It's no secret that a pumped glow engine runs better and it will have
no fuel tank location restrictions. But simply sticking a pump on your glow engine creates
other problems. First and foremost, you can't use a pump on an "air-bleed" carb.
The pump will flood the engine at anything less than full-throttle. So forget the
air-bleed carb application. Then there's the two-needle carb. This is a lot better because
the low-end needle will control the pump pressure to some degree. BUT at mid-throttle the
low-end needle is not functioning, thus the pump runs rampid supplying full-throttle fuel
at half throttle. Needless to say you'll have a very RICH running mid-throttle.
To solve the problem, most pilots insert a "T" between the carb and pump, with
the extra line going back to the fuel tank VIA: oneway valve. This fuel loop (if you will)
lowers the pressure and allows the carb to draw fuel as needed. This method works OK but
the engine has to "suck" the fuel in from the point of the "T". The
farther the "T" is from the carb, the harder the engine has to work to get fuel
thereby it looses some horsepower.
To prove this, I installed a Perry Pump on my OS 1.20 AX engine (in my Super Skybolt). I
first connected the pump directly to the carb and ran it. The topend was supreme pulling a
15x8 prop to 10,100 RPM, mid-throttle was blubbery rich and almost died, while the idle
was good and stable. There was no adjustment that would stop the mid-throttle richness and
still retain the great topend. I then added the "T" connector half way in the
fuel supply line. The mid-throttle was perfect, the idle was perfect, but my killer topend
dropped to 9650 to 9700 RPM. There was no adjustment that would bring back the 10,100 RPM.
While it was running at full throttle I pinched off the bypass line and the RPM
immediately jumped back up to 10,100 RPM again. So, the engine was having to work hard to
get adequate fuel supply at full throttle. I moved the "T" connector right next
to the carb and tried the tests again. The mid-throttle was still good, the idle was still
good, and full-throttle came up to 9950. My conclusion is: the "T" needs to be
as close to the carb inlet as possible to get optimum fuel delivery. There's still a
performance drop as indicated by the slightly lower RPM. It's not much of a drop, but its
there none the less.
Having this knowledge, I decided to get my 10,100 RPM back one way or another. I closely
studied the carb and concluded I could insert a by-pass line right at the topend needle.
And that's precisely what I did. I drilled the carb with a .0915" drill and pressed a
.093" aluminum tube just before the topend needle orifice.
Here's how it did: The idle was perfect, the mid-throttle was perfect,
full-throttle... 10,050 to 10,100. Where there's a will, there's a way.
The Low Pressure Return line still needs a oneway check valve. You may
notice in the photo, that there's a smaller fuel line on the return. This is becasuse the
majority of the fuel is used, and the smaller line generates slightly more back-pressure
at the carb.
This is the VP-30 Fuel pump. It's made specifically for glow fuel and is
very effective. The nipple on the left is the crankcase line, the ones on the right are
fuel in & out. |
 This is a simple setup but very
effective. |
Here is an OS 1.60 FX set up with a pump & bypass.
This engine runs extremely well with this setup. I found it best to set
the Perry pump to maximum flow for this engine as it needs lots of fuel for topend. The
engine uses 2.2 ounces per minute and the pump at full rate is 3.0 ounces per minute so
there's not a lot of fuel being returned at full throttle!
The really cool thiing about pumped engines is, you can put your fuel tank
anywhere and once its tune up, it stays tuned up all the way to the last drop of fuel.
To get optimum efficiency from your pump its best to have the pump line
(on the bottom of the pump) as close to the engine as possible. In other words, keep that
line as short as possible. |