I just wondered how common it is for the timing marks on the alternator rotor to be wrong.
After snapping several of my ankles I decided to go right back to basics and check the timing marks.
I found to my dismay that they are a staggering 23dgrees out. (Roughly)
The rotor was kindly gifted to me by a friend and fellow member. It came off an 850 Commando and I have fitted it to my 750 rather than the original explosive type.
I can't remember if the keyway lines up with the marks but it must have worked on the 850 as no attempt has been made to make other marks. As far as I know all lucas rotor keyways line up with the marks or mark.
Is it possible that some other 750 crank has it's rotor keyway in another position to the Commando? I do remember that the keyway is not aligned with the crank pins, but assumed that this is to put the marks in view of the timing window.
I can't be sure that the crank is right. It was bought from Fair Spares by the previous owner and is not the original for the engine. It all fits and the pistons sit at the tops of the bores as they should at TDC. The only thing of note is that there seems to be a lot of thread showing in the rotor nut, I thought the shaft should fill the nut?
very common...
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Thanks Grant
Thanks for taking the trouble to throw some light on this Grant.
I had read that 2 or 3 degree errors are quite common, but 23 takes the marks right out of the window when it's set at tdc, or even at 31 degrees btdc for that matter.
I'm not really in the mood to take the rotor off again to check the keyways. It is what it is at the end of the day and so long as my new marks are correct we should be in business once the bruising has gone down.
I can't remember if the keyway in the rotor is in line with the marks, but it has been used before with no problems so I'm assuming it's okay. The crank is a different matter, If I recall correctly the keyway is at about 3 O'clock, which struck me as strange before putting the engine together, but assumed there was a good reason for it to be there.
Thanks for highlighting how the rotor nut looks. Even installing the alternator was a trauma. I didn't have a rotor spacer, so popped the stator on the original 1/2" spacers to see where it should go and I couldn't seat the clutch cover, it was held off by about 6mm. I resolved this by buying a standard rotor spacer and then stacking washers under the stator to place it nicely on the rotor. Everything fits now, but not with the standard stator spacers!
The rotor should be nicely central in the stator to get the best of the magnetism into the coils, so maybe the standard spacers don't work every time if they are often not in this position?
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It’s not uncommon at all for the ‘ignition timing degree scale’ on the inside of the primary cover to be inaccurate.
It gets knocked around and can be misshapen quite easily (plus on the MK3, it is adjustable on slotted holes, but that's a whole other story)
However, I have only ever seen it to be up to six degrees or so out, so nowhere near what you are seeing.
So I think in your case we are looking for a different culprit.
First thing to check is your rotor – where is the timing mark in relation to the woodruff key slot?
They should be opposite each other for you to stand a chance of using the timing marks.
I have seen them a million miles out (admittedly on non-Lucas parts) but still, it’s something to be aware of.
The other thing – is the woodruff key slot on the crankshaft in the right place?
I have a bike that has two slots on the crankshaft – it’s a Combat engine that was rebuilt very early in its life due to failed bearings.
…so that is also something to watch for.
Regarding the positioning of your rotor – it is quite normal to see the rotor set back from the stator, and several threads showing up the inside of the rotor nut… so no worries there.