Boring Out 28mm carbs

headcoats · Post by **headcoats** » Sat Jan 15, 2011 4:27 pm

Is it a worthwhile doing it and what is the maximum you can bore them out to ?

Bikemike · Post by **Bikemike** » Sat Jan 15, 2011 8:25 pm

You can't take them out very far.
I have a set bored out to 29.5mm, and you can almost see daylight down the side of the slide

You need to offset the Bore slightly as well, not just straight down the middle.
Apparently it is an improvement, but I have never got round to fitting them

so can't say by how much.
Mike

martin · Post by **martin** » Sat Jan 15, 2011 8:51 pm

i have heard too that it is an improvement, i have a pair 29.5mm that came off of a race bike [same bike that the nice head+barrels came off]
acording to the guy i got the bike from. does make a difference.

Charles · Post by **Charles** » Sat Jan 15, 2011 9:02 pm

ok, it will make a small difference, but the way to go are the 35mm PWKs.

Charles

fred · Post by **fred** » Sat Jan 15, 2011 9:50 pm

ive had a saved search on fleabay for some and they dont come up ,need to save a search for them others that was fitted standard ,still rare as hens teeth

Post by **500bernie** » Sun Jan 16, 2011 2:07 am

Aliens Performance have the current version of the 35mm available....but not cheap!
Cheers
Bernie

martin · Post by **martin** » Sun Jan 16, 2011 11:01 am

i do have a pair of 29.5mm carbs for sale.
i could post up pictures of them to show the difference compared to 28mm

martin · Post by **martin** » Sun Jan 16, 2011 5:04 pm

standard carbs on the left.
29.5mm carbs on the right

smithyrc30 · Post by **smithyrc30** » Mon Jan 17, 2011 1:31 am

Do a search on the fleece for air strikers in the USA.

There are usually several sets on there. not a bad price for 2 new carbs.

Alternatively get 2 kx125G carbs. These should be relatively easy to get.

I think some of the earlier CR125 and RM 125's had them as well, so make sure you get 2 that are the same.

scooble · Post by **scooble** » Mon Jan 17, 2011 12:36 pm

Got mine from the US (I had a saved ebay search for Banshee carbs). mine turned out to be as good as new and cost about the same as one carb over here.

KR-1R · Post by **KR-1R** » Mon Jan 17, 2011 9:23 pm

.
.
I think he wants to put them on an LC?
... I wouldnt be bothered with the hassle of boring out a measly 1.5mm though - chance of person doing it cocking it up and ruining good 28's

bigger carbs (35's) on a Banshee or YPVS have a slight clearance problem with the clutch cover/actuating arm? so choice of inlet manfold and angle of rubbers (or grind case) I believe.

dave32 · Post by **dave32** » Tue Jan 18, 2011 11:20 pm

The 28's even though small in size compared to say an rgv(32 to 34),arent small on flow.
Its possible to get around 65rwhp with them as shown by some of the ss400 bike's of the early 90's.
You can get fairly crisp running over a relatively wide rpm range,you could probably gain a few hundred rpm overev with larger carbs but with the stock ignition it 's pointless.
If your motor is modded accordingly then a set of 91 kx125 carbs(35mm keihins) will be of benefit, maybe.
For a roadbike or even trackday machine the stock carbs definitely aint the limiting factor.
I tried some rgv carbs with ramair filters on my tuned B2 and to be honest there was no noticeable gain,just alot of faffing about with jets and needles to get anywhere near the 28's.
Not a worthwhile mod IMO,only for bar room bragging.
Dave

TwoStroke Institute · Wed Jan 19, 2011 11:40 am

Sometimes small is better other times bigger is better. I never bore the throat of the carb.
A interesting comparision I saw recently was between a 28mm VM Mikuni round slide and a PWK28 flatslide, the PWK had more peak but the humble round slide was only 1 HP down and had way more over rev and area under the curve(came on about 1000rpm earlier).

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Boring Out 28mm carbs

Boring Out 28mm carbs

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