A7 firing on one at low revs- lumpy running etc

[A7Paul]

Hi,  I could do with some help.  I have broken one of my rules and have been fiddling with both the carb and the ignition and am now at a loss as where to go now. Bike will start  but runs on one at low revs and small carby openings, revs fine,but bogs up and dies if revs fall.  Have adjusted points and found different gaps for each cylinder. So I have purchased and fitted a new cam ring (top quality, Ive been told). But this has made no difference.  Points gaps are again different by 2 or 3 thou. (Normal or worrying!) Engine still runs on one at low revs, but goes on two as revs rise.  Have checked magneto pick ups and cleaned slip ring.  New plugs fitted and points also. I had earlier fitted new main and pilot jet and fitted a new air filter and hose.  Have now taken off the filter and hose but no improvement.  What can I do now to find problem .

[bsa-bill]

Hi Paul
2 to 3 thou equates to 4 to 6 degrees difference in timing, depending upon your timing this can be significant, if you had it timed correctly then it might not make a difference but if your timing was out a couple of degrees to start with then it could be the cause of your running on one at low revs.
I'd be inclined to send the cam back and get one from somewhere else

[RichardL]

Paul,

It might be worth looking at this post (and the rest of the discussion) from a10gf wherein he describes bearing alignment/eccentricity in the magneto. I don't know if this is your problem, as I am a long, long way from being one our magneto experts. However, my memory is still pretty good and so, apparently, is my ability to search our forum for buried treasure.

Richard L.

http://www.a7a10.net/forum/index.php/topic,1375.msg9862.html#msg9862

[A7Paul]

Thanks Bill and Richard for your advise on this problem.  I have removed and carefully measured the cam ring and the lobes measure the same, so this is not at fault.
Richards link gives food for thought.  I have never had the mag off and am a bit nervous about stripping it to try and move one of the bearings to centralise the spindle.  Is it possible to just remove the points end (two screws) to get to the bearing at that end and perhaps adjust it a little.  Your advise please.

[RichardL]

Paul,

My disclaimer of expertise regarding the eccentric running of magneto armatures is not for modesty. I can't answer the question you've asked and I know there are people here much more qualified to comment (names come to mind: a10gf, groily, trevnoz, others whom I hope are not offended by my not remembering them at this moment). Even though I carefully followed Erling's (a10gf) work on this, here, I was just playing librarian.

Richard L.

[groily]

Are you saying, Paul, that there's a bit of 'wiggle' on the end of the armature? Ie that you can wobble the contact breaker end up and down and side to side a bit?  A slightly worn bearing can be (re-)shimmed to get rid of wiggle, a dead one will need to be replaced (not the easiest job in the world). A lot of cam rings are sold to people who actually have a bearing problem! A bit of wiggle doesn't usually afflict just one side though . . . so, before you start thinking expensive and complicated thoughts - Is it always the same cylinder that goes funny on you? If so, have you tried swapping the HT pick-ups and leads over, and the plugs, to see whether anything makes the problem swaps sides? I know you've replaced all sorts, but funny things can happen in Lucas Land.
Or do both sides play up - general flatulence rather than one-sided?
On the points gap difference, far better people than me live with 2 thou variations between cylinders; it does equate to a timing difference - which matters more on performance engines than on my softly-tuned A - but even in the best-regulated environments identical gaps can be hard to achieve. Doubt any of my mags is correct to under a single thou, and I doubt many other people's are either unless they have been smart and brave enough to remodel their cam ring.

[trevinoz]

I always "remodel" the cam ring while I build the engine.
Easily done with a degree disc and the head off.

      Trev.

[groily]

I remembered that Trev, was thinking of you while bashing the keys. Of the 4 K2Fs I have at the mo, only one refuses to come close (a thou-odd I live with happily enough) after having done the usual mechanical stuff. The 4th one (a manual job), which is my spare, with new electrical guts and bearings etc, decent end housing but not a new cam ring, gives a 3 thou variation which is a bit OTT. I'd like to dare to give it your treatment one of these days - Do you just dress the peaks to get the gaps the same at 180° or do you have a smart way of checking or profiling the ramps as well? I've never known how critical the dwell is on these things, in terms of getting the full charge, but it must be at least important? Not sure I've ever even seen a fig in degrees for it, to be honest. Do you happen to know by any chance?!

[trevinoz]

Bill,
       The ramps and peaks have to be dressed to get the damned things even.
I use a fine cylindrical stone in a drill.
It doesn't take much but is fiddly as the cam ring has to be in and outed several times.
I guess that the cam ring then belongs exclusively to it's magy.
The slight adjustment to the dwell doesn't appear to affect the operation.
I have no idea what the dwell should be but I will ask the magy bloke next time we have a talk.

  Trev.
   

[groily]

All stacks up a treat Trev, thanks.
If you did get a closed/open ballpark number of degrees, I'd be very interested. Agree with you it probably makes not that much odds, especially given that the same K series guts support singles and V2s as well, but it would be interesting to know (if there's anything to know).
The fuss some people used to make about dwell on Kettering systems . . . you'd think it might be more critical somehow with a rev-related ac LT generator in a maggy, esp at low rpm . . .
Cheers