Yet another Carb. Question

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Jan 17, 2001
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#1
I was wondering if anyone can explain why this happens. I've read that jetting has nothing to do with RPM, but rather with throttle position. What about this though...if I have my air screw out 1.5 turns and twist the throttle to wide open, the bike bogs so bad that it will stall. I'm not talking about wicking it open quickly either, but still opening it faster than the engine can keep up if you know what I mean. When I do the very same thing with the air screw out 1/2 turn, the bike does fine. Why is this if the pilot/AS only effects close to 1/8 turn (or somewhere around there)? Could it be because the main is too lean and that by richening the pilot/AS circuit, it's "helping it out?"

By the way, I finally did a good plug chop and got a nice, dry, tan-creme reading with the main I have in it.

As always, thanks folks!

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fishhead

die you sycophant !
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#2
I think your on to somthing there. The dynamics of jetting an active carb for throttle response are much different than jetting for steady state running. I tune the main for power and tune the needle and pilot jet for best throttle response. You might try raising the needle one clip position when your air screw is 1.5 turns out and see if that helps. But if you are able to tune it up with the air screw most would say "Good to go"

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Sage

dirtbike riding roadracer
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#3
One circuit is lean & one circuit rich, the rich helps the lean pull thru it bad spot, which sounds like the start of the needle, and away you go...........

same deal with a rich needle and lean main, the bike pulls ok, but gives you a lean plug color on a mj plug chop. Thats why it good to start with the main and work your way down but thats hard to do sometimes when your way off with the jetting.

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Sage Wilkinson
sage@sagesdirtandstreet.com

[This message has been edited by Sage (edited 04-11-2001).]
 

Mac

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#4
All the circuits of the carb will affect each other, they are probably right in saying that leaving the a/f screw @ 1.5 and lowering the clip will also richen that circuit giving good throttle response. Jet the main by WOT plug chop and jet the rest of the carb by feel and throttle response. I run my a/f screw @ 0.5 to 0.75 in the winter and 1.0 to 1.25 in the summer. Use whatever works for you.

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Mac
00-KDX-200
 

G. Gearloose

Pigment of ur imagination
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#5
Ditto,
So many things can change day to day, air filter, temp, humidity, barometric pressure,

'On the air screw' is how I ride half the time on our terrain, so I feel it when its not set right.
I think the wording 'start from highest idle' setting has some confused, (or perhaps I am, because that begins 1 to 1.5 turns away from where I settle), This may be responsible for some folk's off-idle bog or stumble. Also I think reving the bike in neutral doesn't simulate under-load conditions, and promotes a too-lean set-up unless you're experienced, but not the occasional hack like me ;)

Being on the 'Slightly rich' side of stoikiometric perfection promotes off-idle torque and stall resistance, that tractor-like feel that we crave to pull through the gear quicker and consistantly without a flat-spot.

I set mine in an unorthodox manner, but under load.

I butt my front tire against a 1' rock or stump to crawl up, and set the AScrew for minimal throttle, clutch, and also stall resistant, then let it roll back, repeat, turn AScrew 1/8 turn, repeat. This translates into torque that pulls 'through the gear' faster from real low, and seat-of-the pants pull, and stall resistance on 'mars' terrain.

Its easy to do ,once warmed up every ride, (but I'm a weirdo). BTW my settings resemble MAC's, on my '91. Russ I'd infer trying the next larger pilot if the air screw liked to be at 1/2 turn; you may net better resolution to fine adjustments.

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Mac

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#6
I agree with Gearloose that the highest idle does NOT yield best throttle response, for me anyway. After the bike is well warmed up I find highest idle then turn clockwise(richer) until my idle just drops a little usually 1/8 to 1/4 turn. I then test the bike by idleing in 1st or second gear and flicking the throttle open, the bike should stand right up :) if it doesnt then I change the screw by 1/8 turns until it does. I also find better throttle response using a 42 pilot at 3/4 turns vs a 45 pilot @ 1.5 turns?? Dont ask me why but the results are unmistakable. Your results may be different.

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Mac
00-KDX-200
 
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#7
I've found that no matter what pilot I use(42,45,or 48) the off idle response when wicking the throttle open in first or second gear is horrible unless the A/S is only out 1/2-3/4 turns. I have yet to find one combination where the bike responds well at anything over 3/4 turns out, whether it's 35 degrees or like today, 80 degrees. I've tried mains from 152-160, the above pilots, and the clip has been on every position except for the extreme #1 and #5. Maybe I'm just after more than what the bike can give. I spent the major part of the day playing around with my jetting and getting on and off my bike and my brother-in-law's KTM200EXC trying to get mine to mimic the off idle response of the KTM. I must say that the EXC has remarkable bottom end power, but the KDX may just have a little bit more on top...hard to say. Only way to know if it's possible I guess is to ride someone else's who thinks that his is a stump-puller. Like someone with the R&B Carb!

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#8
yea baby im glad im not the only one who sits and thinks about this stuff, but it is amazing how just a 1/8 of a turn of the air screw can make all the diffrance, i just keep a large jewelers screwdriver in my tool kit then once im sure my jetting is right its time to play with the airscrew it seems to make off idle throttle response way better by turning it then testing, its worth spending a day doing it,

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#9
re: AS setting to high idle.

That is just a starting point. Throttle response is the tuning method needed to set it correctly (crawling up a tree or whatever). I don't know how the 'hi idle' thing ever got started. If you DO set your AS that way, throttle response will likely be AWful!

An AS difference between 1.25 and 1.125 turns out can change your bike (did mine) from kicking it until you can't breathe (when it's hot) to one-and-go.

RE: KDX200 vs KTM200EXC

Not a personal experience, but my riding buddy's 200 (setup real close to mine..both with RB's plenum divider) has the benefit of an EXcellent rider with decades of experience. A couple weeks ago we were riding with a brand new KTM'er. While my buddy said the other rider was better'n him, he said he could pull the KTM anytime, anywhere. He thought the KTM was pretty anemic.

We're currently running 42-152-AEN/4,AS@.5-.75, throttle valve cut to 7mm). Temps in the upper 40º-mid 50ºs, 2500'el. Both our bikes load up some on extended lo idle runs (long downhills),but otherwise run great!

Anyway, the general point of this thread is 'adjust to suit'. If it's not 1.5 turns out, so what?

Oh..as russ said, 40,42,45,48..150,152,155,158..R1174K, 1173K/L, AEG, AEN..clips wherever.. no matter...1.5 out has NEVER been the case on my bike.

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  • OLD member
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