Spark plugs

Hrvatski

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According to original User's manual for our Viragos we should use spark plugs from Japanese manufacturer NGK: type BP7ES for Virago from xv400 to xv1100 - these are spark plugs without built in resistor (suplement is DENSO W22EP-U) and C6HSA fro xv125 and xv250.

Spark plug NGK BP7ES (xv400 - xv1100) - Φ=14 mm

Spark plug NGK C6HSA (xv125 - xv250) - Φ=10 mm

The corresponding spark plugs with internal resistor have label BPR7ES (of course, letter "R" in label stands for RESISTOR). This additional internal resistance built inside spark plug is intended for alleviating the radio-interference (disturbance and noise) of radio receiver. Since we do not have radios on our Viragos which could be disturbed by spark plugs, we do not need this internal resistor. Moreover, existing original Virago spark-plug connectors already do have its own ceramic resistors of 5 kΩ so additional resistance is not needed. If we used these connectors without internal resistor, maybe then it would be meaningful to use spark plugs with resistors.

DENSO W22EP-U

If one used spark plugs NGK BPR7ES (with internal resistor) in Virago that would work as well. The additional resistance from spark plug in ignition circuit lowers the voltage for spark ignition, but the total voltage is high enough (cca 15 kV) and that eventually would not prevent the creation of spark. The spark is ignited in this case too, but it is smaller, and lasts longer. In some cases that can even be required. If you want to setup the motorcycle to operate with leaner mixture (more air less fuel) then longer spark is more appropriate for complete combustion of mixture.

According to Haynes manual the spark plug cap resistance:

• xv500 and xv535 models - 10 kΩ
• xv700 - xv1100 models - 5 kΩ
• TR1 model - 7 kΩ

YICS

Japanese even experimented with this concept of longer sparks in period 1981-1986 on some of their models - this system is called YICS (Yamaha Induction Control System). For example Yamaha on model XJ700 (in 1985.) used 10 kΩ resistance inside the connector plus spark plug with 5 kΩ internal resistor in order to achieve this effect.

Yamaha XZ550 - carburetor and YICS

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The explanation of letters on NGK spark plugs

NGK (BP7ES)

B --> Thread diameter = 14 mm
P --> Construction = Projected Insulator Type
(K) --> Construction = Hex size 5/8 in, projected tip
(R) --> Construction = Resistor Type
7 --> Heat Rating Number = 2 Hot -> Cold
E --> Thread Reach = 12.7 mm
S --> Firing End Construction = Standard 2.6 mm diameter center electrode
(VX) --> Firing End Construction = High performance platinum
(K) --> Firing End Construction = 2 ground electrodes
(N) --> Firing End Construction = Special side electrode
(-11) --> Gap Width = 1.1 mm (0.044 in)