mid throttle surging 650L

A little help from the ones in the know. 05 650L with JD jet (165, stock pilot), UNI filter out of the air box and smog removed, stock pipe unbaffled, Clark tank, sealevel in Florida. At constant throttle, 1/4 through 3/4, any gear any speed I get a surging. Enough that it feels like brakes are being applied softly over and over. Pulls strong throughout the band, gobbs more power that stock. No stumbling off the line or hiccups during accel or decel. I've searched the forums and can't pin it down ? lean condition (jet too small?), air leaks (no popping through exhaust), UNI too restrictive (in the open air?). I'm way worried that this is hurting the engine - any help is appreciated.

Try adjusting your needle richer and see if that helps.

Stock pilot is def. too lean, even with the stock ex. baffle in place. Try a 55, make sure you get the short one.

Have you drilled the slide and freed the pilot screw?

Most XRL folks find that keihin jets, and shimming the stock needle to work better than the JD kit. No matter what, drill the slide- that'll help the throttle response a lot.

carb mod link:



Try some basic checks first. Is the throttle cable adjusted properly? Too much freeplay can affect the operation. How about the choke? Is it working normally?

Then you mght want to get back in the carb and see if the float is sticking or isn't moving smoothly. I've also heard less than positive comments about using other than Keihin jets. The OEM Keihin jets are longer than the aftermarket ones. This could be part of the problem when riding at constant speeds or steady RPM's. :)

Did the kit you bought come with a lighter spring for the slide? I would guess the the slide is bobbing up and down. Even though you're keeping the throttle constant, the slide is vacuum controlled. If you replaced the spring with a lighter one you might want to try switching back.

My first thought is to go richer, but with the DJ clip at the #3 and a 165 I thought it was pretty rich. My hand held IR temp guage reads from 240 in front of the case to 350/400 degrees in the rear of the case. I've never read anyone using a IR gun to read temps but use it all the time on my r/c car (gas engine, air cooled).

Also, if the pilot is too lean wouldn't that case problems only in the beginning of throttle response? My problem is just at cruising, not idle, or acceleration. Choke works fine - actually better witht the changes as I only need half choke to start and it warms up faster / smoother.

I'm going to drill the slide first, that was the only thing I didn't do because I can't return it to stock. I used the lighter spring from the kit too. This makes the most sense in my mind that vacuum pressure could be pushing the light spring up and down and without the hole drilled out could make it easier / more susceptible to movement.

I'm going to get a couple of jets from the shop to fool around with, and fiddle with it this week end. I had to play with jets on my parttime race car with dual Webber carbs. With that we looked for power with jets then adjust for throttle respose off the line. That took about 2 - 3 weeks of fiddling with it.

Thanks for the help, y'all.

Ok, jumped into it tonight. Just couldn't wait till the weekend. Picked up a 55 pilot (wrong one, returning tomorrow), tore it apart and drilled the slide. Readjusted the throttle cables, put in the xr only temp gauge, and generally neatened it up.

Started right up and quickly idled smoothly. Once warmed up I started whacking the throttle. Quick to rev up, and no immediate bogging. Further up the band and engine stumbles, pops through exhaust and some through carb. Multiple tries of reving and backing down shows no clearing, just the same general roughness. On hard full throttle it bogs seriously at the upper limit as if starved of fuel/air. Kind of choked down.

Stopped here to reflect on the nature of Himalayan yak dung, and gather my thoughs. Since I didn't change the pilot and I have no idle problems this should be again a main jet or mixture problem, yes? Well, no actually. looking at my Uni filter I decide to pull it off to see what the slide and butterfly and such are doing. Engine starts right up and idles perfect again. Whack the throttle and no popping through exhaust. A few high revs and no bogging. Humm. Rev it up and hold to listen for pops, snaps, pings, or other oddities. Nothing - just relatively smooth through the rpms. The fuel vapor standoff occurs at the mouth of the carb and I'm wondering if the Uni seriously hampers air intake.

Throughout the run I have a very small fan to keep me cool going and the bike aired. The temp gauge stays at 200 - 230 the whole time. Obviously not lean by most standards.

So in a nutshell, I think I found it - anyone else using a Uni clamped to the carb mouth? Would a K+N work better? I do not want to reuse the airbox, can't see how an "open" filter won't flow better than a somewhat restrictive box. Thanks for the input to those who did. :)

Not sure if this helps but I had similar symptoms although I did not remove the Unifilter (I still have an airbox). What I did discover however was that the DJ kit sucks. I replaced the jets with Kehein 55p and 158m (I hear the DJ 165 is equal to a K 158) and replaced the DJ needle with the stock one, shimmed of course. I tried numerous attempts at using the DJ needle as the bike did not come with a stock one and the one i ordered was a couple weeks away. I had it running ok but never good. The difference was night and day when the "original" needle was put pack in. Turns out if you compare the two, they have completely different profiles.

I would never recommend anyone get a DJ kit especialy for this carb especially when it only takes about 10 bucks worth of parts to out do a 50 dollar kit.

Hope this helps you and if not perhaps someone else.

UNI filter out of the air box

Where did you find the out of the air box uni filter and how does it perform?

"Stopped here to reflect on the nature of Himalayan yak dung"

pay attention folks, this is an essential step in any carb tuning proccess.

Now that I have put in the DJ kit I hear more and more how the kit blows. Lesson learned.

The Uni I have clamps right to the carb throat - a 2 1/2 inch opening sock type filter I picked up at Cycle Gear. As I posted I think this maybe the problem as it doesn't flow very well. The fuel vapor standoff points to the filter stifling the atomization of fuel. I see this on my race car webbers. A cloud of fuel/air just above the carb throats on hard full throttle. I'm going to look for a K+N slip on. I really want to use the airbox for battery and such.

I am running a K&N clamp on to the carb filter, I'm running a DJ kit, 55 kiehin pilot, 170 kiehin main, DJ needle with clip at 4th from top, and slide drilled, air mixture screw approx one turn out. I am also running a K&N pre charger or filter skin over the K&N filter just to help keep the filter cleaner for a longer time. I am running a FMF megaphone slip on muffler. This setup on my bike runs perfect, I did run a foam filter on the carb before the K&N and it seamed to run fine, I just like the K&N better, easer to service and flows a little better. I have been this clamp on filter setup for about two years, maybe 4000 miles.

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