Motor 4 Stroking like a son of a Gun!!!

Discussion in '2-Stroke Engines' started by Rockspider99, Jan 15, 2014.

  1. Rockspider99

    Rockspider99 Member


    Hello All,

    So I have run about 2 tanks of Fuel with the stipulated oil fuel ratio of 40ML oil to one Litre of fuel.
    The issue I am having is it is 4 stroking and munching through fuel. I have tried adjusting the Needle to the leanest setting.
    Found the Float was sitting very low, 19mm, so set that to 21mm but still seems to be over fueling. I have the HT engine mounted on the back of the bike, from what I understand about 2 stroke engines is the orientation of the motor does not matter as long as the carb is upright, but is there a chance that the motor leaning back like that could be causing the issue.
    Ohh I have also modified the Intake manifold to get the carb level and checked that it is Air Tight. Lastly other thought is that the high oil to fuel ratio could be causing it to 4 stroke?

  2. HeadSmess

    HeadSmess Well-Known Member

    who told you that adjusting the needle would make it leaner? so much dis-information on forum boards. hard to tell whats real and whats :icon_bs:

    you hammered the nail on the head but. its too rich!

    just, the needle aint going to do a thing until you get the main-jet sized right for WOT. adjust it til your blue in the face, it still wont cure the four stroking. take the float back to the level it was at. was the jet submerged still? of course. raising the level just makes it richer still... less height to jet orifice, less vacuum needed to lift fuel, or, with same vacuum(no reason why it should have changed)...more fuel lifted. just like a hose will let more water out if you open the tap a bit more and increase the difference between the pressures at either end. (head or water level and atmosphere whilst in a carb its atmosphere and vacuum)

    more oil makes it leaner, but its not really the best way to get an ideal mixture, as oil doesnt burn (much) and doesnt do anything for making power, just takes up excess space and makes a mess of everything.

    try soldering very fine copper wire in the jet, one piece at a time, making sure you dont bog the hole up, or you will need a new jet or micro/fractional drills. (which are a worthwhile investment anyway...with a pin-vice chuck as well)

    standard jet appears to be around 0.75mm... ive found these engines usually prefer something around 0.55, depending on engine, altitude and various other factors... what suits yours, is the part YOU have to work out ;)
    Last edited: Jan 15, 2014
  3. HeadSmess

    HeadSmess Well-Known Member


    just curious...why rack mount when that frame looks like it would accept an engine quite nicely?

    having all that weight up high and far back is just so awkward... especially when you open a gate or move the bike around the back or something...

    not too mention the exhaust noise being reflected from the ground straight to your ears...

    each to there own, not bagging...just asking? :)
  4. Rockspider99

    Rockspider99 Member

    Hey HeadSmess,

    thanks for advise, I rack mounted it because the carb was just too long, I have made a change to the intake and will try fit it in the frame this weekend. The bikes balance is not so great with the motor so high up and to the back, not to mention my behind getting a little warm.

    Will setting the carb float lower not allow more fuel? The float lifts and shuts off the fuel intake as the bowl gets filled so bending the pins in would keep the intake open for longer?
  5. HeadSmess

    HeadSmess Well-Known Member

    what determines the fuel height in the bowl is those pins, or the fork shaped thing in the carb.

    the float pushes them UP, and closes the valve attached to the fuel intake.

    so with the pins bent down, the float will lift them up earlier, closing the valve earlier, therefore the fuel in the bowl will be lower.

    now, with the fuel lower in the bowl, the distance the vacuum in the throat of the carby has to pull fuel up from the bowl is greater. gravity sucks, remember! fuel has weight! this is basically equivalent to either less vacuum,(which wont change for a given speed anyway so forget about that) or a smaller jet... but note! only a very very very tiny little bit smaller!

    the reverse applies if you bend them UP towards the carb...which can lead to incessant flooding if you overdo it!

    i prefer having the fuel lower. i can live with a slight "miss" when i hit a big bump (the fuel sloshes around and the jet gets exposed briefly) rather than having to turn the fuel tap off all the time and getting crankcases full of fuel ;)

    usually tune with 0.05mm increments, thats what sizes metric micro-drills come in. (theres others but they cost as much singly as a full set of 20 standard ones do...about $20-$30) fractional/number drills are imperial and have slightly different sizes...handy for the ultra fine tune but not really required unless you really get into it. you also need one really good drilling machine to get them accurate enough to warrant the expense! all drills drill slightly oversize. process is simple. fill hole with solder, remove excess solder from inside of jet with drill until it reaches the brass of the jet again, then drill the tiny little jet hole itself.

    the super fine copper wire works just as well... try things like the wires in headphones...just one strand of it at a time! if you can solder it at least without blocking the jet!

    :wacko:eek:h yeah... and it gets a lot hotter if you have to get off in a hurry! :joker:
  6. Rockspider99

    Rockspider99 Member

    Cool, now I just need to read up on how and where to solder on the intake jet.
  7. HeadSmess

    HeadSmess Well-Known Member

    there is no intake jet!

    it is sort of confusing at first.

    there is a needle and seat. that is controlled by the float and sets the fuel height. the barb that the fuel line pokes onto, is the seat. the needle is the bit inside.

    then there is the MAIN JET, which is (usually) what appears to be a small nut screwed into the end of that brass tube that goes through the float in the bowl.

    this is the bit you adjust for the mixture, at WOT or wide open throttle. this requires PLUG CHOPS. search that :p and or just test riding. if it dies with full throttle...too lean, too small. what you have now? too rich, too big.

    then you adjust the needle for easy starting and nice acceleration, it only controls (basically) half throttle and below... if its hard to start raise it. if it idles high then slows down and stalls if you dont blip the throttle... its too rich, lower it.

    i lost the link... search mikuni tuning guide. its simple :)
  8. Rockspider99

    Rockspider99 Member

    he he, thanks, I think my wording is all muddled, I have stripped the carb down about 4 times already in the 2 weeks I have had it.
    "more familiar with WEBER/Solex Carbs, these are extremely simple but with the lack of air full mixture adjustment needles makes them pretty difficult to tune."
    The bike pretty much 4 strokes through the entire range and I am pretty much Sea Level where I live, Cape Town.
    So I will try making the Main Jet smaller with wire first. Also read up that the motors to tend to lean out as they get run in?
  9. HeadSmess

    HeadSmess Well-Known Member

    dont mention the phrase "break in" or "run in" around me :shout:

  10. Rockspider99

    Rockspider99 Member

    Curious now as to why I should not mention those things you told me not to mention.
  11. grinningremlin

    grinningremlin Active Member

    No need for pedal extensions, no burnt legs are two great reasons, full use of legs (can't sprint with legs splayed).High and far back, like where a massive human sits?Rack mounts are very stable, not awkward in the least, the only stability problem is sitting still on a kickstand (tip easy).The sound is behind you when riding.A rack mount can be connected/disconnected in minutes.Nothing to come apart BETWEEN YOUR LEGS.I'm bias, frame mounts look like c.r.a.p. wannabe motorcycles IMHO.My 2 cents,... of course will buy nothing.
  12. Rockspider99

    Rockspider99 Member

    Thanks Grinn, appreciate the kind words, only issues I think I have is that you can't really ride out the saddle, the motor acts as a counter weight and throws the bike all over the place. The Rack with motor is about 12KG in total so it is a fair weight and I am only 1.72m tall and weight 60KG.
    The other problem is my mount is not far back enough so I could not really sit back on the saddle for obvious reasons.

    I am going to try the frame mount option out, see how that feels, main concern there is the heat between my legs, rack mounted the heat is blowing away from me, frame mount the heat is rising between my legs (GASP!).

    Also to update on my over fueling, I think I found the culprit, the little "prongs" that hold the float needle were also skew and stopping the needle from closing off the fuel completely, tested with so old fuel I had. just a little bend and seems better, going to try finish assembling the bike this evening and maybe get a chance to give it a run.
  13. butre

    butre Well-Known Member

    All I gotta say is that there's a reason Harley Davidson doesn't put their engines on the back of their bikes.
  14. Rockspider99

    Rockspider99 Member

    Thanks all for the great advise, six little strands of old headphone wire later and she seems to be running well. Just a little bit of "4 stroking at low throttle (just about 1/3) but from what I read this is desirable for when you going down hill, extend the life of the motor. When passing 1/3 throttle she evens out nicely and ziiinnnngggsssss away. So much more pleasant to ride now.
    Balance is also way better now with motor in the frame.
  15. ez dave

    ez dave New Member

    what is meant by 4 stroking, your two stroke suddenly developed a valvetrain, just curious. i think for a rich condition you might want to go through the carb and adjust and jet if need be but as previously stated do it on the wot since this is presumably how the motor will be ran, i had a couple of these carbs that were way off and imo not even worth messing with so i bought the performance carb and everything was fine for awhile which seems to be the name of the game with these two strokes they last for awhile. 1500mi is my personal best.
  16. Rockspider99

    Rockspider99 Member

    Greetings all, so update on my Little Happy Times Motor, made the Jet smaller by soldering and she purrs much better when the motor is under load. Weird thing now is once up to speed / coasting speed she still 4 strokes. IE when under load she runs 100%, no Load she 4 strokes.