Understanding jetting on 99 YZ400

BillyWho

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#1
Rich,
I'm new to the "dark side" and I don't get how my bike runs great at about 500 MSL, and then I went to Mammoth which is about 7,000 MSL and it ran and felt (power wise) EXACTLY the same.

I added about 15% oxygenated fuel to pump gas at Mammoth, but I can't imagine that stuff works THAT well. I want to get it jetted right for down here, and no I haven't checked the plug (I'll do that this Thursday) which I've heard is the ONLY way since there isn't the typical "quick" check on 2 strokes at the tip of the silencer for coloration.The only thing it does differently down here is it seems to detanate and stall (very seldom) at idle after a hard moto (for me) on a hot day. I'm assuming it is a hair lean, and any suggestions would be appriciated on whether to change the needle or air screw or both. If these things are really this untempermental, I think I'm in love!!:confused:

Thanks
Billy
 
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#2
Billy - 15% by volume of an oxygenate like MTBE will provide an 2.5% additional oxygen by weight. Given the square root realtionship between air density and fuel density it makes sense that this might be enough to get you in the ballpark. As you've found out these engines tend to be fairly insensitive to anything but major changes in atmospheric conditions. It's the advantage that comes from a high compression engine with a clean efficient combustion chamber and smallish high velocity ports and valves. Those Yamaha guys are pretty smart :)

Sounds like you might be lean on the pilot circuit (most are). Depending on the fuel you can try turning the fuel screw out to about 2 1/4 to 2 1/2 turns for sea level to about 1000 ft and work up from there.
 

BillyWho

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#3
It was 15% by volume, I left that part out. It was the KLOTZ brand but I'm not sure exactly what it was called. I guess it does work that well.
I'll tinker with it a little this week to dial it in, thanks for the info, I'm going to go home now and give it a nice long bath to show my appreciation:D . Thanks again:)
 

Tony Williams

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#4
Mammoth YZF jetting

I made the mistake of arbitrarily reducing the jetting from stock several sizes leaner all the way around for my trip last year to Mammoth. Ran like doo doo.

Using the stock jetting ran great at 7000 feet and 75 degrees, which, of course, means it's a bit lean at sea level, particularly cool mornings. The single biggest benefit I've found was the next size larger needle, which by itself, cured the off idle stumble that many of these bikes experience.

If you're pushing 40 years old or more like me, you can tune out the mega mid range hit with jetting also.

Fun to play with.
 

40+

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#5
99 yzf400

Tony; Now that i've got my WR250F stoked out w/the grey wire mod and the YZ timing, it's time to get the trusty old steed (YZF400) running in top form. Would you shate your carb,etc. mods you have , as far as the idle to go stumble AND BACKFIRING PROB that they all have. I've never really spent too much time tweaking it, and it does need it.:) BTW, I'm still with the stock pipe; only mods are a PE suspension set-up and Pro Tapers.
 

Tony Williams

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#6
I'll find my notes

I keep all that recorded in the notes section of the maintanence manual. When I get home, I'll post that info.