carb question

[bowhunter007]

I can't find a thread, doing a search, that explains the carb slide(i.e. it's exact function). I'm confident that I understand pj, mj, needles, etc. I just don't understand how the slide affects the lean or rich condition, of the carb. Yes, I'm tinkering...again. Thanks

 

[bowhunter007]

bump! Anyone?

 

[Jakobi]

The slides role is to meter the airflow into the carb. Obviously with throttle closed the slide is only allowing a very little air to pass under it. The idle screw actually manually lifts the slide to set the idle. Its basically the slide stop. As the needle and slide are both connected lifting the slide by opening the throttle allows more air to pass and also lifts the needle to add more fuel to the mix. The slide cutaway/number is the angle at which the slide base is cut. It affects the velocity of the air as it passed under the slide. Once you begin opening the slide up a certain distance the effect is less noticable as your working on other circuits more so. The little notch in some slides to to assist in air flow at idle by allowing the slide to be closed further.

 

[twowheels]

bump! Anyone?

You knew you had to wait for Jake right?
Basically what he said ...

PlusOnePerformance

 

[Jakobi]

So what tinkering has occurred and what did you find out?

 

[bowhunter007]

So what tinkering has occurred and what did you find out?

Aside from swapping needles in & out, I've just been studying the effects of changes in clip position with different needles. I've found with my riding style, I like to run a smidge on the lean side. I lug the bike pretty good in very slow technical sections. Since I don't multi-task very well, constant clutching is a pain, so I'm "gas on, gas off" with it. The JDRed needle is working very well for this, untill I relearn a few things. Part of the problem is I'm standing, and I'm not comfortable loosening my grip enough to smoothly use the clutch lever under power. I'll work on it at a later time. I'm still worrying over my endurance for long rides. Since I'm predominently standing, I find when I'm tired, sitting down actually screws up my focus & balance. So far I'm only really good for about 4 hrs of hard riding(hard riding for me). I've kept track of my changes, so I know what I've done and I don't repeat. Thank you for explaining the slide. Now the picture is complete. Now I have to resist the urge to play with different combos of needles & slides. It is an interesting theory "Leaner slide w/richer needle" and so on. I need to start using the clutch properly so I richen the mix a bit, and ride a little harder on the pipe.

 

[Jakobi]

I hear you. A needle diameter change is somewhat similar to the slide in that they both overlap also with the pilot.

It never ceases to amaze me how much the power delivery can change even with just one minor adjustment. For instance I went to try Glenns theory of richer = more torque and more power. I went a half a clip richer on the needle and that was all. I did pick up a bit more torque off idle, but I also had a rich to right transition in the lower. More of a hit right off the bottom you could say, but it wasn't a crisp clean hit. Just an annoying burble and then brap. The mid range response was noticably smoother and tractored well gear high half throttle. The top end the same. After lunch I went half a clip leaner again. The grunt off idle was still there but there was very little burble. As soon as I opened the throttle the bike pulled clean, building revs to the mid range. From here the bike snaps hard to life into the top end where it peaks out hard. The mid range hit either lights up the rear end or puts the front end skyward. It wouldn't do this as well with the richer setting. Further to that. The half clip leaner last week, same conditions, same loop, I stopped on the trail and donated a litre of fuel from my tank to a mate and still made it to the servo. It took 8L neat to fill the tank. This week I hit reserve on my own and took over 8L to fill the tank. I know which setting I'm sticking with.

As for the clutch I use it just about all the time. The leaner jetting usually responds better to clutch work as it'll lack some of the torque of a richer set up, and also be snappier once its revving hard. Its just another skill that you dial down, the same as braking or giving throttle. Practice practice practice!

 

[bowhunter007]

My bike is turning into a portable laboratory

 

[GMP]

I'm convinced that these newer ASII carbs are more of a PIA. If the slide is correct (a #7 in a 250 at low to mid altitudes), the needle should not be such a big problem, and you can acheive good torque and runnout without a blubbering mess. Beleive me, I raced this bike a lot this year and you don't holeshot on a bike jetted like a pig. What fouled me up on the '12 was the notched slide, it wanted a RICHER, not leaner pilot. bowhunter, you shouldn't have to screw around with your older carb 250 much, if all is well it should jet pretty easy and you just ride it.

 

[bowhunter007]

bowhunter, you shouldn't have to screw around with your older carb 250 much, if all is well it should jet pretty easy and you just ride it.

You're absolutely correct. I mess around with it, because I can. When it's time to ride, I set things to what works. My biggest problem, I'm fascinated by silly things like carbs, EFI, etc. I took apart a perfectly good carb, on my first car...to study it. I managed to figure it out, reassemble it, and tune it. First time...Lucky

 

[twowheels]

Air Screw, Slide Notches, Pilots

So tell us the relationship between the air screw, slide notches/numbers, and pilots.

All of my 11s, 12s, and 13s save one came with a 7 slide. The other came with a notched 6.5. All other things being equal, although they never are, are you suggesting that the idle air through the notch would allow the slide as a whole to run lower?

Does the notch serve to focus the air at the nozzle, making fuel pickup more precise? What effect would running an equal number, but notched slide, have on the airscrew setting - less air through this circuit, so further clockwise would be required I would assume.

 

[bowhunter007]

So tell us the relationship between the air screw, slide notches/numbers, and pilots.

All of my 11s, 12s, and 13s save one came with a 7 slide. The other came with a notched 6.5. All other things being equal, although they never are, are you suggesting that the idle air through the notch would allow the slide as a whole to run lower?

Does the notch serve to focus the air at the nozzle, making fuel pickup more precise? What effect would running an equal number, but notched slide, have on the airscrew setting - less air through this circuit, so further clockwise would be required I would assume.

I wish I completely understood the relationships between all the parts. If understanding equals clarity of vision, then my vision is very blurry.

 

[GMP]

I was told that the notch increases the signal for the low speed ckt, makes a clean idle easier, and would require a leaner pilot because of this. Made sense. I found my bike liked just the opposite. I kept screwing around with needles and kept the leaner pilot and the bike never had that nice off idle torque my other 250s had and I knew was hiding there somewhere. Finally I said to hell with the "experts" and went to a richer pilot and things shaped up nicely. In the end the bike runs hard with the same jetting the '07 likes with the exception of the notched #7 and 48-50 pilot.(AS1 carbs) Its the internet, how many of these guys actually tested this theory or did they just propagate something they heard elsewhere?

 

[Jakobi]

What you say makes sense to me Glenn. In theory you'd think the notch would simply allow you to use the same sized pilot but lower the slide a bit more allowing for an improved idle. Its not surprising that it also effects more than just idle though. Realistically the idle circuit is the most dificult to tune correctly because everything has to be just right to make it work (pilot, slide, needle diameter, and air screw). Its possible to compensate for one with another to a certain extent ie. lean needle diameter with a richer pilot, or rich needle diameter with lean pilot, but it really is a trade off.

I haven't had the opportunity or desire to try a notched slide myself, but I've been through enough needle and pilot combinations to know which is doing what.

 

[twowheels]

I was told that the notch increases the signal for the low speed ckt, makes a clean idle easier, and would require a leaner pilot because of this. Made sense. I found my bike liked just the opposite.

Jakobi is the only guy on the internet I quote verbatim

All things being equal (and they never are exactly) if your engine is idling at the same speed with no load, it is pumping the same amount of air and drawing that same amount of air in through the intake, whether under/around the slide, through the notch, or through the air screw circuit. The combustion will be better if the air that comes in draws and mixes the fuel completely, not leaking some away and pulling inadequate fuel for good atomization and a proper mixture ratio throughout the charge for that stroke.

To reinforce your point then, it would seem that if the notch concentrated the airflow around the nozzle the signal would be stronger, requiring more fuel as would be provided by the richer pilot. The result should be a more stable idle, and likely a cleaner pickup when the throttle was opened.

 

[GMP]

The notch is on the engine side, just before the pilot ckt hole when the slide is closed. So, it would concentrate flow there, not at the main nozzle, but only if the slide is low enough. I think what might happen is once the slide is opened slightly, this effect is gone and now there is an added area (like a fractionally larger slide #) and it leans out. The idle ckt is more sensitive to the air screw but at the same time I was able to get a good idle with any reasonable pilot. Basically what you notice is a lower slide position for the same idle speed, so your less likely to pull over from the needle.

 

[twowheels]

Carb Science Project

OK, so now the interest is piqued. I would expect that the needle would be solidly in the constant diameter region at and just above 'slide closed' position.

To prove that point I think someone that has a sacrificial carb around should dissect it and post photos with a variety of throttle opening so that we can see needle position and bore opening.

Calling AS-I or AS-II owners ...

 

[Jakobi]

I'll take photos and stuff next time its out. Probably when my smart carb arrives N-Green was going to do some back to back tests with the notched slide out of his mates ktm. They have been doing lots of back and forwards testing. Think hes up to testing his 36-39 oval bored AS2 now.

 

[GMP]

Steve, your right, the only difference then between carbs set up the same is that notch. Slightly more area with same needle straight section in the nozzle equals slightly leaner condition. The bike runs VERY similar with #7 slide/45 pilot, and #7 notched slide/48-50 pilot.

This is why the leaner dia needles (NECH, NEDH)were especially very wrong and gave me fits. Bike likes the richer JD Blue as did all my 250s and hauls ass with it!

Honestly I'm tired of this crap, there are many things I like to do regarding dirt bikes but screwing with jetting is not one of them, too time consuming. Like cleaning filters. I know from experience what the bike should run like and once its there I'm done. Thats why I just bought myself a Smartcarb for XMas.

 

[Jakobi]

I'm with you again Glenn, I've spent endless hours changing brass and needles. Most of my knowledge has come from experiencing what works well and what definitely doesn't so I'm grateful for that. The problem with the interwebs jetting is that just because one setting works with one particular bike by no means means it'll work as well for another. Then you bring personal preferences into the mix and it ends up a whirlpool.

I still enjoy the conversation and the theory and love to hear others experiences though. I'm just slowly realising that my bike is about as good as it'll ever be. I get great traction and torque down low and a wild hit up top that sends the front end skyward or the back end sideways. Seems I spend all my time these days fixing broken parts. Finally snapped the lower chain guide/side stand mount from the frame. Next ride electrical lighting issues, and same ride front pipe mount clean off the frame too. Looks like welding will be the next skill to learn.

In the meantime keep up the good work Bowhunter. I enjoy your updates. It'll be interesting hearing more on the smart carbs once they're rolling around.