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cvk 40 dyno results

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Hi just got my bike back from the dyno. Not to happy with the results before it was 37.5hp now with an fmf header and the cvk40 its only gone to 38.8hp. Its actually gone down 0.9ft lbs of torque to. There is a big dip in power between 4-5000rpm, anyone have any ideas? Also the guy recommended getting a new manifold on the carb said this 1s really tight is there a slightly bigger 1 i can try

thanks

just looking at some figures i took from the graphs

between 4-8000rpm the cvk is 1hp down on my old bsr surely this cant be right?

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I'm sad to hear this Dan. My bike is down waiting on a new tire, so I haven't been able to continue my experiments. I have the KH148 main jet, and the KLX650 needle jet waiting to try. Figured it wouldn't hurt since I'm using the KLX650 needle too. It may not have any effect but I like to tinker.

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You may want to ask Freewheeler about this. I know a while back I saw he posted about the installs he has done. Maybe he has some ideas.

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What is your jetting/elevation? Did you 3x3 and are you using the recommended N1TB needle? Which clip? How is throttle responce? Trying to help.....

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Something has to be wrong. You should get a very decent increase in HP. I got 4HP at 7,000RPM with my CVK.

What main jet and needle did you use? 3x3?

EDIT: Did the dyno have a gas analyzer? If so, post the CO numbers.

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Hi, im using the n1tb needle, think about a 150or152 main jet it was fitted with a 155 and the guy said he went a couple smaller. Not to sure what clip position it was on 5 but he said he fiddled around with it. Throttle response is pretty good not drilled the vacume hole to 3/16s yet it has been drilled a bit though. Yes i have the 3x3 and ride around sea level. Still gettin quite a bit of popping when the revs die down. Mines idleing low whats the best way to adjust this? And at idle if i give it a quick twist on the throttle 9out of 10 times it cuts out, is that any help. Apart from the lack of increase in power the bike runs fine

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Dan,

Did you get the stuff checked out that freewheelers identified in his last post to you?

"check the diaphram on the left side of the carb .... I'll bet it is perferated ... Its function is to stop the decell poping... there is a tiny "O" ring under the housing along with a pretty stiff spring ... so watch for them when you take it apart ... also ... check the diaphram on the throttle slide ... as it may be perferated also ... which could also cause the bog"

It's tough to go thru this when you are having someone else do the work on the carb.

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I did mention it to my mechanic and he never said anything more about it. Also had another guy do the dyno set up, would he not have noticed this when re-jetting the carb? Could this cause it to lose power? Can i ask were the idle adjustment is on this carb

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The ide adjustment is on the right hand side of the carb. It can be reached with a long handled flat blade screwdriver from behind the shock. Freewheelers said he replaced his with the one from the stock carb, so it is more easily adjusted. I just left mine as it is since I don't ever want to pull the carb again.

Pin holes or small rears in the diaphrams could easily be missed if you are not specifically looking for them.

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Thanks thought i was looking at the idle screw looks a right pain to get at. When i got it dynoed he never gave me a graph showing the a/f ratio can ask him if hes got 1 though. Does anybody know the part numbers for the 2 diaphrams? If i can get them cheap enough might as well try them, thanks

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Found this in a KLR FAQ:

Is there a cheaper slide/diaphragm assembly available?

Harley dealers carry a slide/diaphragm assembly, part number 27585-88, for Kehin CVK40 carbs. List price is usually around $40. For a temporary repair, Park Tool makes small, clear, flexible patches for bicycle tubes that will do the job.

Otherwise it costs $90 for a KLR650 version.

The smaller diaphram is this one:

43028A 43028-1062 DIAPHRAGM,VALVE

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Dan,

I just replaced the slide/diaphram with a harley davidson one (part# 27585-88, for Kehin CVK40) which they had in stock at the local harley shop for $51.

I had been trying to tune out what I perceived to be a sluggish throttle response without any results from jetting/needle clip position. My CVK40 had a repaired slide/diaphram, and I don't think it was sealing well. I pulled the repair glue off, and could see the tear in the diaphram, but could see how easy it would be to not notice it if you didn't examine it closely over a light.

I drilled out the harley slide vacuum port to 3/16" (a big hole!) and put it in last week with a KH148 main jet (I'm at sea level, "E" model cams, 3x3, billet exhaust tip), needle on 3rd clip. I had cable tied 2 of the slide springs together to reduce the spring tension, and left it that way.

Wow! What a difference! This thing has very snappy throttle response! Quick quarter throttle snaps provide immediate power. I may return the slide spring to stock since the response may be too aggressive.

I know you've been frustrated with your carb. I suspect freewheelers concern about a torn slide/diaphram may be the cause of your bogging, and the other diaphram on the left side of the carb is causing the decell popping. Your mechanic really needs to drill the slide vacuum port to 3/16" too, and probably won't want to since it is such a big hole.

Hope you get it sorted out soon. You won't believe the difference in power delivery.

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Thanks, got the slide diaphragm on order should receive it in the next few days. Is it easy to change over? Yeah he was a bit concerned about drilling the hole that large, if i do get a shop to fit the new diaphragm i will make sure they drill the hole 3/16

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He won't even have to remove the carb. He can take the tank off, loosen the clamps securing the carb, and rotate it to the left side enough to get the plastic top off. Just tell him to make sure the throttle cable bracket isn't bent from doing this, and that the throttle cable adjustment actually pulls to the stop at full throttle. I hope he used the info in my original post to modify the throttle cable bracket the way freewheelers had told me.

Modify the throttle cable bracket on the carb - cut off the return cable mount, then I'll quote Dave's instructions: "The reason to modify the main cable bracket is to get rid of the extra slack in the main cable ... The area that the cable mounts to the bracket is what needs to be modified ... You have two 10mm adjusting nuts that clamp to each side of the bracket ... The area on the bracket where the bottom adjusting nut clamps ( where the cable leaves the housing ) has to have material removed. There is about a1/2 inch of space in the bracket that the nuts clamp the cable to the bracket. Take a hack saw and remove half of the space about a 1/4 inch from the bottom side of the bracket. This will allow the cable to be raised that 1/4 inch you removed ... and take the extra slack out of the cable. Run the bottom nut right to the end of the threads on the cable and clamp the top nut down ... do your final adjustment on the cable at the throttle housing"

When I kept messing with needle clip positions, I rotated the carb several times, and inadvertently bent the throttle cable bracket which did not allow full throttle. It bent back easily, and I checked the cable adjustment to make sure I got full travel.

I'm more and more impressed with this setup since I got it sorted out, and hope you will too.

The CVK40 dyno chart thread indicated a healthy horspower increase thru the middle rpm range (I think the guy got a 4hp gain at 70000 rpm). The results were not that dramatic at peak power (which you've already seen). But the throttle response is soooo dramatically different. Quarter turn throttle snaps in the lower gears are very sharp and strong, either you'll pull wheelies, or be looking for small bumps in the road in 3rd gear to snap the throttle and get some air. It really is a great setup.

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