Fuel Mixture Octane??

Discussion in '2-Stroke Engines' started by Question about octane., Jul 8, 2016.

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  1. Does it really matter what kind of gas you mix for your bike? What are the differences between the regular 87 octane and the premium gases out there?

  2. Frankenstein

    Frankenstein Well-Known Member

    I wouldn't say it matters unless you're really going all out on mods to make it better. I will say that the difference between 87 at the gas station in town that undercuts everyone by 20 cents and the aviation fuel station which also supplies 87 for cars is that the dirt cheap gas comes with water, rust bits, and little white floaty things. All depends on what you want in your engine really.

    92-93 Oct has been reported by some users to run badly, but the requirements to burn such fuels nicely are higher when the engine heats up and runs faster, if the timing is off just a bit then you're looking at detonation city.
    Peejus likes this.
  3. Peejus

    Peejus New Member

    I ran some high octane gas and the engine was not happy. But.. The engine wasn't fully broken in and I'm sure I screwed up the oil mixture, so I'm quite willing to call user error on my part.
  4. butre

    butre Well-Known Member

    high octane gas often contains more ethanol, closer to the 15% maximum. plus it has a natural energy content lower than lower octane fuel. if you don't have the compression to need it, then you don't want it.
  5. Frankenstein

    Frankenstein Well-Known Member

    I actually mentioned that aviation fuel port, they supply ethanol free gasoline. If you do some searching you can find a website that lists places that sell ethanol free gas. I thought the fuel port was significant enough to mention because the high octane fuels don't contain that irritating ethanol that seems to eff up using certain engines on 97 or even 102 octane fuels. It actually seems to do rather nicely when I spin by for a tank, also helps keep that castor from separating in the morning.
  6. Steve Best

    Steve Best Well-Known Member

    Octane is a component of gasoline that burns slowly, reducing the possibility of detonations and preignition.
    Pure octane has a burn speed value assigned as "100" and is used as a comparison for fuel that is a mixture of compounds.
    Slow burning fuel is great for a 3500rpm aircraft engine, but kills power on a 6000 to 10,000 engine unless you can speed up the burn by some other means such as compression, heat, confined space or combustion turbulence.

    An the engine will make the most power on the lowest octane fuel that it will run on.
    If it will run on 83 octane, it will make less power with 97 in it.

  7. Frankfort MB's

    Frankfort MB's Well-Known Member

    How cool would it be to say that you have a MOTORIZED BICYCLE that runs on 116 octane racing fuel!!! :p
    There's just some times that I want the wow factor instead the speed factor lol
    Frankenstein likes this.
  8. Frankenstein

    Frankenstein Well-Known Member

    All our bikes run on racing fuel, but sometimes not as well as we would hope...

    I upped the compression, better cdi, better plug, this that yadeeyada, so I would appear that my engine could burn better fuel easier, which might explain why it keeps breaking itself at least 3 times a month it seems...

    Lesson to all: less power = less stuff breaking, pick your side.
  9. Frankfort MB's

    Frankfort MB's Well-Known Member

    You've got your bike to last 3 months w/o breaking something?;)
  10. Frankenstein

    Frankenstein Well-Known Member

    No you'll need to read that again, I ride the bike with it breaking at least 3 times per month, at about 10 days of repairs per break that means that on the months with 31 days I get to ride it one time. Not bad
  11. Frankenstein

    Frankenstein Well-Known Member

    If it breaks 4 times I have to reset my time machine to make enough back time to work forward, it's kinda awkward talking to yourself so I try not to do it, especially backwards...

    Where's my meds?

    Crap I must of left them in yesterday, maybe I'll see them tomorrow...
  12. butre

    butre Well-Known Member

    my stuff never breaks, even when my bike is going 50 miles per hour. my kickstand fell off the other day, that's about the worst that's happened. it's just such a pain in the ass to tighten that bolt when you've got two chains. weight reduction as I see it

    build it right and it'll stay right.
    Frankfort MB's likes this.
  13. Frankenstein

    Frankenstein Well-Known Member

    I wish it was that easy, I litteraly followed every rule in the book, down to the dots on the i's, seriously, every last rule and precaution taken and followed. You could swear it was being assembled in a nasa laboratory under the strictest of guidelines with a gun to my head and it's good by Charlie if I mess up. Intense!

    Yet the engine starts up and runs 2 miles then goes all kamikaze on itself and dies for no reason whatsoever. Can't idle it, can't WOT it, can't look at it without it catching fire and the red locktighted bolts falling out.

    It's generally accepted my bike and/or engine is perpetually cursed. It's also believed to be a yard decoration since it sits in the yard next to the fire escape nearly 24/7, myths that it once was like the bicycles and was ridden around the city surrounds it...
  14. Frankenstein

    Frankenstein Well-Known Member

    I do remember a time when it ran though, it ran really crazy ass good too
  15. Steve Best

    Steve Best Well-Known Member

    I'm only running 40mph Butre, but have over 1000 kms on this engine with only one "pedal home".
    After hours and hours of crazy rpm, a piece of wristpin circlip broke off, scoring cylinder and piston.
    I find these motors very reliable and wonderfully easy to hotrod and repair, better than much of the production machinery I worked on over the years.

    As for need for huge octane, I've learned to get by without it.
    I got my license in the 1970s when they stopped selling high octane gas here.
    1960s supercars were selling cheap cuz they couldn't run on the only available low octane fuel. I bought a rusty 1970 11:1 351Cleveland "Q"code Torino for $120 and using lessons learned from Ak Miller, Larry Widmer and others writing at the time, made it breath fire on low octane fuel. This is that motor stuffed into a Ford Maverick (on 83 octane fuel):

    I had a CR250 that needed the same work. Today I have a KTM 125 EXC that I take on long wilderness excursions. In the back-country a lot of general stores only have 83 octane so my 125 is tuned to run on it. I am typically always the smallest bike and yet I run in the front. Don't need no 90+ octane to get a KTM 125 to run 140 kph and keep up with the DRZ400s. I cannot out accelerate the 250 2 strokes, but I can pass them on the long stretches with a higher top speed.

    My point is, power can easily be built out of regular fuel.
    Most amateur "High Octane Motors" I've seen didn't gain anything from the fuel, but used it as a crutch for a poor build. My potential gain for building an engine to use 95 octane instead of 83 is less than 8% more power. Why would I ever go to $$$ 100+ stuff unless I was racing professionally?

    My little 66cc Grubee could power my heavy arse up the steepest hill in this county faster than I could ever pedal on the flat, all on (slightly modified) stock parts and 83 octane. Not even a tuned pipe. What could I gain if I built it for more octane? Not much. Just not as fun to ride because fuel would be a pain.

    To double the hp of these motors is not that hard or expensive. They are reliable if you know your stuff and build to avoid detonation and vibration problems. No special tricks needed.

  16. Frankenstein

    Frankenstein Well-Known Member

    I agree that modding a bike to run best with race fuel is pretty much stupid, unless racing. Question is if you modded a bike too much and now it only runs good on racing fuel what do you do? Can you un-mod it? Seriously I think sometimes I took it too far, even if by mistake, because it litteraly acts out if not using better grade fuels... Is it possible that I simply tuned it while under the influence of 97 ethanol free gas?

    Really honestly I don't know anymore, bike slowly went from beautiful with a capital B to bitch with a double capital b.

    At one point I tried to curb stomp the motor out of the bike, I have witnesses to prove it (that was after a month and some change of repairs, first time in my life I took any machine and litteraly tried to murder it)
  17. Steve Best

    Steve Best Well-Known Member

    Enlarge the chamber slightly while keeping squish area and gap at reasonable limits to drop the cranking pressure and keep turbulence up. Back the timing off if needed and tune the pipe for a more longer hit.

    What makes the bolts fall out is:
    Poor quality hardware. Look for metric grade 8.8 stuff. All you need.
    Vibration. Mount rigid to frame and keep rpm down.
    Incorrect torque. Buy a torque wrench and use it.
    Insufficient rigidity. You may need to back mount plates with shims, or file mounts to profile.

    Idle is all about "no leaks" and reasonable porting. Take too much off the intake side and you will have trouble.

    WOT? Jetting! Do a plug chop to see if you've got it right.

  18. Frankenstein

    Frankenstein Well-Known Member

    I'll give the head compression a shot, a compression tester gives nearly an identical reading on a nearly identical engine.

    The joke behind bolts falling out wasn't that they really fall out, I was being dramatic, I think the bolts are the only problem I don't have. Good point mentioning that if you can helical the mounting points, and the tapped areas for the carb and exhaust, and replace all bolts with high grade steel then do it, this goes for everybody, not a performance mod but you'll end up preventing a world of headaches.

    I got the no leaks issue problem under control, but even with new gaskets and permatex copper gasket spray I still have problems with just everything. Same cylinder I've been using with no problems, after an upgrade to a Reed valve and a bunch of repair work that's when it went to hell. Beats me.

    Jetting doesn't seem right either, everything between 70(what it used to run long long ago) and 87 don't do it any good.

    Probably it's just my bike, I've seen people's engines where half the mounting hardware is broken, things are done bass ackwards, chains are nearly snapped, and sprockets are badly aligned and or missing teeth..
    They tell me it's been like that for a week and didn't have any trouble. Wtf... I'm cursed
  19. Steve Best

    Steve Best Well-Known Member

    KISS - Keep it simple.
    I'm using gaskets with no sealer, and no leaks. No need. That is what the gaskets are for.
    I have cut a cardboard and aluminum (soda can) base gasket, but only to get proper cylinder height.
    I am also using teflon tape for a headgasket as well, to achieve proper squish gap.
    The aluminum headgasket is simple and works fine for about 5 uses. Easier than the teflon to install.
    Grade 8.8 metric bolts are standard automotive grade. Nothing more is needed and more is not better.
    Helicoils are not an improvement. They are an unfortunate necessity when I screw up.
    (Occasionally I do, but not this year so far! :) )

    Deal with each problem by returning it to the "as designed" state. Mods just confuse things.
    Do all mods only to a well working machine and only one at a time to see the effect. When perfected, move on.
    These are mechanical troubleshooting principles, taught in college and engineering schools.

    These motors are the perfect training school to learn this stuff on.

  20. Frankenstein

    Frankenstein Well-Known Member

    I think mine went super nova on itself, everything was fine and then boom, nothing was fine. I mean yeah on the way there I had the hiccups as expected, but just all of a sudden after a good winter without many issues, even after going in rain sleet snow waving at the mailman as a drove by, it just went downhill, and that was that.

    In the process of installing a new engine, the old one seems hopelessly done for.