Low end power mods!

Discussion in 'High Performance Bicycle Engine Tips and Motorized' started by FoCoMoTo, Apr 26, 2013.

  1. FoCoMoTo

    FoCoMoTo New Member

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    Hey guys,

    I've found a few threads mentioning low end power but most people seem to be after speed. I wanted to start a thread to see how many mods we can get in one place that are dedicated to adding low end torque to your bike. Personally, I'd be plenty happy if i could mod my bike so that its top speed is 27 or even 25 (instead of 40-45 that some people are reporting, crazy!!) but so it had enough low end to go trail riding and climb 'semi'-steep or rocky hills. not sure if thats possible but lets find out!!

    Here's what i've done that has worked so far:

    I put a 2-3" extension between my carb and intake, which utilized that vaccuum pressure and extra space to pull more air and fuel into the cylinder per cycle. I found that mod and a great explanation at this link here: http://www.dragonfly75.com/bicimoto/index2.html (i don't have a good pic of this right now but i'll be sure to post one)

    I also upgraded to the nt speed carb with the red filter cap, i'm not sure that did a whole lot but i felt a little difference, that obviously added speed too.

    i eventually (once my engine is fully broken in) am going to get a bigger sprocket too, not sure what size yet, i currently have the 44 that came with my kit.

    So, lets hear what you got! remember to post pictures! feel free to post links to other threads if you think it'll add to the discussion. :)

    Peace!

    dnut
     
  2. WightBoy

    WightBoy Member

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    I don't know if you can squeeze one in that frame but I would look into a shift kit maybe. It will allow the use of your bikes gears. http://www.sickbikeparts.com/ These little engines can only do so much with a single speed gearing setup. Or a cheaper method would be to pedal harder on take off. Remember, these are motor assisted bicycles.
     
  3. FoCoMoTo

    FoCoMoTo New Member

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    I've thought about getting one of those but they are so expensive and the gear system on my mountain bike isn't made for that kind of power, i really don't want to waste $150 bucks. I'm on the fence, i like the idea but i'm skeptical.
     
  4. 2door

    2door Moderator
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    If top speed isn't what you're after but you want to climb hills go for a larger rear sprocket. It's the easiest and cheapest way. You are currently running a 44T. You might want to explore a 48 or even bigger sprocket. Your top end speed will suffer but if you go big enough that puppy will climb trees if that's what you want to do.

    Think of driving your car in low gear all the time. That's what a large rear sprocket will do for you. Your might top out in the low 20s or high teens but you won't have to pedal up hills either.
    Good luck.

    Tom
     
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  5. tooljunkie

    tooljunkie Member

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    i ordered a 50 tooth for my wife's bike.
    not after speed,but the one hill by my house needs a little pedaling
    to climb.
    that and it seems the less it has to work,the engine seems quieter.better engine behaviour at a lower speed.
    and the fact she wont be able to catch me.....
     
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  6. Moto

    Moto Member

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    biggest improvement i have experienced is a high compression head / long header pipe. big sprocket too. a reed valve supposedly does wonders too...
     
  7. aussiedan

    aussiedan New Member

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    for my mtb i found after the usual mods the bigest low end increase i got was matching the tranfer ports to the crankcase.i cheated a little by puting a 50cc top end on a 66cc bottom end,the 66cc studs are space out further than 50cc but my barrel has the steel sleeve so i was able to drill out the stud holes on the jug.

    with a 44t i can ride up fairly steep hills without pedaling and not loose speed over the real bumpy stuff
     
  8. LR Jerry

    LR Jerry Active Member

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    As far as expense goes you get out of these bikes what you're willing to put into them.
     
  9. mew905

    mew905 New Member

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    yepp, I've dumped close to $1000 into mine so far and it feels like it wants to pull all the way to 70 or 80, if I had the proper gearing, you can really feel it pick up at the high end. However if you're looking for good all round power, then port as follows:

    Port timing (for a 66cc).
    Leave all port heights at the stock height, they're perfectly timed for all-round power unless you have reeds.

    Port widths (again for a 66cc):
    (The arc is the distance following the curve of the cylinder, the chord is the straight line distance to the SAME points)
    Exhaust - 70° width, 28.1mm arc, 26.5mm chord (for low end power, make the cuts round-ish)
    transfer ports - 58.5°, 24mm arc, 23mm chord (for more power, JB weld the exhaust side of the transfers to point more toward the intake side, and grind 0.5mm ramps into the piston on each transfer)
    intake port - 70°, 28.1mm arc, 26.5mm chord (for low end power, get yourself some reeds, it boosts compression by not allowing air and gas back out of the intake).
    boost port (reed valved only) - 15.75°, 6.5mm arc, 6.5mm chord (close enough).

    For more low end power, have a long pipe before an expansion chamber (the extended header does this and lowers the power band slightly), jag's site has header calculations on it. the crank's balanced for low RPM anyway which will maximize your low end power and comfort, the stock CDI and carb are good enough for low end (high end suffers though). Just rememeber that as you get bigger gears (44T to a 56T), that low end power you had gets shoved up in the power band at the same speed (that is if all you have is low end power and you switch to a 56T from 44T, the power that was there at say 20kph, is now at 15kph, and 20kph is lacking now (though with such low gears... you probably wont care, because the tradeoff of bigger gears is obviously better torque, potential for wheelies? :O)

    However one thing you will always hear, you can tune as much as you want but it will only take you so far. In the end, compression is your goal. All this porting? allows more air to be sucked in. What's that matter? Well more air in the same amount of space is better compression. Better compression means better bang, better bang means better power... you get it. If you do go for ridiculous amounts of compression, however, get yourself some good quality bearings. SKF or INA are two brands that come to mind. I got some SKF 6202 bearings for the crank and clutch, rated to 33,000 RPM and built for industrial use, so I think it more than qualifies for these motors. Copper coated gaskets from SBP and better head studs (again from SBP, unless you wanna make 'em yourself, the ones SBP sent me just looks like it was originally one big rod they cut to length). Most 66cc studs are M8x1.25 pitch, I have no idea how long (probably around 110mm). In fact, replace all the hardware with better quality steel, my chain was jamming up because the M8 roller bearing screw became stripped, immediately after I just replaced it too. these screws strip so easily it can probably be stripped by hanging maybe 6 muffins off the end of your ratchet. Better hardware (I find metric bolts surprisingly hard to find, but they have bulk bins at Part source) are strong enough to crush the [stock] motor kit parts if you're not careful.
     
    #9 mew905, Apr 27, 2013
    Last edited: Apr 27, 2013
  10. 48ccbiker

    48ccbiker New Member

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    Increasing engine compression helps but in conjunction with the too-advanced stock ignition timing might cause a bit of a heating problem.
    I would consider these mods:
    longer intake and exhaust
    better carburetor ( that mixes fuel better for a better burn)
    adding JBWeld to the fronts of the transfers to direct the flow more rearward for less loss of fuel mixture out the exhaust port.
     
  11. fx-2

    fx-2 New Member

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    I have 66 cc Jet built that way and it works great . I also built my own expansion chamber . It has a DAX Rt carb on a 6" long manifold ,a puch 70 cc high compression head . It also has a jaguar CDI . http://motorbicycling.com/showthread.php?t=43040 Dan
     
    #11 fx-2, Apr 30, 2013
    Last edited: Apr 30, 2013
  12. database00

    database00 New Member

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    Does an expansion chamber help out that much?


    http://motorbicycling.com/showthread.php?t=46766

    is my build and I Just built it.
    Debatting on an expansion chamber, dont need the power but who doesn't want more power.
    I do have a gear kit with different motor then most.
    2 speed motor after I did the gear mod.
    16speed mtnbike but im debating on the expansion chamber.
    is it worth fabricating? does it matter how big of chamber is there a limit?
     
  13. Scott.D.Lang

    Scott.D.Lang Member

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    from my experience on these 2 strokes so far. this is what Id do add a good x pipe a good carb like a RT from dax and a reed kit. even with out a reed port the reeds raise your low and mid range TQ but you will lose some top end speed. I just did a port job on my 50cc dax engine and until then I was very happy with my reeds. now that said I think my problem is the jug base gasket I had to make on the fly for my motor when I get my new base and head gaskets I very well may report how much the reed kit with a proper jug port will work.

    parts I went with
    Dax RT carb
    voo doo pipe
    JNM reed kit

    Im 280 lbs and my bike will pull me off with out peddleing with just a little fethering of the clutch and will run side by side with my sons stock 66cc bgf build. before I put the reeds on he would pull on me till we hit top end speeds.
     
  14. Wild Bill

    Wild Bill New Member

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    I ride steep climbs and tight twisty low speed turns a lot! The SBP jackshaft will get the best low speed torque and not lose noticeable top end. Someone said in an earlier post the SBP jackshaft/rear deraileur/bicycle chain wont take the power, I am running one on a KTM 50 mountain bike without failure so far and they claim 9 HP (But i think its closer to 7). The lower cost jackshaft kit sells for around $140 and you can spend that real quick on attempted power gains and hop ups.

    The only drawback is you have to keep the rear derailleur tuned right, no problem if you are familiar with adjusting rear derailleurs.
     
  15. 48ccbiker

    48ccbiker New Member

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    yeah, you spend all your money on a shift kit, meanwhile neglecting essentials like a better carb and better ignition. a shift kit should be the last thing to consider if after the essential mods you still need some hill climbing power. I dont like the RT carb because it isnt fully adjustable. Best to get the Walbro from JNMotors. made to fit and externally fully adjustable. If you want a torque pipe you can see the free plans at Jaguars site.
     
  16. Scott.D.Lang

    Scott.D.Lang Member

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    JNM is a good place to get things for your 2 stroke. Personaly I like the RT because doing adjustments to it is just a jet change. That said to each his own and who knows some day I may run a Walbro lol. but I agree 100% with you before a jackshift kit you should do everything else you are going to do first. I like all the things Ive done to my bike and as of now the only reason I would go with a jackshaft kit would be to carry my 280 lbs butt up steep hills lol
     
  17. mew905

    mew905 New Member

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    ...I just pedal with it if I end up slowing down too much., but on a 36T my bike will haul me up a hill and I'm 220 lbs. I try to get a run at it because my power's near the top end but it'll still accelerate slowly if it has to.
     
  18. SubieGuy

    SubieGuy New Member

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    Going to have to respectfully disagree with you here, biker. The shift kit, coupled with my STOCK motor, has plenty of power to take me up hills with ease. With that said, I only weigh 160 lbs.
     
  19. 48ccbiker

    48ccbiker New Member

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    I never said it wouldnt work, just that it would be the last thing I would buy.
    Of primary importance:
    The stock carb is pure crap, compression is too low (but you can easily modify a stock head), the ignition system is all wrong. Correct these things and add an intake extension and then see if you need more hill climbing power. A home made torque pipe does wonders also but most people are too lazy to make one.
     
  20. Bossboots

    Bossboots New Member

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    So are you saying at 2 to 3 inches to what already there or 2 to 3 inches all together
     

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