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So how do you jet a two stroke or four stroke for that matter, properly?????

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It looks as if many people are stating that reading a spark plug for jetting adjustments is not possible with todays fuels. So how do you go about jetting an engine. I bought a bike last winter and it seams to run good but I would like to get a RB head job done on it. It is a 2007 YZ250. I am pretty certain I will nee to do some jetting to fully take advantage of the squish band an cc mod. I have not tore into the bike yet. I ordered a new top end to start fresh. I have not even looked inside to see what jets are in the carb. I heard a few people on the forum say that tuning can only be done on a dyno. Seriously, how has a dyno to tune their engines?

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It looks as if many people are stating that reading a spark plug for jetting adjustments is not possible with todays fuels. So how do you go about jetting an engine. I bought a bike last winter and it seams to run good but I would like to get a RB head job done on it. It is a 2007 YZ250. I am pretty certain I will nee to do some jetting to fully take advantage of the squish band an cc mod. I have not tore into the bike yet. I ordered a new top end to start fresh. I have not even looked inside to see what jets are in the carb. I heard a few people on the forum say that tuning can only be done on a dyno. Seriously, how has a dyno to tune their engines?

A Dyno is a compartitive tool, and has nothing to do with how the air/fuel mixture is performing inside your motor.

The easy way is to copy a bike just like yours, with the same mods, that runs well. You should not need to radically change your jetting anyway.

If you want to do it PERFECTLY, you would need to purchase an AF meter, go through the learning curve, and jet accordingly.

http://www.motorcycle-superstore.com/1/4/70/9382/ITEM/Dynojet-Air-to-Fuel-Ratio-Gauge.aspx

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1) use the air screw adjustment to find your pilot first

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.

2) do a proper plug chop using a new plug each time for the main as step 2. the below picture shows why you use 1 plug per main change and also why its called a "plug chop". this mixture ring is perfect in my opinion

plugs_2_1.jpg

3) finally use the needle to blend the 2 sections for a seamless transition

before any of these steps are taken, you need a tight top end, good reeds, proper float level, fresh air filter, and fresh fuel.

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FCR-Tuning Guide

Basic Carb Theory

Air Density

Jetting for Altitude and Weather

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

Be sure to keep notes of your settings. You may find you need to make a change, say if you're riding in the mountains or down at the beach. Returning home, it is a simple matter of referring to your notes to restore things.

Main Jet

Starting with the recommended main, remove the airbox door and go for a ride (bike fully warmed up, 3rd or fourth gear, up a slight hill). Is it better or worse?

If it is better, you need a smaller main.

Go down one size, replace the airbox door, ride. Remove the airbox door and test again. Better or worse? If better, go down a size again. Keep repeating this till the test with the airbox door is worse.

If it was worse with the airbox door removed, tape over 1/3 of your airbox opening, test (airbox door on, of course).

If it is worse now with the tape and was worse with the airbox door off, your main is just right. You are done!

If it seems better, you need to go up a size in main jet. Then test it again (remove the tape). Replace the tape, test again. If with the tape on it is better, go up another size in main. Keep repeating this till having the tape on is worse than with it off.

To finish up and ensure you are set accurately, retest the bike with the tape off (airbox door on), ride it, then remove the airbox door (tape off, of course). Best performance should be with the airbox untapped, airbox door on.

Remember, the main only operates at WOT. Ideally, you want to be in 3rd or 4th gear doing the tests, hitting max revs (just shy of the limiter) for at least 10 seconds to get an accurate representation of the jet status.

Be sure to keep notes of your settings. You may find you need to make a change, say if you're riding in the mountains or down at the beach. Returning home, it is a simple matter of referring to your notes to restore things.

The Needle:

First, confirm your TPS is adjusted correctly per the manual.

To do this, you need to mark the throttle grip. I place tape on the grip and a piece on the throttle housing. I make a mark on each. I open the throttle completely and then make a mark to correspond to that. Then I make a series of marks at 1/2, 1/4, 3/4, 1/8,3/8, 5/8 and 7/8.

Again, in third/fourth gear ride up an stead incline each time at one of the positions, make notes on how it runs at each.

If there are running issues in the first 1/4 throttle, you will need to try a different needle, one where the last letter is different. As you go up in the alphabet, you go leaner.

If there are running issues above 1/4 throttle, that deals with needle taper, length and clip position. Often just moving the clip will sort things. If a different needle is needed (say you ran out of positions) the second to last letter is what is changed. A letter is typically equal to a clip position, so an one letter change is like moving the clip on spot.

Remember, dry sounding or flat (not a bog, just flat/lack of power) is lean. Wet, misfires, heavy is rich

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Setting up the AP

The carb on your bike is a sophisticated, well engineered device. It breaks down the various requirements and controls them with 'circuits'. You have the following circuits:

Idle

Choke (actually an enricher)

Needle

Hot Start

Main

AP (Accelerator Pump)

On a thumper, it takes a breath in, digests it and spits it out and repeats. Your bike is a very high performance machine, capable of running between 1,800 rpm and 12,000 rpm. To be able to accommodate the high rpm, the carb has to be large enough to easily inhale. The carb has to also suck in fuel, it does this by using Bernoulli's Principle (wish you had paid attention now, don't you). The rushing air thorough the venturi (the carb throat) causes an negative pressure, which sucks fuel up past the needle.

But you decide to roost someone and nail the throttle. The engine is still taking little breathes but no the slide is suddenly up. The venturi is now much larger and the air velocity plummets. As everyone knows, if the air velocity drops, the negative pressure at the venturi drops. It cannot suck any fuel up the needle emulsion tube. The bike bogs, the person your were going to roost, passes you, filling your mouth with mud.

To deal with this, a pump was added, much like a squirt gun. It's job is to add enough fuel to compensate for the vacuum loss at the venturi, but just enough. It has to begin to squirt only when the throttle is rapidly opened.

There are four main parts of the AP system.

The Diaphragm

The Spring and the Link

The Leak Jet

The Timing Screw

Almost all bikes these days use the MX variant of the FCR carb and that is the one we will discuss. If you are unsure of which one your bike has,it is easy to tell. If you look at the right side of the carb, is all the Rube Goldberg parts inside or outside of the carb? Inside means it is the MX variant.Every year, the carb has gotten a little more refined and the tweaks get easier.

First, ensure you have a strong squirt. Wearing safety glasses, fuel in the tank, airboot removed, looking in the carb throat, engine off, nail the throttle. You should get a very strong super soaker stream. If not, you need to service the AP before proceeding

These days, nothing has to be done regarding the Diaphragm except replace it if it is damaged. One down.

The spring and the link are easy to deal with. You can get a stiffer AP spring (Merge Racing sells it for about $16) or you can oring the link. The replacement spring is a one time deal, the oring, needs to be replaced from time to time. To install the spring, you really have to pull the carb. The oring, can easily be installed on the bike. Orings cost about $.20 each (#78 in the plumbing section at Lowes). Some wire the links together. Wiring lasts forever, costs about $.03 however, you must ensure there is no bind at WOT or you are going to be replacing that black plastic cam. Some grind the rivet on the Diaphragm and test, grind and test until there is no bind. I have had orings last several years. To each their own. Two down

Now that you have removed the slop from the system, it is time to select a leak jet. A leak jet does just what it sounds like, it 'leaks' fuel out of the AP chamber back into the bowl. The is is what controls the sensitivity and volume of the AP squirt. There is no one size. What leak jet you use depends on how, where and how well you ride your bike. The stock leak jets were selected for "A" racers, those guys that never drop below 7,000 rpm and do cross ups between trees in the woods at 40 mph. Almost all of us are not that fast nor that good. So we need more from the AP. We need it to respond sooner and with more fuel. To do that, we put in a smaller leak jet. But we do not want too small a jet as that will waste fuel and can make jetting difficult if the AP squirts every time your hand pulses because of your heartbeat. What we want is the largest leak jet that will not cause a bog for the type of riding you do.

So.... I recommend starting out with a small leak jet, then increasing in size until the bog returns. Sadly, this can be a pain to do as the float bowl must be removed to swap the jet unless you get an AP cover that has an adjustable leak jet built in. These are pricey, from $125.00 to close to $200. For most riders, these are a waste of money unless you want carb jewelery (they come with stickers), for some that do jet for the day's ride, they are terrific as you can make an adjustment in seconds, just like with a fuel screw. For everyone else, I suggest install a #40 Leak jet if you do a lot of slow single track and are not an aggressive rider (be honest). If you tend to run fire roads or the desert and/or are a faster rider, a #55 or #60 may be a better starting point. Again, once the bog is gone, re-visit the Leak jet selection and try a leaner one. Three down

The AP timing screw. This controls when the squirt starts in relation to the slide position. We want the squirt to shoot down the slide, not slap it in the face nor do we want it to squirt on Thursday when we crack the throttle on Tuesday. In your manual, there are instructions how to set it. Most of these entail slipping a drill blank of a certain size in under the slide and adjusting the screw to have zero or a certain amount of clearance to the black plastic cam. This should be checked and set, if needed. This will be the initial adjustment point. 99.9% of the time, the squirt occurs a little late. Get your bike fully heated up at your riding place. Test ride. If you still have a bog, turn the AP timing screw in (CCW) 1/2 of a turn and test again by riding normally. Do not force a bog. I can make any carb bog if I want to. Do not test revving the bike in your garage. Test it by normal riding. At most, you will have to turn it 2 turns. Often none to 1/2 is plenty. Four down!

Now that the bog is gone, re-visit the Leak Jet and try a larger one. If the bog returns, you can adjust the Timing screw again but you do not want to turn it more than 2 turns from the initial factory setting.

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When I talk about using a dyno to jet, I never mean by looking at the HP and torque curves. Those are just interesting conversation pieces.

The dyno has a wide band sniffer that tells you exactly what your A/F mixture is at the bottom, middle, and top.

With a modern four stroke, being off by a size or two will still allow it to run pretty well.

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If you know what answer you are looking for, why are you asking?

If your not going to help and just add smart ass comments then don't comment at all. To all the others, thanks for the replies.

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If your not going to help and just add smart ass comments then don't comment at all. To all the others, thanks for the replies.

I wasn't being a smartass. I was trying to understand why, when I answered your question, you were insulting.

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I wasn't being a smartass. I was trying to understand why, when I answered your question, you were insulting.
If you know what answer you are looking for, why are you asking?

I wasn't trying to be insulting. If you took it that way then I apologize. I was hoping to keep it somewhat simple and not invest in some more expensive equipment. That was not the answer I was looking for. The other part of your answer was rather simple, but. (Basically you stated that I should copy the jetting of someone else’s tuning of with the same modifications.) If I did that, and they were wrong, and I copied that, then I could be looking at bad consequences, right. All bikes not being equal, that is. That was not the answer I was looking for either. So when you come back with your reply and basically said, if you don't like the answer, then why did you ask? I took your reply as slightly SA, hence my reply back to you. So sorry that I didn't agree with your solution.

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I wasn't trying to be insulting. If you took it that way then I apologize. I was hoping to keep it somewhat simple and not invest in some more expensive equipment. That was not the answer I was looking for. The other part of your answer was rather simple, but. (Basically you stated that I should copy the jetting of someone else’s tuning of with the same modifications.) If I did that, and they were wrong, and I copied that, then I could be looking at bad consequences, right. All bikes not being equal, that is. That was not the answer I was looking for either. So when you come back with your reply and basically said, if you don't like the answer, then why did you ask? I took your reply as slightly SA, hence my reply back to you. So sorry that I didn't agree with your solution.

So, you request advice, then spurn it, without an alternative. My question is, if that isn't the answer you wanted, which means you already know the answer, then what is it.

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I really appreciated the detailed jetting tuning process.

If you have experience with a bike like a KTM 300exc I would appreciate your advice.

I am thinking of getting A 300exc from around 2003/4/5

The issue is I ride at a lot of altitudes - How many different setups will I need to create for the following conditions?

Hollister Hills 400'-1,500' 50 degrees to 70 and in summer 70-90 degrees.

Sierras 3,500 - 6,500' 75-90 degrees

Stoneyford 1,500-6,500 in winter 34-60 degrees,

Summer 65-100 degrees

Will I have to play with the carb every other time I ride or will one setting do for a season if I rotate through these places?

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IMO, you will have to rejet everytime. i know i do.

you can use 1 setting pretty much everywhere, but you will be compromised in certain areas. at "stonyford" ideally youd want to rejet about 1/3 into the ride and again at 2/3's, but thats not really something even id want to do.

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well that was not exactly the answer I was looking for.

Did I miss something here? Where is the insult in that quote?

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well that was not exactly the answer I was looking for.

You jet for maximum horsepower using a dyno. Do a run, change the jetting, do another run. However, a dyno run is typically done at WFO, so you can only adjust the main jet. You could do the throttle marking system and do runs / jet that way too to cover all of the circuits - don't know if the 'pros' do that though.

JayC

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A Dyno is a compartitive tool, and has nothing to do with how the air/fuel mixture is performing inside your motor.

The easy way is to copy a bike just like yours, with the same mods, that runs well. You should not need to radically change your jetting anyway.

If you want to do it PERFECTLY, you would need to purchase an AF meter, go through the learning curve, and jet accordingly.

http://www.motorcycle-superstore.com/1/4/70/9382/ITEM/Dynojet-Air-to-Fuel-Ratio-Gauge.aspx

i was looking at that af ratio guage does it come with the o2 sensor and everything it seemed way too cheap?

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I read somewhere that the chop test is read at the base of the insulator so that eliminated the false readings you get from todays fuel additives. Is this correct?

1) use the air screw adjustment to find your pilot first

2) do a proper plug chop using a new plug each time for the main as step 2. the below picture shows why you use 1 plug per main change and also why its called a "plug chop". this mixture ring is perfect in my opinion

plugs_2_1.jpg

3) finally use the needle to blend the 2 sections for a seamless transition

before any of these steps are taken, you need a tight top end, good reeds, proper float level, fresh air filter, and fresh fuel.

That looks great to me. I started reading up a bit on PLUG CHOP and found a few decent articles but non really with any great detail. Is the test the same for a two stroke and 4 stroke or different? One place I read was to run in 5 gear, pinned for 30 or more seconds. I would think that would be a little fast and require a lot of wide open space. That could mean like 80 or 90 MPH, just guessing. If that's the case, you would need a super long road and a great health insurance policy, just in case. Also, I think that if the carb was lean and you ran this test, wouldn't you burn a hole in the piston??? Would you start a little rich for the first run and then work your way to the leaner side? Where could I read up on this topic to do the chop test safely?

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I read somewhere that the chop test is read at the base of the insulator so that eliminated the false readings you get from todays fuel additives. Is this correct?

That looks great to me. I started reading up a bit on PLUG CHOP and found a few decent articles but non really with any great detail. Is the test the same for a two stroke and 4 stroke or different? One place I read was to run in 5 gear, pinned for 30 or more seconds. I would think that would be a little fast and require a lot of wide open space. That could mean like 80 or 90 MPH, just guessing. If that's the case, you would need a super long road and a great health insurance policy, just in case. Also, I think that if the carb was lean and you ran this test, wouldn't you burn a hole in the piston??? Would you start a little rich for the first run and then work your way to the leaner side? Where could I read up on this topic to do the chop test safely?

IMO that ring test that is shown for that plug chop on a 2 stroke would be far too lean and the engine could possibly overheat and seize. That would cause frustration for all. A good way to start is with just a few jets and a manual.

Set the bike to stock everything including premix ratio (unless you have a ratio you wanna use specifically) put in the stock jet sizes, ride the bike for a little while. On a 2 stroke the factory jetting is almost always rich, so read your plug and adjust accordingly, no need for a plug chop, just grab a flashlight and remove you plug and replace. Obviously if your bike wont idle, or idles poorly but doesnt rev real high your pilot is rich, if you are getting spooge (oil that hasn't burned) out of your silencer some stage is rich, ride it in each range, pilot needle and main, and figure out where it is running worst, and adjust up or down, if its worse go the other direction down or up.

Thats how I jetted my 88 RM125, the first day i rode it idled but loaded up real bad, so i dropped the pilot a size adjusted the air screw and that fixed that, also on that day, there was spooge every where. So, the top end revs felt good, but the mid range was just running like crap, and thats where most of my riding is done. So, i dropped my needle 1 position and the last time I rode it for this season, when all of the jetting was done, it ran like a raped ape. of course my bike is jetted for 24:1 but ratio is mostly personal preference.

After the last ride the plug was a dark tan color, the whole electrode, not just that little light tan ring at the top of the electrode. That would be truly borderline of damaging your engine and running at peak.

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