I have a stock RD 350 that was running sluggish on acceleration, checked timing and points, ok. Adjusted the air screws on carbs from their standard 1 3/4 out to out only 1/2 turn. Performance dramatically improved. What does that mean? What happens when you turn in air screws, less air or more? What is the purpose of this adjustment and what is it compensating for?
When you turn the air screws in, you are richening the mixture; of the fuel that comes from the pilot jet by reducing the air flow through the fixed air jet. This also controls the way the motor responds to throttle when the slides first move off their stops. Lack of changes in running, by adjustment of the air screw can suggest a blockage of the fixed air jet.
Good results at extreme ends of adjustment mean you should change the size of the pilot jet.
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I thought the air jet had nothing to do with the pilot system, being associated to the primary system.
Sorry yes, just woke up, literally!
The air jet controls the air flow through the emulsion tubes.
All else is correct, air correction for the pilot jets.
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yup. you need a bigger pilot jet. you want to aim for 1.25-1.75 turns out on the air screw. Why? to keep the circuit velocity up for proper atomization of the fuel in the pilot circuit.
Also keep in mind, the pilot never 'turns off' after idle on VM's, it is always a part of fuel delivery all the way to full throttle, albeit diminished the wider the throttle is open. The mixture screw is just a metering valve for the air in the pilot circuit. :toot:
Air mixture controls your airflow to your pilot jet and your throttle response from 1/8-1/4. Like Mark said, it contributes to your entire throttle range as well.
If you're under a certain amount of turns, then you should upjet your pilot because you're too restricting too much airflow to the fuel mixture.
If you're over a certain amount of turns, then you should downjet your pilot to compensate for how much air it needs to the current fuel mixture your pilot jet is.
Hope this makes sense and is helpful. A lot of people mix this up with fuel mixture screws which the same thing but with fuel control to air vs air control to fuel.
I'm starting to understand the Dave F. mod a little better -if I read this correctly that the air correction jets that you modify during the DF mod are the same ones that meter air to the screws?
Do the same rules apply to pilot sizing after you add the 2mm replaceable air jets? If you have done the mod obviously you have increased the pilots or the mix would be lean on the pilot circuit. Do the rules still apply about pilot size in relation to the air screw setting?
Is there a scenario where you would want to vary the size of the air correction jet rather than pilot sizing? If you go bigger on AC jet it seems that you would lose fine control with the screws. But the air correction jets are accessible from outside the carb.
This pilot system is an off-idle progression circuit that remains active even after the transition to the main jet under needle control. It has series and parallel operation in sequence. The transition is the place where this system is weakest. That's why we get hiccups just before the power band, coincidentally.
IR8D8R
no, not really.
the primary air jet for the main has zero effect on the pilot circuit, they are totally independent of each other. Dave f mod has zero effect on the pilot circuit -except- maybe that around 1/4 throttle it may need tuning as the circuit coming in (needle) is emulsifying the fuel a bit better. so, any previous compensation for transition into the needle by the pilot off idle is now corrected
the dave f mod basically just speeds up the circuit velocity in the air jet nozzle, hence the smaller jet sizes needed. ie: 250-260 mains on the stock needle and jet vs the 190-200 main needed for a dafe f modded carb.
the big question to me is if these jets in the different stages are accurately performing, why is there only an adjustment to the pilot jet? Why not on the main jet too if you really want to accurately tweek performance? And why any adjustment at all with the staged jets, or adjustments for all of them? The pilot jet operates through the whole staged cycle of operation and it seems to me its for something bigger than just a pilot, idle, adjustment. Its to offer a slight adjustment of fuel/air mixture to compensate for such things as gradual seal wear.
Other parts of the system ARE adjustable.
Needle clip position.
Main jet size.
Total air flow.
I think its fair to say the main jet operates within a tolerance, which is ok for most of us.
Racers may well set the main jet (and the needle position) 'on the day' to fine tune the mixture.
I've also heard of members who run a different main jet all the time, due to altitude.
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to add to Arrows comments: jet tube series and air jets can also be changed. The role of the pilot circuit is so minimal @ wot that its not going to really compensate for anything though.
Exactly. Just look at the difference in air flow for the pilot circuit compared to the air flow at full bore. Totally different.
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Quote from: Still biking on September 22, 2020, 04:54:07 AM
the big question to me is if these jets in the different stages are accurately performing, why is there only an adjustment to the pilot jet? Why not on the main jet too if you really want to accurately tweek performance? And why any adjustment at all with the staged jets, or adjustments for all of them? The pilot jet operates through the whole staged cycle of operation and it seems to me its for something bigger than just a pilot, idle, adjustment. Its to offer a slight adjustment of fuel/air mixture to compensate for such things as gradual seal wear.
To my understanding, the pilot jet needs an adjustments because it needs to compensate for the air/fuel mixture a lot for the lower RPMs (at idle) and make allows the bike the transition from idle speeds to your RPMs range/ throttle position. There are plenty of other areas that can be "tweek'd" for performance, needle positions and sizes. Hell, there is even power jets for your main jets that allow extra fuel. :twocents:
well, if you look, the pilot circuit has two outlets. one under the slide, one after it. the one after it is what controls idle mixture, but they are fed fromt he same circuit that splits. it has the job of providing fuel when the slide is pretty much closed.
this makes the vacuum signal very different than with an opening slide. But, conversely, when the slide starts to open, the signal over the back pilot output (under the slide) increases greatly. this is why when fitting ufo's, your pilot size reduces dramatically. you can see the 2 outlets in the cutaway i did forever ago.
(http://www.2strokeworld.net/wp-content/uploads/general-tech/carb-cutaway-vm28.jpg)
:clap: :clap:
Quote from: Still biking on September 22, 2020, 04:54:07 AM
t....Its to offer a slight adjustment of fuel/air mixture to compensate for such things as gradual seal wear.
Nope. The pilot or slow speed air screw is adjustable to get idle and just off idle jetting spot on. The pilot jet is so small that even a small change in air density is enough to require adjustment on the air/mixture screw.
Main jets air jets and needle jets are all replaceable but rarely need to be changed on a stock bike unless there is a significant change in air density say from sea level to 6,000 ft.
Just to completely screw with your mind, the air: fuel ratio changes as revs rise even at constant full throttle. That's known as the fuel slope and can be changed by changing the air jet size or the height of the spray bar (outlet on a primary type needle jet).
Once you have the jetting right for your altitude and state of tune, the air screw would only need to be tweaked maybe in winter or heat of summer and even then, few of us would bother. If a motor is badly enough worn that jetting needs to be changed, manufacturers would rather that you rebuild the motor, not mess with adjustable air bleeds that most of us would screw up.