Why not 30mm?

[GenuineThumpy]

hey guys,
ive noticed with carbs, that everyone seems to run the OKO's when they want the best performance, but all the threads ive read on here are about people running 26mm's and no higher, i understand people grab the 26's to match the intake but ive also seen people with a bit of port work running 26mm oko's with 28mm intakes,
is there a reason people dont use 28mm or even 30mm oko's are they harder to tune, or they just get better performance out of a 26, i have access to a 30mm very cheap which the head on my lifan is bored out to about 29.5, but just thinking that i havnt seen anyone run the 30mm before, id ask is if there is a reason people dont run them?

 

[XXX-747]

personally ive used both and the 30mm works for me without bein tuned it is just abit laggy down low and less responsive than the 26mm its like having a huge turbo like a big f off diner plate compared to a smaller one it just doesn't come on boost straight away but when it does come on boost hold on coz youl be crossing over all lanes an sh*t :angry-smiley-013: obviously a car and a bike are two different things tbh I just think unless your racing properly an sh*t I think its really upto you try both size carbies and just go with the size that you prefer

 

[zwebx]

Its all about air velocity over the jets/slide if you get a massive carb and then try to jet it all it will do is send a stream of fuel down into the intake no matter how small the jets are.. ergo making it bog and random afr
However if you undercarb the engine you will have better optimization at lower rpm (because of the higher air velocity so therefor making it easier to start and cleaner burning at low rpm (however could be restrictive at high rpm)
Lets be honnest though you want max peak rpm (if your like me and ride it there most of the time) then you want the largest carb you need eg. 125cc 26mm because of air speed.. 150cc 28/26 180cc+ 28
Air speed can be increased.. eg leaving your ports skinny but then that just restricts flow at higher rpm..

 

[GenuineThumpy]

ahk, thanks for the response guys, that clears some things up, will give both ago and see what happens, but agree with you zwebx, im always on max peak so thats where ill want mnost of my power.
cheers

 

[rotn50]

28mm okos had a real bad habit of just being pure *****es to tune, there was even talk years ago of castings internally being bad, I did use a 28 oko once and percervered with it and worked good but low revs it was rough as guts and then went back to a jetted 26 oko and everything was fine.

 

[GenuineThumpy]

26 is sounding like the way to go then. I will still give the 30 ago nevrr know. Mihgt find it better