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Thread Like Summary
Allan G, Ragmanx, Shane in Oz, The Bonneville Shop
Total Likes: 4
Original Post (Thread Starter)
#835241 01/02/2021 8:03 PM
by Ragmanx
Which plug would you prefer for my '68 Lightning ?
Thank you
Liked Replies
#835331 Jan 3rd a 05:59 PM
by John Healy
John Healy
There is much confusion when it comes to plugs. A change in grade of spark plug can be required because of changes made to the engine. Raising or lowering compression is one such example. But change the factory recommended plug because a plug fouls either from fuel or oil is not a wise decision. This especially true if the fouling is fuel related.

Take a Mikuni VM or one of the amal series carburetors. There is one throttle position we use the most. That is between 1/8 and 1/3 throttle. This is where the straight portion of the needle sits in the needle jet's orifice. Because the the needle moves back and forth, sliding on the brass jet, with each change in air flow as the piston draws air into the cylinder. This cause the inside diameter of the jet to enlarge over time. This leads to a rich condition in only a narrow part of the power band. The one most of us use the most: the point where the straight portion of the needle is in the needle jets orifice.

This incidental enrichment can cause the factory recommended spark plug to foul. This is happening not because you are using the wrong plug, but because the real fault is not being addressed.

Why is this important? When you changed the grade of the plug to cure the rich condition at low throttle openings the jetting when the taper takes progresively returns to normal with no change from factory jeting as the slide approaches 3/4 throttle.

So the first time you open the throttle above 1/3 throttle without the bike increasing speed (this is what is called lugging) your hotter plug that was a cure for low speed fouling now becomes a problem. It puts you one step closer to destructive detonation and detonation induced pre-igntion.

Oil fouling can create the same problem. Oil or fuel fouling is not a problem to be solved by changing plug grades! If the factory recommended plug doesn't work, and you foul plugs (either fuel or oil) it is not the plug's problem.

I see another post where someone wants to use a hotter grade plug because it has got cold and he has to use the choke. This is not a plug problem! In a properly jetted engine it is normal to have to choke the carburetor to get the engine running.
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