Spark Plug Grades for JAWA CZ

SPARK PLUGS

This Article is prompted by the latest issue of Old Bike Mart and what I found to be a confusing article by Stan Dibben who has written a book “Spark Plugging the Classics”. 

It reminds us that older systems produce 8,000 to 12,000 volts, suggests newer type plugs may be better than original specification and then says they need 15,000 volts. Surely a recipe for problems if you fit a modern plug? Stan says a modern plug should be better than the old types – if not suspect the voltage supply.

What is clear is that the greater the surface area of the electrodes – the greater the current needed to spark. It is also obvious that the greater the electrode gap – the more current is needed for a spark to jump it. Modern plugs may come gapped at 32 thou or 0.8mm whereas older machines need 25 thou and sometimes a little less.

So check the gap of any new plug you fit. Also compare the new plug with older ones. If the electrodes are wider giving a greater surface area - then it will test your system for every spark. A plug with a smaller diameter centre electrode will perform better than one with a “fat” electrode if low voltage is the issue.

I am far from sure how the plugs with multiple electrodes sparking to the centre, fit this discussion on optimum plugs but I have used them in the past hoping for some improved performance. I have not noticed any difference so tend to go for the cheapest original specification plugs. Usually NGK B7HS has suited my riding style.

What is important is maximising the current to the plug so if I have any doubts I replace the plug lead and plug cap. I always use old fashioned copper core plug leads and again usually NGK plastic caps or a rubber cap on “off road” bikes. For originality some CZ JAWAs had PAL metal plug caps. Importers Skoda used to uncrate the new bikes, run them up on the original Czech plugs and caps and then replace the Czech parts.

Needless to say the coil must be operating correctly – they can begin to breakdown over time. Current to the coil and any earthing in the system must be good too. So clean the plug hole with some cloth to remove any oily muck before fitting the new plug. Periodically I spray everything with WD40.

Some new plugs do not work or may fail after a few yards running. It is always prudent to try your new plugs before setting off or putting them aside as your spares. Make sure you have more than one spare with you.

Do you chuck old plugs in the toolbox thinking you might use them again? Why? There was a reason you replaced them........... so chuck them out. You will have a suitable tested spare available anyway – won’t you..................

The perfect plug in your bike will have a grey to brown insulator and show slight electrode wear on the centre electrode - which becomes rounded as the edges are eroded by sparks.

Reminder  JAWA 350, CZ 125/175/250 is not a model. Those numbers are the cubic capacity or cc. Any parts supplier needs the model number and year. Pete Edwards

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© Chris Baddiley / JAWA CZ Owners Club

Last Updated: 02 October 2011 - Privacy Policy - Terms & Conditions - Email Us