timing differences - how bad and what to do?

[mikeb]

I’ve been adjusting the timing on my ’61 Super Rocket and aiming for 5/16 inch BTDC (7.9mm) or maybe a little closer to 3/8 (9.5mm) as I have a manual advance/retard. I’m using the object vertically down the pug hole method and a buzzer. Problem is there’s a sizeable difference BTDC between left and right:

Left = 9.3mm BTDC = 0.366inches -> 36degrees approx., points gap = 0.012in
Right = 6.2mm = 0.244in -> 28 degrees approx., points gap = 0.013in (maybe 0.0135??)

(Degrees BTDC are taken from this table, if I’m reading it right- http://www.a7a10.net/forum/index.php?action=dlattach;topic=195.0;attach=16426;image )

The right cylinder has a bluer pipe near the head which I guess suggests more retarded ignition. I’ve recently bought the bike – the timing was very retarded and the carb had problems (warped flange, pitted slide…) so I can’t get a read on the plugs yet, other than they are both very black.

First question is how bad is this?

Most of the threads about timing differences also mention differences in points gaps and then aim at the cam ring or its bearing cup.  As my points are about the same I guess that still leaves the cam ring as suspect. I assume the heights are the same so how to tell?

Are there other likely causes?

BTW if you've ever had a yearning to make a magento buzzer look at this -http://www.historicmotorcycle.org.au/magneto/MAGNETO%20TIMING%20TOOL.pdf -  i made it without the kit mentioned and it works a treat. cheaper than taking up smoking to get a fag paper

Thanks
Mike

[bsa-bill]

Problem is there’s a sizeable difference BTDC between left and right:

Hi Mike
To just clarify - are you talking about a difference in the measurement on the stick down the plug hole or the point at which the points open.
The first would suggest pistons are not the same (quite possible)
The second would as you say point to the cam ring, new cam rings are available but many seem to prefer grinding them to correct with a stone.
you could also split the difference and see how it goes

[mikeb]

Bill - I meant the distance before TDC before at which points open. - height BTDC on the left being bigger so firing on left being more advanced.

if its the cam ring the lobe height must be about the same if the points gap is about even, so i guess one lobe may have a different angle of attack... ?? how would I grind that? would not facing the attack slope risk making the lobe lower?

any other suspects?

cheers

[muskrat]

G'day Mike.
  I would have to agree with Bill on ODD pistons fitted. For the difference in timing to be that far out it's a wonder it ran at all. Do a compression test.
Cheers

[mikeb]

muskrat - i did a quick test when i got the bike and both were around 145-150psi - can't recall which side was a little higher and it was a quick and dirty test. with torch down the plug hole i can see '060' marked on one piston so i guess that's oversize. actually idles in a lumpy rough but ok kind of way.
either way, the difference is between TDC and points opening is by each side, so i can't see how that would explain it. and the the pointer down the plughole was vertical so crown shape/differences wouldn't be a factor.
the only obvious difference between cylinders is the exhaust pipe on the right is darker/blue near the head.

???

[chaterlea25]

HI,

First question is how bad is this?

Not good !!!

Either the points pivot cam ring or magneto bearings need attention, maybe all 3
Usually as Bill said the points gap varies from one side to the other, but 1- 1.5 thou would not equal 6 degrees difference
Make up a timing plug from an old plug (nnock the middle out ) and thread the middle to accept a set stud (rounded inner end) and locknut
Firstly use this tool to set up the degree disc.
Then measure the timing difference setting the tool to stop the piston at the required position
( Use the tool on one side only to negate physical differences in piston / head etc)
This will give you an accurate degree setting to work from
There are several long topics on the forum about similar issues
Heres one, http://www.a7a10.net/forum/index.php?topic=3747.0

HTH
John

[a10 gf]

Camring eccentric vs armature shaft\points ? See http://www.a7a10.net/forum/index.php?topic=1375  for some findings\experiments > http://www.a7a10.net/forum/index.php?topic=1375.msg9862#msg9862

My test setup > http://www.a7a10.net/forum/index.php?topic=1375.msg9915#msg9915

Big gain in getting the l\r timing right, a well running, powerful engine, worth (any amount of :O) time and money.

[wilko]

A mate has a v-twin cam ring that was worn so bad that one cylinder was firing 20 degrees late. Anew one fixed that. so they can wear quite radically if not lightly greased periodically.

[beezermacc]

I deal with this issue all the time. Your main problem is the difference in piston heights when the fuel detonates, 9.3 mm on one side then 6.2mm on the other. Normally on an A10 a points imbalance of less than 0.002" is OK because the old girl (A10) is quite forgiving, particularly the iron head versions. The cam ring is not always the culprit, often its the machining of the cam ring housing which may be eccentric or bored crooked to the face. Your points gap imbalance is only slight, maybe 0.0015 which is tolerable. The points gap imbalance, in itself, is not a problem unless its enormous, but it usually indicates the mag timing is out as well. I would make sure everything is clean (cam, cam housing - particularly the face which mates with the mag body, shims, points heel, face of mag body) and check that the cam housing is a tight fit in the mag body, i.e. not capable of slopping about side-to-side. then set the mag up in the vice with a degree disc on it. Another point to note is that the automatic ones are easier to fix than the manual ones because the cams in the manual mags need to be loose to work! Check the firing moments with the degree disc; you'll probably find the imbalance is about 3 or 4 degrees which translates to 6 or 8 degrees at the crankshaft. It is very rare for me to have to stone the cam ring but, if I do (often on Vincents) I put a Dremel sanding burr ( not a stone as these wear unevenly) in a pillar drill and remove the smallest amount of metal off the leading edge of the overadvanced side of the cam ring. Be careful! You don't need to remove much metal to go too far!

[beezermacc]

A mate has a v-twin cam ring that was worn so bad that one cylinder was firing 20 degrees late. Anew one fixed that. so they can wear quite radically if not lightly greased periodically.

It is more likely that the mag was fitted with the wrong cam ring. A Vincent is a 50 degree V twin but other cam rings can be set for different V's

[muskrat]

My timing set up 1/2 way down this page http://www.a7a10.net/forum/index.php?topic=6011.msg41207#msg41207
Cheers

[mikeb]

Thanks for the suggestions so far.  I made a piston stop (spark plug type), fix a laminated timing disk to the right hand end of the crankshaft (not easy that bit) and did some measuring. Basically the left fires about 7 degrees (at the crank) before the right (39deg vs 32deg).

On the cam ring pic below the green marks show where the cylinders actually fire – so the left is a few degrees ahead on the cam ring. (the black line is 180 degrees) The orange lines are the beginning of the visible wear tracks on the inside of the cam ring (from the points heel) and the red lines the end of the wear tracks. rotation is clockwise.

In case it’s a bent points plate I fitting a different one and the cylinder differences BTDC were the same, if not bigger.  As an aside, these points had a smaller gap as the heel is well worn… on the pic below note the orange mark (wear mark) and the green (firing point) are closer together on the left than on the right – so I assume the left lobe is steeper, and so a smaller points gap may be even more advanced on the left than a larger points gap. (? Does this make sense?)

In case it’s a mag bearing, shaft or cam ring housing I rotated the cam ring 180 degrees.In this position it won’t press in completely (coz the slot for the bottom peg is now at the top) but it was mostly in. obviously the heel will not be running fully in the same part of the cam ring. Doing this the right cylinder was now about 4 degrees ahead of the left. That’s a reversal (tho not of the full 6-7 degrees).. and if the housing, shaft, bearing and points haven't changed then it must be the cam ring... right???  for what its worth there's visible wear.

So the cam ring... Would you agree or am I missing something? At this stage I can’t see any reason to take the mag out form the bike for a bench test.... ??

If the cam ring is stuffed, I’ll get a new one. Given the forum posts talking of dodgy cam rings, who is a reliable manufacturer / supplier? I imagine they’ll be in the UK or US. This is a manual A/R.

And as parts take ages in this part of the world, I’m tempted to try grinding the current cam ring -  maybe facing the front of the left lobe with emery paper. In the photo I think I’m going at the bit at the end of the green arrows…. right?

Are you still reading all the way down here? Thanks for your help!

Mike

[bsa-bill]

couple or more lads here might give you a price for a new cam ring Mike but as far as I can see your talking £70 UK plus any tax and travel.
Before parting with that or spending to much time with a stone/emery cloth or whatever I'd think about lifting the head and just confirm that both piston crowns occupy a similar and parallel position in time and space 

Anyway good luck with the honing
 

[wilko]

A mate has a v-twin cam ring that was worn so bad that one cylinder was firing 20 degrees late. Anew one fixed that. so they can wear quite radically if not lightly greased periodically.

It is more likely that the mag was fitted with the wrong cam ring. A Vincent is a 50 degree V twin but other cam rings can be set for different V's

It was a very worn 50 degree cam ring off an AJS V-Twin. One of the bumps had worn down .

[Triton Thrasher]

You can make quite a neat job with a Dremel type tool with a cylindrical stone. Keep it moving up and down the ramp.  As soon as you've visibly removed some metal, put the cam ring back into the magneto and check the timing. You may surprise yourself with the accuracy you can get.

You say it's a manual magneto. Is the fit of the cam ring sloppy in the mag?