BIG carbs VS little'uns

[dave32]

So it seems alot of us think we need to fit 35mm carbs to let our KR's breathe better,is it a mis-concept or fact?
Yes when you look what other engines use the stock 28mm seem a bit lacking but then again it was/is the fastest Road going 250 ever made (out of the crate) so its not that straight forward.
My take (i maybe wrong) ,
The KR motor is very well designed,nothing restricts as such,everythings designed to work together from Iginition/porting/pipes/intake,and this is where i think its problem (if you can call it that) lies,
Its not restricted as such so you CANT just chuck on a set of BIGGER carbs and "unleash the hidden power" as its not that simple.
I think once you start getting over 60hp THEN it may help to fit larger carbs,but until you have got that far it may be a waste of time and your actually not GAINING but LOSING out in some areas,
small Carbs have a high velocity so give good running over a wide range,there also less likely to LEAN out at the top end (like large carbs),
there also less fussy about jetting.
So what does everyone else think?
Waste of time fitting them or essential for Glory?
Cheers
Dave

[Luders]

Essentially, I agree with you Dave.

On a standard engine and airbox, I really can't see there would be much gain.

Once you get the engine tuned and a decent air space to allow the 35mm's to breathe properly, then you will see the gains on the dyno.

[dave32]

Anyone got any DYNO graphs showing the gains on a "well tuned" motor?
MJ43?

[mj43]

2hp across the board

[dave32]

At what level of tune was that engine Mark?

[Luders]

Not a comparison between a tuned engine on 28 and the same bike on 35's, but a nice looking amount of power from Mark's bikes and as he say's 2 hp gain across the range.

[headcoats]

Wrong bike but Mate's 350 YPVS engine made over 70bhp on standard LC 26mm carbs

Pipes (Kenny TSA) and ignition (Zeeltronic) made the biggest gains

[Top-shaggy]

Im no expert tuner like many of you lot. But i do agree with you Dave, KHI designed the KR1/S engine very well. You probably don't need anything more than the 28s on most engines especially for road use.

Of course they also added 35mm carbs to the R models as standard and I trust KHI that this was for a good reason IE racing !

For me the biggest kick is the noise.. The 35s just 'sound' faster to me

[KFOR]

There are 125's running 38mm carbs even

[dave32]

cheers for the replies
Still it would be nice to see what the gain is (if any) when just swapping carbs on a reasonably well tuned engine,say late 50's low 60's hp.
I dont think you would ever need 38's on the KR,a h***a RS 125 or TZ uses that size and there capable of over 45 hp per cylinder (250).
Its interesting though that the KR1 reed block is the same as an early 90's KX125 which run 35's as stock.
Also some tuners used to get good power out Supersport 400 spec KR's running stock carbs.

[KR-1R]

.
.
the 28's are a fantastic carb (nimble and perky) but the thought of dropping those two tiny screws (top) somewhere in your engine when you might make a needle adjustment is one advantage the big PWKs have with jam jar lids.

the KX125 went to 38's briefly in ?92 or ?93 (the KX250 ! was on 38's in '90 or earlier)
however I think the 125 dropped back down to 36mm future years (leaving you ability to bore them bigger if desired on the large body carbs - extra meat)

the cheapest pairs of PWK's are from (certain - somewhat rare) Polaris snowmobiles US$80-150 pair '97 '98... but they are 39mm !...


it would be interesting to get The Flute's opinion on how they may have went

[TwoStroke Institute]

So it seems alot of us think we need to fit 35mm carbs to let our KR's breathe better,is it a mis-concept or fact?
Yes when you look what other engines use the stock 28mm seem a bit lacking but then again it was/is the fastest Road going 250 ever made (out of the crate) so its not that straight forward.
My take (i maybe wrong) ,
The KR motor is very well designed,nothing restricts as such,everythings designed to work together from Iginition/porting/pipes/intake,and this is where i think its problem (if you can call it that) lies,
Its not restricted as such so you CANT just chuck on a set of BIGGER carbs and "unleash the hidden power" as its not that simple.
I think once you start getting over 60hp THEN it may help to fit larger carbs,but until you have got that far it may be a waste of time and your actually not GAINING but LOSING out in some areas,
small Carbs have a high velocity so give good running over a wide range,there also less likely to LEAN out at the top end (like large carbs),
there also less fussy about jetting.
So what does everyone else think?
Waste of time fitting them or essential for Glory?
Cheers
Dave

Think it has more to do with the rpm range your working in. A GP 125 with a 38mm carb just isn't trying Aprilia RSA ran a 41 and 42mm carbs you will need a carb that size to go over 50HP. What is a big influence is intake tract length and keeping the same cross sectional area along the intake.

[dave32]

RPM range,yep that had crossed my mind,seeing as the KR doesn't REV very high was one of the reasons i questioned the Big carb swap.
From experience the difference ive noticed is the engine will rev harder at the top end and hold on to the power before peaking,this was only "seat of the pants" dyno though.
the 88 and 89 KX125 used a 39mm carb,went back to a 35 for 90 ,91 and 92,the 92 is the same as fitted to the kX250 but undersized (from 38 to 35) on the bore size only.

[Luders]

I know Kev should be doing a dyno run on his bike soon with 35's and he did one the other month with 28's, so that should be a good apples for apples comparison for us if he's willing to post his charts.