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I have a 04 400ex with a Yoshimura exhaust and k&n air filter with an altitude of 1,400 I was wondering what jet to run I am running a 160 and it was getting red hot and scared it is not supposed to be like that, please help.
 

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What temperature does steel glow red at? About 850 degrees F
What temperature is the exhaust gas at the exhaust port? About 1200 degrees F
What jet should you run with a K&N filter? The right one to maintain a 14 - 16 to 1 air to fuel ratio when topped out in high gear.
How do you know what is the right size main jet? You check the exhaust O2 with an electronic sensor while running the engine on a Dyno, or use the 'SEAT OF THE PANTS' Dyno.

The exhaust does run red hot, it's just that normally you do not see it unless running in subdued light (in a poorly illuminated building, early evening or at night. Also, the cheap, thin steel of a performance exhaust system will glow brighter and quicker than the heavy steel or the stock exhaust. Install a cast iron head pipe about a foot long and it will never get red hot

Your altitude is not a factor. When you get above 4000 feet, then the atmosphere is lean enough that you neet to decrease the amount of fuel being mixed with the air. It will run like the choke is on above 4000 feet and reducing the main jet to a 142 instead of the std 148 will correct the mixture.

Changing the exhaust had little to no effect on the carburation, but any effect it may have is only at the top of the RPM range of the engine when under load (topped out in 4th or 5th gear), but changing the intake has the greatest effect. Just removing the lid from the air box will result in a loss of performance above 1/2 throttle. Just replacing the quality OEM foam filter with the cheap K&N gauze filter will affect top end performance by leaning out the carburetion. Without the aid of a Dyno to achieve top performance, you get a handful of jets from 150 to 170 (they come in increments of 2.5) and you go up in size until you feel the engine fall flat near the top end, then jet back to the jet it ran the best with. Too lean it will taper, maybe surge a bit and a little choke will improve the way it runs momentarily - too rich and it runs up to near top end and then falls on it's face blubbering like the choke was left on.

Good luck - it takes patience to select the correct size jet. NOTE: jetting should be done using gasoline without ethanol. Ethanol fuel provides a false octane as it requires 1/3 more alcohol than gasoline mixed with air to achieve good combustion temperature. Alcohol burns cooler than gasoline and with ethanol fuel you may have to jet as high as 200 or more to get near the same performance as gasoline. Also, do not use gas whit a higher than 94 octane rating - best results will be with 91 to 93 octane pure gas. BTW - the pipe will still run red hot; it's a thermal engine and it gets it's power from the heat of the burn. If the glow bothers you, wrap the head pipe with exhaust wrap so you can't see it.
 

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Take that K&N off and throw it in the trash. You wont believe the dust it lets by... or if you want to prove that to yourself, put a thin coating of oil inside your intake boot. Snowmobile engine fogging spray works great for this. Run the machine for 4-8 hours on a dusty trail, pull the filter, run a clean finger inside the boot... feel that grit? Thats what the filter is letting by... and going directly into the cylinder.

I've gone back to OEM foam filters. Any HP gain isn't worth the wear a K&N will cause to your motor.

-DallanC
 

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I agree with Dallan - K&N air filters are designed for performance and the designers of the K&N filter accept that performance engines are rebuilt routinely as a cost of racing. I race, but I use OEM, UNI and TwinAir foam filters as they trap more dirt, are easier to clean and oil and cost much less than the fragile, difficult to clean and tedious to properly re-oil K&N.
 
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