Hello.
All the above suggestions have merit.
Here are some of my suggestions you can try.
1) Unless you did a throttle specific chop IN the throttle position/fuel circuit you are testing plug reading is suspect.
Simply removing the plug and looking at it is not an indication of AFR plus, you need to look at the spark plug base ring to determine AFR. Plug reading is complicated.
2) Ambient air temperature/air density have been super high lately, this causes a rich condition. Are the needles you have/tried the same p/n?
If not, use a micrometer and write down the diameter top to bottom of the two.
http://www.sudco.com/Carburetor/MikuniCatalogPg32.jpg
Larger diameter = less fuel = leaner.
Smaller diameter = more fuel = richer.
The I.D. size of the needle jet vs the O.D of the needle is the amount of fuel that can flow through the needle jet to the main jet.
3) I would do a throttle specific to fuel circuit test to determine the exact fuel circuit to change.
0 ~1/4 = air screw/pilot circuit.
1/4 ~ 3/4 = needle position.
3/4 ~ WOT = main jet.
BUT,,,, ALL fuel flows through the needle jet to the main jet (not counting off idle where the slow speed circuit comes into play) so don't forget that the main jet DOES come into play in the 1/4 ~3/4 throttle position.
The needle plays a HUGE part in tuning which is why there are so many variations of needle/clip position (s).
4) Do you set the sir screw each time riding? On both 2 and 4 stroke (although 4 strokes have a fuel screw and the slow speed metering system is completely different) you do need to set the air screw or fuel screw each time you ride to the
highest smoothest idle once the machine is up to normal engine temp, then re set the idle to where you like it.
5) Pump fuel is always suspect if you are looking for stability. I did dyno testing when I was at SuperTrapp/Kerker using different local fuels and found a huge difference in quality and performance.
A dyno room is supposed to be temperature/humidity controlled and have a high air change rate per hour thus a stable testing atmosphere to give consistent results.
I have done allot of testing in poor dyno rooms where the exhaust gasses remain in the room giving false powercurve readings making fuel re calibration a nightmare. You also get a nice headache.
Sunoco was the best pump fuel batch to batch.
Additives in todays pump fuel make it harder to get a proper AFR than 100% race fuel.
ALWAYS use race fuel if possible, race fuel is consistent batch to batch, pump fuel is NEVER consistent batch to batch in quality and additives. Always, always test ANY fuel for water.
I use Kolor Kut:
http://www.grainger.com/product/KOLOR-KUT-Water-Gauging-Paste-8UNU1
6) Keep a log of carb circuit settings per track and note the ambient air temp and water grains. Sand tracks/muddy conditions are of major concern. Sand/mud taxes the machine increasing engine temperature and throttle position.
7) Crude ways to check AFR: do a more air less air test. Simply taping over the airbox inlet will show a quick result.
More air = less fuel =leaner.
Less air = more fuel = richer.
Hot ambient temp/high water grains = more fuel = richer.
Cold ambient temp/low water grains = less fuel = leaner.
For all the 4 stroke racers: Innovate has a fantastic AFR you can use to keep tabs on your machine. Summit racing has them.
http://www.innovatemotorsports.com/products/MTS.php
I have a AFR on my drag bike combined with a weather station I can predict how my machine will run in different ambient conditions.
8) 2 strokes not only have to pass fuel through the fuel circuits but they also have to pass the lubricating oil through the same fuel circuits.
A change overnight in oil manufacturer can change the AFR.
9) Usually, once you set up a fuel system (2 or 4 stroke) you normally do not need to make huge changes spring, summer to fall. The setting of the air/fuel screw is the exception.
So, if your machine is running poorly it is probably something else you did to the machine and not the fuel system.
10) Dirt bikes are not racing where TIME is measured in your 60 foot, or your elapsed time or index. Your set up in the morning may be totally different than the heat of the day where a weather station and AFR monitoring
allow you to make changes to your dial and or fuel re calibration so you can meet your prediction or index. Dirt bikes are forgiving, we do not "feel the difference" in minor ambient changes.
Good luck and I hope this helps.
