yz450 big bore over heating

Seems a little out there but I will give it a try

Why do you think that? It's the correct way to do it. If you are doing it any other way, you aren't doing it right.

The throttle chop method used to be the way to tell something about the main jet, but only the main jet.  Furthermore, if you are using unleaded gasoline, reading the plug will tell you nothing at all about the jetting because the lead in the fuel is where the color came from.  A correctly jetted engine with the right heat range spark plug will produce a bone white plug when using this method, and won't tell you a thing.  The only information you can get from looking at the plug anymore is whether the ignition is OK, and whether the plug is the right heat range.

 

If you want to get serious about rejetting, follow this:

 

http://www.thumpertalk.com/topic/272073-burner-how-do-you-confirm-the-mainneedleneedle-clip-for-a-fcr-carb/#entry2289879

I'm on old info then. My own experience with that method is on a couple 04 bikes and a couple jetskis. It works well for jetskis, and the 04's at least.

That's a old wives tale from the 70's when they started fading out leaded fuel.

Plug chops work just as well with unleaded fuels. We tune our cars on the dyno before each race with wideband air/fuel sensors on each cylinder and read the plugs after each pass. You can read any jetting changes off the plug each pass. I can then look at the data logger from each pass and verify the air fuel ratio. We plug read as a backup to verify the A/F Sensors are OK and not melt down a $30,000 engine.

This is the same for leaded or unleaded fuels.

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