99 yz400f jetting at a mile high

Is there anyone out there riding in Colorado that could help me with the jetting on my bike and have no clue what I'm doing and I get piping on decel and lack of immediate throttle response I have a jet kit for it that has a 48 pilot and main jets are 168,172,178,182 right now I have stock jets needle is two or 3 notches from the bottom

Read: http://www.thumpertalk.com/topic/604117-1998-99-yz400f-user-jetting-database/?p=9395567#entry9395567


If the clip is two or three notches from the lowest groove, it will be too rich, most likely.  The needle has 7 grooves, numbered 1-7 from the top. Start with it in the center (#4) position.


Assuming stock jetting is correct at 1000 feet, you can go by the general rule of thumb that you reduce the size of the pilot and main jets by 1% for each 1100 feet of altitude increase.  So if you are at 5400 feet, that's a 4400 foot gain, divided by 1100 equals 4%.  If you then have a 45 pilot and a 170 main, you would multiply each by .96 to arrive at a 43 (closest size) and a 163 (if available, 162 if not).


A 48 pilot is probably too big at 1000 feet already.

Ok I won't even touch that 48 and I'll try to puck up a pilot but for my main which is a 175 now I did that equation and it was 169 should I throw the 168 in and set clip dead center?

Also I read that running Rich is real bad what are the best ways to recognize if it's too rich

I would rather run a tad too rich than too lean. Foul a plug or burn a piston up, choices....


Where in Colorado?

Edited by morphrider

I live in fort Morgan and always ride ohv areas around here and Greeley and tracks around Denver never really in the mountains and yeah I guess your right a plug is a lot cheaper

I am over in Longmont area. Every bike will be a bit different as far as jetting goes. I just recently did top end on my 250f and went ahead and purchased a JD jet kit for it. Still have yet to try it out due to this wind. Try your main at that 168, clip in the center notch and then play with it. Jetting always takes awhile to get just right.

I would rather run a tad too rich than too lean. Foul a plug or burn a piston up, choices....




Old two-stroke info.  Doesn't apply to four-strokes.  Before a four stroke gets to be too lean for its own safety, it will run so badly that no one would ever deliberately leave it that way. 


A thumper with too big  a pilot will tend to stall a lot on maneuvers where the engine is loaded at lower RPM and the throttle suddenly chopped.  The RPM falls back so fast that it dips below idle and stalls.  A leaner idle settles back more slowly. 


A too rich main will be hard to pick up on unless it's way too big, but generally, you'll end up with a black, sooty pipe outlet and a filthy combustion chamber.  To spot a main jet in without a dyno or a gas analyzer is to keep going smaller on the main until it starts to loose power, stutter, crackle, miss, pop out the exhaust under power, that sort of thing.  You'll be able to tell.  Then go from there back up two sizes and you'll be pretty darn close.


Read:  http://www.thumpertalk.com/topic/272073-burner-how-do-you-confirm-the-mainneedleneedle-clip-for-a-fcr-carb/?tru=GOece#entry2289879

Lol, you know, once you stated that it clicked in my brain. Duh.. good catch.

Ha ok I gotcha I'll check it out I just swapped the main it was a 170 so I put my 168 and noticed a bit of difference decel popping is way better and idles a bit smoother should I keep the tips sensor hooked up or no I've read that on these 400s people always disconnect them cause they are out of sync should I try it?

If the bike runs OK at part throttle, there's no real reason to disturb the TPS, although you can try it both ways and see what the difference is.  With it unplugged, the CDI assumes full throttle all the time, and sets timing based on that and the RPM.  The result (compared to a properly functioning TPS) is that the timing will be more retarded at part throttle and cruising conditions, and the engine may not be quite as "crisp" as it should be. 


The condition and adjustment of the TPS can be tested.  The process is in the manual.

Sick man thanks I'll definitely run some tests both ways and let you guys know and I need to wait for my manual to come but when I check it out you guys have really helped a lot

168 jet is causing back firing and throttle won't stay constant it rises and drops slightly while keeping it at mid rpms what now adjust my needle???

The main jet should only be tested for size by running at full throttle. Any less than that, and you're on the needle down to 1/8 throttle.  Read the procedure in the link I posted in post #8.

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