PA50 top speed restriction?

My PA50 Hobbit has a throttle "sweet spot" where if I open up the throttle a hair more, it totally bogs down and loses power.

Plus I have to have the choke stuck in the "on" position in order to run at peak performance.

Is the carb on this thing (recently overhauled) restrictive of the gas flow? I can only get a top speed of about 20 mph, and thats after about 20 seconds of acceleration.

Re: PA50 top speed restriction?

if your bogging at topend, you might need to gio down one jet size, and perform plugchops to see where your at.

Re: PA50 top speed restriction?

btam -

Do you have a PA50 I, or a PA50 II?

There will be quite a bit of difference in performance.

Search here and in the General and Performance forums. There have been qiute a few discussions about the I and II versions.

Re: PA50 top speed restriction?

Brian Tam /
btam OP

Thanks Cyberian and Paul.

Cyberian, its a 1978 PA50II Hobbit

Paul, what are plugchops? And do you know the stock jet size for the PA50II?

Re: PA50 top speed restriction?

The carb is dirty or was overhauled wrong or was reassembled wrong .. but you will trace the trouble to inadequate fuel delivery.

The photo in the profile is a PA50II.. it will easily cruise at 30 MPH if it's anything near healthy.. and tuned up.

These bikes will do nearly 40 mph with only the addition of a performance exhaust to the otherwise stock bike. There are no restrictions beyond the nice, quiet muffler.

Re: PA50 top speed restriction?

Re: PA50 top speed restriction?

... and the stock Keihin 11.5mm butterfly-style carb came with a #78 jet.

Re: PA50 top speed restriction?

Brian Tam /
btam OP

Thanks Joew.

I have recently overhauled the carburetor. However, I think I may have done more harm than good. I plan to take it apart again, and check out the jet size and make sure I put it back together.

A few questions:

-Is there a way to tell what size jet I have on there?

-How is the 11.5 mm measured from the carb dictating its "size?"

-If the carb needs to be replaced, I know I can get an OEM carb at Power Sports Pro, but are there any mopeds with carbs that will fit the PA50II?

Thanks in advance!

Re: PA50 top speed restriction?

The main jet will be stamped with the number 78. You can see it with a magnifier without removing the jet.

The throttle plate has the number 11.5 stamped on it. Measuring the venturi dimeter with internal calipers confirms it.

There's no reason i can imagine for replacing the carb.. and anyway i'd do whatever possible to repair and keep that original carb (or replace it with an identical one). It's a great, simple carb and, like i said earlier, is capable of fueling that bike up to near 40 MPH without running lean. Performance at lower speeds is trouble free. In conjunction with the reed-valve intake porting a nice, slow smooth idle is to be expected.

Honda knows how to build bikes. I have three PA50 IIs. It's a nice bike in stock condition with lots of hidden potential for more speed just the way they made it.


The carb is hard to access but i still made the mistake of hurrying the carb cleaning job more than once. The result was always the same.. Drop the engine and do it over.

Remove the main jet, push or blow the internal brass emulsion tube down and out and clean up it's cavity above the jet.. REmove the screw that hides the idle jet (its the one below / next to the idle speed screw) All the passages and parts should be soaked and blown out with strong compressed air and examined closely. The bowl must be immaculate. Any old fuel residue on internal parts will come loose and clog something.

Coat carb/intake red valve block mating surfaces with Permatex 2 (non-hardening gasket sealant) to prevent vacuum leaks since the old gaskets and o-rings in the area tend to develop vacuum leaks. Speaking from experience, don't use sealant on the float bowl O-ring because it will squeeze out into the bowl and end up clogging the main jet.

Old Hobbits with old fuel systems initially need extra effort, but once cleaned up and kept clean they are trouble free and will run forever.

Re: PA50 top speed restriction?

Brian Tam /
btam OP

Thanks Joew.

The last time I took the carb off, I didnt seal it. I will pick up some Permatex 2 and do that.

Also, you mention the screw below the idle set screw that doesnt really _do_ anything. You say it hides the needle jet? You want that screw tightened down, right? I dont really understand what that one does.

Thanks again.

Re: PA50 top speed restriction?

I mistakenly said it hides an idle jet... but it actually hides a tiny air-bleed jet. That screw is already tight against the carb body.

Remove the idle speed screw and then that screw can be removed.. and inside the hole can be seen the tiny air jet.

Air comes in through one of the brass tubes in the air horn and goes through this jet and into the emulsion tube area, premixing fuel and air before the fuel enters the air stream..

My point on mentioning it was just to stress disassembly to the degree that all hidden passages and cavities can be accessed and cleaned.


Brian Tam /
btam OP

Cool, thanks again Joew.

Re: Thanks!

you're welcomed.. the PA50s are not rare but not a whole lot of people around here have them. It's nice to see another one could be back in operation soon.

btw, take a look at g.b. pix of his recently completed PA50 project in the General Discussion forum.

Thread is from today named "Check out my ARMY pa50 (pic) "

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