I have had my bike apart and was messing around with the carb trying to get rid of the annoying popping on deceleration. I have a #35 in there at 2 and three quarter turns out, I have ordered a #38 and #40 with the hopes of reducing the popping. Other than the popping the bike runs as well as these things can. Anyhow, I found this somewhere on the net when I was hunting for info.




http://www.dirtbikeworld.net/forum/s...ad.php?t=26074

 

I would say two things.

Adjusting jetting to get rid of lean pop during decel is not smart in my opinion. Even properly adjusted carbs give little pop through the exhaust when chopping the throttle, the pop is not a sure sign of incorrect jetting.
You want to adjust jetting to be spot on when you are ON the throttle, or on cruise, not for best mixture when closing OFF the throttle.

Let it pop and focus on response and glitch free jetting for on-throttle conditions.

Then again, the CV carbs are so forgiving with the mid-throttle jetting that it almost doesn't matter what you do as long as the bike idles well and main jet is set correctly.

Removing the air box cover is another bad idea.
Look at race bikes. They all have very tightly sealed air boxes.
Why? Because airbox helps to make more power.

When airbox has a resonance at correct frequency (RPM) it gives a small boost to the intake and helps to charge more mixture into the cylinder. A closed airbox also helps with the jetting.
The carburetor senses the vacuum in its venturi and that pulls in the right amount of fuel through the jets. With no airbox the vacuum gets bit smaller and accurate metering of correct amount of fuel becomes so much more difficult.

Lot of people say removing the airbox lid "adds more air".
It really does not do that. It just kills the little vacuum that pulls in the fuel.
And that's why you need bigger jets w/o the airbox lid. Not because the engine now pulls in so much more air. It still pulls in same amount if air on each intake stroke but less vacuum means less fuel. Mixture went lean.
So we need bigger jets to pull is same old amount of fuel up from the bowl with less "suction".

--
Mikko

 

Im running super rich and my bike pops like a ****

 

good post Miko. I have a little airbox design software program that has a function to determine airbox opening size. Even with big fudge factor % entered for location of the box opening (under seat etc) you'd be suprised how small it can be and still flow the desired cfm or air.

Resonant tuning can make more power at a specific rpm, but the more it helps the more it will hurt other rpms not at that frequency. So many times boxes are tuned so they resonate outside the operating range of the motor for best spread of power.

 

My bike popped on decel till I got a kouba air screw, and adjusted it per the instructions.. It must have been running a bit lean, because after the airscrew and the tuning procedure, the bike runs absolutely perfect, NO popping on decel, no coughing, hiccups, or hesitations.. It's a frickin mean beast now.

 

Thanks for all the replies.

Mikko has some good points, especially where the vacuum is concerned, and tuning too much to get rid of popping.
Although, when you are trying to make horsepower you need to introduce more fuel which in turn means that more air is needed. My bike pops from the moment I start to decelerate, especially when it is hot, all the way down. I have a #35 pilot installed with 2.5 to 2.75 turns out. I was thinking that I could go to a #38 or 40 and tune it better to decrease the popping. I will install a Kouba screw and play with the jetting. When I installed the pipe and re-jetted I never did actually get a chance to tune the idle mixture and see if I can dial that in better.


I just recently removed the backfire screen and have noticed a change in throttle response. I haven't ridden the bike enough to decide whether it is better or worse.


Happy Fathers Day to all you dads!!

 

If I'm understanding the discussion about the airbox lid issue, I think that normally applies more to multicylinder engines. There are engine and carburetor combinations, and fuel injections setups, that require a huge "dead air" space to funtion at optimum performance. It's also my understanding that a CV carb is more sensitive to this than a slide/needle carb, but doesn't the KLX have plenty of airbox volume and air disruption protection even with the lid off? I'm thinking the KLX isn't going to be hindered by running the airbox lid off...aside from properly matching the air volume with the right jetting. I don't think you'd notice any issue with the KLX unless you ditched the airbox and just went with a clamp-on K&N or such. I think this where the CV carb can get hinky, as air disruption this close to the carb throat can sometimes affect vacuum slide performance.

 

Redpillar, did you remove the backfire screen from the exhaust or the air box. I looked at mine and the air box screen is put in with phillips head screws, not like the '06 which has bolts and I figured I would just wait and remove it later. I read somewhere that someone removed the whole air box to get their '09 screen out. I don't want to bugger the head of the screws trying to get them out. Any advise?

 

The op is on the right track. The dynojet kit instructions are thought out. The suggestion to use duct tape and block off the airbox opening to tune in the jetting is a good idea. IIRC the instructions call for the lid to be removed in stage II jetting. Also remember you paid for the kit, they have a technical assistance line to answer your questions...1 800 992 4993

I did everything I could find (modified header,slip on exhaust, jet kit & filter, free mods,lid off) and needed a 40 pilot 2.5 turns out w/ kouba screw.

 

Not really correct. If the airbox lid creates a restriction to flow, which I think everyone here would agree that it does, then upon intake less air by mass is pulled into the cylinder than if the airbox lid (and resulting restriction) was removed. This is because air is compressible, very compressible. Now if you were talking about an incompressible fluid, like water than the amount drawn in would be the same.

Since we now know that more air is drawn into the cylinder with the airbox lid removed then it followed that at a given rpm the RATE of air through the carb increases, thus increasing the venturi effect and pulling more fuel into the air stream, not less. But if the jet is not large enough then you might get too lean a mixture with the airbox opened up.

I'm not an expert with jetting but I do know a few things about fluid mechanics.

What you mention regarding resonance in the airbox may in fact help create increased momentum with the incoming mass of air through a particular rpm range, thus increasing performance. Don't know whether Kawasaki did this but I suppose it is possible. This would be tied into the specs of the cam which for the most part determine the momentum characteristics of the incoming air into the cylinder.

If an internal combustion engine were to be viewed as a quasi-static process then the intake valve should open at TDC, just as the piston descends and begins to create vacuum in the cylinder. But this is not a quasi-static process and there momentum associated with the process and that changes things if you want optimum performance. Hence variable cam timing which helps optimize torgue across the rev range. At high speeds the incoming air has quite a bit of momentum and will continue to rush into the cylinder even when the piston passes BDC and is on it's way back up. I'm off topic here but the point is the compressibility of air is critical to the design and tuning of ICE's.

- Mark

- Mark