plug chops?

[lonetree]

hi all,
been playing with my jetting a little more, think i have it sorted but i want to do some plug chops to make sure.
so my question is, what is the correct method for doing this? i dont want to waist an afternoon with no result.
I understand that basically you run at a certain throttle opening (was going to look at a little off idle, 1/2 and wot) for a bit then hit the kill switch, is this correct?
do you shut the throttle after hitting the kill switch or hold it where it was?
thanks.

 

[stainlesscycle]

i pretty much stopped doing plug chops - i still do one plug chop after i'm done usually....


anyways - proper plug chop is: bike warm, run to throttle position you want (with bike under load - i.e. riding..can't do this on a stand - no load = inaccurate), hold it for at least 10 seconds, hit kill switch, pull in clutch, maintain throttle position until you come to a stop.. check plug.



i gleaned this from the net, and it's my new favorite method for jetting.. it works great for piston port bikes, and bikes with very few needle options (the gg fortunately has tons of needle options...) still, this method will get you in the ballpark very rapidly.... i do the third gear/slight incline jetting - that way i know there's a real load on the motor..
_________



If you can start it cold with no choke, the pilot circuit is too rich.
If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

The best way to set a pilot circuit is with an RPM guage. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.

On a 2-stroke - You should be able to ride in 3rd gear, throtlle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.


Needle Clip:

On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the otherhand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.

Main jet:

On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.

Advanced Topics:

I will continue to add to this FAQ as time allows. I'll start with the needle because that's the circuit that is 'in play' the most.

The needle regulates the mixture from around 1/4 - 3/4 throttle. Most people are familiar with the clip position, as it's the most common adjustment, but there's much more to the needle. The jet needle is a long rod that fits into the needle jet. On most carbs, both are replaceable with different sized components. As the throttle is opened, the jet needle is retracted from the needle jet and this creates space between the two for gas to flow through. The more you open the throttle, the more the jet needle is pulled out of the needle jet, and consequently the more gas can pass through the increasing space between them. Below I'll outline the various parts of the jet needle.

Length - The relative length of the needle is adjustable by raising or lowering the clip. If you lower the needle (by raising the clip), then the needle sits deeper in the needle jet. This leans out the mixture across the range of the needle. Conversly, if you raise the needle (by lowering the clip), then the needle is further retracted from the needle jet, and this richens the mixture across the needle's range. Needles are offered in various lenghths. If you have a needle which is still too rich, even though it's in clip position 1, then you need to order a longer needle. For example, needle 'A' in clip position 1 is the exact same relative length as needle 'B' in clip position 3. If you had needle A in your bike, and it was still rich - even though you had the clip in position 1, then you could change to needle 'B' and lean things out by going to clip position 2.

Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).

Needle taper - Needles taper from top to bottom. As with all principles regarding the needle, the taper is relative to the diameter of the hole in the needle jet. Tapers are rarely changed, but here's a condition which warrants a taper change. Let's say the jetting is perfect at 1/4 throttle, but becomes increasingly leaner as you approach 3/4 throttle. In that case, you would want a needle with a shallower taper. Conversly, if the mixture is great at 1/4 throttle, but getting richer and richer as you approach 3/4 throttle, then the needle taper needs to be steeper. In my experience, needle taper only needs to be changed when the factory mis-spec'd it to begin with. Under very rare circumstances, big modifications to the motor - such as an overbore kit - will require a change in needle taper.


Remember that jetting needs to be adjusted for every 2000' elevation change and every 15 degree temperature change. If it was jetted right this summer, it's sure to be too lean during the winter. If you rejet it now, when it's cold out, make sure to lean it out a bit in the spring.

 

[Jakobi]

As above for how I jet. By feel. 3rd gear incline gives a good engine load to work with. If you jet it on the stand or at light loads it'll be too lean when punished up a hill.

Plug chops - method as above. Reading the plug becomes debatable. Some say use an old plug as a really lean condition will show white. Others say to use a brand new plug and then cut the threads back to read the fuel ring at the base of the plug. Obviously this costs you $5 a chop so not something you want to do often.

 

[arminhammer]

Spanky's Jetting Guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband.

A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using.

A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.

Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.

The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.

Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.

It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.

Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel.

One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.

Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.

Warm the bike completely, and shut it off.

As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.

Once you have determined (and installed it if it's neccessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.

The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.

Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.

Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begine to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.

The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.

Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit.

Two-strokes have an air screw that you screw in to make the jetting richer, ansd screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.

 

[AZRickD]

I get a close ballpark using most of the seat-of-the-pants techniques above.

I'll do a plug chop once I get everything set. I'll hacksaw the threads off so I can see that pretty 1mm band of gray deep in the insulator pocket.

I tend to jet a little rich because I like that feel (usually).

 

[lonetree]

thank you everyone for the responses.
as for all the jetting theory, well i have done all that and im pretty happy with where my bikes jetting is at, i just wanted to confirm its all good with a few plug chops, but i really wasnt sure if you hold the throttle open when hitting the kill switch...
doesnt sound like its a very effective tool for checking anyways...
o well ,my bike rips, that is all that matters...

 

[AZRickD]

It seems to work pretty well. And it's repeatable.

 

[Jakobi]

It seems to work pretty well. And it's repeatable.

Using the cut the threads back is very solid I think. You just don't want to do too many at $5 each.

The tan colour is a bit harder to read now due to differnt fuels and such, but a lean condition will ash up a good plug without a doubt so I check with used plugs.

Lonetree, definitely throttle in a steady position. Engine under load, I usually go walking pace 4th/5th gear up a slight incline (remembering its throttle position, not rpm). Keep the throttle open, hit the kill switch and slip the clutch in and come to a stop. Pull the plug, burn your fingers, have a peep and then do it all again at a different throttle position. Same applies if cutting threads. Pull it, throw it in a box with the throttle positon marked out and take them home to cut back and interpret.

 

[farmerj]

Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).


I'm trying to understand how the needle affects the pilot circuit at 1/8 throttle, or just above idle. I thought the needle only affected 1/4-3/4 throttle openings, but in practice that is not true. My BEL is not as rich off idle as the CEK. So the diameter (and maybe the taper??) affects the pilot circuit - but how? Is the explanation above correct? Does the pilot circuit cross the needle at some point?

The article above says that the root diameter is the diameter at the pointy end? Is that true? That is different than the Sudco drawing, which measures the diameter at the clip end (http://www.sudco.com/vol33/135-136.pdf) This is info. that I have seen in more than one place. But how could the "clip end" diameter affect the pilot circuit/idle??

As you can tell, there are a couple of things that I do not understand!

Jeff

BTW - The info. you guys have given above is excellent. Thank you.

There is a picture of the inner working of the carb - and more great jetting advice - here: http://justkdx.dirtrider.net (tech tips/carb tuning.)

 

[noobi]

Its mostly correct, except its the fattest part of the needle that they are describing, not the tip of the needle, by the time you are getting the throttle that open you are pretty much WOT and fully using the main jet.

The needle diameter in 4 character needles is defined by the last letter.

From Jakobi

D=2.685mm
E=2.695mm (JD Blue)
F=2.705mm
G= 2.715
H= 2.725
W/I=2.735 (JD Red)
J= 2.745
K=2.755

These are the horizontal values on that Sudco sheet.

Larger diamter = less/leaner needle effect on small throttle openings.

Ideally changing the clip position doesnt affect the jetting at idle. However it can if you have the slide open enough to by pass the idle circuit at which point the needle taper sections is determining how much fuel is pulled through the needle jet.

 

[Jakobi]

Like Noobs said. When the needle is installed into the slide it becomes an extension of it. The first few mm below the slide is all the straight section of the needle (where the diameter is in play). Even with the slide sitting only 1mm open at idle the diameter of the needle is what is controlling the fuel flow through the main. The pilot jet works in conjunction with the air screw to fine tune, and the slide cutaway also determines the velocity of the air coming in.

From there you go into the tapers. This can vary depending on the particular needle and the length of the straight section. For instance the stock needle has a very short straight section and also a very rich first taper. This is why its so hard to tune cleanly.

 

[farmerj]

Thanks for the info., guys - it's getting clearer.

So the needle does physically intersect the idle/pilot circuit?

 

[Jakobi]

Basically you have a hole that joins the throat of the carb, and the bowl together. The fuel is drawn up through this hole via vacuum and then drawn into the engine. The needle sits in the hole, and controls the amount of fuel that can come out. As the slide lifts, so does the needle and as the needle tapers down, more fuel is supplied. At idle the straight section of the needle is sitting in the hole. If its a thick needle, less fuel, leaner. If its a thin needle, more fuel, richer. Then as noobi said, if you use the idle screw to lift the slide (and needle) too high you will effectively be idling on the first taper of the needle (which is not what you want).

Bump the thread later and I'll post a pic of the level I set my slide at to idle.

 

[farmerj]

OK, thanks Jakobi and Noobi.

(1) From what I can gather, the needle does not physically intersect the pilot ciruit, which discharges into the venture through a small orifice that is separate from the main jet.

(2) Needle diameter has some relatively small affect on idle, as diameter determines the amount of fuel that can pass through the main at idle. This also means that a larger main jet can affect the idle to a certain degree ???, though the pilot circuit affects it the most.

(3) The needle taper could affect 1/8 throttle, though it has greater effect from 1/4-3/4 throttle.

So am I right, there?

Practically speaking, I'm having trouble getting my RB carb jetting clean from 0-1/8 throttle. I'm down to a 35 pilot and CEL needle, and it's still a tad rich. The air screw is out about 3 turns, and has little effect. PWK doesn't appear to make a 33 pilot - and I've never heard of anyone going that low! This is on an '05 DE 300.

I have a DF II Reed cage with the reeds set on "extra low" tension (I have some black reed stops that are supposedly "lower" than the normal grey ones set on "low.") Could the "floppy" reeds be requiring the lean jetting? I did not have this problem with the same setup on a KDX 200 - but maybe the 300 creates a different sort of intake pressure that affects the reed function differently...

So I suppose the next step is a larger diameter needle?

Jeff

 

[noobi]

The throttle openings refer to slide height more than actual throttle twist. Have you confirmed that the slide is closing 100% with the throttle closed? If there was some sort of snagging or incorrect cable set up you could have the throttle closed and the slide still open enough to have the needle on a tapered section. I dont know how much straight section the CEL needle has, so I cant comment on how much leeway you have there.

As for the reeds, I gather low tension means they are easier to open when there is a pressure difference across them. So they may be opening to a greater degree at lower rpm compared with standard tension reeds. Depending on which way I think about this problem, you may need leaner or richer jetting to compensate for the reeds.
I dont want to make any assumptions on this, you might find some info on reed manufacturers web sites, Boyeson has jetting suggestions with their carbon reeds when replacing stock resin reeds.

 

[Jakobi]

OK, thanks Jakobi and Noobi.

(1) From what I can gather, the needle does not physically intersect the pilot ciruit, which discharges into the venture through a small orifice that is separate from the main jet.

(2) Needle diameter has some relatively small affect on idle, as diameter determines the amount of fuel that can pass through the main at idle. This also means that a larger main jet can affect the idle to a certain degree ???, though the pilot circuit affects it the most.

(3) The needle taper could affect 1/8 throttle, though it has greater effect from 1/4-3/4 throttle.

So am I right, there?

Practically speaking, I'm having trouble getting my RB carb jetting clean from 0-1/8 throttle. I'm down to a 35 pilot and CEL needle, and it's still a tad rich. The air screw is out about 3 turns, and has little effect. PWK doesn't appear to make a 33 pilot - and I've never heard of anyone going that low! This is on an '05 DE 300.

I have a DF II Reed cage with the reeds set on "extra low" tension (I have some black reed stops that are supposedly "lower" than the normal grey ones set on "low.") Could the "floppy" reeds be requiring the lean jetting? I did not have this problem with the same setup on a KDX 200 - but maybe the 300 creates a different sort of intake pressure that affects the reed function differently...

So I suppose the next step is a larger diameter needle?

Jeff

Spot on about everything mate. If you take the main out completely you will notice it is a bit richer at idle too. The main circuit it effects is WOT though. The needle diameter has the most noticable effect at 1/8th throttle, but also noticably effects the idle circuit. IE for me to sustain the same idle when going up one PILOT size, i need to drop one letter needle diameter (leaner). The bike will then idle the same, but the feeling at 1/8th throttle will change.

As Noobi said also. The position is more related to the slide and needle height/tapers than actual throttle position. I'll post a pic in the next few days of the height I set the slide at before trying to get an idle.

 

[farmerj]

Great info. guys, thanks for the help.

IE for me to sustain the same idle when going up one main size, i need to drop one needle diameter.

Don't you mean that you need to go "up" one needle diameter?

In my mind if you increase the main jet size, you are increasing the amount of fuel it will pass at idle - unless you increase the needle diameter to compensate for the increased main jet hole size. Wouldn't dropping the needle diameter pass more fuel and make it idle too rich?

Sorry for all the questions!

Jeff

 

[Jakobi]

When I said drop the needle diameter I was talking alphabetically, ie from a 38/H to a 40/W. So technically when I said drop, I meant a larger diameter. I also just picked up a mega typo in my last post which I will now edit. I said when going up one main, I meant to say when going up one pilot.

I'm sure the main has an effect on idle as it does across the entire rev range, but I'm not sure just how much.

 

[farmerj]

I'm "with you" now, thanks!

 

[liv2day]

Digging this one up

The first couple posts in this thread should be combined and stickied - there's great, simple advice for yahoos like me that try to remember all the stuff they did a while back.

Jake - can you do that? I think this would be a great help - kind of like the jetting 101 that's on the KTM forum (that helped me get my 300 dialed when I owned it).

I'm going to copy and paste Stainless's post and the other post onto my tablet so I always have it