what's wrong with the ignition on this cylinder?

[Airbear]

Gidday Doctor and welcome to the forum. Lots of good advice there but I will throw in another possible direction by recounting an experience from a few years ago. To mitigate any stress that unnecessary suspense might cause to a delicate soul I will explain up-front that my 74 R90/6 had a problem with her camshaft nose - the points cam was bent and/or had been butchered to the extent that left and right cylinders were firing out of sync by about 30 degrees but it took some finding to make this realisation. I knew the timing chain was quite stretched - I could hear the chain thrashing against the alloy web in the timing chest and I did feel slightly concerned that I wasn't seeing a ‘proper’ double image when I put a strobe on the timing marks on the flywheel. This kind of became important later.

Anyways, if I timed it to one side, the other would either be greatly advanced or greatly retarded, depending on which side I chose to use for timing. My normal timing procedure was to use a test lamp to time statically, and then ride to a friend's place to borrow a strobe to dynamically time to the advance mark. By accident this worked well enough every time and the bike ran well enough for the first couple of years, but one day I must have timed to the 'wrong' cylinder (can’t remember which one now) and the opposite cylinder would not fire in any predictable way. I don't know whether the opposite cylinder was now over-advanced or over-retarded, but the effect was very much as you describe. At the time I had just re-bored the cylinders and fitted new pistons and valves and wanted to have the control to run those new rings in properly. It was very frustrating to have one cylinder not firing or 8 / 12 / 16 stroking and popping at idle, but under power when the revs got up the power would suddenly cut in. The carbs proved to be good and there was a good enough spark. I read up a lot of stuff on the web - an article on Duane Ausherman's site provided a useful test - shine the strobe on the cam nose. This is what I saw.



I made up an indexing disk and used it to check where each side was firing with the test light. I kept rotating the engine and recording the degree reading. The average out-of-sync reading was 31 degrees. This explained why the second image I should have been seeing was not visible - it was way out of the window. I have now fitted an Omega crank-mounted electronic ignition. (beautiful results), but at the time I discovered the out-of-sync condition, my only recourse was to file down part of the offending side of the points cam (lots of trial and error filing and sanding). This worked well enough that I was able to see at least a 'proper' double image in the timing hole and get them to coincide on a good day. I soon got around to replacing the timing chain and that made a good improvement. A crank-mounted ignition system is the ultimate solution. This is a long winded rave but I trust not too stressful to read. Here is the indexing disk - just a bit of paper with a circular array of degree marks glued temporarily to the alternator rotor -



I have heard that it is possible to straighten a bent cam nose by judicious tapping with a suitable hammer but it requires a deft touch and measuring gadgets. I certainly did not feel competent at the time to try this. I hope this story will be useful, if only to discount one possibility. If you have a strobe handy, have a look and please keep us up to date.

[sterob]

Spark Plug. Don't waste time with the other stuff.

My money is on Jean.

Yep, me too. Do the easy stuff first.

[Duane Ausherman]

It does not take fancy measuring stuff to fix a bent cam tip.

Charlie, I am glad that something on my site helped identify the basic problem for you. We ran into this often, so shining the strobe on the advance mechanism became standard. That way one knows instantly. I would estimate that one out of 10, or 20 had this situation, but not as bad as yours.

It is quite common for the "other" mark to be completely out of the window and therefore it doesn't exist and isn't a problem. We make so many assumptions.

[Motu]

This was very common on British twins with a magneto.The points rotated on the armature,and the cam ring was external.It didn't take much play in bearings,misaligned housings or damaged points taper to make a big difference in timing per side.The worse I had was 10 deg on a Lucas,the best 2 deg with a better quality BTH.I used to split the timing,but with 10 deg difference sometimes an engine couldn't take 5 deg over advance.I changed to dist on a couple of engines just to get more accurate timing,and moving to later model twin point ignition was a relief.