The Process of Jetting - 2006 YZ450F
Friday, April 11th, 2008 at 8:02 pm by angrybob
I am having to learn all over how to jet carbs on bikes. It has been since 2000 since I have had to jet a motorcycle. It sucks at first, but I have a better understanding of how stuff works now. For an excellent techie look at what attribute of the carb affects performance at a certain throttle position, click here. I have been using this graph for years now and it is the absolute bomb when it comes to understanding jetting.
I am going through my bike now and trying to sort the jetting. This is the history to date:
1. As purchased (used & stock performance parts with
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Condition - ran like total crap. Idle was variable, but on the high side like it was running lean. Low end bog, but WOT was good.
Hard parts - 165 main, 48 pilot, and no spring over the hot start plunger. No idea on fuel screw.
2. Shipped to Zipty for carb mod ‘kit’ (includes fuel screw, drain plug, & hot start nut)
Condition - wouldn’t start cold. had to feed in throttle while kicking to get the bike started…like a two-stroke. Figured out hot start plunger spring was missing. Ran much better overall, but low end still had a little bog. Everything else was clean and pulled nicely
Hard parts - same as #1, but fuel screw at 2 turns out & MRD full exhaust system.
3. Rode next time with spring over plunger, but plunger was damaged. Spring was installed, but cable not attached.
Condition - still a total bitch to start cold & still needed throttle w/ choke. Once warm, only once or twice it didn’t start on the first kick. Ran the same as above
Hard parts - same as #2 but with plunger spring
4. 10Apr2008 installed 45 pilot, new hot start plunger (system now fully functional), and set the fuel screw.
Started at 2 turns out (zipty recommendation). Needed throttle to start. let it warm up and rode around my neighborhood. Set fuel screw and found out that I was best at just over 1 turn out. Ran very smooth around my neighborhood, but no WOT. This should be unchanged though. Throttle response seemed the best it ever has. Started a lot easier albeit warm. Will try again tomorrow night with cold start.
5. Tonight (11Apr08) Same as #4 but cold start. One kick, idle came up nicely with cold start, and ran very well once turned off. Progress!
Next step = install 42 pilot to give me some flexibility (want 2-3 turns out) for temp and altitude.
The reason for not stopping where I am is fuel adjustability. An ideal position with the fuel screw is between 2-3 turns. This allows you to add or reduce fuel on the fly due to altitude or temperature changes. Since the bike ran better by reducing the fuel amount (closing the fuel mixture screw), it indicates that the pilot was a little too big. A smaller pilot jet will force me to back out the fuel screw to the 2-3 turns area. Then I have the flexibility I want for the mountainous Arizona trails that I ride.
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Great site, I think you might have had an issue with closing tags above.
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