|
Jim W
12-25-2003 21:09:30
|
|
Re: Carburetor in reply to Paul F B, 12-24-2003 09:33:39
|
|
|
Hi Paul,
First thanks for the complement but warning! I am not a mechanic! I have learned most of my tractor stuff by translating it from what I know about cars, which I mostly have picked up over the years by hands-on trial and error combined with lots of discussion with other folks. In other words don't take my posts as gospel - weigh them in with everything else you hear.
Carbs have to do what the engine demands of them, so there are as many different carb requirements as engine designs. As already mentioned the air/gas mixture has to be just right. The carb has to be able to properly use the amount of air flowing through it. The air must be passing through the venturi (narrower area in the carb throat) fast enough to create a vacuum to draw in the fuel and atomize it, yet the venturi can't be so narrow as to overly restrict the total air/fuel mixture to the engine. Airflow through the carb depends on rpm of the engine, displacement of the engine (cubic inches), size of manifolds and porting in the head (assuming overhead cam), diameter of intake and exhaust valves, cam profile (how long is the intake valve open?), whether there is significant back pressure from the exhaust system, and probably other things.
I *think* that a carb can really only be designed to be optimal at one rpm, and at all other rpms there are tradeoffs. Someone please jump in and help me here...
At the one given rpm the airflow can be calculated, the venturi sized properly, and the needle sized to deliver the right amount of fuel. At other rpms we see (especially on a car) things like a second venturi coming into play (two barrel carb), and additional air passageways opening up to better atomize the greater quantity of fuel, etc. I suppose it's not so bad on a tractor where the range of rpms is much narrower. Notice on the 24T2 the instructions tell you to open up the fuel jet some amount if you will be doing heavy work so it's not as refined as on a car which would cope with it automatically (well cars have pretty much switched over to fuel injection now anyway).
I'm starting to ramble. My point is that each engine design is unique, and that is why there is such a vast range of carburetor models and variants.
As an example, for the carbs on my Triumph car, there are 354 different needles available. As I say, my knowledge is really more car-based but knowing this, I think that even an old Fergie must have reasonably precise carb requirements.
Sorry for the length of this but hope it helps,
Jim
|
|
|
