I frickin hate jetting - help on 250RR

[worstell]

As an engineer, I find both jetting and suspension tuning closer to black magic than actual science. It just seems like guess work.

I have a 2016 250 RR set via the factory jetting chart for my riding conditions. I live in Colorado and mostly ride at 7,500 to 10,000 feet at aprox 60 to 80 F. The problem I have is when trying to cross large rocks or logs from a near standstill or very slow speed. I twist the throttle hoping to lift the front end, and the engine fails to respond. I don't know if that is termed a flat spot, burble, or what, but it sucks. I decided to compare the Beta 250RR and KTM 250 XC-W owners manual settings since both use the Keihin PWK 36S AG carburetor:

Beta 250RR: Air Screw=2.5 turns; Pilot Jet=35; Main Jet=158; Needle=NOZK; Needle Position=1
KTM 250XCW: Air Screw=2.0 turns; Pilot Jet=38; Main Jet=172; Needle=N2ZJ; Needle Position=2

I think that the needle designations might be specific to the manufacturer, but the N2ZJ specified for the KTM 250 is the same specifed for the Beta 300 for even lower elevation/colder temps.

So the KTM jetting specs seem to be massively richer across the board, or at the very least across idle and the pilot circuit. But perhaps despite having the same Keihin model number, PWK 36S AG, the carbs aren't identical between KTM and Beta. And if so, then they can't be compared directly as I have done.

This makes me want to go back to fuel injected 4-stroke.

BTW, I don't think it is my technique. Never had any trouble lifting the front end on my 350 XCF-W.

Thanks for any wisdom you all can share on my dilemma.

[hooter]

I'm no expert either, but that sounds lean to me. Try a small increase in the pilot jet size and set your air screw to 1 1/2 turns out. You might need 2 or 3 different sizes of pilot jet to get it right. It's a PITA but the reward is well worth it.

Hopefully there's somebody else on the forum who rides at similiar altitude who went through this already and has the recipe, if so chime in!

[bikesparky]

I don't mind jetting and it's absolutely no guess work!
The KTM jetting chart you want to compare too is a waste of time, different bike and that 172 main is way too big anyway.

The pilot you suggest for the Beta might be a bit small, I rather start a bit bigger and work my way down.
A 38 is a good starting point, run the engine warm and adjust the airscrew. If still too rich go smaller, too rich means you need to open the airscrew more than 2 turns before it starts to run crisp. If too lean, means the airscrew needs to be closed to run slightly better.

After you done that start with a 165 main jet and the needle clip in the middle. A NECJ needle works well on these bikes.
If doughy go a bit leaner, if it wants to go hard but runs out of steam it's too lean and you need to richer.
Lifting the needle (clip lower) goes richer, dropping it is leaner. If the needle needs to be moved to the extremes to run better change the jet. Easy!
Some helping stuff:
https://www.youtube.com/watch?v=zVzqqpgviyI
https://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf

[McKay]

Nice and easy to understand info!

I don't mind jetting and it's absolutely no guess work!
The KTM jetting chart you want to compare too is a waste of time, different bike and that 172 main is way too big anyway.

The pilot you suggest for the Beta might be a bit small, I rather start a bit bigger and work my way down.
A 38 is a good starting point, run the engine warm and adjust the airscrew. If still too rich go smaller, too rich means you need to open the airscrew more than 2 turns before it starts to run crisp. If too lean, means the airscrew needs to be closed to run slightly better.

After you done that start with a 165 main jet and the needle clip in the middle. A NECJ needle works well on these bikes.
If doughy go a bit leaner, if it wants to go hard but runs out of steam it's too lean and you need to richer.
Lifting the needle (clip lower) goes richer, dropping it is leaner. If the needle needs to be moved to the extremes to run better change the jet. Easy!
Some helping stuff:
https://www.youtube.com/watch?v=zVzqqpgviyI
https://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf

[bryanaverill]

I'm also an engineer and jetting is not black magic. It just takes practice and experience like anything else. I also read a lot of tuning papers written by experts and Keihin.

I have jetted many two strokes (street and dirt) and I find the 250RR super easy. I basically agree with bikesparky. I use close to his settings.

My jetting (2016 250RR):
MJ:165
PJ: 35
Jet Needle: NECJ (3). This is a 2006 Suzuki RM250 option needle available from Rocky Mountain ATV for about $12.
AS: 1.5 (fine tune to get a reliable idle)

I ride at sea level and almost never up to 10,000ft. I did ride in Idaho up to 8000ft. At that altitude, I lowered the need to the (2) position (2nd groove from the top). No other changes and the bike ran great.

In my experience, the phenomenon you're experiencing is because of a too lean needle clip position. You might try raising the needle (lowering clip). 158 MJ seems too lean to me, even at your altitude. That will also affect the roll-on response a little.

Don't give up. The 250RR can easily be made to run supper sweet. It's the best part about the bike in my opinion.

One last thing. Although the jetting charts work for most people, I have not had much success with them (KTM, Beta, GG, Yamaha). I use a classic tuning process that has been used for decades and is available online if you do some research.

[nevermind]

The problem I have is when trying to cross large rocks or logs from a near standstill or very slow speed. I twist the throttle hoping to lift the front end, and the engine fails to respond. I don't know if that is termed a flat spot, burble, or what

That is a lean bog. If you are idling ok, lower the clip on the needle at least one position, which raises the needle in the slide. Two-strokes bog when lean and stutter/burble when rich.

[Bandicoot]

My jetting (2016 250RR):
MJ:165
PJ: 35
Jet Needle: NECJ clip 3.
This is a 2006 Suzuki RM250 option needle available from Rocky Mountain ATV for about $12.
AS: 1.5 (fine tune to get a reliable idle)

As above but I would change the Pilot Jet to a 38

[worstell]

Thanks all for the input everybody. By black magic, I mean that it takes a trial-and-error approach, whereas I would like to be able to set it per the jetting chart in the owner's manual and then be done with it. But based on the input from folks here, it seems that it will indeed take some trial-and-error.

As a first step, I turned in (clockwise) the air screw, making the pilot circuit richer. It is now set at 1.5 turns out from fully in versus the 2 to 2.5 turns in I had been running per the manual. That seems to have helped. Now, when I quickly twist the throttle from idle, the bog is at the very least greatly reduced. I followed Jeff Slaven's advice here:

https://www.youtube.com/watch?v=zVzqqpgviyI

But I haven't been able to ride it that way yet, and only tested in the garage.

I might try moving to the second clip position on the needle too as suggested, with the intent of making it richer, particularly when transitioning off the pilot circuit. Yet, given the large amount of spooge I get on my silencer, I think I am already running rich on the needle and/or MJ.

I may try the 38 pilot jet too, but the manual does not call for that until much lower elevations/temps.

I don't expect a larger main jet to address the problem because it occurs at lower throttle openings and before the main jet comes into play.

Finally, I am still puzzled by the difference in the 250 jetting charts between KTM and Beta. Sure they are different bikes, but they have enough in common that I would expect similar jetting.

[Clearvu1]

The air screw simply fine tunes the pilot circuit. Put the 38 in there, run the bike up to temp, slowly and gently turn the air screw in (closed) until it just starts to seat. Notice how the engine runs. You may have to temporarily increase idle speed to keep it running. Start turning the air screw out (open) and listen to the engine rpm. Somewhere between 1/4 - 2 1/2 turns the bike should idle faster. Find the spot where the bike idles fastest and leave the air screw in that position. Readjust idle. When the proper pilot jet is installed the bike should go from rough idle to clean idle and back to rough idle. You can try this with the 35 that's in there and see what happens. If you get the results indicated and the problem persists then you need to move to the needle. Raise the needle by moving the clip toward the bottom of the grooves, go one position at a time. If you have the correct needle you should get good results year round using the middle 3 positions. As stated earlier if you have to go to the very first or very last clip position you should be looking for alternative needles. I like the NEC* series in my 250RR but others will also work.

[iamovru]

I would go to the 38 pilot first. What you are referring to is a lean condition and that is what causes the bog when you twist the throttle. I ran a 38 and 40 in my 09 KTM 300xc all year long in the PNW when I lived there. I have a NECJ and I believe a NECW if you would like to try them.

Marc