24” mountain bike project

Discussion in 'Motorized Mountain Bikes and Road Bikes' started by Greg58, Apr 23, 2019.

  1. Greg58

    Greg58 Well-Known Member

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    Jet numbers confuse me that’s why I say what drill I’m using,,I think the stock 66 nt has a jet at #71, I’m going to a 73 and open it up from there. I remember 2door saying that in Denver he had to run a #73 drilled jet because of the altitude. It has alway seemed odd that two engines the same size and the same location could need different jet sizes.
     
  2. allen standley

    allen standley Active Member

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    Am I correct in understanding the Higher the # the bigger the hole?
     
  3. Greg58

    Greg58 Well-Known Member

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    No, they are backwards. When talking jet numbers the lower the number the smaller the hole is, with drill bits the higher the number the smaller the hole. I think when all of this was conceived the guy was probably laughing.
     
    #43 Greg58, Jun 3, 2019
    Last edited: Jun 3, 2019
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  4. Greg58

    Greg58 Well-Known Member

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  5. allen standley

    allen standley Active Member

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    Thank you Greg.
     
  6. Greg58

    Greg58 Well-Known Member

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    The jet that was in the carb was a #70 drill size or .28, I couldn't convince myself to go smaller than a #72 on the first try. The #72 was much better but still flattens out at WOT, I soldered up a spare jet and drilled it to a #73 drill or .24, it runs the best it has so far. I'll do a plug check to see where I'm at color wise but by feeling no difference in engine heat I don't think its too lean. On the plus side performance was improved, it gained 3 mph up the hill of the road we live on. So sometimes less is more.
     
    #46 Greg58, Jun 4, 2019
    Last edited: Jun 4, 2019
  7. allen standley

    allen standley Active Member

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    Excellent. Thanks for making sense of it for me. I ordered a pin set and will be drilling my own soon.
     
  8. Greg58

    Greg58 Well-Known Member

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    Allen I mic the drill bits in the range I need to use to see what the size really is, mine are fairly close but some are .05 off. The only drill bits I've used fromm the set are 69-73.
     
  9. Jimmy Bloodmaker

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    I've always just bought a spread of jets. the drilling was to much for my fat shaky hands. if i told y'all how many bits i have broken before just dropping the $20 bucks on a 60 -80 5mm jet spread, you'd think i was insane. plus it makes it way easier for my old man hands to just grab a flat head and pop in a new jet. i've become so good at rejetting, i don't even spill a drop from the float bowl anymore. speaking of jetting, i have to rejet my love ( fav bike). i just put a custom pipe on her. WOW!! got my power band to kick in at about 3k rpms and shuts down around 5500 rpms. ik, i'm spinning her so slow. just love this bike, and want her to last. what is your header length greg? i'm trying to find out as much as i can about others 24" builds. what size pipe? the pipe i welded up today has a 23" header, goes from stock 3/4 and transitions at about 7" to a 7/8 pipe for the rest of the run. then to a double 40 degree cone ( front and back are same on the chamber) for the expansion chamber, then it's plumbed with a 12" stinger at 1/2, with a muffler can on the back. i'm lifting the front off the ground if i pedal and hit the gas on take off. i have to pedal then slowly give her some throttle or she wants to jump. curious about if you are gonna be tuning your exhaust anymore or just leave it as it is.
     
  10. Jimmy Bloodmaker

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    i'm sure if we all knocked our heads together, we could rule the motor bike world with our collective parts lists and imaginations. lol ;)
     
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  11. Greg58

    Greg58 Well-Known Member

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    Jimmy I did a lot of pipe building and testing about 5 years ago, the pipe on my silver bike is the best I could come up with, it has a 16" head pipe with a deep bent to not restrict flow, a 12" chamber and a 2", 1/2" I.d. stinger. That's on a 66cc modified engine, totally different than what the 48 cc needs. I'm applying what I learned as a kid in the early 70's working on Suzuki and Yamaha motocross bikes, no two are or respond the same. Trying several pipe types till I find what this bike likes will be what I do, the pipe on it now is a stock 66cc kit pipe, it has a unusual large stinger. This pipe is better than the first 48cc kit pipe I tried, I've cut the 48cc pipe up to start building a custom pipe. I'll post results as I go.
     
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  12. indian22

    indian22 Well-Known Member

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    Jimmy I agree with Greg. Though same dimensions will get you close try, ride and change is the only way I've found that works with selecting expansion chambers. I have two 66cc engines set up identically in every way and one performs much better with the same expansion chamber than the other. Change the stinger length & diameter a bit and it's vice versa on performance.

    Rick C.
     
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