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.

Bigger Tire

Can I get a few MPH out of a bigger tire and will it fit?

Stock Tire, a spare rear rim and tire.

Trying a 2.5 inch tire:

Stock tire has a circumference of 58.25 inches. This tire has a circumference of 61.25 inches.

At 10,000 RPM the stock tire would give us:

38.79 MPH = 10,000 RPM * 58.25 / (14.220 overall engine gear ratio * 12 in/ft * 5280 ft/mile) * 60 minutes/hour

The bigger tire would give us

40.79 MPH = 10,000 RPM * 61.25 / (14.220 * 12 in/ft * 5280 ft/mile) * 60 minutes/hour

This tire would be good for roughly 2 MPH gain, assuming the motor has the power.

Tire is from Treatland.tv. 

Motion Pro spoons to put the tire on. These Moped tires are very easy to put on compared to changing dirt bike tires. I have even done them with my plastic bicycle spoons.

Mocking it up on a spare engine, it's really close.

Really have to think about where the magneto wires are going to run.

Looking at the scooter, I don't think it will work with the back fender on.

Back fender is already tight on the stock tire.

I did have to modify the rear fender a little bit. The back bracket was easy, I just bent it a little bit and bolted it down at a different angle. The front, I re-drilled the location of the bolt for the front bracket, to move the fender up and over one of the ridges on the motor. It clears the back tire, not by much, but it does clear.

Not a lot of room to spare, but after weeks of running this, it hasn't ripped the back fender off.

There was a big benefit. The Honda Express was really sketchy at 50 MPH (Yes, I have the poor little motor spinning over 13K, it's not making much power up there, but enough.) Putting the bigger back tire on it helped. The 2.5-14 tire only raises the back by about an inch, but I think that must put a little bit of pressure on the front wheel, since the stability at high speed improved significantly. This makes me want to find a shock with adjustable sag, that I can experiment with.

Also, some later Honda Express's came with a 40 MPH speedometer rather than a 30 MPH speedometer. This is a direct drop in replacement. With the bigger tire, the GPS and speedometer are pretty close. Even at speeds beyond 40 MPH, the speedometer seems behaved and you can tell whether if you are running 45 or 50.

Faster

I knew this motor felt good.

I didn’t want to GPS it until I got a different spark plug. It was detonating with the protruded plug. Still stock gearing, getting all the miles per hour out of turning higher RPMs. It’s going flat on top, not fading away, which likely means I need to drive more heat into the pipe. So I will retard the ignition to drive some heat into the pipe. I have a bigger rear tire which should give me a couple MPH.

I am running a B8ES plug with an Airsal head, premium gas, I can still hear some occasional detonation, but with the switch to premium gas and the B8ES, I am not seeing evidence of detonation on the plug electrode. I have B9ES and B10ES plugs to try. Temperatures on the head are peaking at about 386F, which I know is high. I am using a Trailtech head temperature gauge from treatland.tv to measure the head temperature. I think I am going to experiment with E85 anyhow, so I will leave this combination.

Athena Kit from Treatland

Putting the Treatland Athena kit on a motor.

Putting a points ignition on the motor for compatibility with the 6 volt system 

Carburetors

The original 12mm Keihin PA carburetor on the NC50 (shown on right), really was problematic for me. The pressed in main jet really limits any easy tuning. The carb didn't respond well to temperature and density changes, so much so that I zip tied a small screwdriver to the NC50 so I can readjust it on days that it didn't want to idle well.

I moved onto the Dellorto SHA 15.15, middle. It does have replaceable main jets and has been much better. The idle air/fuel mixture, you have to adjust by replacing slides, which is unique to my experience. Treatland, http://treatland.tv, has an adapter that will let you bolt this carb directly to the NC50 stock intake. This makes it a very easy swap. The large 60mm air horn on the carb limits the choices in air filters, but again, Treatland has some options and I am using a Mallosi air filter from their site.

I have purchased a Mikuni 20mm VM round slide carb and a Weak Ends intake to use with it. Again, both from Treatland. My suspicion is that I will like this carb since it is similar to many of the Motocross VM carbs that I worked on in the 80s and 90s. The Mikuni has a 43mm air horn.

Right now, I running about 10,000 rpm at 38.6 mph. If we use the Jennings formulas for carb sizing, and the K-Star 44mm cylinder I am using, safe carb bore size comes out 19.6mm.

Starting Port Molds

Starting to make some molds of the ports on the stock cylinder, so that I can take measurements.

Seal off the inside of the cylinder. This is a stock 1978 NC50 cylinder.

I am going to have to do this in two steps, since I have to set the cylinder at an angle to fill the exhaust port. The transfer ports I can lay the cylinder flat. Grease the inside of the port:

Microwave the molding compound. In this case, I am using Composimold:

Pour into the exhaust port:

Fill it up:

On the left is the Composimold exhaust port from the stock NC50 cylinder. I get Composimold from Amazon, http://smile.amazon.com/ComposiMold-FC40-Reusable-Material-40-Ounce/dp/B0096BSQ2S/ref=sr_1_1?ie=UTF8&qid=1438548890&sr=8-1&keywords=composimold. I used to use another molding material, that was a reusable molding compound available in different stiffnesses. It was like a silicon rubber, but turned to liquid in the microwave and reverted back to rubber state quickly. An example of this is the ZXI 1100 cylinder head mold on the right. I still have some, but I am using it sparingly since every time you use it, you introduce some contamination. I don't like the Composimold that much, but it is readily available.

Now the transfer ports.

From the port mapping, I get a rear transfer port width of 18.54 mm as a cylinder segment width. Converting to chord length, this is 17.88 mm. Measurement of the mold, I get 18.02 mm.

From the port mapping, I get the magneto side transfer port width of 19.20 mm as a cylinder segment width. Converting to chord length, this is 18.47 mm. Measurement of the mold, I get 16.51 mm.

From the port mapping, I get the clutch side transfer port width of 19.24 mm as a cylinder segment width. Converting to chord length, this is 18.50 mm. Measurement of the mold, I get 16.67 mm.

From the port mapping, I get an exhaust port width of 29.42 mm as a cylinder segment width. Converting to chord length, this is 26.83 mm. Measurement of the mold, I get 22.02 mm.