Teach me Stroker Tune!

[burnt_toast]

Hey guys, just recently swapped in a stroker motor into my black S, that's been sitting in the garage since 2008... This is my first attempt at working on a stroked out dio.

Figured I'd try this route before rebuilding Corsa. Anyway, this motor is all built by previous owner but somewhat of a mystery. Believe it to be 120cc, 54mm piston for sure. Pipe is stroker specific, but unknown what it is exactly. J header def looks stroker size compared to my shelved YMS or Scorpion. After several serious snags, she's installed and fired up! However carb seems to be way off at anything other than WOT, and even that may be off.

The carb is a genuine Keihin round slide (old school?), measured at 26-27mm. will have to double check.
And current jetting is as follows: 48pilot, 122main

Which seems wayy small, but my baseline was to assemble everything exactly as previous owner had set up to start off with.
SO is the Keihin jetting convention different from OKO's? The main and pilot are a different style.. main is flat head vs hex

Tune questions are:
What is a good range to start proper jetting for a setup like this?
And what kind of CHT temps should I be seeing from a healthy tune?
Lastly, I'm premixing at 40:1, this okay? Or does a stroker setup prefer 32:1?

Input is appreciated. Will try to post up pics when I can.
cheers

[burnt_toast]

p.s. dug up this old thread, has some pictures:
viewtopic.php?f=4&t=10021

[burnt_toast]

dang. anyone?? ive seen strokers running 150-160+ mains w/ OKO carbs.. but pretty sure they were 28-30mm

just want to pick up some new jets to try to tune with this Keihin, that could at least get me in the ballpark.
carb looks just like this one:
http://g01.a.alicdn.com/kf/HTB14VepHVXX ... y-free.jpg
http://s279.photobucket.com/user/high_o ... 2.jpg.html

[burnt_toast]

that carb is probably a little small. Im tuning a stroker right now with a polini 30 and I believe i will be in the 130-140 range. Give it some time there are a few guys on here that have dealt with more strokers than I have. If your air cooled you REALLY need to watch the temps

cool, thanks for reply.

indeed I am AC, hence only blasted up and down street once. been shelved since, until I can properly sort out the jetting.

found logs that said it was a 28mm Keihin, and some research later looks like there are PE27 and PE28 versions, but this one did measure closer to 27mm last I recall. Have yet to verify this..

I'm not after all out performance on this one, and at WOT it felt plenty strong, so as long as I can tune it well and stay reliable at this size, I'd be happy

[burnt_toast]

slight update:
Verified carb ID, it is definitely 27mm, on the money

Also, tried swapping main jet "flute", (forget the actual term) from my spare 21mm OKO and it fit fine, so now should be able to use all my spare OKO type main jets. pilot jets are the same as OKO. will just try bumping up from what's current, until I hear otherwise.

But would still like to know:
if 40:1 premix is sufficient (vs 32:1)
AND what critical and healthy running temps are for 125cc stroker

cheers

[veedubh20]

32mm work good and premix 32:1

[burnt_toast]

so looks like PE27 is a Japanese version carb (makes sense, since this motor was originally built in Japan), while 26/28 are Taiwan..
https://translate.google.com/translate? ... rev=search

and roughly based on this:
http://www.keihin-na.com/aftermarket/pe/
looks like I need to go up on main, and down on pilot.. will try and update.

So lastly, what is critical temperature for a 125cc stroker? And should I run full shroud, if I can make it fit? Currently just has stator/fan side shroud

[deuce217]

I would try to keep that beast under 390f regardless of what you are building (wait for the stroker gurus info on this, but I figured I would give you my 2 cents incase you were driving it around)

The fan shroud is good, as it directs air flow to the target area that needs the cooling.

If you run 32:1 slightly larger jets might be needed as that will make you run leaner than the 40:1 you are running

Ride safe.

[1man8scoots]

Stay below 375 or around. If you are cruising and start accelerating ,and the temp is already at 300, and it starts jumping up 15-20 degrees per temp reading cycle back off immediately and upjet or find the air leak.
I'd run full shrouds on an ac stroker. Even the lc bores likes shrouds on the lower exposed areas. Prevents hot spots. You can also modify your shroud to redirect air flow.
As for the Japan vs taiwan on size it's really a matter that real keihin pwk do not start until size 28. Also a pe is a round slide vs flat slide and jetting and tuning will reflect on this. Flat slides are a better option for performance. Not that a round can not tune well it's the way the air rolls into the venturi that makes the difference.
Tune rich rather than lean is the best advice anyone can give you. Rich you foul a plug and figure out your rich. Lean you melt a piston and maybe ruin a bore then figure out your lean. Rich can always be fixed easily.

[burnt_toast]

Stay below 375 or around. If you are cruising and start accelerating ,and the temp is already at 300, and it starts jumping up 15-20 degrees per temp reading cycle back off immediately and upjet or find the air leak.
I'd run full shrouds on an ac stroker. Even the lc bores likes shrouds on the lower exposed areas. Prevents hot spots. You can also modify your shroud to redirect air flow.
As for the Japan vs taiwan on size it's really a matter that real keihin pwk do not start until size 28. Also a pe is a round slide vs flat slide and jetting and tuning will reflect on this. Flat slides are a better option for performance. Not that a round can not tune well it's the way the air rolls into the venturi that makes the difference.
Tune rich rather than lean is the best advice anyone can give you. Rich you foul a plug and figure out your rich. Lean you melt a piston and maybe ruin a bore then figure out your lean. Rich can always be fixed easily.

great info, thank you!

[ped]

Stay below 375 or around. If you are cruising and start accelerating ,and the temp is already at 300, and it starts jumping up 15-20 degrees per temp reading cycle back off immediately and upjet or find the air leak.
I'd run full shrouds on an ac stroker. Even the lc bores likes shrouds on the lower exposed areas. Prevents hot spots. You can also modify your shroud to redirect air flow.
As for the Japan vs taiwan on size it's really a matter that real keihin pwk do not start until size 28. Also a pe is a round slide vs flat slide and jetting and tuning will reflect on this. Flat slides are a better option for performance. Not that a round can not tune well it's the way the air rolls into the venturi that makes the difference.
Tune rich rather than lean is the best advice anyone can give you. Rich you foul a plug and figure out your rich. Lean you melt a piston and maybe ruin a bore then figure out your lean. Rich can always be fixed easily.

Always great advice.

[burnt_toast]

So after countless combos with the 27mm (almost a dozen.. pilot, main, needle, af bleh..) she would still run like total crap on the bottom end. Like NO idle, and no reponse or power with any throttle unless up to WOT. even then when it would jump into powerband, something just doesn't feel right, and as soon as you're off the throttle, dies.

In an attempt to at least get it to idle, and rule out bad carb, I threw on my spare 21mm OKO with 120/38 jetting. This got it to finally idle, and even rev up some (ON stand). But with load, on ground still NO power.. and will NOT jump into powerband with the 21mm, at least not with this jet combo.

SO investigating further, noticed base gaskets looked like might be leaking so I did a compression test: only 80psi?!? Had only 2 skinny spare base gaskets and replaced them, dropping cyl some over previous fatty base gaskets (~2mm to 1mm ttl) and timing still looked good. comp retest: 88psi...

This leads me to believe I may have a leak on the bottom end somewhere, but I am stuck as to how to check for it. Note top end is pretty fresh, good seal on rings and no cyl scoring. Pretty surprised it even ran w such low compression, but guess the healthy top end made it do so? not sure

Points of possible leakage:
The case is originally from a SYM DD50 I believe, and it has a spigot under the oil pump hole, which used to be connected to fuel tank on the bike it came out of. I have this spigot blocked off with a small fuel line and bolt. It hasn't blown this off and I'd imagine its just for vacuum originally, thoughts?
Case seals.. this engine sat in garage for 8yrs and was not ran before that another 3 or so. How can I check to see if these are leaking?
And what should proper compression for a 125cc stroker be anyway?
Intake manifold is a naraku black, with no signs of leakage.

Not as thorough on this build as I'd like to be, lack of time mostly.. was hoping to just piece things together and try it out. But kinda stumped at the moment

[Meloshameless]

Only sign ive seen of bad crank seals is cracked rubber and small amount of oil making it past seal onto crank shaft on vari side.. and even 88 psi has gotta be low... have u checked squish? Any head gaskets? To many maybe? Or possibly to much base gasket? Take a pic of inside of cylinder @ bottom dead center and maybe 1 man can diagnose....

[burnt_toast]

This comment from bonesv on this thread, has me thinking this is where i might be losing my air..

The af18 motor is designed to operate a floor mounted fuel tank on the Dio and that has to have a pulse from the motor block itself (just below the oil pump) which you will have to permanently disable somehow, insuring there are no air leaks whatsoever.

not actively working on this project at moment, but food for thought when i get back to it. what would be the best way to seal this off?

currently just have some hose with bolt capping off, and hose clamps at bolt and spigot

[MrJumps]

Place a vacume cap on it thats a size smaller than the pipe for a very tight fit. Then heat srink over the edge of it. Thats probably the cheapiest and easiest way to do it. And it dosnt envolve filling with resin. The bolt in the line will leak over time. Vaccume cap is like 2.50 usd and the heat shrink at walmart is 6.50usd or you could use a wire clamp, ziptie, etc.