2 stroke gas mixture and jetting q's (this fits here i hope?)

[oldskool83]

hey guys,

i got a problem, well more so a question. i hope this fits here because i could'nt figure out where to ask a general fuel/intake question.

i built a bike this year and it should run 32:1 but, a mechanic at a dealer told me to go to 40:1. i know 40:1 is said to be used on 250cc+ 2 strokes and 32:1 for 200cc and under.

is 40:1 a really huge difference?
if i mix a gallon at 32:1 i need 4oz of premix
if i mix a gallon at 40:1 i need 3.2oz of premix

this is for a 80cc 2 stroke, so should i just lean the main and pilot jet out alittle and go the next size down runing the 32:1 or find a happy medium like 36:1 and leave jetting stock or still lean it out some with doing plug tests.

please advise what to do or try. i have stock jetting in it now.

125 main
52 pilot
needle at 3rd clip

my motor was rebuild and everything new and fresh, clean carb, air filter, etc so please dont say do this, this, and this, because the bike is fully rebuilt and clean like a restore from ground up, its all new parts, even jets.

thanks guys, hope you can help because other websites are not, also hope this is right for this forum?:(

[Dammit!]

If it's running fine at 32:1, leave it. And how lean you can mix depends on your choice of oil. The new synthetics can run very lean but that doesn't mean you HAVE to run them very lean. I run my 250R at 40:1 with Klotz Super Techniplate. No problems to report in two years on this top end.

[oldskool83]

sorry i forgot to say i am using just regular old 2 stroke oil, yamahalube

[oldsking86]

I run 50:1 in my stock jetted 250R; maxima oil, Runs great

[GPracer2500]

A quick note about pre-mix ratios and air/fuel ratios (jetting)--they are related but seperate issues and should be treated accordingly...

The more deluted the pre-mix ratio the richer the air/fuel ratio becomes. Conversely, the more saturated the pre-mix ratio the leaner the air/fuel ratio becomes. Lets take a 40:1 pre-mix ratio and a 20:1 pre-mix ratio as examples. With 40:1 you've got 40 parts gasoline and 1 part oil in every unit of fuel being metered by the carburetor. That's 97.5% fuel and 2.5% oil. If you change that to 20:1, now you've got 40 parts gasoline and 2 parts oil for every unit of metered fuel. That would be 95% fuel and 5% oil. In other words, with the 20:1 pre-mix you've got less fuel available for combustion which = a leaner air/fuel ratio.

If your carb is jetted correctly for a 40:1 pre-mix and you change to 20:1 you'll have to richen your carburetor jetting to maintain the same air/fuel ratio you had before. The difference isn't huge in the above example--maybe a 2-3% difference which might be 1 or 2 (or 3?) jet sizes. Rejetting for different pre-mix ratios mostly comes up for precisely tuned applications (racing engines or anyone who can realize small difference in jet sizes) or when making a big change in pre-mix ratios (like switching from a 60:1 synthetic pre-mix to a budget oil at 32:1).

"Lean/rich" pre-mix ratios should not be confused with lean or rich air/fuel ratios--two totally different things.

....carry on.

[edited for clarity]

[oldskool83]

thanks, i thought i knew enought to know what i was doing but i didnt. i am going back to 32:1 tonight because thats what it should run, its also jetted for 32:1, alittle rich some say but anothing bad i cant tell when running it. i willl jet it if it needs it then. its just trail bike now for my fiance who;s a 1st time rider.

[ceaserthethird]

40.1 mix is perfect ... i run this on the 250R ( just make sure it's 100% Synthetic OIL )

[TimSr]

This topic has been covered many times, whicha search will quickly show. Just to clarify, proper premix ratio is is determined by choice of oil, not engine size. If you are running Yamalube, I would stick with 32:1. If you run general purpose 2 cycle oil, 20-25:1 is appropriate. If you are running a high performance synthetic, most are bottled in 12.8 oz bottles, so you can dump a whole bottle in a 5 gallon can for a 50:1 ratio.

[TimSr]

A quick note about pre-mix ratios and air/fuel ratios (jetting)--they are related but seperate issues and should be treated accordingly...

The more deluted the pre-mix ratio the richer the air/fuel ratio becomes. Conversely, the more saturated the pre-mix ratio the leaner the air/fuel ratio becomes. Lets take a 40:1 pre-mix ratio and a 20:1 pre-mix ratio as examples. With 40:1 you've got 40 parts gasoline and 1 part oil in every unit of fuel being metered by the carburetor. That's 97.5% fuel and 2.5% oil. If you change that to 20:1, now you've got 40 parts gasoline and 2 parts oil for every unit of metered fuel. That would be 95% fuel and 5% oil. In other words, with the 20:1 pre-mix you've got less fuel available for combustion which = a leaner air/fuel ratio.

If your carb is jetted correctly for a 40:1 pre-mix and you change to 20:1 you'll have to richen your carburetor jetting to maintain the same air/fuel ratio you had before. The difference isn't huge in the above example--maybe a 2-3% difference which might be 1 or 2 (or 3?) jet sizes. Rejetting for different pre-mix ratios mostly comes up for precisely tuned applications (racing engines or anyone who can realize small difference in jet sizes) or when making a big change in pre-mix ratios (like switching from a 60:1 synthetic pre-mix to a budget oil at 32:1).

"Lean/rich" pre-mix ratios should not be confused with lean or rich air/fuel ratios--two totally different things.

....carry on.

[edited for clarity]

Ive heard this theory before, and I totally disagree with the premise. When discussing a fuel/air ratio, the "fuel" is not the gas, and the oil is a not a replacement for air. The "fuel" is the gas/oil mixture. Adding more oil will not make your bike run leaner. By this logic, a 50:50 oil gas mixture will overheat your bike and melt the piston. Good luck even getting it started. Most everyone knows, the more oil you put in your gas, the more prone you are to plug fouling. By this logic, the same oil at 50;1 is more likely to foul plugs than at 20:1. Plug fouling is not a symptom of running lean. Increasing the oil ratio of the fuel will make it run richer, even though the fuel:air ratio has not changed. It does this by changing the characteristics of your fuel. The more oil you add to your gas, the slower the gas burn rate, and the slower the burn rate of the fuel, the richer it runs.

[GPracer2500]

I stand by my comments. I also appreciate a good discussion. :)

The lubricating oil in pre-mix displaces volume that could be used by gasoline. The oil doesn't replace air, it replaces gasoline. The gasoline and the oxygen in the intake air are the two variables of the air/fuel ratio.

Setting aside whether or not a 1:1 premix ratio could actually be combusted--lets assume it could--you would have the same amount of intake air, but far less available gasoline.

Although we commonly associate fouling plugs with a too rich condition, that is not always the case. If your pre-mix solution is overly saturated with oil then you can wet foul the plug. The oil is doing the fouling in this case. You can also foul a plug with unburned gasonline deposites from running your jetting too rich. You can also foul a plug with gear oil from a bad clutch side crank seal. Only one of those three plug fouling scenarios has anything to do with the air/fuel ratio.

I don't like to think of pre-mix ratios as being rich or lean. Those terms are specific to air/fuel ratios and in my opinion don't really apply to pre-mix ratios. It just confuses things to think of both jetting and pre-mix ratios using the same terms. The best way to consider how pre-mix ratios impact jetting is to look at how much gasoline is actually being delivered by the carburetor.

This concept is similar to jetting with oxygenated gasolines. Oxygnates can raise octane rating and help increase the efficency of deflagration (i.e. a more complete burn). But they displace the hydrocabons (gasoline) that is the other component of the burn. This is why oxygenated fuels require richer jetting then non-oxy fuels to maintain the same a/f ratio. E85 (or even many oxy racing fuels) is an extreme example of this. Sure, E85 burns very clean and has other advantages but it also significantly reduces gas milage--because more volume of the fuel needs to be consumed to supply an appropriate amount of hydrocabons.

[3Razors]

Gpracer is right. A 20:1 mixture will lean the bike out as compared to a mixture of 50:1. All top two stroke engine builders agree upon this such as Eric Gorr, Macdizzy, Harry Klemm, ect. Think about it as if you had a cup of chocolate milk and a cup of water both with a straw in each. Which one is easier to drink? The water of course. Now think of the straw acting as the main jet on your carb. Jets need to be adjusted according to fuel mixture.

Also smaller cc 2 strokes need more oil to gas than larger cc ones, due to the fact that they turn extremely high rpm's and need more lubrication/protection.

[jason 32]

try the needle in the 2nd position
if it is rich on top end - lower the main jet
40:1 is a good ratio
even weed wackers - chain saws ect... air cooled motors are using that ratio now

oil to fuel is chainged by the pre mix,not the jets-
unless it is a oil injected machene,then going with a bigger jet will change the ratio because oil and fuel are entering the motor seperately- thus a bigger main/pilot will allow more fuel in--- or less if smaller

but not when it is a premixed bike--- then oil and fuel go through the jets together----- a bigger jet will just add more fuel and oil!!! not just fuel

you can add all the air you want if it is mixed too rich-"too much oil"
it will not burn completly or clean- and will "drool"

not everyone rides the same- but the bikes should run the same!!!
there should be no need to "wring it out"-- except for the warm up process
-- most of the time -too much oil/fuel is used
crisp throttle response from the bottom- to midrange- to the top end
plug needs to be a tan- dark tan color
injected or premixed:w00t: :Bounce :beer

[oldskool83]

well i tried the clip in all 5 different positions, also tried a smaller main jet. my results are nearly all the same, a huge, bog anything over 1/2 throttle and it will just die out.

gone thru 2 CDI's, using the better one but dont know if the one i am using is not working 100% or if i got a jetting problem

[Jason Hall]

Is this an older bike? Is there a chance the stator Is breaking down? Call The Stator Corp, 1-866-478-5712 they will give you the Ohm reading's over the phone. They know there stuff, they rebuilt my 87 CR 125 Stator for 90.00. Also Is the Air filter clean? Is the throttle slide opening all the way? Is the plug boot making good contact with the Inner core of the coil wire?

[jason 32]

try a bigger pilot jet!
what motor is it-
how many turns out is the air screw??