Spark Plug Gap

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Spub
Mach 3
Posts: 94
Joined: Mon Feb 27, 2006 4:01 pm
Location: Pasadena, CA

Spark Plug Gap

Post by Spub » Mon Jul 13, 2009 9:07 am

I have always had a problem with my 900SS idling. It has 40 PHM dellortos, and while it would idle if warmed up, the idle was not real reliable. I started playing with the fuel air mixture and the spark plug gap. Bike started out with a .7mm gap, and I first decreased it to .6mm which did not help. I then increased the gap to .8mm and the idle was much improved. I then enriched the fuel mixture a quarter turn, and the idle seems much stronger and reliable. I am running 2 Dyna 6 volt coils in series and a Lucas Rita ignition. Wires are non resistor copper core, and the plug caps are NGK 5ohm resistors, and the plugs are NGKBP6s.

Has anyone tried adjusting the spark plug gaps on their bikes, and what were the results?
1982 Laverda 180 JOTA, 1975 Moto Morini 3 1/2 Sport, 1993 Moto Guzzi Daytona, 2010 KTM Adventure 990, 1974 Laverda SFC #17188

wdietz186
Cagiva Alazzura
Posts: 703
Joined: Sun Sep 10, 2006 5:40 pm

Post by wdietz186 » Mon Jul 13, 2009 5:26 pm

The 40s aren't really ideal for a consistent idle.Due to the available vacuum signal and their size the low speed mix is delivered in blobs rather than a finely atomized mix.With a good voltage supply the Dyna coils deliver a strong spark which will ignite a wide range of mixtures. The recommendation of the close gap[.020] is to reduce the load on the stock ign. syst. which is weak and failure prone.If you have found a setting that works go with it but don't be suprised if a temp. change or a phase of the moon affects the idle quality.

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