Dual Carb 70cc "blue oni"

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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! :D 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:
 


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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
 
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J

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.
 
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andyinchville1

Guest
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
 
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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
 
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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. :D 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:
 
C

ChrisHill

Guest
All th' restrictions is creating TORQUE....................thusly, causing VIBRATION!!!!!!:mad::mad::mad:
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 :)
 
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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.
 
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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
 
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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.
 
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