Modifying PA-50 Variator

Chris Robertson /

Howdy Folks:

For those of you who are interested in getting a little more top end on your Honda PA-50s, here's a fairly easy modification you can make to your variator. I have personally performed this modification on my PA-50 and it added about 10-15km/h (6-9 mph) to my top speed without noticeably impacting my acceleration.

Although the photos and instructions presented here are mine, I cannot take credit for this idea. I got the idea from a guy named Gangly Jeff. His website is here:


Variators are an interesting way of keeping an engine within its power band at varying speeds (hence the name). On Hondas this works by using two cone shaped pulley sheaves that are pushed together or pulled apart to change their diameter with respect to a belt that runs between them. A second spring-loaded pulley in the rear (picture 1) takes up the slack in the belt as the diameter changes up front. This system works exactly the same way (from a physics point of view) as gearing in a traditional transmission, except there are an infinite number of graduations between "low" gear and "high" gear.

To see how this works, take the cover off the side of the moped and start it on the stand. Rev the engine a few times and watch the variator. Pretty nifty, isn't it?

In order to keep the top speed of Honda PA-50s down, Honda engineered the variator to "stop" at a certain point. The modification described below will allow the variator to continue working a little further than stock allowing you more power at the top end.

The mechanism that forces the pulley sheaves to open or close are simple weights that run on a curved track (picture 4, round thing on right side). As the pulley turns faster they push against the sheave via centrifigal force and force the pulley to close. If they are allowed to travel further along the track, the "gearing" can be pushed a little further.


1. Consulting the first photo. The variator is the pulley contraption at the front of the bike. Take it off the bike. If you can't figure out how to do this, you shouldn't be modifying your moped's transmission.

2. Pictures 2 and 3 are illustrations of what the modified variator will look like. Cut notches just like the ones you see into the outside sheave. You want to make them big enough so that the roller weights (see image 5) can roll a little further into the gap, WITHOUT falling out. Don't even think of trying to do this without power tools! I used a Dremel with a cutting disk to make the notches.

3. Disassemble the variator (see picture 4). Remove the retaining clips. You don't need them, and they'll prevent this mod from working. Clean everything very well. Get all the dirt out of the variator. The variator in this picture is unmodified and obviously needs to be cleaned --so don't use it as an example of "clean".

5. Take the roller weights out and fill them with lead (either solder or pound lead fishing weights into them). You want to make them a bit heavier, but not too heavy or you'll lose a lot of your acceleration.

6. LIGHTLY grease the roller weights and curved tracks inside of the variator before you put it together. Use very little grease or you'll wind up with a ruined pair of pants when you start your ped. Do not put the retaining clips back into the variator.

7. The rear spring loaded pulley (picture 1) should be lubricated at its center. Clean the sheave faces with solvent to remove any oil from them (you only want to lubricate the inner mechanism).

8. Reassemble everything. You might want to fiddle around with your belt tension.

If you try this mod, let me know how it works for you.


Re: Modifying PA-50 Variator

Chris Robertson /

A couple of quick caveats:

Some PA-50s do not have variators. If your PA-50 has two chains (instead of a chain and a belt) this modification doesn't apply to you.

Reverse the order of step 2 and 3 (obviously you take the variator apart BEFORE you cut notches into it).

I forgot to make a step 4.

I suppose I should have done my proofreading before I submitted the message.

Re: Modifying PA-50 Variator

since motobecanes also have variators on them, will this mod apply to them too?

Re: Modifying PA-50 Variator

Chris Robertson /

I don't know. Take a look at the pictures and see if it makes sense to you.

I have a variator on my Peugeot, but this modification doesn't apply to it because the entire engine pivots rather than pulley sheaves moving together.

Re: Modifying PA-50 Variator

is there a special way to view your pics 'cuz they just come up as letters and numbers?

Re: Modifying PA-50 Variator

Chris Robertson /

The attachment is a .zip file. Try right-clicking (not left-clicking) on the attachment link and selecting "Save Target As...". Save it somewhere on your harddrive and open it with WinZip (or the Macintosh equivalent if you're a Mac user).

I just downloaded the attachment myself as a test, and it worked correctly for me.


Re: Modifying PA-50 Variator

i saved the target like u said and it saves as a document not a zip file and when i open it and this is all it is


Re: Modifying PA-50 Variator

It's not just your computer Shmiver, it's the exact same with mine. I'll let you know if I figure it out....

Re: Modifying PA-50 Variator

Ok, just right click the attachment like Chris said, save it to your desired location, when it's done saving go into the location that you saved it to and it should be there. It needs to be unzipped, with mine I just double clicked the file and that started winzip and then it unzipped it.

Re: Modifying PA-50 Variator

i download it and it automaticly unzips it, and it comes out like my attachment

Re: Modifying PA-50 Variator

Chris Robertson /

Hi Shmiver:

The first two letters in the snippet you posted are PK as in "PK-zip". This is a zip file that has not been unzipped yet. Do you have Winzip installed? If you don't, your computer won't know how to handle .zip files. You can get Winzip for free at

If you do have Winzip, save the attachment as I mentioned before. Open up your Windows Explorer (Hold down the Windows key between the ALT and CTRL keys and press 'E'). Open the folder you saved it in and double click the file.

I attached the graphics in this manner because it's the only method I could figure out to attach multiple images.


Re: Modifying PA-50 Variator

i have winzip, i download it and it saves as a html document and all i can do is open it, can't un-zip it

Re: Modifying PA-50 Variator

i finally got it, i had to open winzip first, then using it find and open the file, thanx for the help

Re: Modifying PA-50 Variator

Just one much lead???? should i pack it solid or no??

Re: Modifying PA-50 Variator

Chris Robertson /

I used very small lead fishing weights and pounded them in. When I was doing it, I was trying to fill them, but I doubt I managed to do so. I think the best thing to try would be to fill them with solder and go for a run. If your acceleration seems screwed up, melt some of the lead out and try again. If your top speed disappears, put a bit more lead into the weights.

I doubt that filling them with lead would be too heavy (there isn't much space inside them) --try that first.

Try to keep the weights all about the same of course!

Doesn't work

It (adding weights) doesn't work.

With no more movement available on the ramp mechanism... all the "adding weight" does is makes the pulley "shift" earlier... which actually makes a PA50 slower... Mine would shift so early as to not wind out in the lower range at all... then of course it would be at a lot lower RPM after it shifted which makes the bike slower... not faster.

Don't do anything that you can't easily change back from... try 3 equal weights... you will probably take them right back out... it is a bad plan.

You need more movement in the ramp mechanism.

And franky.... LESS weight will make a PA 50 faster by letting it shift later... after it has wound out more on the lower range.

And I think "gangly jeffs" speedo is about 5mph or more optimistic also.

Re: Doesn't work

Chris Robertson /

First, I described how to increase movement along the ramp. That was the whole point of the mod! I recommend adding weight so the rollers will use the extra space.

Second, I agree that adding weight without cutting notches into the pulley sheave will not make a huge amount of difference, but I can attest that my Camino has faster acceleration than any other moped I've compared it to. Lead can very easy be removed from inside the weights with a couple of seconds under a propane torch so there is no risk involved in trying. How much weight did you have to add to get your moped to "not wind out in the lower range at all"? To do that you'd probably have to replace the roller weights with something heavier.

As for Gangly Jeff's measurements --I'm not using them. It's pretty common knowledge that moped spedometers are ridiculously innaccurate. I always put a digital bicycle spedometer on my mopeds; when properly calibrated they are very accurate. My own PA-50 will hit 55km/h (35 mph) on level ground on a calm day regardless of how fast Gangly Jeff's moped goes. I have verified this speed with a car.

I think you oversimplified the behaviour of a variator with respect to changing the roller weight's mass. Adding mass to the roller weights does not necessarily decrease acceleration, in the same way that subtracting mass does not necessarily decrease acceleration either. A variator is designed to keep the engine in an optimal part of the power band. Changing the weights will affect what RPM range the variator maintains, but depending upon which side of the power curve the variator is already set to, the power available can go either way. I contend that when you're dealing with a twenty year old mechanism, its performance can change from its behaviour when it was brand new.

Fiddling with the weights is a reversible process, and it might make a difference. It didn't work for you, it did work for me. Your mileage may vary.

Re: Still doesn't work

My reply was not to you... Mr Robertson.

And ....You need to relax.

You read several things into it... that I did not say.

One... I did not say that adding or subtracting mass increases or decreases acceleration... what I said was that adding weight to the "weights" will make it shift SOONER... Which it absolutely does.

And in the case of a PA50... that makes it slower because there is less HP available at a slower RPM. once it shifts sooner.

And I never even mentioned your slotted ramps.... Did I ?

Read it again.. my reply was to the guy asking about adding weights... which will not help the acceleration of a PA50.. despite what "gangly jeff" says.

And did I ever say YOU used "gj's" figures......... No.

So... I stand by what I said... gj's adding weights is the wrong way to go.

And his speed numbers are inflated.. (unless his canadian model PA50 is different from a US model)

Re: Still doesn't work

Chris Robertson /

Hi Fred:

I agree that added weight will make the variator increase the gearing sooner, but I don't agree that this will necessarily reduce power.

When brand new, the variator and engine will work together optimally, and changing the weights (up or down) is only going to make things worse. Would you concur that it is possible for an old engine's power characteristics to change or for an old variator to shift sooner than it was designed? I'm no mechanical engineer, but I think that if this were the case it MIGHT help to increase the roller weights a bit.

I agree that Gangly Jeff's speed measurements seem optimistic. However, the poor guy isn't even here to defend himself (as far as I know) so perhaps we should give him a break! I mentioned him only because he was the one who gave me the idea and I thought he deserved credit.

You told Matt (and everyone else) that the idea of changing the roller weights will definitely not work even though it did make a measureable difference for me (and apparently Gangly Jeff). Please don't act so surprised that I defend my recommendation! I might point out that you obviously didn't like it much when I questioned you either.

According to my shop manuals, there is only a North American variation of the PA-50. For the record, however, my PA-50 (like me) is Canadian.

It sounds like you've already cut notches in your PA-50's variator. Did it make any difference for you?

I'm not interested in arguing, so I'll let you have the last word on this.


Weights alone

By the way... I didn't mean to imply that the ramp mod was bad... It sounds great...and I am going to do it.

I was just referring to the weights alone... they will make it slower (acceleration) because it shifts too soon.. hell... it shifts too soon stock... so adding weights makes it worse.

And with no change in pulley spacing.. the top speed will remain the same.

Your ramp slots should change that.

What else have you done to it?

Re: Weights alone

Chris Robertson /

I did a complete rebuild last fall. I managed to find another (non-running) PA-50 and basically took everything apart and rebuilt with the parts that were in the best shape.

I rebuilt the engine, carb, and magneto. I chose the best brakes and wheels as well.

I rewired the moped, but I have a couple of weird problems. Maybe you can help:

1. When braking the horn won't work.

2. I can't get the brake light to work (the bulb is new).

The weird thing about this problem is that I had it before I did the rebuild too! I suppose it's just a bad ground somewhere in the frame, but I've spent quite a while testing continuity to no avail.

One of the things I've wanted to try, but haven't, is putting an air scoop on the air intake to pressurize the carburetor --I can't really figure out where to put it where it won't be in the way. Has anyone ever tried anything like this?



Your motor is stock except for the clutching mods?

The exhaust is stock?



And it does 35MPH?

What did it do before you modified it?

Re: Stock?

Chris Robertson /

Other than the variator, the moped is basically stock. There were lots of things I tried, but nothing really made much of a difference. Before I modified the variator the moped had a top speed of about 45km/h (28 mph). This is with a completely decarbonized engine and exhaust, and spotless carburetor.

One thing that was causing some reduced performance until I cleaned it was the main nozzle (its right underneath the main jet --I didn't know it was there until I got the shop manual; it has 12 pin-sized holes in it!).

There are two versions of the PA-50, the PA-50I (20 mph) and the PA-50II (30 mph). I have the faster version. According to my spec sheets they are exactly the same except for:

1. Compression ratio (PA-50I is 6.9:1, PA-50II is 6.5:1).

2. Air Screw Opening (PA-50I is 1 3/4, PA-50II is 1 3/8).

3. Exhaust port open (PA-50I is 57.5deg BBDC, PA-50II is 65deg BBDC).

4. Exhaust port close (PA-50I is 56.5deg ABDC, PA-50II is 65deg ABDC).

5. Scavenge port open (PA-50I is 45deg BBDC, PA-50II is 47.5deg BBDC).

6. Scavenge port close (PA-50I is 45deg ABDC, PA-50II is 45deg ABDC).

7. Final reduction (PA-50I is 22.767 - 17.446, PA-50II is 22.767 - 13.537).

The final reduction of a PA-50I can be upgraded by replacing the rear hub (or the entire wheel) with a PA-50II's. I haven't a clue if there's any way of re-porting the engine.

Hope this helps!

Re: Weights alone

Ron Brown /


Watch this space.

I do not have time to investigate right now, but I am fairly sure that my Motobecane horn and brake light are on the same alternator winding and that the brake light somehow uses the horn as a path for current. I will check the wiring diagram and let you know.


Re: Weights alone

Ron Brown /


This is a little difficult to describe, but if you have a wiring diagram for your bike, this may help.

On my Motobecane, one of the alternator windings runs the brake lights and horn as follows:

When the brake lamp is not on and the horn button is not pushed, the alternator output is fed through the normally closed horn button contacts and then tied to ground by the two brake light switches.

Operating either brake light switch, breaks the path to ground and the brake lamp lights.

Pushing the horn button routes the power through the horn to the brake lever switch ground, making the horn blow.

If you also operate either brake lever, the horn and brake lamps are in series so you get a half bright light and a half assed horn.

Hey, it's French.

Hope this helps,


Re: Modifying PA-50 Variator

Is there any free mods you can do to an PA 50 1981 for more power and more speed ?

Antony Caron

Re: Modifying PA-50 Variator

on a Pa 50 which size of POD (UNI filter)instaling on the carb

Antony Caron

Re: Modifying PA-50 Variator

Chris Robertson send me for the email please, dont to be able downloads

has then if I raise the needle of the carb is what its will give me of horsepower

Antony Caron

Re: Modifying PA-50 Variator

for some reason my computer tells me the zip isn't a valid archive....

Re: Modifying PA-50 Variator

how I could modify my exhaust origin of my Pa 50 to gain speed and the porting is it nescessary

Antony Caron

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