprocket,  strobe or no strobe. In fact you don't even need to measure the firing point. If your bike is pinking too much you can just move the sprocket back one hole and job done. 

Probably the best thing I've bought for my ES2. 

Don Anson 
Melbourne. 

Submitted by Tim Dekkers on Tuesday, 28 October, 2025 - 14:39

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For this budget project, the normal setup will do. But I definitely have to read up on vernier sprockets, as I fail to see their advantage completely. I'm sure it's me, as people tend to be quite enthusiastic about them.

Submitted by alan_osborn on Tuesday, 28 October, 2025 - 16:38

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Timing light-I sell them. Move on from 'fag' paper, in fact fags being a thing of the past does make this way of timing in short supply.
The idea of moving the timing to stop pinking is quite good until you find your self with a twin.

 



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