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[samj]

Test-more to follow.....

[samj]

OK,system works here goes. 1953 Redditch 350 Bullet running fine -engine cuts out, won't restart. Plenty of fuel,filler cap vent not blocked. Plug out (normally tan colour)but now so white it looks bleached! Look round for other clues-find nothing. After about 10 minutes give one last kick,starts immediately runs perfectly all the way home. Check plug again-back to its normal tan colour. I suspect something in the mag's breaking down but before I go any further has anyone got any ideas and why was the plug so white when it stopped but normal when it ran again? By the way I never reported back on the Honda 550/4 bearing problem I raised last year. All sorted please accept my belated thanks.

[Gwilly]

If its the original mag then as you suspect it must be time for an overhaul, a specialist is probably best for guaranteed refurb..

I would forget the plug for now until the bike is running for more than ten minutes without failure..

[Davie Hall]

I would check and make sure all the jets are clear in the carb, sounds like it starts to run very lean. If it was electrical you would expect the plug to be the normal colour or possibly turning black

[samj]

I should have added that on both occasions I had been riding for about 20 minutes and the bike had been running fine.When it stopped it was as sudden as switching off a coil ignition machine.

[Ian Martin]

I doubt very much if an intermittent fault with a magneto or any ignition system will "bleach" a spark plug. I have always used the colour of a spark plug to determine if the mixture is correct. The lighter the colour the weaker the mixture and conversely the darker the colour the richer the mixture. Given that the bike stops when the plug is light coloured I would lean (no pun intended) towards fuel starvation, possibly a sticking float valve not allowing fuel into the float chamber. Later when it is free again the engine will receive the correct fuel to air mixture and happily run home again giving a correct coloured plug. Years ago I had a 650 Panther which refused to start when it was hot but would burst into life with one kick once it had cooled down. Two issues were at play then. The mag broke down when hot and the Bakelite spacer between carb and manifold had been left off resulting in the fuel evaporating from the float bowl. In the Panthers case the plug remained the correct colour. Also My old BSA 350 did a similar thing to yours and it was traced to a missing filter tube which should have been inside the petrol tank attached to the petrol tap, this allowed flakes of rust and general crap to block the fuel supply to the carb. I hope this gives you food for thought, Ian

[Alan R]

Hi guys---------- try the same test run with 2 other identical plugs ( if you get the same results that takes the first plug out of the equation)....A magneto breaking down due to a temperature fault tends to be erratic rather than like a switch effect.... I tend to agree with IAN MARTIN ie it's an intermittent fuel feed problem...I'm guessing you have an AMAL MONOBLOC fitted ??.......Why not a)Change all the fuel lines for new,... and b) Fit an in-line fuel filter like this period-looking one >>>>>>>>>>>>>>> http://www.ebay.co.uk/itm/like/12116948 ... e&lpid=108& ................... c) Give the fuel tank innards a good clean-out ( Consider having it lined against corrosion etc ??)------and service the fuel tap filter........Check the float level..Very easy on a Monobloc..

[simon]

You could check the points on the mag to see if they have a white deposit on them if so this is a sign of condenser failure which can be intermittent. Also it could be failing windings on the mag which will also start to break down when the mag gets warm. If you can't find the solution in an easier spot I suggest getting someone to put a mega on the windings to check the continuity. If they are good it's time to replace the old cloth condenser with a modern solid state one. I'm assuming we are talking a mag/dyno here with an N1 type mag as opposed to a later SR1 style mag..

[Mark M]

If there is a capacitor problem it means a complete magneto strip and rebuild which although a rare event is a pain. However the clever chaps at Brightspark Magnetos have come up with a neat little fix which can be installed without dismantling the mag. Have a look at their website where there are several clever products for mag users. REgards, Mark

[Dennis C]

Up to now you have been given some good advice and some which is not so good.

Two simple tests you can do on your mag, 1/ Get a mutimeter and set it to the Ohms range place one probe on the spark plug lead and the other to the case of the mag it should read 5K Ohms if not it needs a rewind if it reads OK and it is the original coil get it rewound anyway it is a breakdown waiting to happen. 2/ Kick the bike over whilst looking at the spark plug you should see a good slightly blue spark, get a hairdryer or heat gun and heat the magneto up until good and warm to the touch (at least 15 minutes you are trying to get the armature warm) kick over again and see if the spark is still the same or weak/gone if weak the condensor is leaky. The thing to remember about these old condensors is they very rarely go open circuit causing the points to burn they mostly go leaky and short circuit or partially short circuit the primary coil, heat causes the fault to get worse and cooling allows the fault to decrease, there is no point fitting a Brightspark condensor until the old one has been removed so a strip of the armature is still required unless you are lucky enough to have one of the few armatures where you can reach in with a pair of wire cutters and snip it out of the circuit.

All the above assumes that you have checked/replaced the earth brush and pick up brush which should have been the first things to look at.