Small vs larger pilot jet

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kdxdazz
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Small vs larger pilot jet

Post by kdxdazz »

Still trying to increase my knowledge on tuning carbs, is there a difference between using a smaller pilot jet with the air screw slightly open vs larger pilot jet and air screw further open e.g 35 pilot air screw 1/2 turn open vs 38 pilot 1.5 turns open, if there is a difference then where exactly is the difference
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bufftester
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Re: Small vs larger pilot jet

Post by bufftester »

Yes there is a difference. That's the short answer.

The long answer gets all complicated with flow rates and mixture blah blah blah but basically boils down to:
The pilot jet (PJ) sets the baseline amount of fuel that is metered from the bowl into the carb throat. The idle mixture screw (aka air screw (AS)) sets the baseline amount of air that is mixed with the baseline fuel (set by the PJ). Between those two components is how you set your baseline idle response. Because you are talking about such small volumetric amounts, very small changes in the environment (e.g.: altitude (pressure density), temperature and humidity) effect changes in your baseline response, as will the small differences between engines of the same type, which is why two identical bikes can have different jetting in the same environment.

So you might be able to get it to idle the same with different PJ/AS settings, the response will not be the same. And since the idle circuit is present in all circuits, any response issues will only be amplified by the addition of jet needles, slides and main jets. This is why the PWK (and really all slide valve carbs) are tuned from the bottom up...idle response first, then WOT. Basically you set the two ends and then everything else is toying with the middle area responses. Hope that clears the mud a bit, I tried to proof read it myself and got a headache lol
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Re: Small vs larger pilot jet

Post by kdxdazz »

Appreciate the response and yes it clears it up a bit, I'm still not clear on the rpm that the pilot jet is used up until and it's tie in with the needle diameter, e.g if your needle diameter is a little on the lean or rich side then you can go larger or smaller with the pilot jet
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bufftester
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Re: Small vs larger pilot jet

Post by bufftester »

The pilot jet is active all the time, which is why it's so important to set it first. The interplay of the different circuits isn't based on rpms, its on throttle position. Mark your throttle tube where it's fully closed, then mark where it's fully open. Then halfway between those two marks put a mark for 1/2. Halfway between 1/2 and closed mark 1/4, halfway between 1/4 and closed mark 1/8, and finally halfway between 1/2 and full open mark 3/4.

The idle circuit (PJ/AS) is solely responsible from closed to 1/8. as you transition from 1/8 to 1/4 the end of the needle starts to move up in the needle jet and the slide cutout begins to appear, and around 1/4 open changes to the needle diameter begin to have more affect. Keep in mind the idle circuit is still in use and delivering fuel/air, in addition to the fuel now being metered up through the needle and air from the slide cut out.

From 1/4 to about 3/4 the needle is fully in play and this is where folks start to get a bit confused. The slide cutaway is also in play, but slides are generally a lot more expensive than jets so mostly slides are ignored by the general riding public.
The needle has 3 characteristics: Diameter, Length, and Taper
Diameter is the transition from idle to 1/4 (just above the PJ) and the effects of diameter are most noticed in the 1/4 range.
Length is determined by clip position (top position is leanest, bottom clip is richest) and the length is most noticeable in the 1/2 range. it will have some bleed effect on both the diameter and taper, but primarily is in the 1/2 range.
Taper is mostly felt in the 3/4 range, but is the least sensitive setting of the needle. With all 3 of these you want to avoid becoming too lean (engine will be revvy but lack power) to avoid seizing. Rich settings will make the bike hesitant to take throttle, or feel boggy or lazy.

Finally the main jet (MJ) is from 3/4 to full open. Very easy to set it by doing plug chops (once you have set the other circuits of course).

So to answer your question above, you never change needle diameter to correct for an incorrect pilot. You set the pilot correctly, then use the diameter to fine tune the response at 1/4. Always bottom to top. And only change one thing at a time then ride it. Carb tuning isn't difficult beyond the fact that it takes more patience than most of us are willing to apply, because it takes away from riding time!. This is for PWK specifically, and slide carbs in general. I know there is a lot of info out there about tuning scotter carbs starting with the needle. Fine if you're riding a scooter....
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Re: Small vs larger pilot jet

Post by kdxdazz »

Cheers again. I've recently learnt a lot about the slide and it's effect on the circuit, I ground my number 5 slide into a number 7 and it completely changed the circuit from idle to 1/4. With a number 5 slide aircrew was at 1.75 turns, after grinding it I went to. 75 of a turn
I have a bung welded in the exhaust with which I can connect my wideband O2 sensor, I might check the changes with that on the larger and smaller pilot jets but like you stated I'm starting to get tired of tuning and just want to ride
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Re: Small vs larger pilot jet

Post by TwistedRoot »

@Bufftester that is a great write up. Thank you.
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Re: Small vs larger pilot jet

Post by bufftester »

I've not played much with slide cutouts, but my impression is they give a more overall change in mixture as compared to a needle which breaks it up into three areas. Mostly a 5 or 6 will cover a large portion of 2Ts.
As for the O2 sensor, where is yours mounted and have you compared calculated A/F values to actual readings? In my past sledding days we found that the scavenging effect of the exhaust system confused the sensor because of the intake pressure wave containing a higher concentration of oxygen as compared to the exhaust pulse. With the sensor in the stinger the readings settled a bit, but were always off compared to the calculated values we came up with. Of course this was on multi-cylinder engines, might not mater on the KDX. Plus oil and O2 sensors don't really play well together. Be interested to hear how it's working for you.
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Re: Small vs larger pilot jet

Post by kdxdazz »

[quote=bufftester post_id=204870 time=1576199011 user_id=6936]
I've not played much with slide cutouts, but my impression is they give a more overall change in mixture as compared to a needle which breaks it up into three areas. Mostly a 5 or 6 will cover a large portion of 2Ts.
As for the O2 sensor, where is yours mounted and have you compared calculated A/F values to actual readings? In my past sledding days we found that the scavenging effect of the exhaust system confused the sensor because of the intake pressure wave containing a higher concentration of oxygen as compared to the exhaust pulse. With the sensor in the stinger the readings settled a bit, but were always off compared to the calculated values we came up with. Of course this was on multi-cylinder engines, might not mater on the KDX. Plus oil and O2 sensors don't really play well together. Be interested to hear how it's working for you.
[/quote
I only recently started playing with slide cutouts because I exhausted all other options, I now realise the importance of the slide cutout, jetting close to the equator requires much leaner settings across the board.
I mounted the sensor bung at the highest point on the pipe just by my knee, I strapped a battery to the back of the seat to power the whole set up. At that time my bike was running so badly and rich as well as air leaks at the carb boot that I stopped trying to tune with the O2 sensor until I could at least get it in the ball park. I really only had the sensor in there for a couple of hours at a time so I wasn't worried about contaminating it. In regard to a/f values compared to readings I only used the readings on the gauge.
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Re: Small vs larger pilot jet

Post by Goat »

Very nice! Helps add details to the jet guideline.
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Re: Small vs larger pilot jet

Post by Goat »

Did the O2 sensor really work well? I tried that on my firebird when I put the blower on. Never seemed to properly collect the data. But was just trying to watch a MM rather than use a decent data collector. Hard to pop keep it straight and watch meter at the same time. So ended up enjoyably flogging it on the track & dyno to get it right. Maybe need some more lessons from the 405.
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Re: Small vs larger pilot jet

Post by kdxdazz »

Wideband O2 sensor was invaluable on my drz, it turned 1 year of tuning in to 1 week of tuning and taught me to always be careful of advice you are being given regarding jetting, every bit of advice I was given on thumpertalk was wrong. In terms of using it on a 2 stroke I really can't find any information on the Internet and not even sure if stoichiometric still applies. Made some amazing gains on my bike this week after working on the airbox, will do a short write up once I finished testing and re jetting
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Re: Small vs larger pilot jet

Post by bufftester »

O2 sensor usage between 4T and 2T are different animals. Primarily because reed-inducted 2Ts produce unusual exhaust gas ratios due to the pressure waves developed in the pipe.
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