PA50 jetting/performance questions

alright, so hopefully i'll have my 83 running sometime this week. once i get it going i'm planning to make a few upgrades.

at first i'm going to be running on the stock airbox and exhaust. i'm thinking i'll start things off by bumping the compression a bit... maybe half a point or so. would this generally require a re-jet?

after that i'm going to get (or build) a pipe of some sort (maybe from an ebay pocketbike exhaust), and switch to an open element air filter. i know this will require a re-jet, but does anyone have an estimate as to what size jet i might need?


Re: PA50 jetting/performance questions

Bumping the compression should not effect the jetting, but do your plug chops just to make sure.

Re: PA50 jetting/performance questions

yeah, i will definitely check it afterwards. just was wondering if i should have a richer jet ready to put in or not. sounds like i should be okay, i'll just order one afterwards if it seems too lean.

my friend says he may have a pipe from an old 125 lying around.. would that be too large in volume, or be tuned for too high an RPM to work efficiently on the pa50?

Re: PA50 jetting/performance questions

most likely

Re: PA50 jetting/performance questions

another question...

is it possible to swap heads without removing the jug? like.. is the base gasket generally stuck on there pretty well?

if the bore/ports look good (which they should be, it accelerates nicely and runs at 30mph no problem, all stock) i'd rather not have to mess with the jug/piston/rings while swapping the head.

Re: PA50 jetting/performance questions

Raymond Rexroad /

The head will come off easily, and the cylinder will stay put.

Re: PA50 jetting/performance questions

awesome, thats what i was hoping to hear :)

i just bought a spree head on ebay, read on here that they bolt right on and bump the compression a bit. i havent had any issues starting without using the decomp valve, so i'm not too worried about eliminating it

Re: PA50 jetting/performance questions

I gotta hear about how that spree head goes.... that sounds cool.

Re: PA50 jetting/performance questions

when you say stock airbox...are you implying there's an upgrade?

Also, what are you doing to up the compression?

Re: PA50 jetting/performance questions

I know if you wrap the air drawer/box in pantyhose and dipe it in oil, bang, performance air filter.

Re: PA50 jetting/performance questions

eventually i may use a hole saw to make a nice big hole right through the airbox so i can fit a small cone/foam pod filter on the carb. for now i'm sticking to the stock unit though... might remove that weird rubber inlet thing with the 3 holes in it though.

i read on here that the spree head bolts right onto the express and bumps the compression, and ive also read that PA50 big bore kits work on the express.... so the spree head should fit the PA50. i'll let you all know how it goes, it should be here next week.

Re: PA50 jetting/performance questions

spree head is here :)

might slap it on tonight if i'm bored. supposed to rain for the next two days but i could do a before/after compression test at least.

the head gasket is reuseable on these, right?

Re: PA50 jetting/performance questions


what if i didnt use a head gasket at all, and instead just used some of this stuff:


Re: PA50 jetting/performance questions

can you take a couple of pictures of the spree head? the top and the combustion chamber side, just wanna see what it looks like.

hope it fits!

Re: PA50 jetting/performance questions

Re: PA50 jetting/performance questions

that reminds me of a question i had...

is it going to make any difference what orientation i mount the head in? it appears to be symmetrical aside from the fins.

Re: PA50 jetting/performance questions

anyone? thoughts on the orientation of the head, and possibly eliminating the head gasket?

Re: PA50 jetting/performance questions

you might wanna put the edge where it has the most fins on the exhaust side, other than that it shouldnt really matter

you can leave they gasket it off, it might not seal properly tho. try lapping the head and the jug together using some rubbing compound and put some instant rubber gasket on it, that should seal ok.

looks good btw!

Re: PA50 jetting/performance questions

thats what i was thinking for the head placement actually. the only other reasoning i could come up with would be to line the fins up in the same direction as the ones on the jug... but i dont see that mattering at all.

i think i'm going to keep the head gasket in there. i want it to be reliable, and i feel like i'd just be asking for a leak if i left it out. maybe i'll make a new one eventually with thinner material.

Re: PA50 jetting/performance questions

alright... here are my results so far:

'86 NQ50 spree head onto an '83 PA50-II

upon side by side comparison the spree's combustion chamber is clearly much shallower. the head bolts right on. i was suprised how much smaller it is overall compared to the pa50 head... a LOT less cooling fins. not sure if that might become an issue or not.

compression test results (i have a cheap gauge, it's good for comparison but i dont know how accurate the numbers really are):

stock 120psi

spree head (with head gasket) 130psi

spree head (without gasket) 145psi

the main problem with installing this head on a hobbit is the decompression valve passageway in the jug. with the spree head, there is no material above this hole to seal the head gasket over it... therefore it creates an exhaust leak which seemed to adversely affect my performance, especially from a dead stop. the bike didnt seem much faster or slower out on the road which may also have to do with the exhaust leak. i didnt take it out on the road without the headgasket because i figured it would start leaking pretty quickly.

unfortunately my base gasket seperated on me a little so now i have a slight leak down there which may be affecting performance as well.

for now i put the stock head back on, i need to come up with a way to plug that hole in the jug and also pick up a new base gasket. i may end up having the stock head shaved rather than using the spree head, afterall. the reduction in cooling surface area doesnt seem like a good idea especially since i'll be increasing compression at the same time.


Re: PA50 jetting/performance questions

jb weld?

but that seems to be my answer to everything. Let someone else come to that same conclusion before doing it.

Re: PA50 jetting/performance questions

Really, you think the hole on the jug is not being sealed? Looking at my cylinder the hole that allows the decompression is right in line with the head bolts. So it should be blocked off by the new head. Did you torque down the head? And what are you using for gaskets? With a head that small on your Hobbit, you should be getting some fierce acceleration. Sounds cool though, keep the experiments going.

Re: PA50 jetting/performance questions

the head was properly torqued, to 9 ft lbs.

i've been reusing the stock hobbit aluminum (?) head gasket, as it was in good shape. i had no compression leaks, but i definately had exhaust/air/fuel leaking out of that decompression hole (i could see/hear it). if you look at the picture of the spree head above, you can see how the sealing surface of the head isnt completely square... the decomp hole is left at least partially uncovered, creating the leak. the hobbit gasket does cover the hole completely, but the spree head does not clamp down over it.

when i get around to replacing that base gasket... am i going to need new rings and/or a cylinder hone to get things to seal properly when i reassemble? the cylinder walls appeared to be in excellent condition.. no scoring, and ports were not clogged at all.

as an aside, i must say that i'm having a lot of fun experimenting with this little two stroke. i have a fair bit of experience tuning high performance turbocharged/fuel injected 4 cylinders (in cars), and the simplicity of these mopeds makes things so much easier. you don't get to do much trial and error sort of experimentation when you're making 165hp per liter... things tend to blow up! nor can you swap the cylinder head three times within the course of a single afternoon to test various compression ratios :).

Re: PA50 jetting/performance questions

nah you don't need new rings or the cylinder honing, you only need the cylinder honing if you replace the rings...and you shouldnt need to replace the rings, but might be worth checking them anyway

Re: PA50 jetting/performance questions

Hummm. Sounds like you're close then. Take Portland's advice then and JB weld it. If you fuck up the cylinder then I've got some extra ones, and so does every other Hobbit owner who's put a kit on. I'm wondering though if it's going to be really hard to start once you get the head properly installed.

Re: PA50 jetting/performance questions

I doubt it, my hobbit starts fine without using the decompress lever, but then again that 10% increase in compression might make it a little harder to peddle start.

Re: PA50 jetting/performance questions

i doubt it will be a problem, ive never used the decompression lever to start the thing, and its not exactly hard to start :P

is riding around with a little base gasket leak gonna hurt anything? i've still been using it to get around..

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