What do the pilot and main air jets effect?

7 replies to this topic
  • live_to_ride

Posted June 11, 2003 - 06:58 PM


Could someone please explain exactly what they do? I am completely baffled as to what their function is. I thought that the ONLY air circuit in a carburetor was the throttle, and everything else metered fuel to match the amount of air the throttle was letting through. What do both of the air jets do? :)

Chris :D

  • live_to_ride

Posted June 11, 2003 - 07:04 PM


Well I just looked at the other thread about the main air jet on a wr 450, and apparantly it doesn't have one. If I can't change it, then I don't care what it does. What about the pilot air though? thanks

  • Hick

Posted June 11, 2003 - 08:20 PM


Well I just looked at the other thread about the main air jet on a wr 450, and apparantly it doesn't have one. If I can't change it, then I don't care what it does. What about the pilot air though? thanks

I'm pretty sure the WR does have a Main Air Jet, size 200. There was a thread just yesterday about it.

I'm not going to try and explain exactly how they function, because I'm not exactly sure, but IMO what is important to understand is this:

- The Air Jets work in conjunction with the corresponding fuel jet. If you change the fuel jet (pilot or main) more than a size or two in any direction you probably need to change the air jet in the same direction.

- The Air Jets tend to have a more pronounced affect at higher RPM.

So, what you do is, go by the rough guidelines to determine which air jet to run with the fuel jet you have selected, and then later fine tune the air jet size based on high rpm performance in the appropriate throttle position.

That is just my take, hope this helps.

  • Hick

Posted June 11, 2003 - 08:29 PM


Oops, I just read the last post on the aforementioned thread, I'm surprised the MAJ isn't replaceable.

That is weird, OTOH they have never offered any optional MAJ sizes on any Yamaha WR/YZ parts fiche I've seen in the past 5 model years.

This part is not found on the YZ 450 fiche either. Just the PAJ (size 100), which, on the fiche, is called simply a "Jet, Main # 100." That gives mixed signals, but I'm betting it is a 100 PAJ.

Well, to further explain the Air Jet that, I would think, could be replaced on your bike, I'll offer this:

The PAJ on past WR/YZs was different, probably in conjunction with the different needle tapers and breathing restrictions (corked & covered) between models. Once you undid all of this a larger PAJ, like the YZ, may have been an improvement. On the other extreme if you ride at altitude, like I do, and end up with smaller Pilot Fuel Jets, and don't compensate on the PAJ, you end up with a bike that surges during cruising and pops on decel (too lean, too much air, at high rpms and low throttle settings). The cure for this was a smaller PAJ.

That is pretty much all I know about FCR Air Jets.

Hope this helps.

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  • Taffy

Posted June 12, 2003 - 02:37 AM


the pilot air jet
this is for about 1/8-1/4 throttle. essentially there isn't enough fuel coming up the side of the needle so they send it down a passageway to collect fuel and bring it out on the engine side of the carb slide.

as you know toilet tissue will only absorb so much water before you need more and the same goes for the air that goes through and the fuel it pulls.

lots of air sucks lots of fuel, there are corresponding pilot air jets to go with pilot jets. this keeps the ratio throughout their range correctly.

if they work in a 2,000 rev window from say 2,000 - 4,000 revs you need the curve to be correct.

the PJ and PAJ ratios are established. are you happy with this explanation?

the main air jet
a bike needs a MJ NOT i say NOT for max revs it needs it for maximum power, after that point it gets too much fuel to the red line.let's say 9-11,000 revs. so we bleed air in. this makes up for the fact that the MJ is pulling too much fuel. so the suction through the carb takes air from a 'sideroom' instaed of up the emulsion tube.

depending on how much we want to lean off the MJ we can do it a little or a lot and the 200MAJ is the biggest. ok with that?

in my tests the MJ was 160 and MAJ 200 but by coming down to 160MAJ i introduced less air and it revved hard to the rev line (see jetting Qs) then i dropoped the MJ about 5/8 because it had always been too rich at max power.

i like you, was seeing how FAR it revved and so i was jetting for after the fuel peak, i made it rich for max revs and not max power.

hope this helps



Posted June 12, 2003 - 02:54 AM


In Layman's terms:

There are three circuits in the carb. The Idle circuit, The Pilot Circuit, and the Main Circuit. The Air/Fuel mixture jets control the mixture of each circuit (each circuit has its own set.) As the throttle opens the carb cycles from the idle circuit to the Pilot Circuit and then onto the main. The needle functions as a regulator in all three circuits.

Balance of the jetting is required on each circuit or the bike misses,stumbles or dies.

Thats the basic explanation.

Bonzai :)

  • live_to_ride

Posted June 12, 2003 - 01:35 PM


Let's see if I have this right. The main air jet is to bleed air into the fuel stream between max hp and max rpm to lean the mixture out. The pilot air jet is to mix air with the fuel BEFORE it is introduced into the main air stream at (or after) the slide. This is because the fuel would otherwise be too heavy to bring up the side of the needle? And at this point it is not actually brought up the needle jet, but through a separate passage after the slide? thanks for the help!!

If I'm ever going to cure this hesitation, then I'm going to need all the help I can get in understanding this carb. :D

Chris :)

  • DGrizz

Posted June 13, 2003 - 05:25 AM


I live at 4000'. Most of my riding is between 3000 and 6000 feet on fireroads and single track trails. I'm running a 158 main, 48 PJ, squirt screw retarded 1/2 turn. Standard free mods. Stock muffler with GYT-r. Screw is out two turns. Runs like it's scared to death . No hesitation no stumble. Plug is lite gray.


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