correct carb size

Discussion in 'Performance Mods' started by jaguar, Mar 31, 2013.

  1. jaguar

    jaguar Well-Known Member

    According to the formula for correct carb sizing the 66cc engine with 5500 rpm peak (normal piston port intake) should be equiped with a 17mm carb. If it has the same carb as the 48cc then it only has a 14mm carb. The cross sectional area of the 14mm carb is 32% less than that of a 17mm carb. But if you get a bigger carb then you have to fab your own intake manifold also because the standard manifold is only around 15mm inner diameter.

    ps- the 48cc at the normal 5500 rpm only needs 1mm more in carb throat diameter so it's not as far off as it should be unless its ported to rev much higher.

  2. jaguar

    jaguar Well-Known Member

    If you switch to a reed valve intake then a 66cc engine only needs a 16mm carb. Lukily Rock Solid Engines (who sells a good reed valve) is kind enough to fab the intake size to what you want. So you can buy a bigger carb (Dellorto PHBG or Mikuni VM) and when ordering the reed valve have them make the intake clamp on size to match the carb.
  3. jaguar

    jaguar Well-Known Member

    installing the right size carb

    I think all these Grubee engines come with a 14mm carburetor.
    According to the formula for calculating correct carb size to prevent power loss at peak rpm the 48cc piston port intake engine at the standard 5500 rpm needs a 14.6mm carb. So the stock size isn't far off. But the 69cc needs a 17.5mm. That provides a cross sectional area of 240 square millimeters. The standard 14mm carb provides only 154 which is 36% less. That may be translating to a 36% power loss. Not good. The solution? Well you could use an 18mm carb but the standard intake manifold has only a 16.5mm inner diameter which equates to you only having a 16.5mm carb. But still it's better than using a 14mm carb. (Don't use the billet intake which has only 13.7mm I.D.)

    So you could use a 16mm Mikuni (order with #75 and #80 main jets) for a 30% area increase. ($85 from Treatland) The Mikuni isn't hard to use either since it's clamp on size is 20mm. Just use a 3/4" (I.D.) heater hose from the auto parts store and cut a section 1" to 3+3/4" long, depending on how much you want to enhance low end power. I think all 48c engines should have the long intake length for more grunt. It causes more intake charge to enter the cylinder by a resonance effect. (read more about it) All that you would lack is a pipe 20mm in diameter 1.25" long to clamp the carb to and then fit it inside the heater hose. (available here) I prefer to use the longer offset intake and connect all with a 2.5" hose. That way there is maximum extension but minimal carb movement on bumps. (sources: for 32mm bolt spacing, for 40mm bolt spacing)

    Now if you have raised the exhaust port a bit and lowered the intake port a bit you are getting around 7000 rpm if your top speed (on 26" bike) is 30mph. Too small a carb won't limit hardly any of the top speed on level ground, just the ability to reach it quickly or to maintain it going up a hill. So the correct carb sizes at that engine speed for a 48cc and 69cc are 16.5mm and 19.8mm. So the previous 16mm Mikuni solution is viable for the 48cc but not for the 69cc. You will need a custom intake manifold. With the help of a machine shop that isn't hard to do. Use fiberglass cutting discs with your rotary tool and cut out the flat plate from a one speed front bicycle (20" bike) sprocket. The center of the sprocket is the right size for the intake hole. Then cut 1" off a 1" diameter pipe (25.4mm) and have that welded to the plate. Drill the two mounting holes into the plate. Then use a 1" heater hose to connect to a Dellorto PHBG 21mm carb ($70 from 1977mopeds)
    I'm not sure about the right jets to use in it since I haven't tested it yet. You'll also need a $10 choke lever and $7 choke cable.

    Here is a $52 Dellorto copy that requires a 32mm filter.
    Here's a picture of my 48cc engine with 14mm Dellorto:

    Attached Files:

    Last edited: Apr 4, 2013
  4. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    The Rock Solid reed valve is sized for about a 25 cc engine so putting a
    larger carb on it is kinda pointless.

    The Arrow Motorized Cycles reed valve is correctly sized for a 66 cc motor.

    Arrow Motorized Cycles will also fab intake plates to fit any size carb
    for their reed valve.
    Arrow Motorized Cycles has a very large intake on their race bike using
    a 26 mm Mikuni VM
    if you have room Arrow Motorized Cycles also makes a straight intake
    as opposed to the angled one.
  5. jaguar

    jaguar Well-Known Member

    From my measurements the Rock Solid reed valve has around an equivalent area equal to a 15mm carb so they are just right for the 48cc. The Arrow has a large flow area but it changes the crankcase compression ratio from pathetic to absolutely horrible. It does that by allowing more "crankcase" area between the cylinder intake and the reeds.
  6. jaguar

    jaguar Well-Known Member

    Here's the required carb sizes for use with a reed valve:
    48cc at 5500rpm: 13mm
    48cc at 7000rpm: 14.7mm
    69cc at 5500rpm: 15.6mm
    69cc at 7000rpm: 17.6mm

    For all but the last setup the Walbro 15.5mm carb will do. JNMotors sells it with or w/o reed valve.
  7. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    cylinder intake and the reeds??? Arrow's race engine is case reed inducted.

    Your theories sound good but lets see how well your set up works in the real
    world on the race track.

    Less talking, more racing!!!

    Is JN Motors going to be at the April 13th race with your motor, CDI and exhaust?
    Only 8 days to go!
  8. jaguar

    jaguar Well-Known Member

    >Arrow's race engine is case reed inducted.
    yeah but what people commonly think of when you mention their reed is not their special and costly setup but rather what they commonly sell.

    >Your theories sound good but lets see how well your set up works in the real
    world on the race track.
    I agree. let's see.

    >Is JN Motors going to be at the April 13th race with your motor, CDI and exhaust?
    Only 8 days to go!
    His racer has been in the hospital and may not race. I suggested they see how he feels during practice. If he's ready to stuff dorks into the weeds then he's ready!
  9. jaguar

    jaguar Well-Known Member

    A little analysis here. Mid turn speed on a track is usually half the straightaway speed. So which motor do you think is superior? one that spreads the power out from mid rpm to top rpm or one that runs like an old dog until it gets to within a 1000 rpm of peak power rpm?
    the one that spreads the power out will have usable power coming out of the turns. the other will be pedaling like a schoolgirl to help it get up to speed where the powerband will finally kick in.
    TORQUE will win the day

    OK that's enough on this malarky. keep the thread pure. start another one if you want to debate correct engine design.
  10. Fabian

    Fabian Well-Known Member

    Purchasing parts from Arrow Motorized Cycles seems to be fraught with difficulty as they do not answer their emails. I've tried numerous times with similar result.
  11. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    Fabian, I will try and say this politely, if you want emails answered from Arrow
    you had best pretend to be someone else and the same goes for trying to buy
    anything from them. :D

    Arrow Motorized Cycles answers their email and phone.
  12. Fabian

    Fabian Well-Known Member

    Arrow Motorized Cycles is a vendor selling parts.
    I simply want to purchase some of their items to test - how hard can it be?