Author Topic: spark plug ignition reduction  (Read 264 times)

Offline psahlt

  • Active
  • *
  • Join Date: Dec 2020
  • Posts: 1
  • Karma: 0
spark plug ignition reduction
« on: 01.01. 2021 09:50 »
Hello there. I have a 1952 golden flash 650cc engine left for me by my grandfather. I took it out of the box and collected it. But every 3 hours spark plug cuts. What can I do with magneto coals solid? I'm using a 5 ohm spark plug header.

Online groily

  • Wise & Enlightened
  • *
  • Join Date: Jul 2007
  • Posts: 1202
  • Karma: 19
Re: spark plug ignition reduction
« Reply #1 on: 01.01. 2021 10:54 »
Ideally, you don't want the 5000 ohm resistor plug cap there, but I don't think it is the cause of your problem. You could try without it to see - but 3 hours running to find out seems a really very long time! It is worth trying new spark plugs, though, as they are cheap and easy to change.

Normally, if sparks are going to disappear when the magneto gets hot, they will stop after a few miles/ km, say 20 minutes, and the bike may then start again when it has cooled.  And stop or splutter again as it gets hot again. That behavious would suggest a faulty condenser or a faulty HT winding, or occasionally a faulty slipring (the bit the carbon brushes (coals) run on inside the magneto) or HT pick-up(s) which leak when hot.
The brushes should move freely in the pick-up on their springs - a clean with electrical contact cleaning fluid or similar would solve that probably. If they were stuck solid, though, I'd expect any fault to show up much sooner than 3 hours!

It is also always worth checking the state of the contact breaker and the points gap - but again, three hours is a long time for it to run before it displays a sudden fault.

If the machine had been left standing for a long time, it is more than likely that the condenser has deteriorated, and / or that the HT coil is tired. There are various tests you can do to check that the HT coil is continuous, but no easy test for the condenser.

As a very basic test, you should be able to get a 5 or 6mm spark to jump from the end of the HT lead (plug cap removed) to earth, at kickstart speed, hot or cold. If it won't do that, then further investigation is needed. If the magneto is off the bike, then you should be able to get 5-6mm sparks off it at the rate of just less than 2 per second, turning it steadily.

The thing with these things is that although they are extremely reliable and can work for many years with no attention to speak of, once they start behaving erratically it is often necessary to carry out a full range of tests to work out what's good and what isn't. It's possible, yes, to play a bit, get it to work for a while, then have more trouble  . . . until you are going round in circles.
If the magneto is 'as original' it may well need a proper overhaul, to be perfectly honest.


Online BSA_54A10

  • Wise & Enlightened
  • *
  • Join Date: May 2008
  • Posts: 2113
  • Karma: 32
    • BSA National
Re: spark plug ignition reduction
« Reply #2 on: 01.01. 2021 11:12 »
My first BSA used to do something like that as well.
Turned out to be the wrong carbon brushes in the magneto pick ups which would get too hot then go soft & loose contact
Fifteen minutes or so latter it would be right as rain & off I would go again.
Took a long while to work that one out.
Bike Beesa

Offline KeithJ

  • A's Best Friend
  • ***
  • Join Date: Feb 2013
  • Posts: 242
  • Karma: 2
Re: spark plug ignition reduction
« Reply #3 on: 08.02. 2021 12:14 »
Had a similar issue but with a misfire rather inconsistent.  Turned out to the the HT brushes being too hard.  Tough to establish the cause.
'59 A10RR + Second engine