Honda Model Differences
Ok, let's say that you have a PA50I, and you want to go faster. By far the easier quicker way to get to 30 MPH is to buy a PA50II motor, including its variator. if you would rather upgrade the piece parts, here's what you need to consider:
The crankcase is MOSTLY the same. The PA50I crankcase has little ridges at the bottom of the bore (for the cylinder) to prevent a PA50II cylinder from being bolted on.
See ridge at bottom of case (inside the bore for the cylinder) and ridge at top of case (inside the bore for the cylinder).
PA50II is on the left, PA50I right. the PA50II is 30.25mm gasket to ridge, the PA50I 25mm
In addition to that, the PA50I case has a smaller hole (that accepts a PA50I reed block with a small "nub". The larger nub of the PA50II reed block can easily be drilled or ground off to bolt onto a PA50I case.
PA50I left, PA50II right
The PA50II carb has a 12mm venturi, versus a 10mm venturi of the PA50I version. It also has a #78 jet versus a #60. [A PA50II carb, reed block, and manifold on a PA50I engine will net you 5+MPH]
The PA50II reed uses two petals, flowing through two openings. The PA50I reed uses one petal, flowing through one opening. A PA50II reed block can be modified to bolt-on to a PA50I case. There is a little "nub" on the block that can be ground of drilled off, then it will bolt right on.
The PA50II manifold has a much larger "nub". The PA50I manifold nub is rather puny. The nub on a PA50II manifold can be removed or drilled off to allow use on a PA50I engine.
See photo for differences.
The crankshaft on the left is from a PA50II, with the one on the right from a PA50I. The bushings that are shown in the picture are also different - in every way (inside diameter, outside diameter, and length) - see Drive Assembly section (most people call it a variator) for more info on the bushings.
Piston & Head
The cylinder differences are numerous:
PA50II has different exhaust port sizes/timings. The PA50I can be easily ported to PA50II specs.
The PA50II exhaust outlet, I'll call it the flange area, is larger on a PA50II than a PA50I. For this reason, a PA50II muffler will not bolt up to a PA50I cylinder. This also applies to aftermarket exhausts, too! This means that if you have a PA50I cylinder, you can't just bolt-on an aftermarket exhaust. Now, I have used a 1-1/4" metal hole saw (in a drill press), and have opened up a PA50I cylinder to bolt-on a LeoVince.
PA50II Exhaust Port
PA50I Exhaust Port
I wasn't sure what to call it, but the bottom of the cylinder of a PA50I has a "notch" cut into it.
PA50I Cylinder Base
The PA50I case has a corresponding ridge in this area (see crankcase info above). The intent of the case ridge is to prevent any cylinder without the notch from being bolted onto a PA50I case.
The PA50II cylinder has no such notch.
PA50II Cylinder Base
I have used a cut-off tool to notch a PA50II cylinder, and I have also been advised that one other MopedArmy rider has done the same. So, if you have PA50I case, you can bolt-on a PA50II cylinder, if you take the time to modify it.
This pic shows a PA50II cylinder half way through the process of being notched to fit into a PA50I case.
NOTE: You may need to refer to the parts lists on-line, or linked herein to get used to the terms used. I use Honda terms.
The drive face differs between a PA50I and a PA50II. The PA50I drive face can be modified to accept a PA50II ramp plate. It is pretty easy.
Otherwise, the outside bearing in the drive face has a different inside diameter, and the bushing that rides on the crankshaft has both a different inside diameter and outside diameter. See picture below:
Drive Face Bushings
The bushing sizes are:
This means that in order to use a modified PA50I drive face on a PA50II crankshaft, both the inside needle bearing and the bushing must be replaced using PA50II hardware. Similarly, if you wanted to mate a PA50II drive face on a PA50I crankshaft, both the inside needle bearing and the bushing must be replaced using PA50I hardware.
It is good to note the the outside roller bearing from the PA50II drive face is unavailable in the US at this time. I have called many bearing supply stores all giving me the same info. The HMK1512 is only available in Japan. I do not know about the PA50I roller bearing.
An educated guess regarding the change in bushings/bearings. The PA50II bearing is larger, meaning that it will carry more load. I'm not saying that a modded PA50I bearing will definitely fail. In fact, I've got a modded PA50I myself, with a ported 50cc cylinder that rips. I do however, add fresh grease to the bearing occasionally.
The roller weight set has different Honda numbers. Variator weights are shown below:
|Stock PA50II||14 grams|
|Stock PA50I||8.4 grams|
The external dimensions are mostly the same, but the PA50I weights have much larger holes. The only other noted dimensional difference is the area that the plastic cap sits on. On some weights it is smaller, and on others, it is larger. See picture below.
The weights. left-to-right are: stock PA50II; stock PA50II (weight difference is 0.2 grams compared to other stock PA50II); PA50II (with 1/4" hole); PA50II (with 5/16" hole); and stock PA50I. See the PA50 Drivetrain section for variator weights mods/tuning.
Other dimensions are the same:
I've looked around for aftermarket variator weights, and I have not yet found any. When you look on-line, variator weights are identified by O.D., then width. That being said, Hobbit rollers are 15x16mm. The closest that I've found is 15x12mm. But - the channels of the movable drive face (the outside moving pulley half) are about 18.5mm wide. It would seem that 15x18mm weights would be ideal, but this size, and anything even close doesn't seem to exist.
Variator Ramp Plate
The ramp plate has a steeper slope on a PA50II versus a PA50I. This greater slope allows the variator to push the fixed and movable drive faces closer together - increasing the effective diameter of the front pulley set and thus increasing the drive ratio - meaning higher top speed.
Air Inlet Tube
Both Air Inlet Tubes have three holes, but the three holes on a PA50II are larger in diameter. I've pulled these from my mopeds. The actual function for these tubes is to be a "silencer". When you pull them, you will hear wide open throttle - it will be louder than the stock muffler. Check your jetting after removing the tube.
The muffler on a PA50II has a larger diameter head pipe, and a larger outlet pipe. In the attached pic, a 7/8" wrench will fit, and 13/16" wrench will not. In addition to different header sizes, the internal chamber transfers and the exhaust stinger are much more restricted in a PA50-I pipe as well: see: https://www.mopedarmy.com/wiki/Honda_PA_Exhaust_(Stock)_Cleaning_(or_what%27s_inside_this_black_box%3F)
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