Josef’s A10 is misfiring

[Steverat]

After careful reassembly in the UK and the friendly online advice from many of you including Swarfy the A10 made it back home to the Munich area a couple of months back. Unfortunately not had much use due to an awful misfire throughout the rev range which reminds me of capacitor trouble I had on another bike. A10 fires up sweetly and ticks over without drama, but drive it away and you are greeted with this random misfire. I’m satisfied it’s not carburation because we have tried varying everything in that department without effect. The mag has been recently and thoroughly rebuilt by a seasoned professional who I have used many times before. Have you any ideas for us? How can I check the capacitor in situ on a K2F?

[groily]

I'd check the brushes and slipring first. If the brushes can write like a pencil (many sadly can, despite the vendors knowing darn well they're useless) throw them away and get some harder ones, and clean the slipring with care to get carbon traces off it.
This is such a common  problem with the pattern pick-ups & brushes that are sold widely, that it has to be the first thing to check, especially as it causes random misfiring, worse under load, etc. It has been an epidemic, more like, over recent years!
Then I'd have a look at the contact breaker. If it is a steel backplate unit, make sure the spring blade on the opening point isn't kissing the camring on either lobe. Because the opening point is 'live' on them, the lightest kiss will interfere with sparks if there's a short to earth. This is also a common problem. (If it's a brass backplate, not a problem though, the opening point is earthed.)
Checking condensers in situ is hard as one end of it at least needs disconnecting from the primary winding to be able to read the capacitance or subject it to a leak test. If you do suspect it, then really the mag has to come to bits.

[Swarfcut]

 Thanks for the mention Steve.  This frustration is a minor problem in the grand scheme of things, considering how the bike started off. Esteemed member groily knows a thing or two about magnetos, you may be better off contacting him directly if you find the problem proves too challenging.
 The  steel backing plate kissing the cam ring is a well known foible of the design. My quick suggestion is to run it in the dark, maggy cover off, look for random sparks and also sparking from the HT leads where they run close to any path to earth.

 All the best, Swarfy.

[coater87]

 wouldnt you get quite a bit of arcing at the points if the condenser was bad?

 Lee

[groily]

very probably would Lee, yup.

[lawnmowerman]

I'd check the brushes and slipring first. If the brushes can write like a pencil (many sadly can, despite the vendors knowing darn well they're useless) throw them away and get some harder ones, and clean the slipring with care to get carbon traces off it.
This is such a common  problem with the pattern pick-ups & brushes that are sold widely, that it has to be the first thing to check, especially as it causes random misfiring, worse under load, etc. It has been an epidemic, more like, over recent years!

Bill's advice is spot on. I had the mag rewound but the brushes and pick ups were the originals and the bike was firing all over the place. A clean of the slip ring and new quality brushes, pick ups and acorns transformed the bike's performance and starting.

Jim

[Steverat]

Thank you all for your time and ideas. I’ll try all of them on Monday when we get back to Josef’s. We are having a cool time in Hochgurgl this weekend - the Top Mountain Crosspoint Museum holds the Hand Shifter Hill Climb tomorrow. No BSAs in it as far as I know- pity!

[Steverat]

Pickups and slip ring looking good so proceeded to dismantling points - they are quite badly eroded in the centres. I polished them with a fine needle file . Reassembled-hey presto no difference, machine still misfiring. Any more ideas? Paul the repairer insists it’s not the condenser but what else can it be?

[chaterlea25]

Hi Steve,
Run the bike with the points cover off and see if there is a lot of arcing going on
Pitted points is an almost sure sign of bad condenser
I'm sure you tried new plugs and do not have suppressed plugcaps fitted

On a newly built engine the valve clearances can close up but this does not cause misfiring

John