aaaah ... success !

In the last week my newfound speed seemed to dwindle mysteriously away. (Honda PA50I)

The symptoms ?

Just gets to 25mph.. and kinda bogs there and 'crackles'... it won't rev out... (the same way it acts till it warms up)

Following all the typical advice I give people... I did a fairly quick carb clean... found no dirt or blockages... hmmmm why is it slow then ?

Cleaned the points.

Checked ignition timing... it has moved since I last set it... so I re-set it about 5 times.. till it ended up where it should (a little advanced).... hmmmm .. no help ... why is it slow then?

Checked the air filter... not bad... it still acts lean anyway... dirt would richen it up.. experimented with removing the air filter.. no help... taped the filter up (to reduce air flow)... no help... dang !

This time I cleaned the carb very carefully.... NO HELP !

Awright... slept on it... and I just had the feeling that something ws just stopping the airflow.

The exhaust pipe is a handmade one... several years old... nearly straight thru (except for the easily removed baffle I made for it).

I have never cleaned it because it is so simple.... I suspected maybe the only 'tight spot' in it might be carboned up.

I jammed a bent rod up there and wiggle-waggled it around... while I was at it I cleaned the exhaust port a little... (not bad).


It flies now... that exhaust pipe must have slowly built a carbon brigde over that 'tight spot'.

It goes an easy 35mph on flat ground.

My 0-25 has dropped to 15.5 seconds... (was at 26... then 22... then 19... now 15.5)

If I had reached this point a week ago... I would have ridden to the BBQ (150 miles).

By the way... Chris R. - this is without the PA50II reed valve... when I get that and gear it up (IF I can)... this thing could do 50 maybe.

Re: aaaah ... success !

Matt Wilson /


Do you think cheap gas and oil caused this?


Re: aaaah ... success !

Matt Wilson /

Also how would switching to a reed valve system increase your speed.

My scooter has reed valves, and I know that it allows the air fuel to only travel into the engine.... but why would that help?

And I didn't mean the first post as confrontational... but do you think that a cleaner burning oil would have prevented your carbon build up?


Re: aaaah ... success !

david f martin /

Exhaust port... Hmmm...


Re: aaaah ... success !

Good question.

The fact is I also ride/run other more modern 2 strokes with the same oil... and guess what?... I Never get carbon buildup (say... in my 250cc dirtbike)... the inside of the motor is spotless.

I also run it in an outboard boat motor.

I can't really say why... but remember... it took all of 5 minutes to clean this stuff out... so how bad is it ?

Remember also... this handmade exhaust pipe is over 10 years old.

For 3 or 4 times the price for synthetic ?... I'll clean the pipe every 10 years.

It MIGHT be because I usually ride the ped to the store and back... maybe it never gets hot enough ?

The regular gasloine I run ?... that has no effect whatsoever on this slight carbon buildup.

the reed valve ?.... here is the thing... this PA50I was the 'real slow' model... 21mph was all they did stock... I don't know why Honda made them that slow.

But 2 years later... Honda made the PA50II.. (very similar... but several small mods)

I am sure they had complaints from buyers of "why did you make this thing (the I) so slow ?"

They (PA50II) were near 30 mph stock... and one of the big changes was they went to a much higher flow reed valve... thats what I will be putting in early next week when I get it.

So the PA50I is kind of a special case... with a tiny, restrictive reed valve.

(a modern 50 would have about 6 times as much reed 'flow area')

Re: aaaah ... success !

....heay fred....i picked up a pa 50-2 about a month runs great..i love this thang.....its bone stock and in killer shape..........i plan on putting a proma circuit exhaust on it.........but you seem to know your pa's well , do you have any advice on goes 25 like to go 40-45........thanks........

Re: aaaah ... success !

Matt Wilson /


Is a "higher flow" reed valve made of a softer material so that it flexes open more?

Or are the manifold and reed larger?

I can't think of any other reason that you would get carbon build up (So Quickly).... Air+Fuel+Oil->Explosion If any of these is impure than you would get some byproducts...unless the air or fuel is dirty (unlikely), then I would guess that its the oil...right?

I'm no expert on this, but that's just my general intuition.

And yeah the synthetic is more... but a quart lasts me about a year (Maybe 1000mi) in my scooter.


Re: aaaah ... success !

You aren't reading.

So quick ?

10 years is 'quick' to you ?

(I think you are thinking it 'clogged' within that one week... no... it slooowly built up over those 10 years till it finally started to affect flow)

And the cleaning took all of 5 seconds... (took 2 minutes to find a suitable rod and bend it).

I don't think it has anything to do with 'impurities'... I think it has to do with not getting hot enough because of my short trips.

Did you see that part where I said I get no carbon build up in the other motors ?

The reed valve is at least twice as big in the 'new' one.

(but some companies DO make what you said... reed flappers made of differing degrees of stiffness... they give different power characteristics)

Re: aaaah ... success !

Matt... the reply above under 'vapor' was meant for here.

Re: aaaah ... success !

Do Chris Robertsons 'variator' mod... it will gain you 3 to 5mph (because of gearing).

Casey has the Proma exhaust and the variator mod and he says his does 42 or so... he says his is fast as heck.

Search for ... PA50 variator mod.

(you make 3 slots in the outer housing of the front pulley.. it takes 10 minutes to get it off... and 15 minutes to make the cuts... be careful... don't make the cuts too deep)

Chris R posted about 4 pics of how to do it.

Re: aaaah ... success !

Matt Wilson /

(I think you are thinking it 'clogged' within that one week... no... it slooowly built up over those 10 years till it finally started to affect flow)

Yeah I thought that it just clogged up this week.

I'm glad its running well.

Did you do the variator mod to the bike?


Re: aaaah ... success !

variator mod?... yes... I put the slots in... but remember.. this is a funky old PA50... your scooter is probably already designed better and you might not be able to make a simple mod like this for a gain.

Re: aaaah ... success !

Matt Wilson /


Yeah you cant do the variator mod to my Zuma. I did by an aftermarket variator but it came one slider I've been waiting for a 15cent plastic piece to come from the netherlands....and its coming by postal mail...ugh

Also I was wondering if you think speeds of 40mph could push more air into the air filter. My performance airfilter is shaped like a cone, but because of the plastic panel it faces backwards. I could cut a hole in the plastic and mount the scoop forwards, kind of like an air ram on a motorcycle. Do you think having the scoop face forward would make a difference? I'm guessing no but I thought I'd ask.

I took a picture of it but my home computer crashed. I'll post it as soon as I can get the computer out of safe mode.

Also I'd be interested to see some pictures of your exhasut and baffle if you have any.


Re: aaaah ... success !

Matt sez >

Also I was wondering if you think speeds of 40mph could push more air into the air filter ?

Kind of like an air ram on a motorcycle.

Do you think having the scoop face forward would make a difference? I'm guessing no but I thought I'd ask.

I'd say no... the ram-air scoops on fast streetbikes are known to only start to have an effect at speeds nearing 100mph.. and above... when they start to build positive pressure from the air piling into the airbox.

There are modern fast bikes that still don't use it either... and they are still verrry fast.

So... 'ram-air' is overrated and somewhat of a sales gimmick.

There is also another benefit that might be more important than the 'ram-air effect... that most people don't realize.

Riding around at 'not too fast' speeds .. leaves these motors sucking HOT air pre-heated by the motor and forward mounted radiator... having these 'ram-air' ducts means they are sucking un-heated (much cooler) air from the front of the bike.

Cooler air means more HP.

That may be as important as the 'ram' effect of positive pressure that occurs at higher speeds.

So.. I wouldn't bother with attempting a ram air scoop for a 40 mph scooter.

Pics of my exhaust and baffle ?

I have no pics.

I don't have a digi-camera.

The pipe is like any basic 2 stroke expansion chamber shape .. (a snake that swallowed a rat)... with one diff... because of its length.. I attached the 'stinger' (the exit pipe) at the bottom of the large diameter... (instead of the end like normal).

So what would normally be the 'exit' is a closed-off end.

The baffle is simply a 1/8th inch dia rod with 4 flat washers welded to it at diff spots... it slides inside the stinger.

Without it its just too loud... with it its 'medium loud'... still not as quiet as a stock ped or scooter.

Actually.. my old baffle made many years ago fell out the other day.. I couldn't find it... so I had to make a new one yesterday.

Re: aaaah ... success !

Matt Wilson /

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This is what I was talking about.

I'd heard that the air rams weren't even hooked up on many subarus, so I thought that it wouldn't have any positive benefit... but I thought I'd ask

Did you use the online pipe calculator or did you just model it after an existing race pipe?




I used formulas from books before the internet barely existed.

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