Honda PA50I or PA50M Performance (or lack thereof - and what you can (or can't) do about it)
Before you start the process of tuning your PA50I, you should consider the pros and cons. In the Honda PA50 section of the Wiki, the differences between a PA50I and a PA50II are listed. This list is longer than you think!
In summary though, here's my own two cents. What you choose depends on your desired top speed.
Mid to upper twenties: Depends on parts prices. This speed can be reached by buying/installing a PA50II carb or re-boring a PA50I carb, PA50II reed cage, and PA50II variator ramp plate. So, if you can buy a complete PA50II engine for less than the price of a PA50II carb, reed cage, and variator ramp plate, then do it. Otherwise, buy the piece parts and upgrade your PA50I motor. (Or, if you don’t want the hassle of the parts swapping work, just buy the PA50II motor).
Mid to upper 30’s: If you want mid to upper 30’s, it will be cheaper to start out with a complete PA50II engine, and just bolt on an aftermarket exhaust. You may reach 40 if you combine this setup with a notched variator.
40’s and up: Now you're porting/machining your PA50I 50cc cylinder, or buying a "kit" made for a honda motor, meaning you are going to buy a cylinder and exhaust. Either way, the decision whether or not you should start out with a PA50I engine or a PA50II engine boils down to the cost factor – the same as described above. If you can buy a complete PA50II engine for less than the price of a PA50II carb, reed cage, and variator ramp plate, then do it. Otherwise, buy the piece parts and upgrade your PA50I motor. The reason: the crank and block are essentially the same. The differences (motor wise) then boil down to the carb, reed, manifold, ramp plate, and rollers.
I’ve personally pursued both paths, having upgraded a PA50I and a PA50II. In my case, it was cheaper (at that particular time) to buy a PA50II carb, reed cage, and ramp plate. But – it was all a matter of timing and patience. When I was shopping for a complete motor, they were hard to come by, and expensive. I’ve seen a number of “long blocks” for sale on ebay lately (the carbs being sold separately most of the time).
NOTE: The PA50II variator will not "bolt-on" to a PA50I crank (the outer bearing has a different inside diameter). A PA50II ramp plate can be put on a PA50I variator - if the variator is modified first. Go here for more info: PA50I variator using PA50II ramp plate
If you want the full story on the model differences, go here: Honda PA Model Differences
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