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: