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The Oko 24 Rock Test

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Ok as discussed many times in past posts (and at Cope's requests) today I fitted an OKO 24mm to my 06 Sherco 250.

Firstly, after removing the OKO24 from my 125 I made a note of the current settings....M:112, P:138, and JJH needle (middle clip) then slapped it on there using the slide (and JJH) from the Keihin, which went straight in.

Started her up, warmed her up, adjusted air mix (leaner) 1 3/4 turns out, raised the idle and took up some slack on the throttle cable......Hmmm. she seem to be running nice....so let's take her up on the rocks :rotfl:

I rode over some very familiar sections and I have to say the results were excellent!!! .. For starters there was FAR MORE CONTROL over the power delivery yet the power was still there when it was needed. Secondly the power was consistent and reliable and I found myself pulling up and landing the front wheel with a great more accuracy then I have ever had before. If I was competing "C" Grade this is the carb I'd want on my bike. And to be honest after 2 hours of riding I was finding it hard to give this carb back to the 125. :wacko:

Ok the down sides....

1. Although the engine will still rev clean to the highs it does take a split second longer to get up there. I did a few small splatters but I didn't find it limited the bikes abilities in any way. If fact I found myself with greater control of the bike when it landed on top of the rock.

2. When I rode some flat track stuff I put her into 5th (at a lowish speed) and whacked the throttle wide open. The engine struggles for a bit longer (than with the PWK28), but it still manages to pull it's self into the power band.

But here's the "big one" (for me than is) .....at no time did the engine hunt at all :D:hl::D .....so I personally confirm Cope's suspicions that a 28mm Keihin is too big to tune to perfection on a late model Sherco 250.

Such was the improvement that my money is now going on a 26mm OKO soon. But again, if I were competing I would be buying the 24mm.

Make of this lot what you will!!.

Best of balance.

Neo

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My XiSPA's 250 and 280 come with the OKO 28 and they run great. I fitted the very same OKO to my 1972 250 OSSA Mar, same jetting as the XiSPA 250, raised the needle one clip and it runs absolutely perfect.

cheers

bob

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Any idea what jetting and needle is in the Xispa's?

Do they look like these with the balance tube connected form inlet to bowl? No lower tube on the bowl?

Edited by copemech

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hey,

with the mussy trials comming now i wil try an 24mm oko on my 4 stroke (a lot of people run them on ariels and bsa so i guess this should be fine.

regards bob

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Hi Cope,

Yes identical to the one pictured.....Do you know what that balance tube is doing?....I've only seen it on the OKO 24mm.

The 24m OKO is often used on Thumpsters (Honda 4 banger copies) and I picked my one up from that market place about 12 months ago. OKO's are also popular on scooters too and stuff is available for a lot less money than you normally pay for Keihin stuff.

I've purchased clear bowl too for my one. It 's odd but the rubber bowl seal swells in the first few weeks of use but is fine again months later.....Also, even though the vent in on the muffler side this carb rarely drips and keeps itself as clean as a whistle. :D ...so there's no need for all that tube work I have on my Keihin PWK 28.

My XiSPA's 250 and 280 come with the OKO 28 and they run great. I fitted the very same OKO to my 1972 250 OSSA Mar, same jetting as the XiSPA 250, raised the needle one clip and it runs absolutely perfect.

Borus, I appreciate your feedback but I am specifically referring to the Sherco 250 (and poss 290). XiSPA's will have their differences to the Sherco's. I know the header pipe and exhaust box are different not to mention the possibilities of port timing, compression etc, etc. Honestly I'm not knocking the 28mm in any way, it's a great carb, but I think it's application is borderline on a 250 Sherco.

Go for it bobtlr. :rotfl: ...let us know how you get on?

Best of balance.

Neo

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Hi Cope,

Yes identical to the one pictured.....Do you know what that balance tube is doing?....I've only seen it on the OKO 24mm.

The 24m OKO is often used on Thumpsters (Honda 4 banger copies) and I picked my one up from that market place about 12 months ago. OKO's are also popular on scooters too and stuff is available for a lot less money than you normally pay for Keihin stuff.

I've purchased clear bowl too for my one. It 's odd but the rubber bowl seal swells in the first few weeks of use but is fine again months later.....Also, even though the vent in on the muffler side this carb rarely drips and keeps itself as clean as a whistle. :D ...so there's no need for all that tube work I have on my Keihin PWK 28.

Borus, I appreciate your feedback but I am specifically referring to the Sherco 250 (and poss 290). XiSPA's will have their differences to the Sherco's. I know the header pipe and exhaust box are different not to mention the possibilities of port timing, compression etc, etc. Honestly I'm not knocking the 28mm in any way, it's a great carb, but I think it's application is borderline on a 250 Sherco.

Go for it bobtlr. :rotfl: ...let us know how you get on?

Best of balance.

Neo

Ok, collecting thoughts here. Firstly, The balace tube on the intake side seems to me to be mostly emmissions related, and with the (what seems) elimination of the overflow tube on the bottom of bowl, that just means fuel will get recycled to the inlet and flood the bike if things get upset. I am not sure one needs either, really, as other carbs like the dellorto do not. There could be some other influence on bowl chamber pressure with that positioning on the intake side that effect fuel flow.

Secondly, if given a pref, I would not run ANY, tubes on the bowl vents on Any carb, their job is to provide atmospheric pressure into the bowl, which works against the manifold vacume to manage things. As they are basically a direct route into the bowl, the tubes just act as crap deflectors for any dirt or water getting IN. Add they will still spill fuel on sloshing and tipover. I run the very short tubes as crap deflectors only, so I do not neccessarily have to clean the carby every time I wash the bike or get into dirty/ dusty conditions . I am not cleaning the carb after every ride, as the pros do.

Thirdly, if one takes notice, there is basically little difference in the install between bikes. Just as one can take a carb off the 125 and put it on the 250, one can usually take the carb off a Gasser and put it on the Sherco. The carby just sits there and gives a certain amount of fuel for a given amount of air run through it. The stoich ratio remains a constant for an engine to perform properly upon what it recieves in most all cases.

Next, the smaller bore of the 24mm carb as compared to the 26 or 28mm versions would tend to increase the velocity of the given amount of air run accross the control valve (slide) and ports that are sucking the fuel up from the bowl. Running a bit faster per stroke, also tends to be more consistant, therefore more manageable. The limiting factor being the total available flow(WOT) to the motor. A balancing act in itself!

All depends upon what the rider wants of the bike! Control is the name of the game in the sections! Predictable response is key! The 26mm dellorto is just a generic standard, simple and works, and I still do not need all it can provide on the 2.9, thus many wanting to tone them down. Specially some older models which were quite brisk!

As the Xispa was developed with the Oko, I would still be interisted to know what they are shipping with, and if it works, per Borus. As it is all relative it seems.

And with Bobtlr, the only known caution that I know might be the diffuser height(that thing that stands in front of the needle) for the 4T, requireing a change out from the original. Not sure? Measure the heights!

I do think the new carb is better, may not be worth the cost though.

MC

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Hi,

We also test a OKO/KRP 24 mm in a 250 cc GasGas with similar results: increased feel at low rpm and quite less power at high (the complete report at: http://www.todotrial.com/ttactual/taller/c...-keihin-krp.htm ). In our opinion, the 24 mm carb is just a little small for 250 for those that ride big stuff, but it is great for classic riding. A more equilibrate combination for all users is a Keihin PWK 28 sleeved to 26 mm.

BTW, the ''balance tube'' is a third jet that injects gas at the ceiling of the diffusor. Is a small jet (10, in comparison with pilot 45 or main 125) that affect only at WTO and can be eliminated for trials use.

In any case, almost any carb in the range (Keihin, OKO/KRP, Dell'Orto VHST, Mikuni VM) is better than the standard Dell'Orto PHBL.

Cheers,

JM.

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Wow!!...this is great information Marlintec.....your website is fantastic :rotfl: ...but I keep forgetting to check back there. :D

Can you please tell me how a sleeve is fitted to a Keihin PWK 28? What is the sleeve is made of and how is it kept in place? ...Are there any disadvantages to doing it this way?

And Could I use the same principle to make it a 25mm Carb?

Best of balance.

Neo

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Any idea what jetting and needle is in the Xispa's?

Do they look like these with the balance tube connected form inlet to bowl? No lower tube on the bowl?

OKO 28 mm

XiSPA 250/280 - main 125 / pilot - 45 / needle JJH / middle clip

OSSA 250 - except needle moved up one clip (may try a 48 as the air screw is just a 1/2 turn out) normal is 3/4

OSSA 350 - except pilot - 48

major improvements on the OSSA'ss, OKO's don't seem to be a finicky as the Keihin, basically set and run

i realize these are not sherco's but it gives alternatives for the OKO - all bikes run very good.

best regards

bob

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Uhhhhmmmm, good stuff there! Seems there are indeed others that have taken interise in these things!

Of course, I would have been on the forefront if I could have gotton Neo off his ass in time, mind you I am as cheap as they come, therefore I have not had either the time or dosh to run this experiment! :D

Marlintec is very good on these things, pity my Spanish is not as good as his English!

Tuning different bikes to different riders has been the issue with me, as many, including myself have had difficulty adapting to many of the high performance bikes out there. My '07, 2.9 is butter smooth, with some massaging of the carb, reeds and timing. Still has a bit of an aggressive midrange hit. Finding that proper balance of power with the larger motor is even more difficult when one gets into the 270, 280, 290, 300 class bikes it seems. How does one find a good working compromise for ones old tired ass?? Not to mention a newbie?

I have always considered the Jap carbs to be better mixers, yet the 28mm K carb was just simply more powerful accross the board when I tried it against the 26 Dellorto, just too much, thus the interist in the smaller carbs. The entire next question IS can the 24 OKO supply good controlability and power at the lower rev ranges to keep a 2.9 rider happy and full of controllable torque in the lower rev ranges that us punters seldom exceed in the sections! RELIABLY! ?

I think things are looking very positive here, with the 24mm being a very progressive yet power limiting device. I do not know many that can ride a 2.9 that takes advantage of the 26mm Dellorto power. Maybe we will flatten the power curve here a bit?

I suppose I shall get one to play with, seems good to go if the initial response was positive.

Thing is, it seems to me to be easier to bore one out to 25 or 26 than it is to reduce a 28 to whatever? Maybe I need to search for a reamer as well? :D

Could 25 be the majic number? Doubt that would be accross the board!

Then we get into the 4T thing, that is another topic of which I share great interist and little money! But I do think there may be great potential there?

Ask Marlintec if he has sleved down a 28 to about 25 on the newer SPS carbs to get a result? That would be good, maybe? :D

Edited by copemech

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Hey Cope,

If you buy an OKO24 I don't think you'll be disappointed. :D ......but next time don't forget to bribe me with some of those Carbon Fibre swing-arm guards of your :D

But in addition to the other points Ive notice that the new Dellorto VHST they are fitting in Europe is also 26mm. The OKO dealer here is OZ is also selling kits based on the 28mm and 26mm OKOs

Best of balance.

Neo

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just fitted the 24mm on my 4 stroke

big differance in lower revs, a lot smoother bike idles better starts good (on the kehin it started gos also no change there)

still a lot of power but more spread out over the revs, it feels less agressive and doesn't spit when the throttle is opened to fast.

this is how sherco should have selled it and a 28mm for the cabastany and the racing!

really a good carb just have to see what it does on a real trials test track (only tried my sections in the garden where you only use low revs)

regards bob

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Thats' interesting news Bob.....let us know how she goes on the trials test?

Also, What jets and settings did you use?

Best of balance.

Neo

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i have no idea what jets i bought the carb for my tlr 250 but i will keep it on the sherco and buy 2 more (for the honda and my bsa)

i just fitted it like i got it and it seems to be good i will look what jets and post it soon.

regards bob

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