ET: Engine (Twin) Timing

[MichaelOrgzey]

As I posted already a while ago I'm replacing the large idler and boss and doing some other bits and bobs on my Rapide. I know that a lot of people don't rely on the original timing marks but I just checked them since the correct marks are all present. Turned the engine over and found only the half time pinion out by one tooth.. see pictures. The bike ran and started well before checking. Any thoughts on that would be much appreciated.
Cheers, Michael

 

[davidd]

It sounds correct. When all the marks on the gears line up, the ex valve and the inlet valve will be at the same lift and that lift will occur at 4 degrees before TDC on the overlap side of the cam, not the side of the cam where you adjust the valve clearance.

The marks on the ATD and the small idler are somewhat irrelevant. If the cam timing is correct, ie. the marks on the gears line up, you will then have to check the ignition timing. The accuracy of that timing depends on the taper of the self-extracting nut in the center of the ATD and not on how the gears are lined up with each other. The taper on the self-extracting nut can shift, but it is rare. Check with a degree wheel that the spark occurs where you want it (this might have been 36 or 38 degrees BTDC. If it is not where you want it, put it right and you should be fine.

David

 

[MichaelOrgzey]

Hi Davis, cheers for the input.Yes, I'm aware that I will have to check ignition in the end, I put the marks on the small idler and ATD only for my reference. But the thing that puzzles me is that the half time pinion is not lining up on the original mark but is one tooth late. You can see the correct mark (single dot) on the half time pinion being not in line with the mark on the large idler. Since Big Sid build that engine I wonder if that was done on purpose. Now I have to decide if I stick with the "incorrect" settings or if I line up the order marks. As I mentioned the bike ran fine with the HT pinion one tooth out. Since I'm using the original timed breather this would obviously mean that it's timing would now be slightly earlier as well. I will check it with a timing disk.

 

[MichaelOrgzey]

Sorry,David, spelling mistake!

 

[lee_812d]

It was probably done like that on purpose to get the overlap mentioned as close to 4 degrees as possible. With the 5 different key ways on the half time pinion it is possible to move the timing in steps of 1.5 degrees but that might mean using a different tooth and adjusting 4 steps depending on which way it needs adjusting. On my Comet I could have it at either 3.5, or 5 with the dot one tooth out. So I put mine at 5 (hopefully I chose the best way).
I think at 3.5 it would have been another tooth further round with no dot - can't get my head round it now. When I started typing I was originally thinking it would be lined up correctly but that can't be because the keyways would take it about 72 degrees.

 

[MichaelOrgzey]

I was pretty convinced it was done on purpose, great to get some confirmation!

 

[oexing]

Is it an effect of the hunting gear method ? Turn your engine a few revolutions and watch marks if they still align at all. I was thinking you have to set up all dots and lines on ALL gears as a first action and not turn the engine in between as the hunting tooth design only ligns up at the start. You´d have to do a lot of turns till all marks lign up again.
My thinking was to have marked cam gears and corresponding marks on the engine case at suitable places set at equal lift according to clocks placed on valve spring cups . So then you can replace half time gears and large idler which will not have marks at all when new.
Anyway, when you got marks like in my photos you just have to find TDC at overlap resp. 4 degrees BTDC , lign up the cam mark with case mark and engage the half time gear in best position, testing all 5 slots in it. When turning the crank 360 degrees plus 50 for V-twin the front cam marks should lign up with marks you did before at equal lift for front cam.
Certainly your marks will be unsuitable when you change camshafts so then you do fresh marks on the new cam gear assembly.

Vic

 

[lee_812d]

Once the large idler is replaced you may need a different size half time pinion so you will need to install that as described above as it will have no marks. There are quite a few threads on here about various similar methods of achieving it.

 

[lee_812d]

As I have nothing better to do I made a simple spreadsheet of the alignment of the large idler mark to the half time pinion per revolution (basically the remainder each time when the HT has 48 teeth and the idler 79). As we know it takes 79 turns to get back to the start. To get 1 tooth out either way occurs after 28 and 51 turns (at 51 turns the cam will be 180 degrees out as well). So after either 28 or 51 turns from 1 tooth out, the idler and HT will be aligned but as the cam advances by the same number of teeth that will now be 1 tooth out from its markings as it started aligned.
That is probably not much help apart from to say that the marks won't necessarily line up however many times it is turned.


Corrected version (thanks to chankly bore's post below)
Based on the remainder each time when the HT has 24 teeth, cam 48 and the idler 79. It takes 79 turns to get the HT back to the start and 158 for the cam. To get 1 tooth out either way on the HT occurs after 23 and 56 turns (the cam is 1 tooth out at 28 and 51 turns of the cam).

 

[chankly bore]

Well, there is a bit an error creeping in here. The half-time pinion has 24 teeth and once the large idler is spun all marks go out of line, so the "misalignment" of the half-time pinion alone is not a function of the hunting tooth. In any case, Big Sid probably got it right and if the bike goes well then best leave it alone