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Discussion Starter · #1 ·
I need opinions on my tuning: I've got a 2000 400ex, original motor, wrapped header, oil change. Rebuilt the carb and adjusted valves. After a lot of trouble with backfiring through the carb (signifying really lean) I'm running a 42 pilot jet and a 150 main jet. This helped a lot, but I'm still about 3 turns out on the fuel/air screw and a hair on the idle adjustment. It still coughs tons through the carb when cold, and nearly runs right with choke on during a cold startup. But once significantly warmed up (like I need to throttle it for quite some time to get it hot enough) runs great, idles great, and the plug looks good. Lots of power, feels great. I haven't messed with adjusting the needle, but it is in the stock position. I have no leaks around the carb on either union. I'm kind of at a loss here, and tentative to up pilot jet anymore sizes since it is already pretty big. Opinions?
 

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If it runs great hot, what's the problem? You are not supposed to run hard when cold or just warm. Ever heard of a "cold seizure"? It happens when the engine is run hard before it is at normal operating temp.

You didn't mention any mods to the intake, but I'm guessing you have chanhed the air filter to low restriction and probably removed the lid. Yeah, you might have to go up to a 42 on the pilot jet if you made changes to both the intake and the exhaust. A wrapped header just keeps heat in and it's a cheap way to disguise a crappy head pipe - changing the muffler screws things up, but not as much as changes to the intake. I put a stock muffler back on mine to get it to idle right and it runs better with the increased back pressure. Starts right up, but needs choke and throttle play till it gets up to temp.
 

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Discussion Starter · #3 ·
Well that's the thing, my airbox lid is still on and I'm using an oiled uni foam air filter, stock muffler. I don't quite understand why I've needed to go so rich to obtain normal performance. It seems to me that you can obtain similar results by adjusting the needle like that of adjusting the fuel screw too.
 

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The slide needle is for mid-range. It controls fuel from about 1/4 to 3/4 throttle opening and at a steady speed.

Sometimes it's the fuel - higher content alcohol fuels require richer jetting. Try using non-alcohol fuel or at the very least premium fuel from a brand name retailer (I like BP), but I use VP Racing MOTOSPORT 93 - it's $64 plus shipping & hazardous materials surcharge (totals about $120 for 5 gal, but the hazmat charge is capped at $150, so I always buy six 5 gal pails each time I order it. IT IS AWESOME! Lasts for 2 years uncapped and over a year in the tank. The throttle response is instant and it goes farther per gal than the pump junk.
 
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