Two-stroke oil premix ratios: Is less more? Is more less? Is more more? Is less less?

Discussion in '2-Stroke Engines' started by spoom, Jul 3, 2009.

  1. spoom

    spoom Member

  2. give me vtec

    give me vtec Active Member

    very good article. I will save everybody the trouble of reding this thing... less is indeed less. However they used castor oil, and the article was done in 1978... could it be assumed that things have changed?
  3. Pablo

    Pablo Motored Bikes Sponsor

    It always depends on the oil. Things have very much changed. I believe that article has been posted here before.
  4. give me vtec

    give me vtec Active Member

    im gonna stick to 25:1 it seems to run fine with very little oil spewing out the exhaust...
  5. spoom

    spoom Member

  6. RMWdave

    RMWdave Member

    32:1 is what i run in nearly everything, i wouldnt run any less oil than that ever. when i fire up the new engine ill be burning 5 liters of something like 14:1 at low rpm with a couple "ring seaters" closer to the 4l mark.
    once i feel the rings have seated i will lean it out gradually untill its comfortable at 28:1

    edit: BTW that second article was a little better read than the first its true that thicker mixes will give more perormance. my pocketbike is nearing the 30hour mark and i just ran a bit of the 14:1 through it and its definitly got the balls back of its midlife!
  7. spoom

    spoom Member

    One more for the road, from Spanky over at

    Pre-mix 101

    OK, looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded on the subject, but I'll put in my $.02 here anyway.

    There is a prevailing myth that less oil is better, and that the oil in the fuel is what lubricates the engine. Both are wrong.

    *less oil is better* People think that if they have a plug fouling problem or a lot of spooge, they need to run less oil. Wrong! Both problems are caused by rich jetting, and have nothing to do with the mix ratio.

    *the oil in the fuel is what lubricates the engine* The engine is lubricated by the residual oil that builds up in the crankcase. All the oil in the fuel does is replenish this oil.

    The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. If the ratio you run leaves enough residual oil in the crankcase to cover about 1/8" of the bottom of the crank wheels, then you are fine. If you don't have that much residual oil in your crankcase when you pull the top-end off, you aren't running enough oil for your riding style and conditions.

    With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 26:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.

    To understand why the residual oil is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine, but it must get to the parts to lubricate them. The way it gets to the bearings and onto the cylinder is by being thrown around as a mist by the spinning crankshaft, and the droplets are distributed by the air currents moving through the engine. Ever wonder why there are two small holes in the transfer port area of the crankcase, right over the main bearings? These are to allow some of the oil droplets being flung around inside the engine to drip down into the main bearing area.

    Some of the oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1, you simply can't jet very lean without risking a seized engine due to oil starvation, unless you're just putt-putting around on trails without putting the engine under much load.

    With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained.

    One small point. No one ever broke an engine by using too much oil.

    __________________________________________________ _______________________________________________

    Pre-mix ratios and power production

    I have run Dyno tests on this subject. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (a modifed '86 YZ 250, mine) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel, and the burn-characteristics of the fuel with that much oil tended to be poor. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.

    Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.

    As a side note, I no longer run 26:1, I now run 32:1. I'm not a young man any more, and I just can't push as hard as I used to, so I don't need as much oil now. 32:1 is enough oil to do what is needed for me now, since I'm getting slow...

    Now let the flames begin! I love a good debate!
  8. artmaker

    artmaker Member

    Oh spoom THANK YOU!!!!! This was very helpful.
    Just one question...
    <quote>The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. </quote>

    This may sound really stupid, but I'm new to this stuff. Where exactly is the crank case?
    Do you have pictures of what it should look like?

    (Ok, stop laughing!)
  9. RMWdave

    RMWdave Member

    crank case is always the lower half of the engine, logically, where the crankshaft resides!

    to check it you need to remove the spark plug, remove the head, the cylinder and you could try a rigid piece of paper from there to put between the crakshaft lobes to touch the bottom (like dipstick)

    its really a waste of time unless your a factory backed racer lol

    *a little tip, if your uneasy about your mix ratio it never hurts to add a little tiny squirt of mix oil under the piston (to the crankcase ;) ;) ) about 3ml is enough.

    remove the intake manifold via the two 10mm nuts, inch your bike forward to turn the piston to TDC and you should be able to see the bottom edge of its skirt, once you can see UNDER the piston add your oil and reassemble.

    i do this every 10 hours or so, the easy way to do it s to use a medicine syringe, you can get one at your local pharmacy for under $5 and its a must in an engine builders toolbox.
    Last edited: Jul 17, 2009
  10. artmaker

    artmaker Member

    I'm not, so do I really need to do this???

    I bought my bike already assembled. But it was new enough that it hadn't had it's "break in period" yet. I was told after it's break in distance, (Which was last summer) The mix should then be 21:1.....
    I THINK! I can't find that old email.

    Now yesterday it just seemed like going up hills it was not quite as perky as it was. That's what prompted me to look this up. Perhaps that mix I think I remembered is wrong. (Darn brain farts anyway)

    IN any case, at some point I'm going to need to take this thing apart for one reason or another. I need to learn my terminology. And I don't. Not a clue.
    I did once remove the flat thingy, to put cardboard inside the thingy, to help reduce noise.
    See what I mean? (I told you all, stop laughing!)

    So... if someone has the patience, I know the spark plug. Lets go from there.
    The cooling fins part, what's that called? What's in it? I do know the piston has to be under the spark plug so that's got to be in there. Anything else I need to know?
    That flat thingy, I THINK the clutch plate was in there???
    And that's about it. No clue what the skirt is, or where exactly the crankshaft is. Tried googling two stroke motors but all that comes up are lawn mowers.

    The whole motor seems to have an oil leak and I can't quite pinpoint where. LOOKS like where the exhaust is attached to the motor but it's hard to tell. Tried tightening everything, helped a little, I don't get black splatters on my leg anymore. Much.

    What is good to clean a motor with? The fins have got so much black goo on them now, it's impossible to see if or where anything may be leaking.
    When do you know IF the exhaust is building up crud? And if so, how do you clean it?

    Aside from struggling up hills, it seems to be running fine. I read the post above about really using the motor. I do tend to go distance at a comfortable 20 or so BUT.... I also let er rip at least a few times every ride.

    I asked my mechanic what oil mix I should use, his reply? "they are all different." Swell!
    I need to see what MY motor needs and it seems that's based on use and checking.
    Gotta learn how to check. Maintenance is better than replacement right?

    So... who's up to educating me here???
  11. give me vtec

    give me vtec Active Member

    I didnt do any of this... I ran a really lean mixture of about 14:1 for the first gallon. Varied the throttle for that gallon, and then ran the **** out of it. I beat it up pretty good on a daily basis, and it is 105 degrees here. I kinda wanna to see how much these engines can handle and just how much you can beat them up. I run the regular echo 2-stroke oil from home depot at about 25:1... nothing special.

    No problems yet... I will let you guys know just how much I get out of this thing before it fails. I cant imagine one of these engines being subjected to a harsher environment than I am subjecting it to, except maybe racing in the desert.

    I am quite surprised at how stout these little engines are.
  12. give me vtec

    give me vtec Active Member

    Last edited: Jul 18, 2009
  13. artmaker

    artmaker Member

    Hey thanks for that link. They GOUGE on shipping though.

    $2.40 cent part, I was going to order it.
    Came to check out, NINE DOLLARS to ship one flat light little gasket?????
    So I sent an email asking about this. (Some sites automatically post shipping but it's wrong and will correct it.)

    Well.... I get this rather snippy letter back telling me how padded envelopes cost 1.50, that it takes time to put cardboard around this piece, how standing in line at the PO is more time.... blah blah!!!!

    I sell too! All the time. Padded envelopes can be had for .50 cents each if you look. And small boxes (better, no need for the extra cardboard) can be found at for TWENTY CENTS EACH!!!!
    I ship clay which is heavier and easier to ruin than a gasket, materials AND postage is rarely over 3.00. I'd pay that. Not 9.00! But gees. The sob story about cost... what BS!

    I'll find a gasket somewhere else. Surely this isn't the only place.

    HOW do people stay in business doing that??? I could have been a repeat customer who refers people too. Not now boy! NO way.
  14. Pablo

    Pablo Motored Bikes Sponsor

    We just added our exhaust gasket to our site today. $2.49. Shipping should come up as $2.00
  15. srdavo

    srdavo Active Member

    artmaker.... you (or anybody) can buy high temp gasket material at any auto parts store. (Napa etc.)
    make your own gasket.... scissors/utility/exacto knife & a paper punch.
    cost= pennies
  16. RMWdave

    RMWdave Member

    actually, the best way to make a gasket is to use a ball peen hammer. just tap lightly around the edges!
  17. artmaker

    artmaker Member

    Hey Pablo THANKS!!!! Already bought and paid for.

    That guy at Dax is nuts! Really nuts. look. Copy from his email to me.

    Lol.... 20 bucks? Yea right.

    I've read about making your own, but for now I'd rather get one ready to go. Maybe after I've actually taken apart a few things I'll try that. But I don't mind buying a pre cut part for a few bucks.

    Thanks everyone.
  18. mattysids

    mattysids Member

    SBP is wonderful about their prices and respond very courteously to emails

    I got a head stud kit, other engine hardware kit, exhaust gasket, and plug shipped for $8

    totaled at $30

    try there for parts!
  19. artmaker

    artmaker Member

    Yes I agree. They were very nice. As my motor ages I'm SURE I'll be ordering parts there again.

    BTW.... got those tires and they are FANTASTIC!!! I've already put more than 100 miles on them. NO problems at all.
  20. crackers

    crackers Member

    This is for rmw dave, what is a ring seater and what is the 41 mark your talking about?