Jump to content

YZ250X Off Idle Flat Spot

Recommended Posts

I have a 2017 YZ250X, I feel like it is running really well except for a flat spot when accelerating from just off idle.  It doesn't really feel like lean bog and definitely doesn't sputter like its rich, its more just a lack of snap right off the bottom.  Mainly I want to figure out if this is normal before I drive myself crazy trying to correct it.  Here is my setup

GYTR +8.4oz Flywheel

RB Designs Head Mod

172 Main

NECJ Suzuki Needle Clip 3

#8 Slide

48 Pilot

AS 1 Turn

Everything else stock

500-1000ft

70 Degrees

Compared to what I've seen posted my pilot seems rich, but I've tried a 45 and the 48 appears to be right.  I tried a 170 main and the plug was a little whiter than I cared for.  The bike sputtered like crazy with stock jetting, RB head mod and Suzuki needle made a world of difference.  This one area is the only thing that now seems a little off to me.  I did the RB head mod, Suzuki needle and GYTR flywheel at the same time.  It has crossed my mind that maybe this is normal, especially with the heavier flywheel.

Thanks in advance for all your thoughts.

Edited by Shykerry

Share this post


Link to post
Share on other sites

Is your timing stock? Advancing it a touch may improve that. I notice that a wee bit on my YZX but only over about 6000 ft. Maybe a by product of the lower compression??

Share this post


Link to post
Share on other sites

try removing the heavy flywheel, i had a 9oz on mine for about 10min and removed it...

Share this post


Link to post
Share on other sites

Yes, timing is still stock.  I will give the OEM flywheel a try just to see what difference it makes and I'll try to come up with an indicator to check timing.

Share this post


Link to post
Share on other sites

Put the stock flywheel back on and did not see an improvement.  Checked ignition timing and it was set at .008 from factory, set at .004 (1 degree retarded) since the only ethanol free fuel here is 91 octane.  Still feels the same though.  Put the GYTR flywheel back on and am going to just ride it.  Plug is nice and mocha brown, I think the "problem" is in my head.  Thanks for the ideas though.

  • Like 1

Share this post


Link to post
Share on other sites

Have you done the powervalve mod? Removing one of the 0.8mm washers made a huge difference for me. I have had the opposite effect with the Suzuki needles and have had better luck with the stock needles and a 50 pilot for more off idle punch. The Suzuki needles worked amazing in my KTM 300 but gave up a lot off the bottom and seemed very inconsistent in my 250x especially when compared to the stock Yamaha needles.

Edited by mcgradybrandt

Share this post


Link to post
Share on other sites

Set the float height to 6.5 or 7 mm.  Many came from the factory set at 3.5 mm WAY too high, it makes the bike impossible to jet.  Like playing with shock clickers but the sag is way off, the clickers will not fix the incorrect sag issue.  Set your floats correct then test again.

Share this post


Link to post
Share on other sites

Thanks for the replies.  I set the float height to 7mm when I started messing with the jetting.  I haven't tried Yamaha needles since I had the squish corrected.  After this weekend I may give them another go just out of curiosity.  Thanks!

Share this post


Link to post
Share on other sites

For what it's worth, the reason your 48 pilot jet is better than 45 for your setup is likely the leaner #8 slide, which covers the entire range that the pilot affects.  I also have the #8 slide (and RB head mod) in my 16 YZ250 and the 48 pilot seems spot on for me.  Let us know if you have any developments in your testing :thumbsup:

Share this post


Link to post
Share on other sites

Power valve mod. Remove one .8mm washer on the spring side of the collar, or replace both .8mm washers on the spring side with one 1.0mm washer from the standard YZ.

Then remove the heavier flywheel and see what you think about it.

Edited by 218

Share this post


Link to post
Share on other sites

I've been surprised in my ability to tune the power with the needle clip.

 

When I installed a heavy FWW and went to a leaner needle for summer I started on the 3rd clip=dull and fouled plugs, went to 1st clip= too snappy for my liking in single track, now on the 2nd clip and still quick but not TOO snappy.

No setting lean bogged.

Share this post


Link to post
Share on other sites
For what it's worth, the reason your 48 pilot jet is better than 45 for your setup is likely the leaner #8 slide, which covers the entire range that the pilot affects.  I also have the #8 slide (and RB head mod) in my 16 YZ250 and the 48 pilot seems spot on for me.  Let us know if you have any developments in your testing :thumbsup:

Do you have any 1/8 throttle burble? What elevation are you at?

Share this post


Link to post
Share on other sites
13 hours ago, Kinger317 said:


Do you have any 1/8 throttle burble? What elevation are you at?

It's pretty crisp there until the weather gets hotter, but granted it's a track bike and I don't spend much time in that range outside of the pits.  Dead of summer I'll run a 45 but usually its the 48.  I'm at about 5000 ft consistently with this bike and the tracks around here.  I'm sensitive to hanging idle and/or any sign of a lean bog when revving the engine, so tend to stay slightly on the rich side of the line if I need to make a choice.  FMF Fatty + PowerCore 2.1, rest of motor (other than squish correction and 9oz FWW) is stock, including reeds.

My 125 is more of a chore since I ride both track and woods (8-9000 ft) with that bike, and it's pretty sensitive to elevation changes.

EDIT TO ADD: I used to have a tough time jetting in that lower range when the bike was new (one adjustment to correct it would cause another issue elsewhere) which is what prompted me to get the #8 slide to begin with.  Been pretty smooth since then.

Edited by Tankslap
  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×