Re: Throttle While Cranking

Posted by Ron in Nebr on February 24, 2005 at 18:22:00 from (65.169.97.55):

In Reply to: Throttle While Cranking posted by PAULIH300 on February 24, 2005 at 14:15:24:

If you ever get a chance, hook a vacum guage up to a regular automotive type engine. You'll notice that it has high vacum at idle(throttle shafts shut), but if you crack the throttle wide open the vacum will drop off to almost nothing. The pistons going down "suck" a certian amount of air into each cylinder and with the throttle blades closed, it creates a vacum in the intake. Throttle blades open past a certian point, it's basically atmospheric pressure doing all the work. On an engine running wide open, the pressure in the intake tract will be equal to atmospheric. Which is why an engine will have less power up in the mountains than it will at sea level. A turbo creates a positive pressure. Anyway, sorry for going into "engine theory 101"...back to the original question.

A tractor engine is the same way- throttles closed will have a fairly high vacum in the intake system. Throttles open and it wont hardly pull a vacum at all.

So, why will a carbureted auto engine start up with the throttle open? Because an automotive carb has an accelerator pump that squirts a charge of gas into the manifold. On a tractor, with an updraft carb and no accelerator pump, the only thing getting gas out of the float bowl on the carb is the vacum created by the pistons going down in the cylinders. Open the throttle, lose the vacum, and- no gas. Unless you have the choke shut, which will do the same thing and usually give even MORE gas since it shuts off the intake tract on the upwind side of the carb.

Sorry so long-winded!

Follow Ups:

• Re: Throttle While Cranking PAULIH300 18:29:32 02/24/05 (0)

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