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Pilot Air Jet?

By WGP,
in WR400F/426F/450F

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Morton

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As I understand, pilot AIR jet is used in the same circuit as the pilot jet (fuel regulator) and is most affective for 0 to 1/4 throttle. WR's have a smaller stock pilot air jet than YZ's because YZ's need more air during low rpm's. This is because WR's have an air cut valve (ACV) which allows more air flow to the pilot jet circuit. Stock size is 80 for WR's but if you disable ACV, then you should use a 100 as does the YZ to lean the pilot circuit....clear as mud :devil:

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Grand Master

bg10459

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Not exactly.

The pilot air jet (PAJ) controls vacuum at the pilot jet (PJ). The bigger the PAJ the less vacuum you will have at the PJ. So, increasing the PAJ will pull less fuel through the same PJ, effectively leaning it out. If you wanted to use a larger PAJ you would have to also increase the PJ to compensate for the reduced vacuum.

This has little to do with the ACV, though. The YZ's use a smaller PJ because they don't have an ACV. With the ACV on the WR's there are two air passages to the PJ which are normally open. One is closed by a vacuum signal on decel, which cuts the air supplpy to the PJ in half, richening the circuit to stop the decel popping. Since there is only one air passage to the PJ on the YZ's, they only get half the air, requiring a leaner PJ (not quite half. Since they use a leaner PAJ they must increase the PJ to compensate)

There are many methods to disable the ACV on the WR. All of the methods prevent richening of the PJ on decel by either blocking the extra passage closed or open. If you block it open, you will need to keep the stock WR PJ/PAJ configuration because you always have air through two passages to the PJ. Only if you block it closed you need to switch to the YZ PJ/PAJ specs because you have cut your air supply to the PJ in half.

In a nutshell, unless you block the ACV closed, don't mess with your PAJ.

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  • 2 years later...
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ohioryder

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I am confused bg10459...you have made two contrasting statements about the pilot air jet circuit.

Not exactly.

The pilot air jet (PAJ) controls vacuum at the pilot jet (PJ). The bigger the PAJ the less vacuum you will have at the PJ. So, increasing the PAJ will pull less fuel through the same PJ, effectively leaning it out.

In a previous post you described the pilot air jet this way:

Your pilot air jet is #22 and controls the vacuum signal at the pilot jet. As you increase the PAJ, you increase the vacuum at the PJ, which would result in an overly rich pilot. You would then need to lean the PJ back out to maybe a 42.

I am confused and want to learn/understand how the carb works and all its circuitry. Can you please clairify which statement is correct.

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Grand Master

grayracer513

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The function of the pilot air jet is a bit more complex than has been laid out so far here, and for the most part, there's no reason to modify one unless the carb came from a completely different vehicle.

The pilot circuit, in simple form, consists of two drilled passageways. The first runs vertically from the float bowl to the carb main airway on the engine side of the slide. The second runs from the air side face of the carb parallel to the main airway and intersects the vertical passageway. There is a jet at the air side entrance to this second passage which is the pilot air jet, and a jet at the float bowl end of the vertical passage which is the pilot fuel jet, or slow jet. The fuel screw is positioned above the intersection of the two passages, so that it controls a mix of air and fuel flowing up into the air way.

When the engine is running at idle, vacuum created by the obstruction of the main airway by the nearly closed slide is applied to the idle discharge port, the top end of the vertical passage. Atmospheric pressure then pushes air through the pilot air jet and fuel from the float bowl up through the pilot fuel jet up into the discharge port in response to the vacuum.

The amount of fuel delivered to the engine at idle is the total amount of fuel through the pilot less the amount of air through the air jet. So it is true that the larger pilot air jet will cause any given pilot jet to behave leaner overall. A bigger PAJ weakens the vacuum signal applied to the pilot jet by allowing more air to flow through that side of the passage. But the function of the air jet is actually to keep the idle mixture corrected across a wide RPM range, so the engine will respond smoothly as you roll the throttle open from completely closed at progressively higher RPM's.

The FCR tuning guide discusses this more clearly, and is worth reading.

My question regarding the air cut valve is that since it is only effective at high vacuum levels (high speed, throttle closed), why bother with it?

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Grand Master

bg10459

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I am confused bg10459...you have made two contrasting statements about the pilot air jet circuit.
Ahh' it's been so long I confuse myself. ? I should stop doing that.

Based on what gray has posted, I was right two years ago, but forgot and have been wrong recently.

....So it is true that the larger pilot air jet will cause any given pilot jet to behave leaner overall. A bigger PAJ weakens the vacuum signal applied to the pilot jet by allowing more air to flow through that side of the passage.
My reasoning went wrong when I thought the smaller PJ in the YZ was due, in part, to the larger PAJ, but it's really because there is no ACV. It seems the YZ's PJ would be smaller still without the larger PAJ. :ride:
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ohioryder

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Ahh' it's been so long I confuse myself. ? I should stop doing that.

Based on what gray has posted, I was right two years ago, but forgot and have been wrong recently.

My reasoning went wrong when I thought the smaller PJ in the YZ was due, in part, to the larger PAJ, but it's really because there is no ACV. It seems the YZ's PJ would be smaller still without the larger PAJ. :busted:

That's ok bg10459...I just wanted to know which was correct as im trying to learn the fundamentals and am having a **** of a time jetting my bike now:banghead: .

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  • 6 months later...
Grand Master

grayracer513

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What would happen if you added a lager PAJ and didn't close up the ACV? (I am currently running a 48 pilot with the stock 75 PAJ and ACV en-abled)
The biggest thing that you might notice by tweaking the air side of the pilot circuit is the transition from throttle closed to throttle slightly opened while at higher engine speeds. If the PAJ (overall, including the ACV and any other air sources) is reasonably close to ideal, then the mixture will be close to right at 1/8 - 3/16 throttle throughout the speed range. But errors in PAJ size become exaggerated the faster you run the engine, so if the PAJ is a little too lean at idle, it can be quite a bit too lean at speed under a light load. A lean condition could manifest itself as an erratic misfire, stuttering, or exhaust backfire under these conditions. This "zone" of the throttle is also influenced by the root diameter of the needle, but there, too, if the fuel metering is wrong, it will stay consistently rich or lean throughout the speed range. If the PAJ is wrong, lean gets leaner and rich gets richer the higher the revs are.
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wondermuscle

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Posted (edited)

For example......

Running a 48 Pilot jet with the fuel screw all the way in is still a bit too rich and starting when warm requires the use of the hot start. BUT a 45 pilot jet with the fuel screw all the way out is still too lean, pops on decel. One might try a 48 pilot jet with a a 100 pilot air jet lean out the pilot circuit?

Am I onto something here Gray or still out in left field?

OR.... Should I try messing with the needle position first? Lean it out a bit, move the clip from 4th to 3rd position.

Edited by wondermuscle
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Grand Master

grayracer513

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Posted
For example......

Running a 48 Pilot jet with the fuel screw all the way in is still a bit too rich and starting when warm requires the use of the hot start. BUT a 45 pilot jet with the fuel screw all the way out is still too lean, pops on decel. One might try a 48 pilot jet with a a 100 pilot air jet lean out the pilot circuit?

Am I onto something here Gray or still out in left field?

OR.... Should I try messing with the needle position first? Lean it out a bit, move the clip from 4th to 3rd position.

The fact that it pops on decel is not a solid indication that it's too lean. It's somewhat in the nature of this engine to do some of that when jetted correctly.

Changing the needle position will have no bearing on the matter. at such low throttle openings, the taper of the needle is no yet in the metering orifice of the needle jet, so nothing will be changed by it. It is the interaction of the pilot jet and PAJ with the needle jet that has the most influence in this zone, but it is the root diameter (fat part at the top) of the needle that controls this, not the taper.

If you try to lean the idle circuit with the PAJ, you may find the bike doesn't run right at a light cruise, or during roll-on, roll-off maneuvers. Then again, you might find out you've corrected a problem.

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wondermuscle

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The fact that it pops on decel is not a solid indication that it's too lean. It's somewhat in the nature of this engine to do some of that when jetted correctly.

Changing the needle position will have no bearing on the matter. at such low throttle openings, the taper of the needle is no yet in the metering orifice of the needle jet, so nothing will be changed by it. It is the interaction of the pilot jet and PAJ with the needle jet that has the most influence in this zone, but it is the root diameter (fat part at the top) of the needle that controls this, not the taper.

If you try to lean the idle circuit with the PAJ, you may find the bike doesn't run right at a light cruise, or during roll-on, roll-off maneuvers. Then again, you might find out you've corrected a problem.

I should have noted that with the 45 the bike feels lean if the fuel screw is not at least 3 turns out the idle hangs.

I'm just going to take one of my 45 pilot jets and turn it into a 46.5 pilot. Problem solved. ?

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wondermuscle

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Yep, been there done that and I'm not happy with either pilot jet (45 or 48), thus I'm looking for a better solution.

I'm going to try a to find the appropriate size drill to make a 46.5 Pilot and I will also try the 48 with the 100 PAJ. I'll let you know how it goes, might be a few weeks.

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Grand Master

123BigcoopDawg576

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The fact that it pops on decel is not a solid indication that it's too lean. It's somewhat in the nature of this engine to do some of that when jetted correctly.

Changing the needle position will have no bearing on the matter. at such low throttle openings, the taper of the needle is no yet in the metering orifice of the needle jet, so nothing will be changed by it. It is the interaction of the pilot jet and PAJ with the needle jet that has the most influence in this zone, but it is the root diameter (fat part at the top) of the needle that controls this, not the taper.

If you try to lean the idle circuit with the PAJ, you may find the bike doesn't run right at a light cruise, or during roll-on, roll-off maneuvers. Then again, you might find out you've corrected a problem.

Just wanted to give a quick update on my results with disabling the ACV and installing a 100 PAJ

IMHO the bike was a lot more responsive. I was able to tune the fuel screw with great succes. I did notice some slight back firing on hard deceleration in lower gears but nothing to indicate the bike was lean.

The only issue that I have no, is the dreaded off idle bog...After talking with PBDBLUE, we both agree that the 75 PAJ and ACV enabled help mask that bog which never existed.

I ordered a Merge Racing AP spring and will spend next weekend fine tuning the AP. I may even drop the clip on my needle a hair to richen stuff up a hair.

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