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I was just packing up the Flyer (Lomax 223) this evening for a trip tomorrow when, just as I was turning it round in the drive it started massively misfiring. I could keep the engine running (just) but it would fire a few revolutions then nothing for a few revolutions then pick up again for a few more.

After checking the obvious things (plug and coil connections, compressions - not a stuck valve) I checked the points. I gave them a quick go with a bit of folded emery and there was an improvement. The engine will now idle but still misfires when speeded up. I watched the points in motion and they spark quite merrily (small blue sparks) brightening up with higher engine speed. My question is, is this sparking normal or is it the sign if a failing condenser? If it's normal I might just try giving them a better clean in the morning before I go. If not, I shall have to try fitting the spare condenser (and maybe points) I salvaged last time I changed them. That would be a better solution but I am short of time.

What I don't understand is why it should happen so suddenly when the engine had been running perfectly. Any advice would be most welcome.

Ff
An occasional small spark is OK, but if it's like this poor thing, then your condenser is kaput and that amount of arcing will burn out the points in short order.

http://www.youtube.com/watch?v=ro9xMG_VMYU

A quick fix is to install another condenser next to the coil, earthing its casing and attaching the lead to the one running to the points box.
Depending whether the original condenser has failed in 'open' or 'closed' mode, its lead may need to be cut to isolate it...

Ken

Success!! Swapped out the condenser, cleaned the points and Bingo! back to 100%! Minimal sparking when running (last night it looked very much like the video you gave me the link to)

A few lessons learned. I thought I had worked out a foolproof method of testing a condenser. Put a meter across the condenser (disconnected) set to highest resistance scale (20 megohms on mine). A healthy condenser should show initially a minimum resistance as it takes current from the meter and charges up. In a couple of seconds the resistance appears to increase to infinity as the voltage built up on the condenser matches the meter voltage. Reverse the leads and the same thing happens again. Perfect in theory, but in practice both the faulty condenser and the replacement passed the test (although the "faulty" condenser took a little longer to charge up - not sure if that is significant, I would have thought it meant a higher capacitance). My conclusion is that the condenser was OK at low voltage but breaking down at higher voltages.

The other difference is that before my work this morning, levering the points apart with ignition on produced no spark - now it does produce a small spark. This suggests that the points might not have been making a good contact. I put a meter across them when they were cold (ignition on) on a voltage scale and got 0.8v. If there were no contact between them I would have expected 12v and if a perfect contact 0v. Added to the fact that running improved after cleaning the points suggests to me that the mechanics of the problem was that partial breakdown of the condenser caused arcing which burnt the points resulting in the misfiring. Just cleaning the points might have restored good running for a time but would have recurred.

Anyway, thanks Ken for your help

Ken as ever comes up with the goods. As an aside, I make a point of changing the points and condenser (and oil of course!) as a matter of routine every 3000 miles. For the nominal cost it can save a lot of grief. I also carry a spare set of points and a condenser in the car.

Condensers have caused my problems in the past , I have had them faulty from new. My opinion is:- always carry a spare but if its working don't fix it .
I ended up fitting a DG electronic ignition which is great but I now carry a spare points box and points and of cause condenser

Regards Ray