Questions #1

Ryan M. posted a couple questions in the comments this summer:

Hi I'm setting up a very similar nc50 to yours with an Athena kit and vm20. What jetting size did you settle on for the Mikuni? On another note did you notice the massive size difference in depth of the side transfer ports compared to the casing. Did you have any issues with leaks around the gaskets here? Did you adapt your case port in any way.... there's not a lot of meat to work with there! 

• Main jet is a 172.5
• It has whatever needle it came with, I did drop it down one position to make the needle a little leaner, so it is in the fourth clip.
• Pilot jet is whatever it came with.
• The air screw, I just play with until I got the best off idle response to opening the throttle. Weather affects the idle characteristics a good bit.

The main jet of 172.5 is rich for the engine, but I am using it as a bit of a bandaid against two bigger problems, head temperature and a mild case of detonation. The head I am using from http://treatland.tv, the Airsal 47.6mm Head has small cooling fins compared to the stock. Adding horsepower, adds more heat and in an air cooled engine, much of the heat from the combustion chamber is dissipated to the head and to the surface of the piston. I try to keep the peak head temperature, as measured at the spark plug using a Trail Tech Temperature Gauge, under 400F, even on the hottest days in Atlanta in stop and go traffic. When I first put the combination together, I could hear some bits of detonation and see some bits of carbon stripping from the plug. I went rich on the main jet and went to premium fuel. I have had some readings over 400F, but for the most part it is hanging in there. It's a 172.5 because it was a 180 and then next two smaller sizes I had were 172.5 and 165. Now that the weather is cooling off, I may drop it to a 165 and see how things are. When I pull the plug to look at it with the 172.5, its black, no white areas, so very rich. My gut says that 165 will still be rich.

For the most part, the low and mid range on the carb almost useless. There is so little time on these little scooters that you are at part throttle. I find that if I am stopped, I am mostly idling, and if I am moving, I mostly roll on full throttle and hold it there.

I tend to like the pilot jet rich, since you go from full throttle, full RPM to stopped, closed throttle, which deprives the engine of both cooling air and the generous lubrication it was getting at full throttle because of the oil mix. This has always been a problem for two strokes on long tracks and on the street. Leaving the pilot rich, gives it a little bit of oil, a little bit of fuel cooling as you are stopping at a stop sign.

Over the summer, I added a second base gasket, just to try to lower the compression a little bit and raise the exhaust port. I now feel like that was a mistake, I am battling the head bolts coming loose and I am re-torquing them every couple of weeks. I think the extra base gasket just makes the whole base gasket, cylinder, head gasket, head, too spongy and it works itself loose over time. Longer term, I want to heli coil the cases and maybe go to a 7mm diameter stud.

Have I seen leaks around the base gaskets, the transfer ports? No. When the head first started loosening up, I saw sponge coming out of the metal head gasket near the exhaust pipe.

Have I seen the mismatch in the side transfers vs the case? No, but you make me want to go look at that.

Banjo Brothers Grocery Bag Pannier

There are times when I take one of my mopeds to the store, just because I enjoy riding them. For my green 1978 NC50, I have had a Banjo Brothers Grocery Bag Pannier and I recently ordered one for the 1981 NC50 and it is almost perfect for the Honda Express. This one is slightly different, but the best part is the hanger clips are a little bigger and fit perfectly on the thicker bars of the rear "Luggage Carrier" (Honda's words).

I really like this thing. I am thinking I might buy one more, in case they change it or discontinue it. Of course, this just shows one of my hoarding problems.

I bought mine from Amazon:

https://www.amazon.com/gp/product/B003D4EYL6/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B003D4EYL6&linkCode=as2&tag=edt11x-20&linkId=82fbd09deb7617675a9dc829ca939d14"

Note this is an Amazon affiliate link. I don't really expect to make any money off of this, but I just want to start understanding the Amazon Affiliate program. So this blog seems as good of a place as any...

Quick Top End List

One of my cousin's is going to do a top end on a PW50 and asked for any input I would have. I was thinking about it a bit and this is just a quick list of my thoughts and what to watch out for.

• The Honda NC50s, many mopeds and scooters, the PW50, etc, have aftermarket cylinders that are available so cheap, its not even worth thinking about the old cylinder, piston, etc. A whole PW50 top end kit, including piston, rings, cir clips, gaskets, upper rod bearing, piston pin, spark plug, cylinder and cylinder head was $49 on Amazon. And if you are mostly stock, these engines are so slow, so low compression, that can live with some cheaper components.
• One of the keys to engine life is keep dirt out of the motor. Be meticulous about cleaning everything. Flush the bottom end with some fresh Kerosene or Mineral Oil, or Vegetable Oil to try to get any loose dirt out of the bottom end. Spend time making sure the gasket surfaces are clean. In general my simple two stroke philosophy is -- keep dirt out of the motor, use high quality oil, spin it fast.
• Get a good set of appropriately sized needle nose pliers to put the cir clips in. Its worth buying a good set.
• If you mark the piston putting the cir clips on, take a fine metal file and just lightly file any high spots.
• Chamfering the ports on the new cylinder with a small file. Too many of the new cylinders I have seen have ports with sharp edges. I just do enough, so my the skin of my finger doesn't catch on the ports as I wipe past them. There are good YouTube videos on this, such as:

https://www.youtube.com/watch?v=UJbtzxtPpSs

• If its a multi ring piston, make sure you know the order of the rings, try not to mark the piston up when putting on the rings.
• Lettering, or dots on the piston rings should face up toward the top of the piston.
• Usually the piston will indicate which side should be the exhaust port side. There will be an arrow pointing to the exhaust port, or an "E" or the part number will be on the exhaust port side of the piston. If you put the piston in backwards, it will often run, but not well. Some pistons are completely symmetrical and some of those don't have any marks.
• Put the ring in the cylinder and measure the ring end gap with a feeler gauge. If its 10 thousands or less, you will where out the cylinder prematurely. It only takes a couple strokes of a file to set the gap. I have had a couple of cylinder kits for the NC50 come with too small of a ring end gap.
• Put some oil on the upper bearing, piston pin, and rings before assembly.
• Make sure the ring has the ring gap aligned with the alignment pin in the piston. It will destroy the cylinder if the gap and the alignment pin are not matched up.
• New gaskets go on dry. You don't need to do anything to them.
• If you have a torque wrench, you really want to torque the cylinder per the manufactures specifications. Then torque again, after you have heat cycled the engine a couple times.
• I usually break the engines in with three heat cycles of about 10 minutes each. The first time you start an engine with a fresh cylinder, it heats up noticeably quicker than normal. So I try to be gentle the first heat cycle.

Putting a new top end on, is something that almost anyone who has any mechanic interest should be able to do. A lot people can probably do it in under an hour, just a few things to look out for.

Two Speed Clutch

The original Honda Express, NC50, was a single speed, oil bath centrifugal clutch, transmission. In 1981, they changed to a better centrifugal clutch, a bit larger, but much lower weight shoes. In 1982 they introduced a two speed transmission. The way they did this was replace the clutch with a planetary transmissions with two clutches in the same package as the 1981 clutch.

It is a very cool little package. I hope some engineer got an award for this clutch and transmission. So much of the NC50 is really simplistic, it is surprising to see something this elegant in the engine. It reminds me of the inside of a mechanical watch.

Here's a comparison of the three different clutches, I have. The 1977 through 1980, the 1981 single speed and finally the two speed clutch from a 1982 engine:

All three of them have a 14 tooth primary sprocket. This means that the final gear ratio is the same as the single speed clutches, it just introduces a lower "first gear".

Does the two speed transmission help? Not really. I have a stock 1982 Honda Express NC50 with the two speed transmission. The first gear helps it get off the line a little quicker, but then the shift to high gear takes a second or two, introducing a lag. Then it doesn't go any faster since the final drive ratio is the same.

The next motor I am building, is the 1982 NC50 that has this clutch. I haven't decided whether I will keep the two speed clutch or switch back to the 1981 single speed clutch. I am concerned that 10+ horsepower could shred some of small parts in the planetary transmission.

Here are some more pictures of the 1982 clutch:

Steeper Gearing

The little green Honda Express NC50 will do about 50 MPH but to do this the engine has to spin about 13K RPM, which is about double the stock configuration RPM. I've also gained a couple MPH by going to a bigger tire, from a 14x2.25 to a 14x2.50. The 14x2.50 tire is about the limit, because cases protrude in front of the tire. The options at this point for more speed is either to continue to spin the motor faster or modify the internal gearing.

Inside the transmission case, from the crankshaft there is a 14 tooth sprocket on the clutch that drives a chain to a 30 tooth counter shaft sprocket. Looking at the counter shaft sprocket it seems to have enough metal to tolerate cutting all the teeth off and making a smaller sprocket.

I only have a small mini-lathe to cut a sprocket, but it looks like it is possible with patience if I buy a rotary table for my lathe.

There are a bunch of different sites that describe how sprockets are made and the geometry of sprockets. One site I liked a lot, since I could easily understand the geometries:

http://www.gizmology.net/sprockets.htm

Measuring the internal chain:

• Roller Width measures 4.75mm
• Roller Diameter measures 4.97mm, probably meant to be 5mm
• Roller Pitch varies from link to link, but the outside to roller to outside of the roller looks like 13.47mm. The actual pitch would be the outside roller to roller measurement minus 1/2 the roller diameter on one end and minus 1/2 the roller on the other end, or 13.47 - 4.97 = 8.5mm pitch.

The NC50 Parts Manual lists the chain as Part Number 23100-147-003, Chain Drive (DID270H-79L). 

Looking up that chain, DID270H:

http://www.did-coltd.com/english/products/pdf/s_102-103.pdf

• Roller Width is 4.75mm
• Roller Diameter is 5mm
• Roller Pitch is 8.5mm

From Ebay I was able to order a handful of "GENUINE DID 270H OIL PUMP DRIVE CAM CHAIN CONNECTING PL LINK RIVET". 

From Amazon and Ebay, it looks like there are a number of small chain rivet tools. I am not sure yet which to buy, or what tool is a good one.

If I cut all the teeth off the counter shaft, it looks like I have about a 75mm diameter blank to work with.

From the gizmology site:

Outside Diameter = P × (0.6 + cot ( 180° ÷ N) )

Original counter shaft sprocket has 30 teeth.

Outside Diameter = 8.5 * (0.6 + cot( 180 / 30 )) = 85.9720978609

Measuring the sprocket, I get about 84.37mm

If I do 26 teeth, that gives me:

Outside Diameter = 8.5 * (0.6 + cot(180 / 26)) = 75.1037981132

It looks like I will attempt 26 teeth.

The overall gearing ratio is given in the manual as 14.22 to 1. If we look at the stock gears, the primary sprocket on the clutch has 14 teeth:

The counter shaft has 30 teeth on its sprocket and 11 teeth on its gear:

And the output shaft has 73 teeth on its gear:

This gives a gear reduction of (14 / 30) * (11 / 73) = 1 / 14.22 or one revolution of the tire for every 14.22 revolutions of the motor.

If we cut the countershaft gear down to a 26 tooth sprocket the gear reduction would be

(26 / 14) * (73 / 11) = 1 / 12.3246753247

With a 2.5 inch wide, 14 inch tire, and assuming that I have enough horsepower to still pull to 13K RPM at 52 MPH with the steeper gear ratio, this would give a top speed of

52 * 14.22 / 12.32 = 60 MPH

Having enough HP to still pull 13K RPM is a pretty big "if". The HP is falling off pretty seriously at that RPM and friction, aerodynamic drag, effects of the oil being thrown around in the crankcase due to the increase in output shaft gear speed are increasing. Also, I am assuming the current engine will pull to 52 MPH. I know with the 2.25 inch wide tire, the NC50 would GPS on flat ground at 50.5 MPH, but I haven't re-GPS'd it with the larger tire.