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Checking for Gas

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Driving along in your car or truck, it's pretty much second nature to keep an eye on the fuel gauge to make sure you don't run out of gas. Gas gauges aren't terribly accurate, but at least you have a feel for how much farther you can go, and some modern gauges leave even less to chance, sporting digital readouts that tell you exactly how many miles you can go before you're on empty. But what about old engines with no modern technology helping out?

Some engines are easy to judge, with external fuel tanks that you can agitate, listening for the presence of gas, or you can simply remove the fuel cap and peek inside to see how much fuel is left. But not all engines are so easy, as this thread shows. The thread started with the following, seemingly simple question:

Can anyone figure out an easy way to check the fuel level on a 5 HP Economy?

A straightforward question on the surface, but in the dark winter months, when cabin fever strikes, the most basic query can result in a flurry of mails to the Stationary Engine List, with suggestions ranging from the obvious to the obscure. I'll leave it to you to decide when the writers have their tongues firmly in cheek.

Here's a good tip we use out in the bush. Carry a length of small diameter rubber hose, maybe even garden hose. Poke that down into the tank (it should be flexible enough to go around any bends) then blow into it. If you hear bubbles, then there's plenty of fuel. This is not definitive, as if the fuel level is low you may not get many bubbles. In that case suck on the hose - if you don't get a mouth full of fuel, then the tank's empty. Oh, it helps to have a willing mate, plied with booze, to do these tests - that way he won't notice the after taste.

A variation on this may be to use small diameter clear plastic tube. Put it into the tank down to the bottom, cover the open end with your finger and then draw the tube out of the tank. The fuel level in the tube should be the fuel level in your tank.

I'm thinking of a device modeled after a tympanometer, which is used to determine if there's overpressure, vacuum or fluid behind the eardrum. This'll actually be simpler than the tympanometer, because all we have to do is measure the air volume in a closed space.

Put a port in the tank closed off by a small speaker driven by an amp of known, constant power. It'll be putting out a very low frequency at fairly high volume so there will be a lot of excursion of the speaker cone/ diaphragm. Next to it in a branch port is a pressure transducer/ microphone that measures the pressure fluctuations in the enclosed space produced by the known air displacement of the speaker cone, which is acting like a piston in a cylinder bore. This acoustic impedance is directly related to the air space in the tank, which gets smaller as the tank fills. (Can't fill it to the transducers!)

You can do lots of calculations to predict the results at different volume levels, or better, just calibrate it by running it with different known volumes of gas poured into an empty tank.

Another workable alternative is to affix a simple whistle to a port in the tank, with the whistle bore opening into the tank. When the whistle is blown, the frequency produced will be a function of the air space remaining in the tank, which is a Helmholtz resonator.

You certainly have laid out some nice technical solutions to the gas tank problem. A solution that came to mind as I read yours is to use a piece of clear vinyl tube, insert it into the gas tank with a loop outside hanging below the tank and with the end higher than the tank. Suck on the tube until gas appears and is lower than the tank. Then, watch as the gas climbs to the level of that in the tank. If a depth marker with volume equivalents is affixed to the outside of the engine, the volume of gas in the tank can be easily determined. The calibration of said marker is simple. Put a known amount of gas into an empty tank, enough to start the siphon level process, then, without removing the siphon, keep adding known quantities of fuel and placing corresponding marks on your depth marker.

This is getting pretty serious. Why not just weigh the engine periodically? Then add fuel till the weight is back up to where it should be.

Four methods checking fuel level come to mind:

1.) Use a sight gauge. That's what I've got on the Hoyt-Clagwell.

2.) Put a petcock close to the top of the tank. When filling the tank, open the petcock. When fuel comes out, the tank is full. That's what I've got on the Fairbanks-Morse Type T.

3.) Take off the fill cap and tip the engine over. Angle of tip is inversely proportional to the amount of fuel in tank.

4.) Fill it up every time you look at it. If it doesn't start, fill it up. If it rains, fill it up. After taking out the garbage, fill it up. If you sneeze, fill it up. If your wife hollers at you to quit fooling around with that infernal, dad-blasted, dirty old junk and come in to dinner, fill it up.

My Hercules S has the type of filler tube with two 90 degree bends. I was sure there was plenty fuel in the tank when I wanted to fire it up yesterday. Here are my comments concerning your suggestions for checking fuel levels.

1.) My Hercules wasn't designed with a fuel gauge, and none of the previous owners added one. I won't either.

2.) See #1.

3.) I tried that, but water spilled out of the hopper and almost hit the filler tube. Bad idea.

4.) Great idea, but I  really wanted to check out that check valve anyway. Clean the plug, remove and replace the needle valve, then back it way out and try to flood the engine. What?? There was some gas in the line above the check valve. Still, it wouldn't hurt to top off the tank just to be sure. The sound of gas dropping to the bottom of a tin tank told me I had just been playing with my Hercules for the last half hour, not really trying to start it.

Amazing, just a little gas in the tank and she fired right up. At least my Cushman X has a straight pipe that I can use to check the fuel level. I reckon I will just always top off the tank on the Hercules before I fire it up.

How about long strips of cardboard, or an old, metal, flexible dipstick with something on the end to show liquid level? Maybe some paint or something semi-absorbent that will show the liquid level but will dry out quickly to do other checks.

Don't think it'd work on a Hercules. There's that first 90 degree bend that's pretty sharp - and then it would still have to fall down into the tank.

How about a leather thong with a nut or large split shot on the wet end?

Here's an interesting approach that worked for a friend of mine who lives in West Virginia. He drilled a series of 1/8-inch holes 1-inch apart in the side of the fuel tank casting starting near the bottom. He could tell the level in the tank by determining the topmost hole that had fuel running out. Hope that helps ...someone?

I've got it! A small round cork like you use for fishing. Tie a string to the cork, put the cork in the tank and let the string hang out the filler hole. As the fuel level goes down the string gets pulled into the tank. Calibrate the string accordingly.

I should head for the shed.

Seeing as how this is an enclosed tank, possibly we could design a little hammer that would go up and down the tank, tapping it. Or I could blow across the filler and listen to the sound ... like the little brown jug. You think maybe I should get a tank first?

With a bit of luck, by the time you read this groundhogs around the U.S. will have emerged, failed to see a shadow and spring will be on its way, a harbinger of warmer sheds and an approaching engine season. Here in England, where we don't have the luxury of groundhogs, we'll just have to keep checking outside!

Engine enthusiast Helen French lives in Leicester, England. Contact her via e-mail at: You can join the Stationary Engine List on the Internet at:

'I'm thinking of a device modeled after a tympanometer, which is used to determine if there's overpressure, vacuum or fluid behind the eardrum.'

'I've got it! A small round cork like you use for fishing-Tie a string to the cork, put the cork in the tank and let the string hang out the filler hole. As the fuel level goes down the string gets pulled into the tank. Calibrate the string accordingly.'