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ORIGINAL: Rubber 21
I have put the carb on the bike and its starts right up no problem and idles.That is with the jets that came with the carb.I might need to put a smaller pilot jet?
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Well, you can try driving it. There's a common misconception about carbs and that includes with hot rodders of cars. Bigger isn't better unless you have more cubes and/or want to operate at higher rpms.
A carb's operation is about velocity in the venturi. (the opening where air flows through.) Think of a soda bottle that's almost full. You blow hard across the open top and you'll draw liquid up and into your airstream. The faster (higher the velocity) of the blown air, the more liquid you pull.
A smaller venturi will give you a higher velocity (a given amount of air will have to travel faster to go through a smaller opening) and pull "harder" on the jets.
The jets get their "signal" (vacuum) to give fuel based on the velocity of the air flowing through the carb. (Through the venturi.)
You can google all of this, but the point is that the bigger your opening/venturi the more air you will have to pull to keep the velocity up. You can get there with more cubes and/or more rpms.
Too often in the search for more power a newcomer will buy a bigger carb. Unfortunately the low end punch is lost. The engine bogs upon sudden wide open throttle.
For normal street driving of the type that won't excite the cops into action, smaller may be better. You can use the smaller venturi for more velocity, and then increase jet size to add more fuel.
When a fluid, in this case air, increases in speed, it thins out. This is what creates the low pressure area to pull the liquid out of the neck of the pop bottle and it's what pulls fuel out of the jets. It creates what we call vacuum although it's not true vacuum but rather reduced pressure.
At a given modest rpm, your engine can displace just so much air and that will travel through the carb venturi into the engine. The smaller the venturi the faster the air will travel and therefore the lower the pressure. Lower pressure is like vacuum; you might think of it as "sucking" on the jets.
An oversimplified and incorrect but useful way of looking at it is this. If you take a deep breath the air will come in at a given speed. If you could take in that same amount of air in the same time span but through a drinking straw, the air would have to scream through that straw. The pressure in your mouth would drop, feeling like a vacuum because of the restriction of the straw. If you had "jets" in your mouth with a liquid behind them, that liquid would be pulled on hard. Not so without the straw. Making any sense here?
Anyway, give it a try. I think you're better off getting a smaller carb or even just tuning the stock one but you can see how you do.
JB
Putting a Mikuni Hsr 45 on a standard Tc88
