Well I finally got my 351 kit installed. I only have about 100 miles on it so far, but I love it. It's not even broke in yet, but I can feel that it pulls much harder than before. Anyway, my question is about jetting.

Before the 351, I had a FMF Q4, Uni filter, and KDX snorkel. My husband and I did the jetting ourselves--38 pilot and 125 main (genuine Keihin jets, if that matters). We did nothing to the needle. More than 50% of my riding is at 4,000 to 9,000 ft., thus the reasoning behind keeping it a little on the lean side. Overall it ran good, maybe a little sluggish on mid-range acceleration.

So when I took the bike in to my mechanic to have the 351 kit installed, I asked him to tune the carb slightly lean, due to my mid to high elevation riding. I gave him a 40 pilot, and a 128 and 130 main (all Keihin jets), but told him to order other jets if he didn't have the right combo. I also gave him a Kouba fuel mixture screw with a t-handle to install.

To my surprise, he ended up leaving in the 38 pilot and the 125 main. He said all he needed to do to get it to run good was to raise the needle. He said that raising the needle would help with the mid-range power.

So here's the problem. Now I am still breaking in the 351, but I've found that the bike runs excellent, up to 6,500 rpm. At 6,500, it starts surging. It's not an occasional surge, it's like as if I were opening and closing the throttle a little constantly.

So what would cause this? I'm guessing he tuned the bike in the lift, and didn't actually ride it, or else he would have noticed it. I may swing by the shop Friday and see what he says, but what do you guys think? My first thought is that we should put a larger main jet, maybe the 128. I know the main jet is for the higher rpm range, but I'm not sure at what rpm it actually is relevant. Anyway, sorry to be so wordy, but I'm hoping you guys might have some thoughts on this.

 

I think the 125 might be a bit small. I have a 132 w/40 pilot and it runs perfect. I'm lower elevation than you though so maybe a 128 or 130 would be good.

Also - it's already broken in

 

So you think the main jet is the culprit?

And yes, I think I will really ride it this weekend. I decided to do a more traditional break in, although I did read about the theory that's been posted here & on other forums.

 

What's your throttle position at the surge? Do you get the surge at the same rpm regardless of throttle position?

 

Yep, I would say your main jet is a little too small as well.

I am at a higher elevation, and found that a 128 dyno jet main and 35 pilot worked best for me.

A 128 dyno jet translates into a about a 138 keihin jet.

 

I put the BB carb on my KLX [when it was still a 250]and took out Bill's T~handle fuel screw and put the trick fuel screw on the right [in pic below]. Riding in the lower RPMs it was fine but chasing some of my buds on them orange bikes it was falling off in the upper revs[7000 or so]. Thought I was running out of gas and put it on Res
sameo sameo. Any way I put the screw that was sent with the carb back in and it has been fine ever since. It is a 351 now and runs great.

The T handle that BB sent with the carb has the same taper as the one on the left that also came with the BB carb.

 

135 main was LEAN for me with the webcams.. (cvk).. needle was too lean as well (dynojet) (I just slapped the TM36-60 back on love that carb.. )

 

Thanks for posting that. The surging I have does feel like it's running out of gas, so maybe I will look at the Kouba fuel mixture screw. I think I will probably go up to a larger main jet as well.

 

I believe you'll find that the Kouba screw has the exact same shape as the stock one, so performance should be identical. I recall seeing turnsleft's pic before, and I think the red screw came from another bike (TL - please correct me if I'm wrong here).

However, Kouba's T-handle makes it SO much easier to adjust. I followed Dan's suggestion and using a sharpie marked a line on the screw that matches up with a line on the carb when the screw is seated. The markings make it easier to count turns and perform fine adjustments.

 

Hi IDRIDER, I got that fuel screw off the shelf at local shop. I had one just like it in a 05 450 YZF that worked great. When my KLX was surging I put it on RES and it ran better? Called BB and he said to check the gas line under the tank to see if it was pinched[not]. Mine hasn't missed a beat since I reinstalled the BB needel. My 520 has Kouba's screw and it runs good also.