Voge spark plugs and electrode gaps by model

The spark plug is the least expensive component capable of making an engine run badly. The specifications below are those printed in the owner manuals.

Types and gaps

Model	Plug	Gap (mm)
DS900X	LMAR9J-9E	0.8 - 0.9
SR4 Max	LMAR8J-9E	-
650DSX	DR8EB	-
SR1 / SR1 ADV	TORCH B8RTC	0.6 - 0.8
300DS	NGK or Bosch B8RC	-
DS625X	-	0.8 - 1,0
DS525X	-	0.8 - 1,0
500R	-	0.8 - 0.9
SR450X	LMAR8J-9E	0.8 - 0.9
SR3	CR8E	0.7 - 0.8
125R	CR9E	0.6 - 0.7
300 Rally	BR8C	0.6 - 0.7
SR16	B8RTC	0.6 - 0.8
525RR	CPR8EA-9	0.6 - 0.8
250RR	LMAR8J-9E	0.8 - 0.9
250 Rally	LMAR8J-9E	0.8 - 1.0

An empty cell indicates that the manual held here does not print that value in a legible form. In such cases the owner's own manual and the plug already fitted are the reference.

The heat range

LMAR9J-9E and LMAR8J-9E differ by a single character, and that character denotes the heat range. The DS900X takes the colder 9 and the SR4 Max the 8.

A plug that runs too hot for the engine becomes an ignition source in itself: the mixture ignites ahead of the spark, which is detonation, and detonation is what damages pistons. A plug that runs too cold fouls at low engine speeds and misfires.

A plug that matches the thread is not necessarily a plug that matches the engine.

Replacement intervals

In the maintenance plan printed in the manuals the plug is inspected at most services and replaced at the fourth and the seventh. For the 300 series engine the chart calls for the plug to be cleaned and its gap checked at every service from 500 km onward, and the plug cap cleaned every 4,000 km.

Setting the gap

The gap should be checked on a new plug as well as a used one. Plugs are gapped at the factory but are also transported in packaging that is not always handled carefully.

1. Measure across the electrode with a feeler gauge.
2. Adjust by bending the outer electrode only. Levering against the centre electrode cracks the insulator, and a cracked insulator misfires.
3. Fine wire and iridium plugs are not adjusted at all. The centre electrode is too fine to survive the operation and these plugs are supplied pre-gapped.

Fitting

The plug is threaded in by hand for its full length before any tool is used. If it does not turn easily by hand the operation should be stopped: the aluminium cylinder head cross-threads far more readily than the plug, and the result is a cylinder head repair.

Tightening follows the figure in the manual for the model concerned. Where no figure is given, established practice for a used plug with a gasket is finger tight plus approximately a quarter turn; a new plug takes approximately half a turn beyond finger tight to crush its gasket.

Reading a used plug

• Light brown or tan: mixture and heat range are correct.
• Black and sooty: running rich, or subject to excessive idling and short journeys.
• White or blistered: running lean or too hot. The cause requires investigation; fitting a new plug alone does not address it.
• Oily: oil is reaching the combustion chamber, which points to piston rings or valve stem seals.

The old plug is worth examining before disposal. It is the least expensive diagnostic available on the motorcycle.

Related pages

Valve clearances for the same engines are listed in Voge valve clearances, and the service at which the plan calls for replacement in what Voge asks for at each service. Idle speed and other engine data are in idle speeds and engine data.