The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: GeoffsA10 on 22.10. 2012 14:30
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Hi All, I have recently bought a 1952 plunger A10 - great bike - but am having a persistent misfire.
I have changed the plugs ht leads etc and points - have also taken the head off and checked the valves but still the same. Basically it has a misfire on the left cylinder when running slow and when hot just stops altogether. It is easy to start from cold after only a couple of kicks but very difficult when hot. Looking at various threads on here and the first question I asked myself was - if it is misfiring on only one cylinder can it be the magneto at fault as it would surely affect both cylinders. Various topics viewed here suggest that it could well be the magneto so that will be my next task to get it looked at. Any other suggestions though ? Also the valves seem pretty good but the exhaust valves are tighter (not TIGHT but less play ) in the guides than the inlets (when cold obviously) is this normal ? Could they be tightening up when hot to give the misfire ? Any help is much appreciated
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A compression test will confirm the valves shutting as they should. Magneto problems can occur on one cylinder only, mine did earlier this year. Same thing as you.
When you time the ignition it is done for the right hand pot, the left hand, in theory will be timed correctly IF the mag is set up right. Check the points gap to make sure they open the same both sides (12 thou) if they are the same put a new slip ring in as the chances are it is contaminated with carbon. Also change the brushes as they are probably too soft.
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Metalflake...Many thanks for the tip. Compression is good and I have just ground in the valves although they hardly needed it so no problems there. Everything is pointing to the magneto so I will check it all out. Thanks again. Geoff.
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G'day Geoff, just to confuse the matter, the tappet/s on the left might be a little close and when hot hold the valve open a tad. Give them a little more clearance (2 or 3 thou more) and see if it improves when hot. Just to rule out a possibility.
Cheers
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Hi Muskrat....good thinking I will definitely try that too....before overhauling the mag methinks !! Thanks