2 Stroke Oil Mixing and Carb Jetting - Tuning 101 Basics - DBC Podcast Ep 73

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Fuel/Oil Ratio vs Fuel Air/Ratio

- Fuel/Oil ratio is the amount of oil you are mixing in the gas.  It's in your gas can

- 32:1 is a lot o oil - thick fuel

- 50:1 is less oil - thinner fuel

- 80:1 is even less oil - thinnest fuel

- Pick something that works for you and stick with that.  We'll fine tune the mix with our fuel/air Ratio

- Fuel/Air Ratio is fine tuned inside the carburetor with Jets and needles.

- I've got oil running down my silencer and I"m running my bike 32:1. I guess that I'm running it too Rich, right?

- Well yeah, but it's not because of your oil mix.  You are too "rich" but you need to take fuel away from the bike by allowing less of it to enter the cylinder of your bike.

Denser air (more of it) makes your bike fuel lean (less fuel) because now you have more air/pressure but the same amount of fuel

Density altitude is what we are really tuning for.

- Aviation - Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases.

- Air pressure is reduced at higher elevations - Elevation goes up - bike runs " fuel rich"

- temperature effects  our carbs - temp goes up - bike runs "fuel rich"

Pilot Circuit - idle up to about 1/8th

- air screw for fine tuning

- 3/4 of turn out to 2.5 "turns out" is a fine range

- If you have to run the bike at less than 3/4 of a turns out, you will need a bigger pilot jet

- If you need to run it more than 2.5 turns out, then you need a smaller pilot jet

Needle Circuit 1/8th to 3/4 throttle - Where it's at!

- uses clips to fine tune here

- Moving the clip UP the needle will allow it to drop down further in the carb which restricts fuel (makes it leaner)

- Moving the clip DOWN on the needle keeps the needle higher which add fuel (makes it richer)

Main Circuit 3/4 throttle and up

- there is certainly some cross over here and I'm not saying that it will ONLY effect you at 3/4 throttle, but this is where it mostly comes into play.

How to tune?

- Looking at the spark plug

- Black or wet means there is too much fuel - it's too cold

- white means that it's too hot - not enough fuel

- golden brown is right in the sweet spot

- Riding the bike and getting a feel for what is happening - Get it hot  before changing anything. 

- Find out which jets you have in your bike

- Find a jetting chart for the bike

- Buy the jets and needles that go in the direction you need (more or less fuel)

- Start with the idle jet - is it running smoothly here - bogging or pinging could be a lean condition

- Play with the air screw to tune

- Now swap out the needle or move the needle clip position to either take fuel away or add fuel

- often times for me, I need a leaner needle than stock.  Probably 2 steps leaner

- then I fine tune by moving the clip up or down

- Now I got a couple steps leaner with my MAIN jet and by that time the bike is likely running really well. 

- sometimes this might require a little fine tuning with the needle clip position

[ryantaylor]

This podcast contains some great content and it was explained in a way that anyone can understand and utilize. Such an important topic to address! Keep up the good work 👍

[endurox]

I also only recommend changing one setting at a time and test ride each time in between. If you change more than one setting at a time, you have zero idea which change made the positive or negative change.

[rexo]

Hi Kyle I really enjoy your videos. Something that seems to never be mentioned is flashpoints of oils. Everybody since way back when has been constantly wondering why their two stroke exhaust leaks out oil. They invariably talk about the jetting being too rich, the quality of the pre-mix oil or not getting down hard enough on the throttle. I thought about this and came to a conclusion after I switched from KLOTZ BeNol to Amsoil Dominator two stroke mix.
Since I was twelve, I have had my share of two stroke bikes and have always had the proverbial (spooge) coming out of the exhaust. I briefly thought it might be the jetting but every bike could not be off. I have always had precise jetting and it ran great, just spooge. Everybody said, “They all do it:'' After I switched to Amsoil Dominator, I noticed I did not have any more spooge coming out. The premix ratio was the same, 40:1. (Since then, I mix it 60:1 for my 07 KTM XC-W as well as my 2002 KX 500) I then read the data sheets for each oil including several other two stroke oils. I noticed Amsoil Dominator had a flashpoint of 194 F., KLOTZ BeNol had a flashpoint of 229 F. I came to the conclusion, the lower the flashpoint of an oil, the likelihood of it burning up expeditiously. I have not tried any other oils besides Amsoil in a long time, simply because if something is working flawlessly why change it.
My conclusion is, the lower the temperature a flammable liquid vapor ignites, means the oil will burn up, hence no spooge!

[CraigSmith]

You may not learn it from a book, but having a book can help a lot. If it has pictures it will help even more so that you will know what it looks like before you take it apart and you will know where these jets, rods, and adjustments are. Really you should use everything at your disposal to help get the job done. Great subject Kyle and some great information.

[dirtfreek]

I got my first dirt bike at the age of 18. It wasn’t until my mid forties for me to really get serious about my jetting and learn how to get good at it. I like to ride at each specific throttle position for each circuit, feel how it runs whether rich or lean and dial it in from there. It takes focus but it’s been very rewarding for me.

[jasonstiffey]

Awesome pod cast ! I have been struggling with the exact issues you discussed and I really understand much better now how to properly tune and jet my KTM thank you !

[SpaZ]

Baseline..


1: Point of action it to adjust the idle curcuit first, most carbs will allow this whithout the the man jet even installed, the reason is, that only the pilot and nedle jet is active at this point..

2: set needle clip 2nd from the top,

3: Start with pilot one turn counter clockwise from fully seated..

4: When the engine is heated adjust the pilot/Air screw til the engine runs the highest in idle and turn it clockwise 1/4 or til you hear the engine starts to idle lower.. Idle circiuit done..

5: Main jet, start overly rich and gradually reduce its size til the engine starts to 4 stroke but not during high loads at WOT..

6: Run the engine as low as it will go 0n 1/3 to 1/2 half throttle, if it bogs out, lower the needle clip 1 groove to raise it..

7: Redoo step 5 at WOT

You should be in the ball park, of the engine being just a tad rich but not runnig the risk of being to lean to cause detonation or have sezure in hot or high load conditions even at low to mid throttle..

PS. Deside your oil/fuel ratio first before jetting, and stick to the same brand and mix it as differintiating alters the fuel/air ratio..

Hope it helps

[abramgalvez]

Hi Kyle I've been listening to your podcast for a while its helped me out a lot

[acesnyder]

Kyle, Ace From PA, rebuilt top end, noticed bog in higher gears, adjusted needle to lean out fuel/air, now idle and instant Crack on throttle bog, going to adjust air screw and perhaps idle jet. Thanks for the info, great job!

[torque-aholic]

Great job explaining very rare that I can understand

[freudz]

Hey man, what's up! Thank you fot the information. It's my first 2 stroke dirt bike and I was having some issues with my jetting and fouling plugs. Now I think I have the know-how to tune my bike. Just downloaded the service manual and have all the data I need to start tunning for where I ride (I live almost at see level...like 40 meters above..I'm like 12KM from the beach so it's almost stock. Some mountains here but highest I can go is like 150M above...). Will go stock first, with recommended fuel mixture, jets and air screw position. Then I will go for a ride and see what it feels like. The manual has all the information regarding carb adjustments for temperature and altitude and correction factors. So now I know what jet sizes I need and how to tune it right. Thanks again bro. Really helped me a lot. Nothing like jumping on google and search all the information regarding my bike. Cheers from Portugal (Europe)

[Grant]

It's been a finicky time trying to tune our huskys. Thanks Kyle!

[nitrorice]

Thanks for the explanation...i totally get it. I am a A amd P mechanic fresh out of school and i felt like i was back in school witj this video. Keep up the great work!

[endurox]

Kyle, I wanted to take a moment and thank you for thought provoking videos. A while ago I caught your video on 2 stroke oil premix. You and I had an email dialogue. After our dialogue I began doing a ton of research on 2 stroke premix. I would like to say this is what I found. A couple of extra ounces more or a couple of ounces less will have a negligible affect on gasoline viscosity. You claim that the reason you run a higher ratio of gasoline to premix is due to the lubrication properties of the premix. This isn't the major reason you have success running less premix to gasoline. The greatest reason is the RPM's at which the engines spend most of their time. What I learned was that even the new fuel injected KTM's inject almost zero oil premix when idling. Then as the RPM's increase so does the amount of oil that is injected. This is to protect the engine parts at extreme temperatures and force. Now in a dirt bike that requires you to premix the oil and fuel, you don't have the luxury of variable Oil to fuel ratios, so you have use a ratio that best works for your bike/conditions. Since you're doing a great deal of single track riding the engine isn't spending most of its time in the upper RPM range most of the time. The engine isn't screaming nonstop like an MX bike does. Albeit the ratios you run work for you and your bikes, it isn't as much a condition of the grade of premix, but more a condition of the type of riding and RPM range you run at. At higher RPM's you need a thicker film of lubrication on the bearings, rings, etc. When a manufacturer recommends a premix ratio, the engineers select this premix ratio based on max RPM and insuring extending engine life. In short you're able to use less oil with strong results due to the lower RPM's your engines mostly run at and not the type/grade of oil you run. The other thing to consider is how well the rings seal especially at higher RPM's. The premix plays an important role here. The less oil in the fuel the thinner the film of oil on the cylinder walls/rings and that reduces compression especially at higher RPM's. Anyway, I wanted to share what I learned, and I did all of this research due to you getting my brain going. Also AFR is AFR, Running a thinner fuel or a thicker won't change the amount of lubrication. I say this because if the engine begins getting more fuel, or too much fuel, you'll need to lean out the fuel air ratio. If the engine is getting less fuel or too little, then you'll have to fatten up the mixture. At the end of the day the ideal AFR is somewhere between 12:1 to 14.5:1 Air Fuel Ratio. You can't run leaner without melting something down and you can't run richer without affecting performance negatively, You can't run a "thinner" fuel and believe you're getting more lubrication. The volumetric efficiency of the engine is static for arguments sake. That said, you'll need "X" amount of fuel for each revolution of the engine to maintain a specific Air Fuel Ratio. The mass of the fuel is more important than the volume. XX parts per million of air to YY parts per million of fuel. That said the volumetric efficiency of the engine is static, so you'll always need "XX" parts per million of fuel. The only variable is air density. The greater the air density, the more fuel, and vice versa. You'll reduce your engine life in no time running less oil to fuel regardless of the brand of oil premix if you let that engine scream. You're not getting additional fuel in the crankcase with a "thinner" fuel, unless of course you're running a fatter fuel mixture to air. I hope this makes sense and am happy to have further dialogue.

[DrizzBarrowsx]

Marshmallow reference was the best!!! Lol . Solid helpful info for jetting . Thank you .
My 96 RM has been a pain with fuel delivery. That needle and altitude nugs should get her back to life

[prestonking]

Playing with needles is the best thing you can do that is spot on indeed. I was turning my bike 03 kx250 into a woods bike and could never get the bottom end power I needed did governor mod flywheel weight played with timing on stator gnarly pipe ect ect. Then I did some research and got a few different needles necw necj ect I believe been a few years and they were from suzuki and yamaha and man what a difference before even fine tuning it instantly got beter response and grunt of the bottom as before it was not transitioning on to the pipe smooth at all and i tried alot of tuning with the kx needle. Then I got the new needle and tuned it in no more acting like a 125 where there's no power until hit the pipe it is now a great woods single track machine tuned for colorado high elevation minimal spooge and can even go down to the dessert moab from riding at 10000 feet in the high rockies and just richen air screw and raise needle 1 notch runs like a top.

[bhowell]

Thank you for these videos, they are very helpful and well done. Keep ‘em coming brother.

[dwhxyz]

Differences in oil ratio mix will not have any noticeable effect on the fuel mixture and jetting - try it on a dyno to see for yourself. This is because the percentage change in the mix is actually quite small - e.g. 1.25% (80:1)
, 2% (50:1) 3.125% (32:1) 4% (25:1). Its best to use the recommended mix ratio as specified by the oil manufacture in their tech data sheet. The mix ratio and quality of the oil is normally dictated by the flash point of the oil. The higher the flash point the better chance the oil will have of spreading itself around the engine before being burnt but the higher the flash point also increases the chance of fouling plug(s) and black exhaust mess. Personally I would never run anything lower than a 2% mix with a decent oil like Motul 800 off road in a dirt bike or 3% mix with Motul 800 road racing/Castrol 747 for high bhp road racing bikes (i.e. Yamaha TZ250). I would stay away from 80:1 / 100:1 ratios/low flash point oils like Amsoil.

[austincoleman]

This is my favorite podcast Mr Kyle haha so sick because I have a two stroke and the jetting is a pain in the ass always has been I need to practice because practice makes consistency

[kevinconnors]

I wish I saw this video before last weekend. My 300 has been running lean checked the plug def a little too lean so I cleaned the carb and jets left the 38 pilot in there and the 162 main slid the clip down a slot seemed to run much better bike is running alot cooler now plug looks perfect.