K2F Uneven Spark on 650SS

Joined
Jul 19, 2013
Messages
71
Hi to All, I've had my K2F competition Maggie back in for a while after rebuild, and the right hand cylinder timing is miles advanced. The bearings were completely flogged out before rebuild, so could this have caused the points cam ring to wear unevenly?
The right hand cylinder pings when going up a hill, so I want to fix it before I put a hole through a piston.
I have not put the degree wheel on yet but I would say it's probably 20 degrees more advanced than the left cylinder, and it can't be tuned at idle, only runs on the left cylinder at idle.
I was reading about Sid Lawton and his preparations for the Thruxton 9 Hour and he said he always had to file the points cam ring when preparing a bike to get the cylinders to fire 180 degrees from each other! And this was on new stuff from the factory! Although he was probably talking a matter of 1 or 2 degrees.
Is this the only cure for this problem?
Thanks
 
First, check point gap on both ramps.....should be .012 - .015. If one ramp is way more, your mag rebuilder should be told of it and he should make it good. Stop and get the gap problem rectified before doing anything else. I have measured a 4 degree difference in timing for each .002 difference in point gap.

If gap is within spec on both ramps, get a degree wheel.....there is a link for one at the top of the forum.

If you remove the center securing screw on the point block, you can buzz when the points open by judiciously attaching your meter at selected places on the block...a little experimenting should make it clear where to attach meter leads. If you have a newly made cam ring (CNC machined), your points should break within 4 degrees (I would expect less, but I would accept 4 degrees as good enough).

If point gap is within spec on both ramps, and points break more than 4 degrees off 180, the cam ring is worn or out of spec.

If your cam ring is not new, and if you have to return the mag to the rebuilder to fix the gap, then get a new cam ring.... no point in centering the mag about a worn cam ring.

Slick
 
Ah ok, that makes sense. so if the points gaps are off, then the cam ring is not centred properly?
What do you mean by "you can buzz when the points open"?
Thanks Slick
 
If you have a volt -ohm meter, most will buzz or beep when there is continuity between the leads ( when set on minimum ohms position). When the points are closed, there is continuity, and the meter will buzz or beep. When the points break, the buzz will go silent ( or the meter needle will go to infinity ohms). I have been able to determine the point break to within one degree by observing the meter and turning the engine with the cam shaft or main shaft nut. It helps to remove both plugs so you are not fighting compression.

If you do not have a VOM, get one....some retail for as little as $10 , and they are an indispensable diagnostic tool.

Slick
 
Ah yes of course, sorry, that's a better idea than the tobacco paper way!
I've got a good Snap On Multimeter so I'll let you know how I go.....
Thanks for that!
 
The tobacco paper is .002 and introduces 4 degrees of timing shift ( retard). When using the VOM, I set the timing 4 degrees earlier than recommended by the paper technique.

If you are running higher than factory compression ratio, you may want to retard max advance from factory spec.

Slick
 
Cigarette papers at 2 thou are quite thick ?
Some of them are claimed to be down to 20 microns

The points spin.
Thats going to complicate things ??
 
texasSlick said:
If you have a newly made cam ring (CNC machined), your points should break within 4 degrees (I would expect less, but I would accept 4 degrees as good enough).

4 degrees out of a newly made CNC-ed cam ring ??
Can't say I've bought too many of these, but that doesn't sound very accurate at all ??
 
Rohan said:
Cigarette papers at 2 thou are quite thick ?
Some of them are claimed to be down to 20 microns

Agreed! I am not a smoker and the paper i measured was most likely not best quality.i do not refute that thinner papers are available, but the ones I had when making my measurements were consistently .002. If you have .001 paper, change 4 degree timing shift to 2 degrees. 20 microns would be only 1 / 100 th of the paper thickness I measured. That seems VERY thin.

It is easy to measure the timing change due to the paper using the VOM method....buzz the points and find the break point, then insert the paper under the rubbing block, and measure again. I found 4 degree change with .002 paper.

I see you have posted again before I could reply.....suggesting that a CNC machined cam ring should be better than 4 degrees.....as I said...I would expect better, but considering a variation due to cam ring centering ( point gap difference) and machining allowance (cam ring machining difference), plus measurement variation (I found I could get +\- 1 degree difference between measurements), then an overall difference of 4 degrees is not bad and not all due to the cam ring.
I would still like to achieve 2 degrees or less, but do not want to discourage or frustrate anyone by setting a goal out of reach. The final authority on the matter is to put a timing light on the engine, measuring both cylinders.

Finally, it should be understood the magneto does not fire exactly when the points electrically break, there is a delay that may be several degrees of crankshaft rotation, but this delay should be the same on both cylinders.

Slick
 
What is your method for putting a timing light on the engine?
The only way I can see is to remove primary cover and attach the degree wheel to the alternator nut, but it hits the exhaust so you would have to make up something to move it out from exhaust?
What is your system?
 
Tbolt:

I have never felt the need to put on a timing light. If I had to, I would make a mark on the alternator rotor, and a mark on the stator that corresponded to specified timing advance. Of course, you have to run up the engine with the primary cover off. And, of course, you first have to do the static set up with the degree wheel in order to get the marks.

I have a 6 or 7 inch Dia. degree wheel and when attached to the rotor nut, there is no interference with the exhaust pipe. I remove the stator and let it hang. It is also necessary to remove the stator mounting studs to avoid interference with the degree wheel. Then using a short bolt that fits the stator stud holes, I attach a piece of skinned solid electrical wire to be my marker.

PM me if you need more info, and tell all what you found to cause such a timing mis-match.

Slick
 
I can verify the tobacco paper stats. I just timed my 850 Commando static and thought I had it spot on, both cylinders. Was still getting some kickback so I broke down & purchased a timing light. When I put it on the pointer I was at 32 degrees. Backed it off to the correct 28 degrees and all is good now.
 
If you are static timing them, then the advance-retard mechanism comes into play, and this adds a whole another level of complications.
Assuming it will reach full advance is not the same as KNOWING it will reach full advance ??

How are folks working out the real spark timing at full advance, if not by strobe timing them ?
There is no other way ??
 
P.S. How do you static time an 850 - the advance-retard system is behind the points, so don't even have access to it.
If you don't lock it in the fully advanced position, then this just ain't gunna work.....

??
 
Rohan said:
If you are static timing them, then the advance-retard mechanism comes into play, and this adds a whole another level of complications.
Assuming it will reach full advance is not the same as KNOWING it will reach full advance ??

How are folks working out the real spark timing at full advance, if not by strobe timing them ?
There is no other way ??

Agree 100%. I just never bothered to strobe check, The Atlas ran fine, started on first kick, and was the king until 100 hp rice rockets came along. It would be a dog if the advance mechanism were not advancing.

Strobe timing also ensures the AAU returns to zero advance at idle speed, but this can be checked with the timing cover off...manually advance the AAU and check for any sticking or failure to return to zero. Too tight mag chain can cause sticking, as well as weak return springs.

Slick
Slick
 
Static timing at full advance - Loosen center bolt and apply a large washer to contact the cam. Turn the cam against the spring & tighten it down. That procedure is outlined in the service manual and it proved to be quite accurate except for the fact that the paper width left me overly advanced. Using a multimeter to detect points separation would have eliminated the paper width issue. I have the multimeter, but I needed three hands to hold the probes and move the back wheel :cry:
 
ommief said:
i have the multimeter, but I needed three hands to hold the probes and move the back wheel :cry:

Get a pair of lead wires with small alligator clips on each end....these will eliminate the need for two of the three hands....
 
Re; I was reading about Sid Lawton and his preparations for the Thruxton 9 Hour and he said he always had to file the points cam ring when preparing a bike to get the cylinders to fire 180 degrees from each other! And this was on new stuff from the factory! Although he was probably talking a matter of 1 or 2 degrees.
Is this the only cure for this problem?

Sadly, an all too common problem on the K2F magnetos.
The other method, as I have previously written on this website, is to obtain some .003 & .005 tho. Magneto bearing shims. Cut them into quarters and insert into the right corner of the points end plate, to get it to fire 0 & 180 degrees with the aid of a timing degree disc, battery & bulb method.
This was also a common practice of correcting these wayward magnetos.

Whatever you do, do not ride the bike until you have corrected the magneto, or you WILL feck up the piston on the side that the timing is out-don’t ask me how I know :!:
 
Bernhard said:
Re; I was reading about Sid Lawton and his preparations for the Thruxton 9 Hour and he said he always had to file the points cam ring when preparing a bike to get the cylinders to fire 180 degrees from each other! And this was on new stuff from the factory! Although he was probably talking a matter of 1 or 2 degrees.
Is this the only cure for this problem?

Sadly, an all too common problem on the K2F magnetos.
The other method, as I have previously written on this website, is to obtain some .003 & .005 tho. Magneto bearing shims. Cut them into quarters and insert into the right corner of the points end plate, to get it to fire 0 & 180 degrees with the aid of a timing degree disc, battery & bulb method.
This was also a common practice of correcting these wayward magnetos.

Whatever you do, do not ride the bike until you have corrected the magneto, or you WILL feck up the piston on the side that the timing is out-don’t ask me how I know :!:

For a road bike, I would not file/grind inside of cam ring to correct 1 - 2 degrees mis-match....I doubt anyone could sense the difference going down the road, plus there is +/- 1 degree measurement variation (more if rotating the engine by the wheel), so a 1 - 2 degree mismatch may only be less than 1 degree. Even strobing the engine can hardly detect that much difference. If you feel compelled to correct a small mis-match, it is better to grind .001 - .002 off the OUTSIDE of the cam ring on the side firing early, then apply a .001 - .002 shim on the opposite side. This ensures the ramp surface is smooth and has same profile as other ramp.

The trick of applying a shim under a cormer of the cam ring housing does work to correct a small mis-match. However, there are two caveats....any more than a small (few thous) and the bearing can be stressed, and there is the possibilty of shifting the housing off center when loosening the screws, making the situation much worse.


Back in the old days, when we were all running Lucas cam rings made on worn out WW1 machinery, no one but the racers were concerened about timing mis-match. The old bikes kept on running, some better than others....point is...we tend to worry about some things too much!

Slick
 
After using a a timing light on my Atlas and 650SS I would never rely on the cigarette paper method again. In my opinion there are too many tolerances that you need to take into account with the static method (slack in the mag chain, timing creep when the auto advance is tightened, etc.) to make it accurate. Every time I have methodically set the timing statically and checked it with a timing light I have been off at least 5 or 10 degrees. I use a self powered timing light that just clamps over a spark plug lead. I made a timing disc the right size using a print off the web glued on to a old DVD. A bit of wire off a alternator stator bolt to set TDC and you are good to go.

I had a problem with my Atlas differing about 8 degrees between cylinders with associated pinging issues. I used a fine grinding stone on a Dremel to match the two cylinders and it made a big difference. I think the shim method would be good to try first.

Good luck!

Paul R.
 
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