426 is revving out on top end, need advice

7 replies to this topic
  • kfrosty

Posted November 09, 2001 - 03:51 AM


I made a post before about the BK mod and how I backed it out because I couldn't get the bike to run as good as stock. Well, it's been two or three weeks since I've backed this out and even though my bike runs better now than I could ever get it with the mod, it still doesn't pull all the way through on top end like it used to before I messed with it.

I thought it was me getting used to the bikes power but I let a person who road the bike before I tinkered with the mod ride it again yesterday and the first thing he said was it wasn't pulling as good on top end as it used to.

Meaning, on this particular track there was no reason to ever get out of second gear. You could wind the bike out and it would pull harder and harder. However now, it's like it gets to a certain RPM and the power becomes flat. It's not too bad but nothing like it was before I messed with the mod.

The thing that has me puzzled is that just making the mod and undoing it shouldn't have made a difference. When I made the mod, I changed the main from 162 to 168. I did turn the pump timing screw a half of a turn but tried to get it back out to where it was and I watched the stream and it's about the same duration as it was stock. The only other thing I did was turn the pilot screw to about 2 3/4 turns out. However, undoing the mod I went back to 1 3/4's out.

The bike runs ok and I still make good lap times but it's just a little frustrating knowing the bike was running awesome and now it's dropped off because of tinkering.

What I would like to get answered if possible, is if anybody has any ideas of where to start. I would assume it's still a jetting problem and if I played around with it long enough maybe I would eventually hit it. My problem is now I only get like an hour or two on the bike during the week and it's so dusty here now I don't want to work on the bike at the track. (Live in a neighborhood and there is no where close to test around my house.)

For these reasons I don't like to do a lot of tinkering because I race about every weekend there is a race so I don't want to make changes and end up having the bike run worse on the weekend.

Oh yeah, I took the main back to 162 but with the cooler weather I went to a 165. I checked the plug and it looked to be a good color after yesterdays ride.

Any advice would be greatly appreciated!



  • yzernie

Posted November 09, 2001 - 04:23 AM


>Did it run better with the 165 main??
>Did you change the needle position after the mod??
>Is your air filter clean??

Usually, the lack of rev indicates a rich situation. Try it with the 165 and if it is better, try again with the 162. Changing a main jet at the track is not bad with respect to allowing dirt and dust into the carb.


  • kfrosty

Posted November 09, 2001 - 10:14 PM


Thanks Ernie for the response. I checked the plug last night and that's the only thing that I can figure to tell if the bike is running rich. The ceramic in the middle was almost perfect. Just a touch of black so it could have possibly been a tad rich but shouldn't be that bad. The needle I moved to 3 for the mod and back to 4 (Stock after undoing it.)

Can you please verify as to whether you can gauge by the plug as to whether you are running rich or lean. I was under the assumption you can but thought I read on here that you could very well?

Also, the temp has been changing drastically around here. It will be 30 here in the mornings and high 60's low 70's in the evening.

I'll give the 162 a whirl again this weekend. I am correct though that as the temp drops the bike will tend to run leaner, correct?

Again, thanks for you help! I really appreciate it!


  • yzernie

Posted November 09, 2001 - 01:20 PM



You can get a very general reading from the plug. Unlike my YZ125 (or any 2 stroke), I don't rely on the plug for my jetting. You have to go by how the bike runs.

You are correct, the cooler it gets the air becomes more dense and the leaner the jetting will be. Temperature does also come into play with jetting...but again, not as bad as for a 2 stroke.

The only way to figure out which jetting works best is to keep playing around with it until you find the right combo. As I change jetting, I write it down because my little pea brain can't always remember all of those numbers and positions!!!


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  • Shawn_Mc

Posted November 09, 2001 - 03:12 PM


What kind of gas are you runnig? The reason I ask is, pump gas is the most inconsistent stuff in the world outside of a womans mood. Reading the plug on a fourstroke that is running pump gas is precarious at best. On race gas, the plug should look clean at WOT. Partial throttle plug should have just a tad of tan on the porcelin. The ground wire shouldnt have any soot on it, nor the plug body (threaded part). You may want to think about how long its been since the valve springs have been serviced. They dont last forever, especially at 11,500 rpm. Good luck

  • yzernie

Posted November 10, 2001 - 07:56 PM


Originally posted by Shawn Mc:
pump gas is the most inconsistent stuff in the world outside of a womans mood.


You got that right. That is funny but.....true!!


  • motoman393

Posted November 11, 2001 - 10:31 PM



When was the last time you checked your valve clearances? Maybe this is what is causing your problem...and the BK mod is "masking" your problem (thus your problem may have been misdiagnosed) This happened to me on my XR400... I just got through putting a Edelbrock Qwiksilver carb on my 97 XR400... and the next ride my bike was running like crap (of course I thought it was the carb, since I just put it on) But after many long hours of tinkering with the carb, I gave it up and found out it was an ignition problem (my CDI was going bad and needed replacing)! Well after I replaced the CDI my bike ran better than ever before.

If you havent checked your valves before, they are very easy (I can give you a link to directions if you want?)! I check mine every 3 months or so...and I am going to adjust them next week during Thanksgiving break (because they are borderline of being too tight)!

You also might want to check the "duhh" things like: dirty air filter, clogged petcock, bad gas, dirty carb, intake leak (possibly in the intake boot...my intake didnt seal very well when new so I ground down the little brass spacers inbetween the clamp with a bench grinder and they sealed much better afterwards), and make sure your accel pump is still squirting gas (they can get dirty and wont shoot a stream like normal)! You also mentioned "since I just removed the BK mod my bike should run like it used to right"...yeah it shouldnt have changed this is why I am thinking it is a diff. problem! Unless the weather drastically changed in your area (IE got much cooler?) I was just tryng to give you a diff. perspective on your problem...because I have had things like this happen to me, and they turned out to be obvious things that you dont think about! Hope this helps,


  • motoman393

Posted November 11, 2001 - 10:33 PM



I forgot to say that if your valves are out of spec and are tight your bike most likely will not be able to hit the rev limiter! Later,


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