Sophomore Rider
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Joined: Jun 9, 2013 7:36:35 GMT -5
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Post by freefour1968 on Sept 22, 2013 1:21:11 GMT -5
Hey everyone . I think I have decided on what to do next to my retro. I'm going with the 18mm carb . It's already set up for 50-70cc scoots . I was thinking about a bbk or carb . Old school hot rodder knows the more you can squeeze out of a motor the better but when you go over the design limits of the internals you can have problems . Although I have thought of getting a bbk the benifits of the low torque are off set by the lowered max rpm . Your moving more mass on parts not necessarily designed for it .Yes you can put taller gears to make up for the speed and eventually have a bigger bike so to speak but then your putting more strain on the parts than what they were designed to do. A bigger carb , better cam ( either for lower torque or higher rpm ) and a good exauhst are makeing the motor breath better and more efficient while still keeping the stresses within tolerance . My scoot does 64 on the flats with no problem add a slight decline or tail wind and it will do 76 without fail, also a slight incline or head wind and it steady at 60 . That's kph not mph lol. Adding a better/bigger carb and it will give me what I think I'm looking for . Of course this is from learning from hot ridding cars in the past and building rat bikes . What's your comments ?
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Post by JerryScript on Sept 22, 2013 3:56:20 GMT -5
A bigger carb is not going to give you much performance increase. I started by changing from stock to a larger Keihin carb, adjusted the needle and tuned the air/fuel mix, and the only improvement it gave was to prevent bogging down during acceleration.
I just finished installing my 50mm (83c) BBK, and it makes a big difference with acceleration (haven't even tuned it proper yet). I didn't take it out and open her up for a WOT ride yet, too windy tonite for a true test ride, but I was reaching 40 without going past 3/4 throttle, and kept up with cars when taking off from stoplights without having to go WOT (my main reason for the BBK).
You can prevent loss of top speed with a BBK by changing your roller weights and/or your clutch spring. It's really just a matter of experimentation to get what you want.
Ask around these forums, and you'll see that a BBK is fine on these motors, if done properly. Good luck with your upgrades!
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