Why do ya have to run a fuel pump on a animal.and ya dont have to on a 2cycle.is it bc the distance the fuel has to go.i mean that would be my first guess.i was just wondering it this morning.its my first four cycle open motor besides running flatheads for a yr 12yrs ago.the clones have killed the open racing in my area.now its all small block open
Animal tilly
- Thread starter Bradley71
- Start date
The fuel pump helps maintain a more consistent fuel supply.
At idle, the pumper stack is not moving as much, which is fine until you flip the throttle blade open.
The fuel pump helps get the fuel needed for torque and acceleration.
The tilly also is pulsed only from the intake, which is every other revolution. Usually, the extra fuel pump is pulsed from the crankcase, which also lends itself to more consistent fuel delivery.
The pumper would maybe do okay after rpms are up.
Most 2 strokes idle rpm is nearly wot rpm for 4 strokes.
Hope that helps.
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ABR #69
Member
Unless the engine has a tank built on it and it's gravity fed into the carb, it will have a remote fuel pump no matter 2 or 4 stroke.
Now, the reason it's pulsed off the intake vs the crank case is that the clean pulse from the intake track vs possible oil contamination from the crank case. But, I've ran both with good results.
The oil system on the single cylinder engines without an oil pump is a splash style, meaning that it suspends the oil through the engine in sort of a vapor, pulling a pulse in that environment will pull oil in the pump. It just requires more maintenance on the pump and watching to ensure the crank case pressure doesn't blow the diaphragm, which it often will if you're running an open engine that isn't vented well.
Now, the reason it's pulsed off the intake vs the crank case is that the clean pulse from the intake track vs possible oil contamination from the crank case. But, I've ran both with good results.
The oil system on the single cylinder engines without an oil pump is a splash style, meaning that it suspends the oil through the engine in sort of a vapor, pulling a pulse in that environment will pull oil in the pump. It just requires more maintenance on the pump and watching to ensure the crank case pressure doesn't blow the diaphragm, which it often will if you're running an open engine that isn't vented well.
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I have wondered the same thing . Why couldn't you pulse off the crankcase ?
Oil volume could be a issue .
Still gotta replace diaphrams either way . Of course if it ain't broke don't fix it ..
As long as pump is above pulse port on crankcase so oil can drain back to engine, the oil is mostly a nonissue.
The tilly pulses from a small hole in the intake mounting flange. I suppose, with a little creativity, this could be connected to the crankcase.
Would this be enough?
A mikuni sbn uses similar sized pumper diaphragms, and feeds 3 carbs from 1 pump to 700 cc 2 strokes, so it should be possible to run a small block open from the carb alone.
But, as stated, no need to reinvent the wheel, as long as you are rolling along.
A lot of other things will net better power vs simply losing a fuel pump, which is relativity inexpensive.
I own sbn carbs both with, and without pumper stacks.
VKE recommends not using an external pump, even on methanol, for their sbn pumper stack carbs setup for gx390s.
Jon Brogden
Member
Sometimes Tillotson used an external pulse , like in an HR 181 , just by putting a fitting in the pulse plate and not drilling all the way through from the flange side, they stopped about half way. We used an external pulse plate when putting a 360A on a KT 100 to pulse off its crankcase . The fuel pump will work great with an intake pulse if you use a pump like the Walbro that has a return diaphragm spring to help it on its weak cycle.