Main Jet size for 250X with 250R exhaust?

Those of you who are running the full 250R stock exhaust on your 250X, what main did you go up to?

My bike has the F2 racing jet kit with snorkel removed, stock exhaust baffle removed, and no other airbox mods. The kit uses a 133 main and the bike is jetted perfectly.

Last night I installed a full system with the older PMB endcap. I ride sea level to around 7,000 feet (at the most).

I'm taking the bike out to my riding area tomorrow night to mess with the main before this weekend. I have a 145 and 143 main I can try.

I'm thinking about starting with the 143, but I figure if someone already has their main jet dialed in, I'd rather save some trial and error.

The guys in the 250R forum are saying that their stock mains were 160 and 170 which would seem like I'm wasting my time with a 145 and a 143...

Thanks in advance.

155 without the airbox cut open. 160 with the top of the airbox cut out.

Wow, 155... I'll pick one up on my way out just to be safe. Thanks.

why not cut the air box? My mods are below and I jetted my bike for tahoe and jd jetting told me to use a 158 or 160. Att sea level to 3,000ft I run a 168 in the winter.

With that said I also think a 155 would be good.

155 without the airbox cut open. 160 with the top of the airbox cut out.

I originally ran a 138 main with the needle clip on position #4 (at sea-level)with a stock airbox. It ran OK, but was a little rich. I'm running a 155 and position #5 with a CCC cut airbox, and it is just about perfect. I might try bumping the main a size or two to see what happens, but it is pretty close now. I did throw in a 168 main once just to see what would happen, and it wouldn't even run at WFO - just blurble and bog.

JayC

I originally ran a 138 main with the needle clip on position #4 (at sea-level)with a stock airbox. It ran OK, but was a little rich. I'm running a 155 and position #5 with a CCC cut airbox, and it is just about perfect. I might try bumping the main a size or two to see what happens, but it is pretty close now. I did throw in a 168 main once just to see what would happen, and it wouldn't even run at WFO - just blurble and bog.

JayC

Interesting.....do you think the smog stuff on the Cali bikes allows them to run a little more rich? Also, we don't have the humidity in Cali like some parts of the country so that plays into it as well.

I just rode the bike out at Baldy Mesa with the 143 main and the bike was crisp from just off idle to 5th gear pinned. I found that it fell a little flat between the low end and the mid but I can live with it through this weekend until I have a chance to try some different mains.

I will probably go with a 145 just to richen it up a little bit since I will be riding in slightly higher elevations. I was worried that a 143 would be way to lean but I didn't note any kind of lean condition. The bike was crisp and there was nothing that would indicate poor jetting.

Why I don't cut the airbox...

My F2 powerback/desmog kit is designed so that you only have to remove the snorkel in the airbox and not cut the airbox open any further. I went with the F2 kit specifically for that reason as I prefer to keep as much dust as possible out of the airbox.

Interesting.....do you think the smog stuff on the Cali bikes allows them to run a little more rich? Also, we don't have the humidity in Cali like some parts of the country so that plays into it as well.

I don't know about that. We DO run crappy E10 here in New England, which requires slightly richer jetting (in theory), so you lucky folks that still get to run real gas should be a couple of sizes leaner. I remain confused by all the folks that talk about running mains in the 160+ range, much less the guys that have 150+ mains in there with stock airboxes. You can't open an airbox more than mine, and I took out the backfire screen too, and my bike literally won't even run with a main in the mid 160s. I'm betting there are a lot of 250Xs out there that fall flat at WFO.

JayC

all stock 2006 250x. i added the twin air filter and i use a 160 main .

I ride at sea level to 1000 feet. I have a 2007 250x with open airbox, twin air filter, backfire screen removed, R exhaust, and I run a 42 pilot fuel screw 2 1/2turns out and 162 main . I just got back from crested butte we were riding 9,000-12700 feet I put a 150 main in for the trip and had the fuel screw 1/2 turn out and it ran perfect.

chicken, your 133, 143 reference would imply that your kit is either 1. not using kiehin jets. 2. they are kiehin, but the kit has proprietary numbering, 3 or both.

Either way it's possible that your jets ARE as big as some being used by others.

I just rode the bike out at Baldy Mesa with the 143 main and the bike was crisp from just off idle to 5th gear pinned. I found that it fell a little flat between the low end and the mid but I can live with it through this weekend until I have a chance to try some different mains.

I will probably go with a 145 just to richen it up a little bit since I will be riding in slightly higher elevations. I was worried that a 143 would be way to lean but I didn't note any kind of lean condition. The bike was crisp and there was nothing that would indicate poor jetting.

Why I don't cut the airbox...

My F2 powerback/desmog kit is designed so that you only have to remove the snorkel in the airbox and not cut the airbox open any further. I went with the F2 kit specifically for that reason as I prefer to keep as much dust as possible out of the airbox.

TC,

I too had the F2 kit and was not that impressed with it. Since a majority on this board use JD Jetting, as well as myself on other bikes, I thought I would give it a go. WOW!! The bike really came to life with the JD kit and the airbox modded to his spec. I also tried the F2 jetting with a modded airbox and pipe and I will tell you: don't waste your time. You won't be able to get it close. The filter does not seem to get any dirtier, and even if it did, so what? Filters are easy to change on this bike.

Mods so far:

Leo Vince full Ti exhaust w/ 93db insert

JD Jetting kit

Air injection pump removed

Overflow bottle removed

Scotts stabilizer

Backfire screen removed

Pro Taper Bars

Work Connection radiator braces

NiMH battery from Ebatt

ODI Lock On grips

Cycra Probend handguards w/ shields

Hyde Racing skid plate

D756 front tire

D952 rear tire

Brake snake

TM Designworks chain guide

This is just a guess... I have never looked at a F2 kit, but I would bet it has a richer needle, and that is why it can run such lean main jets.

I also ran a JD kit in my 07 250X, opened the airbox, pulled the inner removeable plug out of the muffler and ran a 158 main in summer 160 to 162 in the winter.

The bike ripped! JD was the hot set up, followed the directions exactly.

chicken, your 133, 143 reference would imply that your kit is either 1. not using kiehin jets. 2. they are kiehin, but the kit has proprietary numbering, 3 or both.

Either way it's possible that your jets ARE as big as some being used by others.

I was thinking the same thing. But, I picked up the 143 and the 150 from Precision Concepts and it said Kiehin on the reciept.

I bet I can run a leaner main because the pilot is richer.

This is just a guess... I have never looked at a F2 kit, but I would bet it has a richer needle, and that is why it can run such lean main jets.

I bet you're right.

TC,

I too had the F2 kit and was not that impressed with it. Since a majority on this board use JD Jetting, as well as myself on other bikes, I thought I would give it a go. WOW!! The bike really came to life with the JD kit and the airbox modded to his spec. I also tried the F2 jetting with a modded airbox and pipe and I will tell you: don't waste your time. You won't be able to get it close. The filter does not seem to get any dirtier, and even if it did, so what? Filters are easy to change on this bike.

I actually am really happy with the F2 kit. With the stock exhaust system, baffle removed, the bike was a little torque monster without any kind of hesitation or indication of poor jetting. I wanted to go with F2 because those guys work closely with Steve Hengeveld who has a lot of hours in the same terrain I ride, not to mention Henge helped develop Honda's X.

I was really nervous about putting the full R system on and I held off until I knew I'd have a long enough period between enduros to put it on and screw around with the jetting. However, last weekend at the national in Nevada, I was getting beat up on the hills and in some of the really sandy slot canyons.

What I am hoping the R system will help me do is spin up the real wheel quicker at the base of a bad hill and then stay in the fattest part of the pipe longer, without revving out near the middle and the top of a bad section.

I put the system on, went with the 143, took it out to my riding area and tried it out. As far as I can tell I'm jetted pretty good. No poping on decel, no backfires, the bike is crisp.

The place where I get killed in enduros is when you're going crossgrain in a section and all the sudden the ribbon just takes a hard left or a right straight up a virgin hillside. With the stock system, I realized I would go into lug mode too early.

I put myself in a couple of scenarios yesterday where I could stay in the middle to the upper end of the power on a hill and not rev out.

Anyhow, I'm happy enough to ride the bike this weekend, but still confused. LOL. I can't wait for FI!

I have a feeling that it's an apples and oranges thing with the way JD and F2 approach their jetting recipes.

No poping on decel, no backfires, the bike is crisp.

No popping or backfires is indicative of a correct pilot jet. Crisp throttle response is typically indicative of the needle and sometimes the pilot. What happens when you are holding ~1/4 throttle then abruptly wack the throttle wide open then hold it there?

JayC

Rear wheel spins up faster than it did with the stock exhaust and the bike pulls a little bit longer before signing off.

I'll probably fatten up the main to 145 and call it good.

I'm racing the national enduro this weekend in Wickenburg. I'll look into airbox mods down the road I spose. Can always use better power.

I don't know about that. We DO run crappy E10 here in New England, which requires slightly richer jetting (in theory), so you lucky folks that still get to run real gas should be a couple of sizes leaner. I remain confused by all the folks that talk about running mains in the 160+ range, much less the guys that have 150+ mains in there with stock airboxes. You can't open an airbox more than mine, and I took out the backfire screen too, and my bike literally won't even run with a main in the mid 160s. I'm betting there are a lot of 250Xs out there that fall flat at WFO.

JayC

wish I had an answer...LOL. I guess every bike is different, gas, weather, humidity, elevation plays into it all. I was suprised when James Dean e-mailed me back and said I could either run a 158 0r 160mj at 5,000ft-7,000ft with my current mods. I've thought about also moving my needle to the 6th clip just to see what happens.

I only rode 30 some miles of the 80 mile national enduro last weekend as I holed my clutch cover on the first loop and could not continue.

As far as I can tell the bike is jetted pretty well with the 143 main. I'm going to put the 145 in and ride it just to see but I don't see myself going any bigger on the main, contrary to the 160+ mains a lot of guys on this forum are running.

The only thing I noticed on Sunday was if I were in a technical section and I chopped the throttle closed and rode the clutch the bike would stall. This happened a couple of times in the second special test on the first loop. I was planning to adjust the idle at the check in for the next special, but there was no point in doing so as by that time I realized I had holed my clutch cover and lost most of my tranny oil...

Like I said, I'll try the 145 just to fatten it up a bit. But I assume that I can run a leaner main because the F2 overall jetting mix differs from JD. That and you factor in that I don't have a modified airbox which means less airflow enabling a leaner main...

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