Wednesday, February 2, 2011

The gas tank looks good, but it's so SMALL!

I won't be using the gas tank that came with the bike because there's a large dent in it and it's full of crud, not to mention a paper wasp nest that is about four inches across...but I will be replacing it with a unit that is as identical as I can find.

Being such a tiny tank, this bike won't travel very far between stops. Having said that, those frequent stops might not be a bad thing, considering the marked lack of rear suspension...

I doubt I'll be riding this bike distances where that will come in to play, but if I ride it often, I don't want to worry about filling up the tank each time I hop on. Last night I was pondering this, and think that I may have come up with a solution. The theory seems sound enough to me; I'll just have to test it out later.

Here's my thinking:

My SV650 has a vacuum operated mechanical fuel pump; it's "powered" by a vacuum line that is tapped into the throat of one of the carbs. The carbs on the SV are actually HIGHER than the lowest part of the gas tank, so gravity can't be used to pull fuel from the tank. The pump only "runs" when the engine is running, and it doesn't have to push the fuel very far "up" against gravity.

I won't be using the oil tank that's currently in the bike. I'm taking inspiration from another builder and will be mounting a cylinder-shaped oil tank to the front down tubes. It'll look much like this:

So now I'll have a big open space under the seat, much like his bike. Not only that, but the mounting of the battery doesn't flow as well. Why not turn the oil tank into another three quarts of fuel capacity? Great, right? I can run a 1/2" ID fuel tubing down from the top tank to the oil tank. Then run a 3/8" ID "burp" line from the oil tank up to the top inside of the top tank so that it'll fill faster. So far, so good, right?

The challenge is to get that fuel from the oil tank to the carb. That's where the SV's fuel pump comes into play. First, though, a bit more on that fuel pump. It's small. I mean tiny. That's great for hiding it. Here it is:

One line runs to a brass fitting pressed into the carb throat; it's a vacuum line. One runs to the fuel tank, and the third runs fuel to the carbs' float bowls.

My initial worry was that it wouldn't be able to "push" the fuel far enough to reach the carb on the Shovel. Then I got to thinking...The Shovel's displacement is twice that of the SV. Not only that, but it has only one carb feeding both cylinders. Considering this, I bet it'll draw enough of a vacuum to push the fuel that far.

So...having gone through all that, I won't really start trying to fab up a "test unit" until this summer at best--gotta have a running engine first. I'm thinking that after I get the engine up and running, I'll pull the carb off, drill a hole in the intake side throat, and press in a short brass tube. I'll run a line to the fuel pump (placed approximately where it'll be mounted), then run a line from the pump down to a small bucket or similar container that will sit where the oil tank is now. Then a line up to the carb to supply fuel.

Once that's done, start kicking the heck out of the starter to see if it will draw fuel up into the carb. If it works, well, I'll be able to add about 40 miles between fill ups!

My next concern is whether the pump's pressure is greater than what the carb's needle valve can seal against. I believe it can seal up to 1.5psi.

We'll see. Maybe.

1 comment:

  1. Great solution. That's good thinking...
    I like your idea!! Best of luck Brad.