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coffee
06-15-2009, 06:09 AM
Guys,
Does anyone have a formula for selecting a carbi for a 2 stroke?

random0082
06-15-2009, 06:57 AM
the 28mm is more than enough for most RSs kept as 125cc

coffee
06-15-2009, 07:05 AM
Not saying it is not, am still interested in a formula if anyone has one for a 2T. Can not find one when googling for a 2T only 4T.

random0082
06-15-2009, 07:17 AM
Find the Dellorto tuning manual (search for it online, youll find it), im sure there is some info in there

coffee
06-15-2009, 07:30 AM
Thanks, will have a look for that then

coffee
06-15-2009, 07:38 AM
Random,
Found:
http://www.ducatimeccanica.com/dellorto_guide/dellorto.html
Will read through it and see if it says anything about sizing a carbi to an engine. Either way here is a tuning manual for all.

Micah / AF1 Racing
06-15-2009, 08:28 AM
For 125cc cylinders carb sizing for peak power is based on the revs at which peak power will occur and other variable like port size and duration (or what the peak power will be).

Real GP 125cc engine use carbs in the 41-42.5mm range and make about 50 bhp at 12.5-13.5k rpm.

"customer" race 125cc machine like the Honda RS125 and the Aprilia RSV125 or Yamaha TZ 125 make peak power at around 12.5k rpm and use a carb size in the 38-39mm range.

Street 125cc cylinder be it on a 250cc twin or a 125cc single will make peak power in the 10.5-11k rpm range usually and the carb size of preference has been 34mm for some time. The RS250 Aprilia street bike and the RGV250 Suzuki street/cup bikes used 34mm Mikuni carbs.

28mm carbs still flow plenty of air and fuel, more importantly they are about right for off the pipe operation too. Notice how you can go wide open on the Aprilia RS125 at as low as 4k rpm and will just pull through the entire rev range? This will not be the case with the same cylinder and pipe combo and a 38mm carb. 28mm carbs I have used down to 50cc for true max power racing engines but these motors made peak power in the range of 13,800-15,200 rpm with NOTHING below say 11.5k rpm. 28mm carbs were used on the Yamaha RD400 and due to the design of that cylinder and the air cooled limitations it was about the right size for that engine.

This is by no means the bible, just some observations from well over a decade of tuning two strokes for street and track use. You can scale from the above descriptions to meet your goals. Unless you already have the porting tools out, a wicked pipe built and a programmable CDI something in the 38mm range will do nothing but cause you headaches.

As carb bore area goes up porpotional to cylinder volume the required tuning becomes more precise. At 38mm and 125cc a 10 degree difference in intake air temp or even a small change in humidity or air pressure will require different jetting to work properly. This is due to a weaker vacuum signal at the venturi for a given rpm and throttle position. 34mm is still fairly stable, requiring maybe 2-4 jets for perfect year round operation on a track oriented bike and maybe 2 jets (winter and summer) for a street bike.

Hope this helps.

The_Reaper
06-15-2009, 03:40 PM
D=k√c*n

where:
N = rpm at peak power
c=engine size in litres
K= a constant
(this varies between 0.8 and 0.9 for racing application and 0.65 to 0.9 for enduro)

coffee
06-16-2009, 12:50 AM
Thanks guys,
Yes makes sense and a great help, both of you.

coffee
06-16-2009, 06:34 PM
Guys,
Could you please try explain to me what the constant represents and how it effects the equation. I got 22.1mm Carbi based on 8,000rpm and a 0.7 constant.
Is the constant representing the amount of time at peak?

tekebird
06-16-2009, 06:44 PM
Micah.

Happen to have a starting point for a 34mm with the arrow exhaust.

waiting on the reeds but as soon as I get them would like to do the upgrade.

The_Reaper
06-17-2009, 07:27 AM
Guys,
Could you please try explain to me what the constant represents and how it effects the equation. I got 22.1mm Carbi based on 8,000rpm and a 0.7 constant.
Is the constant representing the amount of time at peak?



i don't know how the constant is gained. work it backwards with a standard 28mm carb and peak power at 8000 rpm.

i'll type up the section of tuning book when i get home.

coffee
09-16-2009, 04:42 PM
Guys,
Back to the selction of the carbi. Ok, this is the parts purchased so far.
VF3 Reeds
HPI Dual with software for adjustment
Polini 154cc
JM Corsa
Pnuematic Power Valve (Not purchased yet)

I was further looking to go pre mixed and be done with the pump and tank. I would be wanting to see 13k rpm though a street bike with odd track day. So 38mm looks the go. The bike is an 08 model Euro 3 spec and normally runs oil injection at carb. Has anyone else run the bike as premix, is there anything I need to keep in mind when making a selection for this. Does anyone also know of a manifold for a 38mm carb?

Stephen

ThisEndUp
09-17-2009, 06:47 AM
I would be wanting to see 13k rpm though a street bike with odd track day. So 38mm looks the go. The bike is an 08 model Euro 3 spec and normally runs oil injection at carb. Has anyone else run the bike as premix, is there anything I need to keep in mind when making a selection for this. Does anyone also know of a manifold for a 38mm carb?

Stephen

I'm not sure why you want to spin the engine that high.
If I were you, I'd tune the bike for a wider, more user friendly power band. Especially since it's primarily going to be a street bike.

There's a thread on premix...scroll down a bit. :cheers: