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Flex Jet for 2015 crf250x

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Is a Flex Jet worth installing to allow for easier starts when cold? My bike is all stock and really do not want to modify it other than this.

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No, but it makes adjusting low speed mixture much easier as the engine temps change because the  bike does not have a thermostat to control engine temps.:banghead:

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It's also EXTREMELY useful if you ride mountains or other places with big elevation changes.  2000+ feet change in elevation can make for miserable stalling.  Having a quick way to adjust is priceless in those situations.

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+1 on altitude changes.   That's really the only reason you'd want to mess with the adjustment.

Quote

My bike is all stock and really do not want to modify it other than this.

 If the bike has never been re-jetted, it needs to be and Jetting is not considered a "modification".

 The CRF's come stock are jetted very lean.  Too lean to run well.

Jim.

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Posted (edited)

If I leave my airbox and exhaust stock, what jets would you recommend for easier starting? I am not looking for more power.  I did read other posts on jetting but it  appears most are interested in more power and modify the exhaust and airbox.

Edited by liberty2701

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Posted (edited)

My '09 250X is pretty much stock, but I poked 6 1" holes in the top of the air box and removed the snorkle.   Backfire screen is still in, and exhaust is stock and still has the baffle in.

I bought a JD Jetting kit, which has a custom needle and with that, I run a #158 main.  I still use a #42 for the slow circuit.   I ride at sea level and temps in the 60's - 80's

Jim.

Edited by Jim Dettman
little more info added to clarify what was said.
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stock main was a #130 by the way; quite a difference<g>

and the bike runs far better.   More power and it runs quite a bit cooler.  

Jim.

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I read about the JD Kit. Rather than spending $80 bucks on a kit., could I just buy a slightly larger main jet. They are like $3. I ride at 600 to  1000 ft elevation. 

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So you'd be at sea level.

The thing with the JD kit is that it comes with two custom multi-taper needles; one for hot/warm and one for cold (below 60 degrees), and the jetting and adjustments you should used based on that. I'm not pushing the JD kits, but those setups are dyno tested, so you know they are correct.  $80 sounds like a lot, but it can save you a ton of time.

If you are not going to go that route, then you will need to do some work. 

I don't think anything has changed for 2015, which means it has the "nail head" needle and is nonadjustable.   So you will need to replace the needle.

Most will run a NCYS needle/4th groove, which is the needle for the full CCC (Closed Course Competition) mods.  That will most likely be way rich.  The stock needle for the earlier models ('04,'05, '06) which is adjustable was the NCVT (part number 16234-MEN-671).  You'll probably need a #150 or there about's for the main.  I would get a #148, #152, and a #155 as well and see which works the best.

And I would at the very least remove the snorkel and poke some holes in the top of the air box.  If you don't do that, you probably should leave it just as it is.

There is BTW a jetting thread in the FAQ at the top of the forum:

Where you can look and see if there is anyone with a setup already done.   Just a warning though; you'll find a lot of variation and you will find that almost all have modified the air box.  If you don't do that, you are going to go through a lot of trail and error.

Jim.

 

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Thanks. I might just leave it be. I was looking for an easier cold start solution.  Plenty of power for my old body.

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 Understood, but proper jetting lets it run cooler as well.

<<I was looking for an easier cold start solution.>>

 Try twisting the throttle once (and that's a quick twist, not a slow roll) and letting go.  Then push the e-button.   See if it's easier.  If not, then try 2x next time.  No more than 2x though.

  You might also consider a larger cold (choke) jet if it run's OK for you otherwise.   But try the throttle twist first.   I need 1x on the 450 to get it going when it's cold.

Jim.

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Posted (edited)
On ‎4‎/‎29‎/‎2019 at 4:48 PM, Jim Dettman said:

 Understood, but proper jetting lets it run cooler as well.

<<I was looking for an easier cold start solution.>>

 Try twisting the throttle once (and that's a quick twist, not a slow roll) and letting go.  Then push the e-button.   See if it's easier.  If not, then try 2x next time.  No more than 2x though.

  You might also consider a larger cold (choke) jet if it run's OK for you otherwise.   But try the throttle twist first.   I need 1x on the 450 to get it going when it's cold.

Jim.

I started it this morning at 40F. I twisted the throttle, hit he e start and nothing. I twisted 2 times and hit the  e start and it fired and stalled.  Did it again and it fired and ran like  champ. It never started so easy. Thanks

On the running cooler comment. I am not a racer just a casual trail rider. Would the jetting add longevity to the top end or is it something I will never notice?

 

One more question on the main jet. I need to order something online and adding  a jet to the order is like $3. Would there be any benefit by going up one or two sizes on the main or am I wasting my time? Sorry to beat this to death. 

Edited by liberty2701

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Posted (edited)

As mentioned earlier, in stock form the CRF250X is jetted quite lean on the main jet

which makes for both poor power and, a hotter running engine.

 

Even if you plan on leaving the airbox and exhaust stock

consider a #142 main jet to be the very minimum size to switch to,

even a #145 wouldn't hurt as a first test.  (stock should be a #130)

 

Edited by mlatour

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Agree with Mlatour....something around the 142/145 range is where you'll end up without cutting the air box or removing the snorkle.   As I said earlier though, I would do both and then start with a #150,  

I know your not in it for the power, but it lets the engine run cooler, and that is less stress on the motor.  Valve life will certainly increase as the thing that kills valves is heat and high revs.   There's also less heating/cooling as you ride (so your not expanding /contracting as much).

 Of course there's no hard numbers on any of that and is it worth it?   Well, both my bikes are re-jetted, and like you, I didn't do it for the power.  I just wanted a well running bike.

Jim.

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Posted (edited)

Thanks for all the advice. 

Edited by liberty2701

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I posted this on another post but it applies here also.

 

I decided to mess around with the jetting on my 2015 CRF250x.  I installed a 142 main jet, opened the top of  the air box with two 1 1/4" holes and added the 1mm spacer to the stock NLAT needle. I did not even touch the mixture screw. Holy cow,  it started immediately and has way more power. As soon as it starts I can remove the choke and it runs fine. No hesitation and amazing power. I am glad I did it. 

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