Jetting advice
#1
10-21-2006 | 07:48 PM
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Jetting advice
Soon I will be putting my 331cc cylinder on - hopefully mid this coming week. As most here know, I also have the Keihin FCR 35mm pumper carb to install as well. At the request of several folks here, I am considering putting back on the stock CVK carb so that I can seperately assess the performance effects of these two upgrades instead of installing them both at the same time. I may do that at least long enough to break in the new cylinder and rings for a few heat cycles.
However, IF I put the CVK back on, I think I need to change up the jetting just a little. I'll be having 80cc more volume after all and I think my current jetting might be just a tad lean. I think things were spot-on when I jetted and adjusted for the 250cc cylinder. I did not change the jetting after moving to 300cc and I was I observed was a slight stuttering at the mid-range when the throttle was steady. If I opened the throttle, the stuttering went away and I got good drive, but at 4000 - 5000 RPM or so, with steady throttle, there was a bit of stutter.
Starting and idle were fine. While my settings aren't quite dead-on, they are pretty close. I would be fine with leaving them as is if I stayed with 300cc, but in moving up to 331cc, I'm wondering if the mid-range will suffer a bit more.
My current settings are: #35 pilot, #125 main, clip on 2nd ring on the N1TC needle, fuel screw 2.5 turns out.
I suspect that going to the higher volume 300cc cylinder has leaned out my mixture a bit as the engine is pulling more air and fuel in, but without changing the jets or needle settings, there's not enough fuel to compensate.
I've never claimed any particular expertise in carb adjustment, so I'm asking for some advice on what to change. Here's what I think, but I would appreciate any alternate thoughts. I think the pilot is probably OK at #35 since the idle circuit seems OK. I think maybe the #125 jet needs to go up a few and perhaps raise the needle to the 3rd ring. At that point, I'd need to readjust the fuel screw, probably start with 2 turns out.
At my disposal I have a #132 main jet, a #130 main jet, and a #40 pilot. I don't need any other parts for adjusting the needle height or fuel screw. So I'm thinking of replacing the #125 with the #130 or #132 and making the needle adjustment to the 3rd ring.
Or should I just leave things as they are.
Any advice is appreciated. I'd like to get the CVK tuned well for the 331cc in order to do an fair comparison. But I don't want to spend a lot of time on it since I'm going to be putting on the FCR shortly afterwards anyway.
BTW, I'm pretty sure the Keihin FCR 35mm pumper is going to provide a more dramatic performance upgrade than the additional 31cc in displacement, this based on reports from others. Bill_P over at ThumperTalk describes it as so:
I believe his bike has 300cc displacement and in another thread he indicates that he would gladly take that + FCR 35mm pumper over his buddy's 340cc + CVK.
So based on that and many other reports, I'm pretty sure I know what to expect with what each mod does. I'm still not fully committed to putting the stock CVK back on, but if I do, I at least want it jetted and adjusted correctly.
However, IF I put the CVK back on, I think I need to change up the jetting just a little. I'll be having 80cc more volume after all and I think my current jetting might be just a tad lean. I think things were spot-on when I jetted and adjusted for the 250cc cylinder. I did not change the jetting after moving to 300cc and I was I observed was a slight stuttering at the mid-range when the throttle was steady. If I opened the throttle, the stuttering went away and I got good drive, but at 4000 - 5000 RPM or so, with steady throttle, there was a bit of stutter.
Starting and idle were fine. While my settings aren't quite dead-on, they are pretty close. I would be fine with leaving them as is if I stayed with 300cc, but in moving up to 331cc, I'm wondering if the mid-range will suffer a bit more.
My current settings are: #35 pilot, #125 main, clip on 2nd ring on the N1TC needle, fuel screw 2.5 turns out.
I suspect that going to the higher volume 300cc cylinder has leaned out my mixture a bit as the engine is pulling more air and fuel in, but without changing the jets or needle settings, there's not enough fuel to compensate.
I've never claimed any particular expertise in carb adjustment, so I'm asking for some advice on what to change. Here's what I think, but I would appreciate any alternate thoughts. I think the pilot is probably OK at #35 since the idle circuit seems OK. I think maybe the #125 jet needs to go up a few and perhaps raise the needle to the 3rd ring. At that point, I'd need to readjust the fuel screw, probably start with 2 turns out.
At my disposal I have a #132 main jet, a #130 main jet, and a #40 pilot. I don't need any other parts for adjusting the needle height or fuel screw. So I'm thinking of replacing the #125 with the #130 or #132 and making the needle adjustment to the 3rd ring.
Or should I just leave things as they are.
Any advice is appreciated. I'd like to get the CVK tuned well for the 331cc in order to do an fair comparison. But I don't want to spend a lot of time on it since I'm going to be putting on the FCR shortly afterwards anyway.
BTW, I'm pretty sure the Keihin FCR 35mm pumper is going to provide a more dramatic performance upgrade than the additional 31cc in displacement, this based on reports from others. Bill_P over at ThumperTalk describes it as so:
After 3 months of screwing around with this completely bogus FCR pumper, I finally got the KLX running like a dream. It's a wheelie machine with zero bog and tons of grunt. The 13T and trials tire make it almost impossible to keep the front end down. The KLX is FUN again.
... [description of the taffy mod which was key to dialing in the FCR]
The bike is perfect right now. Ton's of low end - much more than the WR and has a lot more "hit" like the WR than with the CVK. It's kind of like a cross between a KLX and a WR now. Really sweet and very controlable on the bottom end.
Can't wait to get out and ride it again!!!!!! Special thanks to LTK300 for his sacrificed drill collar that made the Taffy Mod work!!! Yahooooo!!!!! I couldn't wipe the smile off my face all day long!
... [description of the taffy mod which was key to dialing in the FCR]
The bike is perfect right now. Ton's of low end - much more than the WR and has a lot more "hit" like the WR than with the CVK. It's kind of like a cross between a KLX and a WR now. Really sweet and very controlable on the bottom end.
Can't wait to get out and ride it again!!!!!! Special thanks to LTK300 for his sacrificed drill collar that made the Taffy Mod work!!! Yahooooo!!!!! I couldn't wipe the smile off my face all day long!
So based on that and many other reports, I'm pretty sure I know what to expect with what each mod does. I'm still not fully committed to putting the stock CVK back on, but if I do, I at least want it jetted and adjusted correctly.
#3
10-21-2006 | 08:48 PM
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Joined: Sep 2006
Posts: 6
From:
RE: Jetting advice
I have not tried this process, but it sounds like a good way to determine if you're jetted correctly and what to change.
http://www.iwt.com.au/mikunicarb.htm
http://www.iwt.com.au/mikunicarb.htm
#4
10-22-2006 | 11:19 AM
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RE: Jetting advice
I run a 128 main and stock pilot and I'm still at 250cc. I have no flat spots or bogs. If you ran a 125 main with 300cc, I would assume a 128 would be about perfect for a 300 as you stated you had a tiny bog or flat spot. But a 130 would not be much of a difference so it will probably work well for both 300 and 330. But what I'm getting at is that you needed a bit more gas after your first conversion and a 128 would probably have worked well. So in essence, you really were the guinea pig for me because I plan on doing a 300 conversion next spring and now I really don't need to worry about adding a larger main jet although I may try a 130 just for comparison if it does run bad.
BTW, I had some spare time yesterday so I took the airbox lid off just to see how it would run after my Uni-filter install of 2 months ago. It ran like crap. It had a major midrange bog. I enlarged the stock snorkel hole and removed the snorkel altogether after I put in the Uni-filter and this must be about right for air intake as removing it fully really made it run like unowot.
BTW, I had some spare time yesterday so I took the airbox lid off just to see how it would run after my Uni-filter install of 2 months ago. It ran like crap. It had a major midrange bog. I enlarged the stock snorkel hole and removed the snorkel altogether after I put in the Uni-filter and this must be about right for air intake as removing it fully really made it run like unowot.
#5
10-22-2006 | 12:36 PM
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Joined: Jul 2006
Posts: 1,985
From:
RE: Jetting advice
Are you going to remove the air box lid altogether?
Or stick with the modified snorkle?
That would probably alter what jetting to start with.
I would think 140 with the taffy mod.
That is just a guess based all the stuff I have read on this and other forums.
Or stick with the modified snorkle?
That would probably alter what jetting to start with.
I would think 140 with the taffy mod.
That is just a guess based all the stuff I have read on this and other forums.
#7
10-22-2006 | 05:38 PM
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From:
RE: Jetting advice
ORIGINAL: Iowaguy
I think the taffy mod is for the pumper carb. He's trying to figure out jetting for the CV carb.
I think the taffy mod is for the pumper carb. He's trying to figure out jetting for the CV carb.
Still the question of air box lid is a consideration.
#8
10-22-2006 | 06:08 PM
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RE: Jetting advice
I plan on leaving the airbox lid on. I do have the KDX200 snorkel as well as a cut down stock snorkel for greater air flow. I don't have any objections to removing the lid, though - other than it increases the intake noise and as Iowaguy pointed out if you are already jetted close with it on, removing it will throw things off and you'll need to revisit your carb settings in a more fundamental way. My settings are close, I just want to tweak them a little so that if I put the CVK back on I'm not pessimizing any reports using it.
I agree that the #130 is probably the way to go and I have made that swap. I also raised the needle by putting the clip on the 3rd ring instead of the 2nd and adjusted the fuel screw to 2 turns out instead of 2.5. If those setting don't feel quite right, I will remove the airbox lid and retest to determine if the problem is too rich or too lean and go from there. But hopefully it will be just about right. I don't really want to spend alot of time on the CVK when I have this nice pumper here wanting to be installed.
I agree that the #130 is probably the way to go and I have made that swap. I also raised the needle by putting the clip on the 3rd ring instead of the 2nd and adjusted the fuel screw to 2 turns out instead of 2.5. If those setting don't feel quite right, I will remove the airbox lid and retest to determine if the problem is too rich or too lean and go from there. But hopefully it will be just about right. I don't really want to spend alot of time on the CVK when I have this nice pumper here wanting to be installed.
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