Jetting Jettting Jetting

[REDRIDER311]

I have a 2005 CRF250R with a WB Carbon Pro, Twinair Powerflow kit, and will only use U-4 race gas. My question is if anyone is using a JD jet kit with the settings different than what comes in the directions. 175 main, red needle with clip in the 5th position, 1 3/4 turns out on the fuel screw, and a 42 pilot. I race in NJ,NY,PA from April thru October. Also, do I have to jet it differently if I throw in a 55 leak jet.

[krony506]

With the U-4 you may need a 80 main jet

[skipn8r]

do I have to jet it differently if I throw in a 55 leak jet.

The leak jet, and the rest of the AP circuit, only adds fuel while you're opening the throttle and for a short time after; so it seems that if the rest of your jetting is currently correct, changes to the leak jet size would not require other jetting changes.

[CRFDAD]

Your set up is just fine. I have ran that spec all last year and it was fine with U4 and Vp C12/Hi-Test unleaded 50/50 mix. I actually like the latter better because the bike starts better after a lay-down than with oxygenated fuels. The bike was raced all over the Southeast on and off the mountains and the only adjustment I would make is to the Ziptie fuel screw from cool mornings to warm afternoons.?

[REDRIDER311]

thanks for the info... I was thinking about starting with the 180 on race gas and adjust from there.

[Cory05]

I run the exact same setup w/out the jet kit. 175 main. 45 pilot. needle in 4th slot. We are at sea-level and that jetting was dialed in by honda-tech's at the winter olympics race in november, its spot on.

[yz400fer69]

I am having a tough time jetting this bike for some reason. I am running 92 octane pump fuel with a WB Aluminum race pipe, I removed the filter screen and run a WB or UNI air filter on and the JD jet kit. I have the Red needle in the 5th clip position with a 172 and before I removed the screen it felt super rich in the higher RPMs and had a bad bog off the bottom, so I took the screen out and it starts great now (1st kick when it is cold...) and the bog off of the bottom is now gone and the throttle is very crisp, but at about 1/2 throttle and up it feels like it is bogging out? Anyone have any ideas?

Dave

[Cory05]

1/2 throttle is the main jet, something isn't right because I've experimented with main jets from 150 to 175 and never had a bog in the mid like that. Hopefully someone else can pinpoint the problem cuz that doesn't make sense to me.

[yz400fer69]

Ya me neither....

It actually isn't 1/2 throttle, it is more like mid RPM's... Depending what gear I am in. Kind of confusing to explain. But I was wondering if that was the Main Jet or Needle... I don't want to drop the main jet because JD reccomends a 172 with a stock bike and I am running the 172 on a non stock bike and still noticing a bog. I wonder if I am now lean...

Maybe I will go up to a 175 and see if that helps at all.

Dave

[Cory05]

A "lean" bog is when the bike actually shuts off for a split-second, I had problems with it when I was too lean, when you are at low rpm's or when you land off a jump and then hammer the gas wide open, the bike shuts off for a split second before coming back to life.

[CRFDAD]

Try a 175 main. You need to get the main right before you try anything else. Then try a 170 for a few runs. The bike will let you know which way to go from there.? I've have tuned two different bikes with the same pipes, JD kits, airfilters, etc, that required 2 jet sizes different from one another to run their best. Experiment.

[thor_386]

If its around 3/8 to 3/4 throttle, the needle position is a contributing factor. I do the trial and error thing. Try raising the needle a clip, take it for a test ride, if it made it worse go the other way. All it takes is time..........

[Cory05]

If its around 3/8 to 3/4 throttle, the needle position is a contributing factor. I do the trial and error thing. Try raising the needle a clip, take it for a test ride, if it made it worse go the other way. All it takes is time..........

The needle clip is only for right off idle, its the least that you would notice. At least thats what I was always told.?

[skipn8r]

The needle clip is only for right off idle, its the least that you would notice. At least thats what I was always told.?

It's been my experience that needle position is pretty important and has a major affect in the throttle position range we all use most: 1/4 - 3/4 throttle. Once everything is jetted right, needle position will have a major impact on engine performance and spark plug color/life during most "normal" types of riding. Once I had my XR400 jetted perfectly, I tried an experiment with the needle. I decided to raise (richen) the needle by three clip positions and ride around the block using everything from idle to WOT. As before the change, all was fine at idle and at WOT, but almost everywhere in-between, the engine would "blubber" and there was a major rich bog. Before the test, the plug was fine. After two trips around block (1/2 mile??), the plug was so wet, it fouled. I immediately changed the needle back and put in a new plug and all was well again. Just my experience.

[Cory05]

1/4 to 3/4 throttle is the main jet.

[skipn8r]

1/4 to 3/4 throttle is the main jet.

You are correct in that from about 1/4 throttle through WOT, there is fuel flowing through the main jet. But the fuel flow through the main jet in the 1/4 - 3/4 throttle range is obstructed in a controlled way by the needle. As you open the throttle through that range, the needle is pulled out of the needle jet and obstructs the main jet fuel flow less and less. Above 3/4 throttle, fuel flow is pretty much controlled by the main jet along with small contributions by the pilot circuit. Basically, the needle circuit controls the rate at which you transition from the pilot circuit alone to the main circuit along with the pilot circuit. There's a good diagram for the FCR at the above link. From the same article:

"The idle or pilot circuit consists of the pilot (fuel) screw (PS), the pilot air jet (PAJ), and the pilot jet (PJ). The pilot circuit controls the mixture at idle and up to about ¼ throttle. The needle circuit consists of the throttle valve, jet needle and the needle jet. This circuit controls the mixture from ~1/8 throttle to 3/4 throttle. The throttle valve cutaway controls from ~1/8 to 1/2 throttle and the needle taper controls from about 1/4 - 3/4 throttle. The main circuit consists of the main air jet (MAJ) and the main jet (MJ). This circuit controls the mixture from ~1/2-2/3 throttle to wide-open throttle (WOT)."

[thor_386]

You need to go to carb school cory 05

[oldnbold]

You need to go to carb school cory 05

I agree.

Take the carb apart and move the needle in and out and it'll all make sense.

[ramz]

Check it out:

CRF250X Jetting