Off topic - wr250f low end power loss

4 replies to this topic
  • Mark_Cantrell

Posted March 10, 2002 - 10:10 PM


Sorry about wr250f questions here but you have better jetting answers here. I have already posted this in the 250f forum.

stock exhaust, vortip, cut grey wire, YZ cam timing, cut throttle stop, no lid or snorkel, just cleaned carb, stock accel pump w/reasonable squirt duration <1s, stock PJ/PAJ/MAJ, down to 165 from stock 170 MJ, stock needle 2nd clip pos, screw 1 1/2 out

I am now happy with my jetting except I have a bog/loss of power/no acceleration on the bottom end. Guessing from 2000 to 4000rpm. The effect is just like a rich condition and is very severe.

The problem is that the condition exists if I am at any throttle position. I can go slow, put it in 5th and hear/feel the bog for a long time, until it builds speed out of this range. It is in the slow bog range longer at lower throttle positions but the condition is the same at any throttle position.

I don't think it's the accelerator pump because in the fifth gear test, I can leave the throttle at a fixed position, any fixed position, long enough for the accelerator pump to not be a factor and it still bogs.

I am running a Vortip now but the stock baffle had the same problem, just over a larger RPM range.

It could be cam timing, ignition timing, or others. I have a Vortex but wanted to get the bike set up better for a fair before/after before putting it on.

Taffy on the WR426 forum has changed PAJ for something like this, but that's new territory for me. I would hate to spend a few weeks playing with the PAJ and it not be related.

Any ideas?

  • Taffy

Posted March 10, 2002 - 09:43 PM


give us any airbox mods, the std carb settings and #s.

your exhaust we know.


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  • Mark_Cantrell

Posted March 11, 2002 - 03:27 AM


Airbox is stock, but running with the lid and snorkel removed.

FCR-MX37 (37mm)
MJ - #165
MAJ - #200 (stock)
PJ - #42 (stock)
PAJ - #75 (stock)
Needle - OBELP 2nd clip
PS - 1 1/2 out
Cutaway - 1.5 (stock)
Leak jet - #60 (stock)

Ohio, 800' asl, 65 degrees (on the day I made last changes)

Taffy, always glad to be able to ask you questions. I hear people posting referring to the pilot screw as the pilot air screw. On logic diagram (WR250F manual p 7-4) it shows PS drawing from bowl. In article, it says all keihin except FCR the screw is an air screw (and on airbox side) but on FCR it is fuel screw (and on engine side). Is it a fuel screw (in leaner, out richer) or a fuel air screw?

Another thing, many postings and tech articles (like tuning tips on exhaust pages) indicate the bikes are shipped lean for EPA reasons and will need enrichening, especially when opening the exhaust and airbox up. My bike is definitely snappier leaner (smaller jet, higher clip position). Am I getting the snap I want but running on the ragged edge of a too hot combustion chamber? It doesn't seem that way (no popping, shearing sound, plug fine), but a little concerned.

My fevered imagination (no evidence) is leading me to an ignition problem, even though it sounds like a jetting problem. I will check the TPS for range and the coil resistance later today, just to be sure. I may even slap the vortex on before working out jetting just to see.


  • Taffy

Posted March 11, 2002 - 11:22 PM


your needle is too low if you're on c2. you should be on clip 3, 4, or even 5.

that's all you need do. if you lower the MJ 7/8 (say 168 to 160) you must lift the needle to make sure this area isn't affected as well. after all you only meant to alter the top end not the midrange.

like wise if you went from 165 to 172 you would lower the needle one clip to reduce the extra fuel at midrange.

as far as the names go. the pilot screw is underneath. this adjusts the mixture of the pilot jet and the pilot air jet.

the pilot jet is in the floatbowl and the PAJ is in the pillbox slit at the entrance to the carb. this can be swopped for an adjustable screw which i have always called the pilot air (jet) screw.


i hope that's as clear as mud!


  • Mark_Cantrell

Posted March 12, 2002 - 04:28 AM



Thanks for the suggestion. As soon as it quits raining, I will try raising the needle. The plug center ceramic was very white when I pulled it last night. I read this to mean it was very or perhaps too lean in at least some commonly used throttle position. When idling in the garage, it also heated rapidly and seemed hot with mixed idle and mid range revs (listening for pops and stutters through the air filter).

I realize mechanically what the needle does and how it works. But it's unclear how to approach using it. I had always thought of it affected by but somewhat independent of the main in tuning. Now it seems this is misplaced.

For instance, assume a properly jetted bike is moved from sea level and 70 degrees to 7,000 ft asl and 70 degrees. I assume the pilot circuit would have to be leaned and the main circuit leaned. For the main cicuit, would the main jet change be sufficient (the needle just providing the same adjustments for varying throttle positions it did at sea level) or would the main jet and needle both need independent adjustments?

I would have assumed the latter, but now am beginning to feel that the needle and clip position would be the same. This would mean that the needle is adjusted to the bike and the main adjusted to the conditions. Of course, in either case the pilot (and perhaps others) would have to be independently adjusted.

As you can see, I am trying to understand instead of (or at least in addition to) just getting numbers.



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