When I picked up my Scram from the dealer last week I bought a couple extra plugs. The mechanic told me get some "hotter" running plugs for the colder weather. He said they burn up a bit more oil for the cold weather riding conditions.

I swithched from a Brand NGK no.6065 BR8ES to a 3961 BR8ES. Does this mean anything to anyone? Why exactly do I need to change plugs? Will the quad run the same?

Thanks,
Brian

 

Spark plugs serve two primary functions. The first is to ignite the air/fuel mixture in the cylinder. The second is to carry heat away from the combustion chamber.

Generally speaking, a "hot" spark plug is one that operates at a higher internal temperature than a "cold" spark plug. A 'hot" plug conveys LESS heat away from the combustion chamber, because it holds more heat within itself. And a "cold" plug conveys MORE heat away from the combustion chamber, because it holds less heat within itself. The designation of "hot" or "cold" does NOT relate in any way to the intensity of the spark generated.

In order for a spark plug to function correctly the operating temperature can neither be too hot, at which point the insulation breaks down and the plug misfires, or too cold, at which point carbon deposits form on the electrodes, i.e., the plug is fouled, and the plug misfires.

For most applications, spark plug operating temperature should be between 500 degrees C (932 degrees F) and 850 degrees C (1562 degrees F). The degree to which a spark plug is either "hot" or "cold", is measured by the heat-range number. This is a relative number. Every one unit change in this number represents an approximately 70 (158) to 100 degrees C (212 degrees F) change in operating temperature.

In the case of the two NGK spark plugs you mention with "different" part numbers, there is really no difference at all. Both are BR8ES plugs. Let us look at what those code numbers mean:

B = 14mm thread size
R = resistor type plug
8 = heat-range number - a relative number between 2 (hotter) and 11 (colder)
E = 19mm thread reach
S = standard 2.5mm center electrode

If you were to follow your mechanic's suggestion to run a "hotter" plug in your Scrambler, you would have to buy a BR7ES plug. If you wanted to run a "colder" plug, you would have to buy a BR9ES plug. Also as a general rule, it is not a good idea to go up or down more than one heat-range number from stock.

If your Scrambler is air-cooled, it might make sense to run a "hotter" plug in extremely cold weather to reduce the chance of fouling and consequently misfiring. But if your Scrambler is liquid-cooled, and thus runs at a nearly constant temperature as regulated by the thermostat, then I see little sense in what the mechanic told you. It is nothing more than mechanical folklore if you will.

Why not wait until January when the really cold weather is here and see how your Scrambler runs? If it runs OK, then the stock BR8ES plug is fine. If it doesn't run so well, then pull the plug immediately after a long hard run and "read" it. If it shows signs of fouling, then go to a "hotter" BR7ES plug.

Army Man

 

WOW! Now that's one great answer. Thanks for the explanation. I thought the plugs were the same but wasn't too sure.

Now all I have to do is memorize the codes and numbers...either that or I will print them out and post them on my "Scrambler board"!

Cool...or should I say hot!?

 

One problem Sarge there is no thermostate in a 400 liquid moter it is a free flowing system. But like you said run it and check the plug.But instead of changing the plug run a bigger jet in the carb and raise the needle height and you will be fine never lean it out in cold weather Or you will be asking for trouble. Lean to the rich side for saftey.

 

Congratulations, great post. Really informative and clear.
One detail.
When I got spare plugs, I asked my dealer a pair of hotter plugs (for winter). I used to run BR8ES, and he gave me BR9ES(which I thought was exact before reading your post). Following what you said (unless I got something wrong), he should have given me a BR7ES...right? Who's wrong...me, the dealer, or you?
PS When I say hotter plugs, I mean plugs than keep more heat (or dissipate less heat).
Who went wrong?

 

Zorro,

As stated above, a BR7ES plug is "hotter" than a BR8ES, which is "hotter" than a BR9ES. Your dealer erred.

The ceramic insulator on the BR7ES plug is longer than the one on the BR8ES. Therefore it will hold more heat internally than will a BR8ES or BR9ES.

While the difference is slight from one heat-range to the next, it should be readily discernable to the naked eye between two heat-ranges. Since you already bought and paid for a BR9ES plug, I suggest you buy a BR7ES plug and visually compare the two. You should be able to see a difference in length between the two ceramic insulators.

The shorter of the two ceramic insulators will belong to the "colder" plug, and the longer of the two ceramic insulators will belong to the "hotter" plug. Lower heat-range numbers mean "hotter" plugs, higher heat-range numbers mean "colder" plugs. Try a visual inspection of the two plugs for yourself, I'm confident you'll find that I am correct.

Army Man