Honda NA50 and NC50 Express Tuning

This page is a long overdue account of my experience tuning the Honda Express range of mopeds. Although it may have  a single sided swingarm and NC50 designation, it is far away from an NC45/NC30, so if you are looking for NC30 info, this isnt the best place for you.

Many thanks to Rick Moran who contacted me since originally posting, and providing pictures plus ideas to take forward as shown at the Bottom.

The Honda Express Word according to Me
NOTE: This may all be wrong, but its not far from the mark.

The Honda Express is a low cost, primitive (compared to modern mopeds) two-stroke, air cooled Moped built by Honda in the late 1970s - probably 1978. The engine used a small reed valve, points ignition without fan cooling, and employed a single speed auto transmission, with a design speed of 25mph - stated on a headstock on the plate. The engine was a stressed member, single sided and acted as the swingarm, in common with other mopeds/scooters.
A notable feature was the engine incorporated a wind-up clockwork spring that the user gently wound up, then pulling the lever would release teh spring energy and start the engine - a poor mans electric start. This allowed the use of 6V electrics, which were terrible even for 25mph at night. The front suspension did not use any damping and the rear shocker (singular) was also terrible, with adequate tiny drum brakes front and rear. The real advantage was the minimal frame giving an overall light weight due to design.
The early express is characterised by a straight frame spar and did not have indicators fitted as standard, with a high headlight.

In about 1981, Honda released the NA50 Honda Express Deluxe. This model was essentially similar, but the main engine differences were the omission of the clockwork starter for a conventional kickstart, and a dual speed auto transmission. Cosmetics were a bent frame spar and this model had indicators fitted, with a low headlight and a front basket option for addedd coolness.

Honda then developed the SH50 Honda City Express in about 1983/4, which uses a fan cooled engine with no real common parts with the earlier models (as far as I know) and will not be discussed further here.

Objective

Why would anyone wish to tune a Honda Express? you may ask.

Many reasons. I originally rode a Honda NC50 Express in 1989 making 20 mile round trip each day to work at a motorway service station. The bike was 1979 so already 10 years old. Though slightly ridiculed by passing school kids as I rode up the hill past the Comprehnsive school (especially with my black and red leather with Suzuki RGV500 on the back in Tippex), it was cheap costing 70 pence to fill up, and achieved something like 100mpg - I regularly used to forget to fill her up, then ran out of petrol as there is no reserve!
It was also good dice with a bunch of youths from a local village (Tickhill) who had Kawasaki AR50 and Yamaha DT50, full motorcycle style mopeds with gears. The DT50 and AR50 were the kings of mopeds at a time before the Aprilia RS50.
Fast forward 10 years, and I got involved in moped racing at the BMF, a cheap form of racing. At the time, the two stroke air cooled automatic class was under populated, since most people liked to run either ran air cooled gearbox mopeds like the AR50 or modern MBK50 water cooled engines - latest rules have Class 1 for HRC RS125 frames with 50cc engines in them! The Honda C70 was also allowed to race with the Air cooled 50cc auto class class if I remember correctly. So the lightweight build and underdog status of a Honda Express got me interested, as there was potential to surprise (and potentially embarass) a few people.

Method

It is first necessary to review the Honda Express, in both tuned and standard form.

Port heights are expressed as height above piston at BDC, and may be converted to degrees using the utility Here

A MOTA V4.44 simulation was run, producing the following graph:-

The power is a little optimistic at over 4.5bhp, but the advantage with simulations is to look at the change. The actual engine did not like to rev - a 10mm small end also implied it was not meant to rev hard, so I guess the peak power was designed to be around 7000rpm . The specifications stated the Express had a peak power of 1.8 or 2bhp whereas the MOTA simulation did not include any losses and calibration was not verified.

Exhaust
Brian Zenk provided a diagram of the muffler - it is a simple plain type with multiple baffle chambers, probably to knock out certain resonant frequencies that are particularly offensive.
On the first Honda Express, the exhaust port in the cylinder was very rectangular even at the muffler connection, changing little from the cylinder port dimensions. Opening this out to match the round diameter of the exhaust gasket inside diameter, which was 20 or 25mm, improved the breathing of the standard engine, making it go 30mph on the flat!
Brian also reports that changing the muffler outlet for larger diameter also improved performance.

Modifications
For the moped racing experiment, it was clear the ports were too conservative and a true multi-angle expansion chamber was needed. The easiest way to do this was to fit a 1mm spacer, plus two 0.25mm gaskets either side, under the cylinder, then machine the top of the cylinder so the piston was flush at TDC, and use a copper gasket to set squish at 0.75mm.
The stock Express cylinder head was very small, but a larger, better finned head was located in the box of bits that came with it, and machined to give a 50% squish band with 5cc volume including the gasket. Higher CR was not tried as care was needed since the air cooling was not forced. The later Deluxe Express has an Automatic choke apparently, and a much bigger head, so try use that, or If you were brave, one could be machined up quite easily....
The Exhaust port was also further modified, but the transfers left alone. The principle was also to fit a 15mm Mikuni carburetor from a different moped (Suzuki FR50), as the Express item was only 12mm Khein and used pressed in jets, and the reed cage was modified with a custom rectangular carbon reed and windows enlarged/streamlined.

The resultant specification was now:-

Port heights are expressed as height above piston at BDC, and may be converted to degrees using the utility Here

Chassis

Chassis modifications were minimal. The fuel tank was turned around and mounted between the driver's legs just like a normal bike - very easy to do using the same brackets. A rear mudguard was fitted for safety, and some drop flat bars from another Honda were fitted. A single CB175 shock was fitted to the rear. The Lights were removed, engine run on Premix (Since it was always full throttle or closed throttle), and the horn doubled as the kill switch, which probably wouldnt get through more thorough scrutineering.

Expansion Chamber Design

An expansion chamber was designed using the software on this site - you could use the Free MOTA Expansion Chamber software too, which is based on my software. An estimated exhaust belly gas temperature was 550C, which with hindsight is too optimistic.  Dimensions are shown:-

Obviously, when building the actual pipe, this starts from LP01 as EXD is the cylinder port and distance from piston face to exhaust gasket flange of 20mm. The pipe is quite long compared to race bike pipes, as it is designed to work at 7500rpm, not 14,000rpm.

Results

The MOTA V4.44 software gave the following Print:-

The simulation showed at least a 50% increase in power!

The various changes were made to the moped taking lots of careful time, machining, grinding, filing and attention, and an expansion chamber made by welding two flat pieces of steel together, then inflating them with a pump - a jet wash pressure washer could be used now. The expansion chamber was blown up straight but bent in one plane, then cut and rotated to fit. See this email for details, with more info Here and Here.No parts were cut out otherwise the lengths would have been altered. Unfortunately, I didnt take any pictures...

The engine was noticibly faster, even without fitting the 15mm carb and running on the standard 12mm Khein carb with fixed jets. There was even a trace of a power band! Not bad on an 1978 NC50 with single speed auto transmisison!

Then an idiot (you know who you are, Stu) who was on the team crashed the NC50 while in the car park trying to show off, and crushed the exhaust - I lost enthusiasm then as fitting the exhaust to fit the bike was difficult and had taken many hours. This made me realise the exhaust would not have been suitable for race day as it didnt survive a single crash, never mind Moped Mayhem, and I wasn't looking forward to making another exhaust that was better tucked in. I stressed that the engine needed jetting up, but my team told me I had 'messed with it enough now'. I threw my toys out of the pram and said I wasnt developing the NC50 anymore, So Stu, after all my hard work, just bodged on old Honda C90 exhaust.

Race Day

Given the Honda C90 exhaust was bodged on, a tuned cylinder, bumped up compression and no rejetting (these modifications would have typically needed richer jetting), on the race day we managed about 30 laps, then the engine seized. It was very fast for about 25 laps, out accelerating some FS1e and other geared bikes. But then the heat fade started to develop, and I knew what was inevitable.

If the expansion chamber had been tucked in better, and the 15,mm carb fitted and jetted to suit, who knows....

Further Work

If I were to do this again, I would want to do the following things:-

• Use 12mm Small End Conrod
  The stock NC50 and other small Honda mopeds use a 10mm wrist pin - this is small, but other models use 12mm which is better for high revs. Conversion kits are available for other honda mopeds, so find one that fits the NC50.
  
• Find Largest Cylinder Heads
  This was done for the moped, but I dont know what head was used. The stud spacing of the Express is common across many Honda moped ranges. Increasing compressin can increase temperature, so to get rid of the excess heat, more fins are required. Find a bigger, heavily finned head from a different model. And/Or, try and get the combustion chamber inner ceramic sprayed to reduce heat absorption from the hot gas. The Express II has an automatic choke and bigger head than the Express I, see pictures (Thanks to Rick Moran), although it will need re-profiling. 

   

  

  
  
• Measure Exhaust Gas Temperature
  At least two expansion chambers would be needed - the first one with a guess at the various section temperatures. The chamber above was designed with teh same temperature for all sections, when it would be better to use different temperatures for different sections. This when one has been built, actual temperatures could be measured from the chamber sections, and a new chamber re-designed with real data. This is tedious, but the first one would not need to fit the bike perfectly.
  
• Optimise Inlet
  The stock Express has the carb mounted on a long stub, between 3 and 4 inches long. This may need adjusting for length since it could be tuned for 4000rpm, but more importantly, the gas must turn through over 90 degrees from going through the carb to going down the manifold. This is real bad as the air effectively has to stop and turn a corner. Additionally, if a four window V type reed cage could be fitted, this could improve breathing, but may not be needed for an engine with a target speed of 8000rpm.
  
• Optimise Carburettor
  When making any modifications to two strokes, it is imperative to check the jetting! The stock Express Khein carb is not really meant to be adjusted, so it is important to fit an adjustable carb - I chose 15mm Mikuni (but didnt fit it), but upto 18mm may be appropriate. I would not want to go bigger than 20mm on an engine that revs less than 10,000rpm.

Rick Moran's Case Study

Rick Moran sent over details of his tuned Express.  Rick agreed with several of the points above, developing a very nice Job, summary details below.

• Express II bottom end
• 4 Petal Reed Block (Yamaha LT-100)
•  
  Yamaha LT-100 20mm Mikuni Carb
• Honda ATC110 Inlet Manifold
• Metrakit 70cc big bore
• Promo Expansion Chamber
• K&N Clamp-On RC-2290 Conical Filter

Nice Pictures:-

This page was last modified on Tuesday, January 05, 2010 08:12:01 PM