Dual Carb 70cc "blue oni"

Discussion in 'Performance Mods' started by sunofjustice, Nov 22, 2007.

  1. sunofjustice

    sunofjustice Guest

    Always wanted to do a dual carb bike engine............ ever since I got my first engine from kingsmotorbikes. Addind dual carbs to an engine is an old hotrodder trick to achieve volumatic efficiency with a v-8 automotive engine. A v-8 can run reasonable well with a 2bbl......... a four bbl........or even a 6bbl (tri-power) ............but, th' engine has EIGHT cylinders. Some where along th' intake side some cylinders lean out or run rich. In th' case of our china-fire injuns.......its intake side is really being CHOKED off (or restricted)................... which is a FUEL ECONOMY trick to get mileage and probably to keep th' revs down. All th' restrictions is creating TORQUE....................thusly, causing VIBRATION!!!!!!:mad::mad::mad: A boost bootle can "cover up" th' fact your engine is STILL restricted .............but a bigger intake........bigger carb............will show bigger gains but, at a bigger cost. This is th' reasoning that influenced my ultimate vision of my "blue oni" (70cc mtb). I intended to run just a single carb to get th' bike functional.............after break-in..........THEN install th' second carb!:cool: This build I wanted to imply HIGH concepts.......LOW dollars. Th' stock carb is 12mm( i think).......th' stock cyl head is roughly 20mm already............add TWO 12mm carbs.......you get 24mm! :grin: Should work just like a 4bbl.......use one carb for low speed/ cruising ( primaries) , second carb .........high speed/ power-blasts (secondaries). The only crux of th' matter was th' intake manifold .................which I just recently solved.........use a pocket bike DUAL EXHAUST!!! Cut off what you need.......NO WELDING. CHEAP. CHEAP. CHEAP.:D Besides, I got a bunch of extra carbs and manifolds left over from failed happy times ........why not recycle?:rolleyes:
     

  2. 5-7HEAVEN

    5-7HEAVEN Guest

    :cool:Brian, two 12mm carbs equals one 15mm carb.

    The area of a 12mm surface = pi X radius(r) squared = 3.1416 X (6mm X 6mm) = 3.1416 X 36mm squared = 113.0976mm squared. Two carbs = 2 X 113.0976mm squared = 226.1952 mm squared.

    The square root of 226.1952mm squared = 15.0397mm or one 15mm carb.

    This is equivalent to saying two 12" pizzas are not equal in size to one giant 24" pizza.

    HOWEVER, if each carb is rated at 120cfm(cubic feet/minute), then two carbs WOULD rate at 240cfm.

    Myron
     
    Last edited by a moderator: Nov 23, 2007
  3. jpilot

    jpilot Guest

    Sorry Myron, you're on the right track but your math is incorrect.

    You have to divide the 226.1952 figure by pi THEN take the square root THEN multiply by two to get the equivalent diameter.

    So the actual equivalent of two 12mm carbs is one 17mm carb in terms of cross sectional area.

    I agree that the flow capacity will be roughly doubled.

    Bryan, the Chinese two strokes actually depend on a slightly restrictive intake tract/carb to improve performance. Because of the huge overlap in valve timing on a 2 stroke, the piston forces gasses out of both the exhaust and intake on the upstroke. A slightly restrictive intake tract/carb provides some resistance to reverse flow and, therefore, keeps more air moving in the proper direction through the engine. Reed valves can virtually eliminate the undesirable reverse flow but at the cost of adding another restriction to flow in the intake tract.

    Two stroke tuning is completely different from four stroke tuning so the things that work on a chevy are not directly transferable to our little Happy Time engines. But hey, give it a try! Experimentation is almost always a learning experience. Even when the experiment is a failure we at least learn what won't work and if we manage to sort out why it failed we have learned something.
     
  4. HI All,
    I love the idea of dual carbs....but the one concern I would have is along the lines of intake flow capacity.....by adding 2 carbs we would need to ensure that the intake manifold is not too restrictive and can flow the volume of 2 carbs as they both come fully open....this would suggest an intake flow roughly double stock?...If so I am wondering how this would be accomplished given the relatively small intake area in the stock cylinder (maybe mild porting....I noticed that the port opening inside the cylinder is narrower than the opening where the intake manifold bolts to the cylinder jug).....as far as the need for intake restriction to help prevent intake reversion through the carb , I believe that is where the boost bottle would come in handy. Who knows?....maybe it would give some benefit by just splitting the intake manifold and adding another carb....definitely try it and let us know what you find...I can hardly wait.
    Andrew
     
  5. 5-7HEAVEN

    5-7HEAVEN Guest

    :cool:Y,know jpilot, I DID have that 17mm figure originally. When i rechecked my work, somehow 15mm was inadvertently substituted. It was late, I was tired.

    You are correct.

    Myron
     
  6. sunofjustice

    sunofjustice Guest

    Thank you 4 your input fellas! Carb operation wise.............you would pretty much cruise about on ONE carb (jetted slightly rich)...............but, when you want some more power..........you open up th' other throttle.........letting in MORE fuel and air. (probably stock jetting) The MAIN part of th' intake manifold (which bolts to the engine)...................would consist of a portion of a "happy time" EXHAUST header(1-1/4" in length. Exhaust port size AFTER DREMMELING: is 1" by 3/4" diameter). Its alot bigger than a stock intake........this would then mate (via silicone hose and clamps) with a portion of pocketbike DUAL EXHAUST PIPE (about 3" in length. TWIN PIPE diameters: both are 3/4")............the "y" portion of the pipe would then mate with TWO shortened "happy time" intake manifolds (via more silicone hose) ..............finally, hooking up to the duece carbs. Fundamentally, its just an inexpensive alternative to buying a bigger carb. :grin: Also, I thought it would match th' boy-racer look of the "blue oni"(70cc mtb). I REALLY wanted DUAL exhaust to go with it.............but it would be rather troublesome trying to route it without welding. Regardless..........th' home-made glasspack DOES sound just like a v-8. Awesome rumble!:cool:
     
  7. ChrisHill

    ChrisHill Guest

    I have always found that poor quality flywheels that aren't properly balanced, and engines that are not mounted securely cause excess vibration..not torque.
    Torque is what gets you up and moving. Trust me, you want torque...it's a good thing :)
     
  8. sunofjustice

    sunofjustice Guest

    Ah........torque gets you MOVING, and some is good.....but, if your peddling to help overcome inertia you are gonna be using HORSEPOWER alot more than torque. Th' main reason for mentioning that torque is a problem is.......I installed a bigger intake and carb(both 20mm) on three engines in two years time. Every one of these engines with this set up.........RARELY tried to VIBRATE itself silly. Throttle response was very good with SMOOTH idle as well as power. If it wasnt for trial and error ..........figuring out jetting, using 32:1 gas/oil mixes , running the engine in 20 below temps WITHOUT adding more oil............( i admit was boneheaded)( the upper bushing worn out)........these engines might have lasted alot longer.
     
  9. sunofjustice

    sunofjustice Guest

    Also....I probably should of said ...." all the restrictions are creating BACK PRESSURE.....resembling torque". A small intake on ANY kind of engine would generate alot of torque. But, the "happy-time" engines DO have fairly LARGE port openings that use SMALL manifolds..........thusly causing an imbalance. "stock is a compromise" sunofjustice
     
  10. jpilot

    jpilot Guest

    It seems that some are a bit confused about the relationship between torque and horsepower. Torque is the twisting force that an engine produces to make things go. The more torque you can get out of an engine the better....always. Horsepower is a measure of work done per unit of time. The formula to calculate horsepower is:

    Torque (in lb/ft) X RPM

    5250

    If you're pedaling to help overcome inertia you're using energy from that ham sandwich that you ate to help make up for lack of torque from your engine.

    Good torque at low RPM with a Happy Time or any engine working in a drive train that has only one gear ratio is a really good thing.
     
  11. sunofjustice

    sunofjustice Guest

    ah. sooooo youre say n that you rather zip along at TAKE OFF SPEED (which is where your torque really shines.......) versus actually attaining the " holy grail "of a respectable top speed? Besides, torque curves usually DROP OFF when your rpm increases. I'll admit.......alot of torque is fun.........BUT you can get that from a sprocket as well. And dont forget.........these engines WERE'NT designed for pedalless take offs. You 're HELPING the engine by pedaling that little bit. NO offense......but I'm NOT lazy. And I DONT eat ham sandwiches.....ITALIAN SUBS.......now thats CHOICE! Also, lots of torque works BETTER in CARS........until your tires keep breaking loose OR you wear them down after a short time. Torque is great......but HORSEPOWER is greater. IF it wasnt more useful.............folks would advertise it MORE! What do you look at first (other than price,displacement,or stroke orientation) on any potential engine you want to use????????? HORSEPOWER.HORSEPOWER.HORSEPOWER. 'Nuff said.
     
  12. 5-7HEAVEN

    5-7HEAVEN Guest

    :cool:Sorry to disagree, Brian, but

    TORQUE is king, especially when trying to push a 250-pound loaded bicycle uphill!:shock:

    FWIW, if we played at or below our engine's maximum torque range rather than its maximum hp range, our engines would last MUCH longer.

    Myron
     
  13. sunofjustice

    sunofjustice Guest

    5-7, Ill admit riding slower equates to lasting longer..........but these engines ARE kinda weak to begin with.........aluminum ISNT very stout ..........particularly the thin chrome lining for the cylinder.......the SOFT piston........and other pertinent nomenclature that I refuse to elaborate on at this particular time;) I'll STILL have to disagree......remember.......these engine kits ARENT designed for PEDDLE LESS takeoffs. Alot of Torque .....in this application ........loosens .........parts very easily as well. There ARE only four, 10mm nuts and bolts holding everything in place, no?:rolleyes: And ........we ARE riding on only TWO wheels with a contact patch roughly the size of a leprechaun's shillelagh.:grin:
     
  14. sunofjustice

    sunofjustice Guest

    Also, once (or maybe if?) I get this carb combo working.............I'm just gonna hover around 18-20 mph with this bike. Until I find Something BETTER than the "happy-time" engine. I PREFER 30-40 mph..........but NOT with this engine. I COULD try the more upscale 2-stroke oils.........but I dont like the "fickle" nature of these china-fires. Either my pocketbike/ happytime solution..........or a new frame mount, 4-stroke kit will do. G'day, mate
     
  15. 5-7HEAVEN

    5-7HEAVEN Guest

    :cool:Brian, I believe a frame-mounted GX50 HONDA or 2.8hp TANAKA 47R w/ improved WHOPPER STOPPER kit would fit your bill.

    Me? I'm stuck on my modified 47cc pocket bike engine with either STATON drive or "RODGER BOX" 7-speeds.:shock:

    Myron
     
  16. sunofjustice

    sunofjustice Guest

    Thank you, myron.......I already considered the ghx50 honda .......I STILL like the thought of japanese engineering. But, the over all size is a concern. I'd put it on a beach cruiser (one of two more projects I'd like to do) where you got plenty of room to work with. The Grubee gearbox was SUPPOSED to have been redesigned .........I think. Plus, arent they centrifigal ? Give gas go of some sort? Ill have to do a site search.......but......I dont see too many folks USING this setup. Staton has some pretty amazing stuff.......but......their gear-boxs are PRICEY. Unless, you buy a cvt or torque converter..........WHICH.....requires a 20" wheel. I wanna rock a 26" wheel . Pardon my naivete, but what is a "rodger-box"? Hope its cheap and doesnt require welding.
     
  17. sunofjustice

    sunofjustice Guest

    Sorry, about getting OFF TOPIC.........but I got a vision of dual-carbs working flawlessly........and a failed engine really puts a monkey-wrench in da gears. I just want to FINISH my project. GRRR!
     
  18. Hi Son....any pics of the dual carb set up?
    Thanks
    Andrew
     
  19. gone_fishin

    gone_fishin Guest

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