Gas Tank Battles -- Round 2

The part of my NC50 that I have had the most problems with is the gas tank. After I got the NC50, I pretty quickly discovered that it would leak gas into the oil tank. The oil tank is just separated by a piece of metal inside the gas tank and my understanding is that it is fairly common for there to be leaks between the two. I disabled the oil pump and went to premixing the oil. I have always been a fan of premixing the oil, because it simplifies jetting when you don't have the oil pump doing its own thing to inject oil at its own rate. However, the oil site started leaking gas. No surprise, since the rubber seal is 40 years old and we weren't putting ethanol in the gas back then. Also even when I was running with the oil pump, the oil cap tended to want to pop out, even though the vent appeared to work by blowing through it. So I bought a second gas tank, punched holes between the gas and oil chambers, rust treated the tank and sealed the oil site with a supposedly gasoline resistant glue. Slowly, the gasoline ate through my gasoline resistant glue. I machined a interference fit aluminum plug to replace the oil site. This solved the oil site problem.

Then the gas cap fell off one day. The stock gas cap just has these two sheet metal tabs that hold it in place and I had damaged one of them. So I found a different design Honda gas cap with an on/off switch, that fit the tank, but had a much better holding mechanism. After several heavy rains, I found that water was getting into gas tank. Not much, but I don't want water in the gas. My thought is that it was getting down through the On/Off switch. I switched to an NU50 gas cap which doesn't have the On/Off switch. Unfortunately, this gas cap appears to pressurize the tank since now fuel weeps out of the oil cap.

Now I have switched back to the second, better designed, On/Off gas cap and I built interference plugs for the oil site, the oil cap and the oil tank outlet. I had a automotive vacuum cap on the oil tank outlet, which was working, but I could feel it getting softer and I think the gas with ethanol was taking a toll.

Here are some pictures of making the interference plugs for my spare tank. I will test it for a while and then do the original tank.

Here you can see the bubbles in the "gas resistant" tank glue that I tried to use seal the oil site. The gas was leaching around the oil site rubber and interacting with the glue.

The spare tank gas cap flange is not in the best shape. Somehow, I need to work on straightening it.

A view down into the oil tank.

A view of my vacuum plug that I used to seal the oil output.

Oil site with the glue removed.

I am going to order a new petcock. The one that is on the spare tank is an original Honda petcock. Most aftermarket petcocks come with a filter tube that is too long for the NC50 tank.

Oil site hole diameter is about 19.89mm.

Oil cap hole is about 23.77mm.

I just machined an interference fit plug with a little bit of a lip on it to act as a stop. Cleaned both the plug and the tank hole. Put some gas resistant epoxy on the plug and pounded it into the tank.

Another picture of measuring the oil site hole.

Close up.

Same thing here. I machined an interference fit plug with a lip on it. Put some gas resistant epoxy on it and pounded it into place.

Big rubber hammer to get it into the hole.

Finally the oil outlet.

I took a piece of bar stock that was pretty close and machined a taper on it.

Tapering the bar stock in the lathe.

Finish plug.

The epoxy I am using:

Setting the plug in place, ready to pound it in.

The oil outlet plug pounded down into the tank.

Tank is ready other than I am waiting on a new petcock. We will see how this tank works out.

After several weeks of running this tank. I am pretty happy. My NC50 has to sit outside, so I have to worry about rain getting into the tank. I have switched back to the NU50 gas cap and even though it seems to pressurize the tank slightly, everything is staying sealed.

I plan to modify the original tank the same way.