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Hi all,

This is my first 400ex although Ive been around quads and dirt bikes most of my life. Did all the typical stuff changed fluids, adjusted valves etc etc. Next threw a new plug in it and ripped it around to see how its running. Doesn't pop on acceleration or deceleration and starts on the first push of the button. Looks to me like its running lean should I bump up the main and the pilot or just the main because it idles fine? And would bumping 1 size be sufficient or jump up 2? Thanks in advance (photo below)
4335
 

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Looks rich - how much time is on the plug? You have to run at the speed you want to check for about 5 minutes steady and then cut the ignition before coming to a stop and then check the plug. A plug check will only be accurate if you are using 93 octane non-ethanol fuel. Ethanol leaves no deposits so plugs come out white regardless of fuel ratio or spark plug heat range.

You change the pilot for idle, slide needle for mid range and main jet for full throttle - if you can run full throttle for a minute, you can check the plug for color, but for mid-range and idle it will take about 5 minutes.
 

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Looks rich - how much time is on the plug? You have to run at the speed you want to check for about 5 minutes steady and then cut the ignition before coming to a stop and then check the plug. A plug check will only be accurate if you are using 93 octane non-ethanol fuel. Ethanol leaves no deposits so plugs come out white regardless of fuel ratio or spark plug heat range.

You change the pilot for idle, slide needle for mid range and main jet for full throttle - if you can run full throttle for a minute, you can check the plug for color, but for mid-range and idle it will take about 5 minutes.
Running 89 octane ethanol free as higher octane without ethanol isn't available in my area (Reno). Thank you for the advice I appreciate it, I was having trouble getting a good reading from it because the black around the plug would indicate rich to me too but the white tip made me think lean. I may try it with a new plug again for something else to compare. It runs great so Im mainly looking to make sure I don't hurt the engine, should I be looking for a tan color on the plug when its tuned correctly?
 

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Run for about an hour under normal conditions with a new plug and check it - porcelain may still be white, and tan to light black on the end of the threaded shell is normal - reading plugs has changed since lead was removed and other products blended in to take the place of lead including ethanol. You can run rich without damage - just wasting fuel.
 

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Run for about an hour under normal conditions with a new plug and check it - porcelain may still be white, and tan to light black on the end of the threaded shell is normal - reading plugs has changed since lead was removed and other products blended in to take the place of lead including ethanol. You can run rich without damage - just wasting fuel.
Will do, thank you very much for your help I really appreciate it. I have the occasional thought to run 100 octane because typical pump fuel just isn't as good anymore but I know there's a lot of pros and cons.
 

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You will lose power with 100 octane unless you advance the spark timing about 5 degrees and bump the compression up to 13:1 - I have found that the engine starts to lose power at 95 octane - I use VP Racing SEF 94 octane which is locally available through our NAPA and Tractor Supply.

Using too high an octane will in addition to causing a loss of power will cause the exhaust to run exceedingly hot and raise engine temperature and can cause detonation (similar to running diesel fuel in a gas engine) - it won't hurt anything under low speed and low load conditions, but the damage occurs when trying to accelerate or pull a load at moderate engine speed. Running too low an octane may cause pre-ignition which left unchecked, can burn a hole in the piston or cause piston seizure. Try to stay between 91 to 94 octane
 
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