Adaptors and manifolds for larger carburetors

Discussion in 'Performance Mods' started by jaguar, Apr 8, 2013.

  1. jaguar

    jaguar Well-Known Member

    Please see my new page on the subject at

    Having a quality carburetor jetted and sized right makes a ton of difference in how a bike runs.
    It starts better, idles smoother, accelerates crisper, has better peak power, and doesn't foul plugs.

    My last dirt bike was a Honda CR250 and my riding buddy had a 250 Gas Gas. We were at 3000 ft elevation and mine was jetted perfectly whereas his was still jetted stock which was for sea level. At that elevation he was running rich and he occassionally had to be pulled back to our cars after fouling the plug. I tried to tell him but he wouldn't listen. I don't see why anyone wouldn't want their ride to be correctly jetted.
    Fabian likes this.

  2. jaguar

    jaguar Well-Known Member

    Using the engineering formula for determining correct carb size I came up with this chart. Stock carb size on the Grubee engines is only 14mm. For anything other than a stock non-modified 48cc engine that size will limit top rpm power. I am recommending the 16mm and 18mm Mikuni carburetors. They work really really good.
    5-7HEAVEN likes this.
  3. 5-7HEAVEN

    5-7HEAVEN Active Member

    In search for more power, I had two carb adaptors made for my twin-engined MB. Since I was running ADA S1 pipes on the Mits TLE-43 engines, they were running a bit lean. That carb experiment was scrapped when I found more powerful engines and shift kits.
  4. jaguar

    jaguar Well-Known Member

    What engine did you go to?

    A shift kit is an external bandaid for an unsolved internal problem. For less money than a shift kit costs you can have a good running powerful reliable bike that doesn't need to shift. All it takes is a little time and effort to get these Grubee engines right.
  5. 5-7HEAVEN

    5-7HEAVEN Active Member

    Using friction drive and 1.25" roller, the CY460 and Tanaka 47R engines still dropped out of their effective torque range when climbing steep hills. When that happened, they were dogs. Rpm and torque dropped well below operating range. These engines need high rpm, especially the 460. The shift kit, when geared properly is an excellent way of keeping them in their torque range, especially with ratios as low as 46.36:1 in First gear and 15:1 in final drive. Hills that bested these engines can now be taken with engines screaming or with engines well below redline. JMO, shift kits conquer mountains with proper gearing and the right engine.I'm building another center-frame 460 engine w/shift kit, this time w/8-speed internal hub.
  6. Fabian

    Fabian Well-Known Member

    Jaguar, i believe you are completely wrong in your assessment of a shift kit, furthermore, i say that a shift kit is absolutely vital to make a motorized bicycle a viable transportation device.

    How do you get up these kind of inclines without a shift kit?




  7. jaguar

    jaguar Well-Known Member

    Fab, you wouldnt believe the amount of grunt my 55cc w/reed,torque-pipe had. At 8500 ft I would go flying full speed up hills I couldnt climb before. If you have something thats simple that works good then you have no need for something more complex. And dont say the torque pipe isnt available. I have two for sale at a bargain basement price of $60. Headsmess is going to start making regular expansion chambers at a price of $150!
    Your hill that you went screaming up at 6000rpm at 10mph would be a piece of cake for my 55cc. sorry to brag but its the truth
  8. Fabian

    Fabian Well-Known Member

    You got it wrong Jaguar. I was screaming up that hill, not at 10mph, but 5mph.

    Sometimes a man can stretch truth past the breaking point of keeping a statement truthful :whistling:
  9. jaguar

    jaguar Well-Known Member

    the typical response to someone highly successful is dislike and even hate. without a monetary purpose I had no motivation to demonstrate absolute proof of my success with this engine. If someone is intelligent and creative then reproducing my success would be no great achievement. The plans for my torque pipe is posted for all to copy. Lazylightning made one and here is what he said: "The pipe
    really adds power. A LOT! 2 or 3 times more I would say. It works from the very start and I have a 35 tooth rear gear on the bike. It's really nice."
  10. Fabian

    Fabian Well-Known Member

    In fact sometimes my bike is crawling over obstacles at maybe 1 or 2 mph; like the method used by those who ride trials bikes.
    Sometimes screaming up hills just isn't possible, no matter how many pieces of cake you have and a single speed system cannot do much screaming in such an environment, no matter how effective the pipe may be - not to say that a tuned pipe isn't effective.


  11. jaguar

    jaguar Well-Known Member

    you have your reality and I have mine
  12. Fabian

    Fabian Well-Known Member

    I agree with you completely.
    My reality applies to the laws of physics.
  13. HeadSmess

    HeadSmess Well-Known Member

    stomp stomp stomp, kicking up dust...

    i found out theres a mikuni vm17!

    nice price ;)

    all my maths says that the port is 20x15.5. and its actual cross section is 20x12.5.

    compensate a little for the radius corners and it compares favourably with 17mmID tubing.

    i managed to squeeze the stock carb on to the handy bit of tube i found, i almost had to redo it when i forgot about the silly studs being so close together!, and then i cleaned up my mikuni 18 and remade a nice manifold clamping ring thing. which works this time. yet to see if it holds on.

    i need an air filter. im thinking i might just get fancy with some alloy plate and make one. clamping that on WELL is the issue.

    now i recall why i gave up on intakes a few years ago :rolleyes:
  14. 5-7HEAVEN

    5-7HEAVEN Active Member


    I share your opinion. My shift kit conquers the hills that bested my friction-driven 5hp 460 engine.

    Every time.

    Since I installed my shift kit, my engine has NEVER lugged down below its optimum operating range.
  15. MusiCALpuLLtoy

    MusiCALpuLLtoy New Member

    christ! just create a small vacuum leak. problem solved
  16. moto

    moto New Member

    A simple way to adapt manifolds at home is to glue small sections of tubing over the oem manifold or even a machined manifold. To use my 21mm phbg I simply jbwelded a short piece of handlebar over my maniac intake and it works perfect!
  17. scott.d.lang

    scott.d.lang New Member

    When it comes to climeing I have to say both jag and Fabian are right but better then either of them is to mix the 2 ideas do all you can to make your engine run through a broad rpm range and have a shift kit also. Ive never ran a shift kit and for city driveing in Chicago that is fine but I have done many things and have many things planed for the schwinn build Im working on now for my engine. that said if I still lived in Tennessee I would do all the porting and everything else then Id add a shift kit so my motor did not have to work hard on all the hills.
  18. jaguar

    jaguar Well-Known Member

    I lived where there were fairly steep hills and my bike, if it could, would of laughed at the idea of a shift kit. Falling on its side laughing it would of mumbled something about "legs on a snake" (an old Zen way of saying something is not necessary).
    But if the hills were extremely steep or the bike had a trailer to haul heavy things then a shift kit would be OK.
  19. HeadSmess

    HeadSmess Well-Known Member

    lol, here in australia, some of our snakes do have legs :)

    and some of our lizards dont... :jester:

    i wouldnt mind a three speed at the maximum, then i could cruise at 70 and still climb my mates 45 degree driveway.

    i cant see a reason to have any more than that. especially when its still just a rudimentary derailleur type transmission.

    gimme a centrifugal auto any day :)