My CV40K swap- questionable techniques inside!

I decided to swap my 36mm BSR for a really really cheap CV40K fleabay carb I got for 100 dollars. It appeared perfect, so I got the N1TB needle from the KLXr, and the KLX pull knob choke from the 250/300 bikes. I also got a 147.5 Keihin main, to replace the stock 145. I drilled the slide to 1/8" for starters, per the master CV40K thread.

Below I'll show some tricky stuff I pulled in case anybody wants to follow my lead in any of these situations.

First, I HATED the location of the idle adjuster, and the BSR adjuster isn't long enough to be of any use. So, I dug through my pile of random streetbike parts I collect, and fished out a super long idle adjuster, which I cut to size:


Then, I cut off the business end of the CV40k adjuster, and line bored it to the size of the flexy cable:


I used some dull bolt cutters to crimp the threaded bit onto the flexy cable in three spots, and threaded it on with the stock spring:


It looked like this. The adjuster pokes out between the starter motor and Regulator Rectifier when installed:


In the next portion, I focused on the fitment of the carb to the intake tract, and the position of the carb within the frame of the bike. I knew I didn't like the collision with the frame and the top of the carb, and how far the boots had to stretch. However, I also wanted to spend $0 to fix these problems, as I was only into it $140 for all my stuff.

First, I decided to fix the fitment of the nose of the carb and the extreme stretch of the intake manifold at the same time.

Originally, the carb is held back by the intake rubber, against the frame:


I fixed this by shaving the rubber lip down and the clamping surface at the same time, in the sketchiest possible way I could imagine:


It turned out perfect, and I even added a little seal lip which clicked into place on the carb nose!


This made the stock clamp usable, and moved the carb forward!


While I was in there, I figured I might as well clean up the lip so it didn't form an edge after the fuel/air exited the nose of the carb:


Lastly, I decided that the stock intake boot would be used to save money, and went to modifying it. Without some of these mods, there is significant distortion of the boot, and a very un-smooth entry of air into the carb:


First, the boot didn't even seat right on the bellmouth, and one side was held further back than the other:


I fixed this by trimming the boot the same way on both sides, and clearancing the top and bottom in the same fashion to clear the carb body:



Now the boot sat firmly against the carb, and the intake clamp was centered on the lip of the bellmouth. Success!

I installed the carb, and decided after a nasty warm beer and much muffled cursing that I would rather defecate uncontrollably in a public place than do that again. I used the stock KLR fuel line, as the stock DRZsm line doesn't reach:


It fired up first crank, and idled perfectly. I pre-adjusted the butterfly with a wire feeler to match the BSR. Dead on! Test ride tomorrow. Current weather: Hail.

awesome man! i swear i checked my intake boots/clamps and decided they were all pretty well matched up and smooth but you got me thinking now. although, i dont want to pull it to double check:bonk:

Nice job, excellent write up!

Sweet!! Super clean, man. Let us know what the difference feels like.

Nice write up, looking forward to your ride impression.....

Ride test,the bad first:

Low end throttle response is dismal compared to the stock BSR carb. I have the hole at 1/8, but I know I can't go back on that if I drill it. I ordered a legit official KLR spring to use as a starting point, as I think I have a DJ spring in there now. Testing will ensue in 3 days when this comes in

Part of this could be due to a mind trick in the throttle ratio. The throttle throw is longer on the CVK because of the different butterfly control linkage on the carb. The throttle-tube to butterfly-wheel ratio was different on the BSR carb. A SSW quick-turn should fix this. I like touchy controls and high-ratio responses to user input as a personal rule anyway, so might as well.

I dislike single-position chokes. That is all.

The good:

Pulls hard to the limiter everywhere, noticably faster in general, seems to be worth the money. Will REALLY be worth the money if I can get low-end response a little better than the 36mm BSR.

It doesn't backfire on decel like the BSR would, no matter what I did. I hated that. It sounds SO beautiful now with the coast enrichment setup like it is stock on the CVK.

BSR: "duuududdudduddud POP! dududuuudududduuuu.... POP!

CVK: "duuuudududuuududuududududuuddudududuuuu..."

i have the stock klr spring, slide drilled out to 3/16's and the throttle response is great, a good bit better than the stocker. i'd like to look into the quick turn throttle also as i have to make a conscious effort to twist the last little bit to WOT.

I seem to be seeing bigger is better up to 3/16ths at least, but I'll stuff that KLR spring in there first to stay careful, and daydream about throttle throw solutions that involve the least expenditure of money possible. I know that there is such a thing as too big of a hole, and that there are vac. slide carbs (like the stock BSR, according to Eddie) that are on the verge of opening "too fast."

I just don't want the slide to open too fast. It may be that it is doing just that right now with the (presumably) softer DJ spring in there. Not sure. What it feels like sometimes is a case of the improperly-tuned "FCR bog" caused by a poorly set up accelerator pump, which I have experienced and cured on my FCR'd bikes. Not that this carb is anything like an FCR or has a pumper, but knowing what an off-idle lean bog feels like might help me figure through this.

Fixed and finished! I took my yz250f out for a run yesterday, and I decided it had a wonderful throttle ratio. I then went about replicating this on my CV40k. I ended up using a throttle wheel from my old streetbike parts pile again. It was just a little smaller than my BSR wheel, so it gave me about a 65 degree throttle throw at the handgrip. I really like this a lot:


I also drilled the slide to 3/16" with the DJ spring and N1TB needle. It is faster and more responsive everywhere. Now I get to find the new limits of my new machine! :smirk:

as is evidenced by my crazy bracketry, i am now able to run a pullback cable, which is excellent.


Edited by jimmyhoffa

Great write up JH!!

Keep a 42PJ around just in case. If you goof around with float height (like I did), the CVK will pop on decel.

Current set up:

150MJ, 42PJ, N1TB @ 3

(Slide predrilled .195" dia, came that way)

Hotcam Stg 1 Intake cam

SSW/MRD Exhaust

Hooligan Mod w/93 Katana rims (BT-003RS)


Have to trim down a hex drive, can't get the exhaust cam caps off.

Looking at an emulsion tube from a HD site.

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