?? 30mm flatslide or 30 mm mikuni for 140 ??

[slick 50]

hey guys just need some advice if anyone has used the 30mm keihin copy flatslide carb on the 140"s ,??
is it any better than the 30mm mikuni carb ??
i am upgrading my 13 y/o sons bike to a 140 and i would like to know what you guys think of it ( the flatslide carb that is compared to the mikuni ,
cheers , any advice would be good ,

 

[Le|Garage]

the pwk copy 30mm flat slide is WAY over kill on the 140cc and you'll never get it running right.. Try running the pwk26 (oko 26) it should work alot better. The mikuni "30mm" isn't really 30mm i don't think because it has a oval bore..

 

[MITTZ]

what about these flat side copys that maylands sell ???
Mayland Performance Spares

 

[slick 50]

thanks for the responce so far guys ,
i think ill end up getting the oko 26 for the stock motor ,,
i am slowly building up a worked 140 ,
with race cam & matching springs ,
inner rotor kit
i have a spare head so that will be port & polished
probly get bigger valves for it too ,
and a hi comp piston ,
ill get the oko 28 for that ,
cheers ,

 

[Mack]

yes the 30 is way too big. The 28 can be tuned to suit with some effort and good results, but the 26 is the best match.

 

[Cactus Jack]

Yes , Mack and Le Garage definitely know what's what !

It's the fake "30" mm 26 mm Mikunis that cause all the confusion in peoples' heads .........

 

[MasterSpoon]

out of curiously does anyone know if there would be much benefit going from a mikuni "30mm" 26mm carbie to a oko 26mm on a 140cc?
I think we came to the conclusion in another thread that its a XY motor
has a IRK fitted aswel

 

[themountain]

out of curiously does anyone know if there would be much benefit going from a mikuni "30mm" 26mm carbie to a oko 26mm on a 140cc?
I think we came to the conclusion in another thread that its a XY motor
has a IRK fitted aswel

No...OKO is the one to go for!!

 

[MasterSpoon]

now i'm confused mr mountain
so the OKO is the better carbie (which I had gathered already)
but I won't notice enough of a difference going from a mikuni to a OKO to bother?

 

[themountain]

The OKO will be much easier to dial in !

 

[Cactus Jack]

Flat slide carbs are supposed to give a better bottom end hit and better throttle response over an oval bore round slide carb . OKO's are actually a semi-flat just like a Keihin PWK . Full flat slide carbs are expensive since they need roller bearings to stop the slide from sticking open and to allow it to operate freely and smoothly .

CRF150R's run a 32 mm Keihin flat slide carb ....... BUT the carbs have an accelerator pump to overcome bogging and hesitation plus flat spots .

Most carbs rely on pure vacuum and intake air velocity to pull fuel thru the jets and enable it to adequately atomise into the air stream ......... The bigger the carb , the quicker the vacuum drops ..... So down low , you'd need to slow down the rate at which the slide is initially lifted to minimise that sudden loss of velocity and vacuum otherwise you'd need to mechanically override the resultant momentary lag in fuel delivery when the throttle is whopped open (just like Honda has done) by way of forced fuel delivery aka accelerator pump . Fuel injection also does the same thing .

Since the intake manifold throat on most pitbikes is 26 mm ...... there's no point in going bigger than that in the carb size anyway ........ all that going bigger does is create the above described problems that can't be properly tuned out merely by messing around swapping jets .....

Incidentally , a bigger carb also requires a bigger intake manifold and exhaust pipe to maintain the flow balance right thru the engine .......

 

[Cactus Jack]

yes the 30 is way too big. The 28 can be tuned to suit with some effort and good results, but the 26 is the best match.

Mack , the posts you put in this thread are so good that I've got it saved in book marks and refer people to it all the time .......... BTW , how's that shoulder going ?

Also , Maylands hits the nail on the head as to why sellers have to exaggerate HP figures and engine specs ..... the REAL people creating the problems for themselves are knucklehead buyers who are unrealistic in their expectations and go for the best sounding "hype" ........


http://www.miniriders.com.au/forum/mini-chat/12446-can-you-tell.html

We've been thru all this before .... LOL

http://www.miniriders.com.au/forum/tech-talk/11264-new-keihin-replica-pwk-30mm-flatslide-carby.html

 

[aussiemonster]

I'm not a guru like cacti and the big mack but,some of this may help.
I have a motovert rx 125.
It has a 146 big bore/high compression kit,the lifan big valve head (27mm inlet valve and 23mm exhaust valve) with basic porting and clean up.The manifold,adaptor,head etc is all port matched.It does have a restrictive exhaust because it is road legal,but i have uncorked it to a degree.
I bought the 26mm oko carb,manifold,air cleaner and throttle cable kit from dhz($89 + $11 postage).Any bigger will just make it harder to tune etc,i even think i should have gone with the 24mm oko,as being a road used toy mid range and throttle response is very important.
I have done alot of drag racing cars and bikes over the last few decades and in some ways it is easier to set up a strip only engine,because it is at open throttle for 95% of the time,but a street engine has to perform in alot of areas,and not be a pain in the ass.I think 24 to 26 oko will be the way to go.Even one needle clip position change made a significant change to how my bike runs,in the mid range etc.
I cant remember the formula off hand but there was a carb cfm chart for street strip v8's around and it is an eye opener to say the least.I think books by david vizard cover alot of this type of stuff and i learned alot over the years by putting his info in to practice.
He and alot of other engine mod guru's say the biggest most common mistake they see is over camming and over carburation for a given combination and it's intended use,food for thought for most of us i think.
Tell you what cactus and mack are a very good source of knowledge,i printed alot of what they have stated and read it over and over until i had a good idea of what they were trying to convey,when i put into practice,it has all worked out very well for the application that i have.i thank them sincerely.you cant go wrong doing the same.cheers dave perth

 

[Cactus Jack]

LOL ...."Guru" .... reminds me of the dvd .... I'm the same as you , been into modding cars and engines for drag racing but started off racing self modded XR75's at a minicycle club then moved on to modded XL350's and TT500's ...... I've read all the SA race tuning books over and over again until I just about know and understand everything off by heart . Bill Jenkins of Jenkins Competition is the REAL guru along with Smokey Yunich . I also followed and read every article about Pops Yoshimura that I could get my hands on plus read every single article on XR mods in the American Minicycle Action mags . I hung on to my old race XR and used it as a test bed for experimenting with porting , bore kits , pipe tuning etc over the years . Most people can't believe how good it goes and sounds .....it's currently getting a full rebuild ........

It's no good of knowing something and not passing the info on ....... just look at what the Egyptians achieved without any fancy machinery ....... but they never passed on the specific info on how they did it ......

The 26 mm OKO that everyone on Planet Minis and mountain are referring to is the VM-26 round slide copy ........ incidentally , I just bought another OKO kit ... a 26 flat slide this time .......

DHZ has just listed some round slide 26 OKO's ......

NEW VM26MM REPLICA PERFORMANCE CARBY KIT 140CC 150CC - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes, Boats. (end time 13-Jan-08 23:03:14 AEDST)

 

[Mack]

I have found (probably because they are expensive), a good exhaust is one of the most commonly left out items by guys chasing performance. You really will make it hard on yourself adding bigger carbs and messing with cams and head porting with the crap stock pipes. Even the bigger dia stock pipes as on some of the new bikes. There is a lot of extra performance to be had in getting the exhaust to flow well. It will bring all your changes to life and the tuning becomes much more positive when the engine "feels" like it wants to go and is just waiting for you to get the combo right. Even better than a bought pipe (for anyone who can TIG or has a mate who can) is to make your own, being careful to purge the pipe as it is made, so there is zero penetration. Also to match the bends and straight sections carefully. A hand made pipe like this even with a common can is so much better. If you are handy enough to make your own cans then you will see even better results and also 10 times better and quieter sound. A design I have found to be very good with these engines is to step up to 48mm in the pipe just before the can and the can shape to be round with a conical entry and exit so that the pressure waves have a very smooth transition through the can without any abrupt changes like hitting any flat sections internally. I pack the entire can right down into the cone ends around perforated 48mm stainless pipe which is a perfectly butted to the main pipe. Deep and quiet and very good performance. With a pipe like this I can get a stock 140 to run a 28 OKO D slide and keep up with modified bikes who are using a lesser exhaust.

BTW- Cactus the shoulder is good. Healed up well. I only had a very small tear in the end which has healed, but the thing that threw the doctors was that the internal swelling and bruising was so bad it made it seem far more serious. I swam, rode and signed for a cricket season to get it back to strength. Better than ever now.

 

[Cactus Jack]

Great to hear the good news about your shoulder mate ! And thanks for the excellent info on pipe and muffler design .....

 

[slick 50]

once again i must say a special thanks to all that have replied to this ,
you guys are a world of knowledge and information ,

it has been an excelent read so far and i have learned alot ,
cheers , Slick 50 / Rick

 

[Le|Garage]

Ah yes, btw! if anyone needs size 38, 42 or 45 slow jets for the oko28/pwk28 i can help you out