Which Carburetor?
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cass38a
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26mm would be what I would get. The maximum size of your carby only comes into it when you have it held wide open AND your engine is at maximum revs. If you ride anything like me this is only a very small % of the time. If you are going up and down in the rev range and wanting to accelerate fast off the line then a smaller carby will be more responsive than a bigger one.
In summary in a race between a 26mm and 30mm the 26 will jump off the line better and will not tend to bog down and it will only be when you have a very long strech where you have it pinned for ages will the 30mm go better and catch you.
In summary in a race between a 26mm and 30mm the 26 will jump off the line better and will not tend to bog down and it will only be when you have a very long strech where you have it pinned for ages will the 30mm go better and catch you.
1fifty1wheel
Well-Known Member
i've had 26, 28 and 30mm oko's on a 140 and also on my 150.
for the 140 i'd keep away from the 30mm oko, especially if you plan to use a 1/4 turn throttle. if your riding alot of tight technical stuff go with the 26 if its more open and flat ground i'd suggest a 28.
for the 140 i'd keep away from the 30mm oko, especially if you plan to use a 1/4 turn throttle. if your riding alot of tight technical stuff go with the 26 if its more open and flat ground i'd suggest a 28.
Cactus Jack
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I'd say DHZ was talking about a "30" Mikuni (actually a 26 mm oval bore) being better than a "26" Mikuni (actually 22 mm oval bore) ..... Molkts are only a 24 mm round bore ...
We'd already worked out in past posts that a 26 mm round bore carb would be the best overall for 140's since the manifolds are 26 mm I/D and so is a stock BVH head port .... and 1fifty1wheel has further verified that info to be sound thru actually testing 26 and 28 mm OKO's ..... He's also verified what has been previously said about the OKO adaptors only being 24 mm I/D and needing to be opened up to match the bigger carbs to either the stock 26 mm or ported manifolds.....
Mack liked how 28 mm OKO's went up top ...... but in later posts he said he found that ported 26 mm OKO's oval ported to 30 mm acted like a small carb down low but went like a big carb up top ..... He oval bored them to 30 mm by grinding 4 mm upwards but that was on a cammed and ported engine with a 58 or 59 mm bore .....
We'd already worked out in past posts that a 26 mm round bore carb would be the best overall for 140's since the manifolds are 26 mm I/D and so is a stock BVH head port .... and 1fifty1wheel has further verified that info to be sound thru actually testing 26 and 28 mm OKO's ..... He's also verified what has been previously said about the OKO adaptors only being 24 mm I/D and needing to be opened up to match the bigger carbs to either the stock 26 mm or ported manifolds.....
Mack liked how 28 mm OKO's went up top ...... but in later posts he said he found that ported 26 mm OKO's oval ported to 30 mm acted like a small carb down low but went like a big carb up top ..... He oval bored them to 30 mm by grinding 4 mm upwards but that was on a cammed and ported engine with a 58 or 59 mm bore .....
OK, I'm after a carby size between 26-30mm so I'm kinder looking at a 28mm.
If I get a 28mm carby will I have to get a manifold that is 28mm in diameter and will the head of the engine where the manifold joins will it have to be 28mm to?
If I get a 28mm carby will I have to get a manifold that is 28mm in diameter and will the head of the engine where the manifold joins will it have to be 28mm to?
Cactus Jack
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Yes .... you're not going to get 28 sq/mm of air to flow thru a 26 sq/mm hole ......... what WILL happen is that there will be a reduction in air speed and vacuum thru the carb venturi and a delay in airflow response / reaction off idle and down low in on and off the throttle situations (aka annoying hesitation/ bog/flat spot) .... You'll have to consciously control how quickly you open the throttle from idle and low revs ......
Carbs rely on airspeed and vacuum to pull fuel thru the jets .... that's why they bog if you whop the throttle open to full bore from idle ...... the bog is caused by a sudden lean out .... the engine gets a big gob full of air without any fuel atomized in it ..... so the light goes out momentarily ...... and only recovers if you back off .... or the revs increase to the point where the fuel starts getting pulled up from the float bowl ....
Accelerator pump carbs were designed to overcome the above described short falls ... it's actually the atmospheric pressure that takes it's time to get moving ....... atmospheric pressure boosters (superchargers) get around the airflow lag time .....
Carbs rely on airspeed and vacuum to pull fuel thru the jets .... that's why they bog if you whop the throttle open to full bore from idle ...... the bog is caused by a sudden lean out .... the engine gets a big gob full of air without any fuel atomized in it ..... so the light goes out momentarily ...... and only recovers if you back off .... or the revs increase to the point where the fuel starts getting pulled up from the float bowl ....
Accelerator pump carbs were designed to overcome the above described short falls ... it's actually the atmospheric pressure that takes it's time to get moving ....... atmospheric pressure boosters (superchargers) get around the airflow lag time .....
Cactus Jack
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What carb did it have ? A phony "25" mm Mikuni that has an outlet bore of 25 mm but inside thru the venturi is 18 mm wide x 25 mm high with a 20.5 mm slide ? Effectively making it equal to 22 sq/mm in venturi area at full slide ie = to a 22 mm round bore carb which just so happens to be equal to the 110's head port diameter !!!!
Due to the fact that it's an oval bore carb ...... it will act like an 18 mm round bore carb until the slide is lifted higher than 18 mm ........ The same Mikunis that are sold in OZ and on ebay as "25" mm carbs yet have 22 stamped on them and are sold in the US as what they really are ..... 22 Mikuni oval bore carbs .....
Man , the chinese have most certainly got some people fooled with their mathematics ......
A round bore carb like an OKO is genuinely the diameter thru the venturi that the carb is sized as ....... a 26 OKO , VM-26 Mikuni or PE-26 Keihin will act as a 26 mm carb right off idle .... whereas a "30" mm Mikuni will act as a 24 mm or smaller carb due to the fact that they have a 24 mm slide .... I haven't got one here ATM to measure .........
So the difference between a "25" mm oval bore mikuni and a VM-26 Mikuni is HUGE ......
A "30" mm oval bore Mikuni only becomes a 26 sq/mm carb when the throttle cable has been pulled 30 mm ....... but a genuine VM-26 is the full 26 sq/mm carb when the throttle cable has only pulled 26 mm ...... so they snap open quicker even with a stock throttle .......
Anyway , I've posted all this excessive info so that you can't say you haven't been told ...... so read it again before you go posting .....
"I've fitted a 28 mm OKO and it needs to be tuned ..... I can't understand why it's got flat spots etc does anybody know how to ........."
Due to the fact that it's an oval bore carb ...... it will act like an 18 mm round bore carb until the slide is lifted higher than 18 mm ........ The same Mikunis that are sold in OZ and on ebay as "25" mm carbs yet have 22 stamped on them and are sold in the US as what they really are ..... 22 Mikuni oval bore carbs .....
Man , the chinese have most certainly got some people fooled with their mathematics ......
A round bore carb like an OKO is genuinely the diameter thru the venturi that the carb is sized as ....... a 26 OKO , VM-26 Mikuni or PE-26 Keihin will act as a 26 mm carb right off idle .... whereas a "30" mm Mikuni will act as a 24 mm or smaller carb due to the fact that they have a 24 mm slide .... I haven't got one here ATM to measure .........
So the difference between a "25" mm oval bore mikuni and a VM-26 Mikuni is HUGE ......
A "30" mm oval bore Mikuni only becomes a 26 sq/mm carb when the throttle cable has been pulled 30 mm ....... but a genuine VM-26 is the full 26 sq/mm carb when the throttle cable has only pulled 26 mm ...... so they snap open quicker even with a stock throttle .......
Anyway , I've posted all this excessive info so that you can't say you haven't been told ...... so read it again before you go posting .....
"I've fitted a 28 mm OKO and it needs to be tuned ..... I can't understand why it's got flat spots etc does anybody know how to ........."
TardTerror
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i ordered an oko 24mm off ken last week, Got here the next day as expected. I realised that he's changed the manifold adapters instead of having to use a nut and bolt the screw threads straight into them and they are aluminium up to where the carby joins then its rubber. The inner diameter of the adapter and manifold he gave me are all 26mm but there is a lip (easy fixed with some sand paper)
just seems like a better idea then the nut and bolt type
just seems like a better idea then the nut and bolt type
Cactus Jack
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LOL ...... I should have checked to see if you had re posted before posting the above ....
Simply stated ..... find a 26 mm diameter socket or something and IF you can't push it right thru the carb from both sides with only the needle jet stopping it ..... then you ain't got no 26 mm carb ....
Take a really good close look at the flat pad on the top of the flange on the right hand side rear of your carby and see what it says .... IF it says 22 .... then it's a 22 mm Mikuni ....... if it says 24 , then it's rated at 24 mm ....... Both my "25's" have 22 stamped there and the "30's" have 26 stamped there ........
Simply stated ..... find a 26 mm diameter socket or something and IF you can't push it right thru the carb from both sides with only the needle jet stopping it ..... then you ain't got no 26 mm carb ....
Take a really good close look at the flat pad on the top of the flange on the right hand side rear of your carby and see what it says .... IF it says 22 .... then it's a 22 mm Mikuni ....... if it says 24 , then it's rated at 24 mm ....... Both my "25's" have 22 stamped there and the "30's" have 26 stamped there ........
Cactus Jack
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The venturi is the choke area thru the slide where the carb throat is narrowest .
To do a manifold properly ... the best way is to sand and polish it smooth to get the surfaces straight , round , and parallel with zero dips ..... then give it an evenly textured surface by bead blasting , hand sanding or acid etching ... Acid etching is "fluid" compatible and the easiest to do ...
The idea behind not leaving intakes polished is so that the textured surface stops fuel droplets from beading up on the port walls ... like how water forms droplets that cling to a car wind shield ..... You'll notice that the glass in most showers isn't smooth either for the same reason .....
The theory sounds logical .... BUT !!!!!! I polished the intake port of my XR75 engine and after running it for several years then pulling it down ...... it was still polished like the day I first did it .... IF fuel droplets had been clinging to the port walls then it makes logical sense that they'd be discoloured and stained to a dark brown by baked on carbon .... The fuel would have cooked and stuck from the heat .... but there is only a slight bit of browning directly behind the valve stem and in a low spot that I missed on the side of the port floor ....
The port was hand done and finished by wet sanding with fine wet/dry sand paper .......
To do a manifold properly ... the best way is to sand and polish it smooth to get the surfaces straight , round , and parallel with zero dips ..... then give it an evenly textured surface by bead blasting , hand sanding or acid etching ... Acid etching is "fluid" compatible and the easiest to do ...
The idea behind not leaving intakes polished is so that the textured surface stops fuel droplets from beading up on the port walls ... like how water forms droplets that cling to a car wind shield ..... You'll notice that the glass in most showers isn't smooth either for the same reason .....
The theory sounds logical .... BUT !!!!!! I polished the intake port of my XR75 engine and after running it for several years then pulling it down ...... it was still polished like the day I first did it .... IF fuel droplets had been clinging to the port walls then it makes logical sense that they'd be discoloured and stained to a dark brown by baked on carbon .... The fuel would have cooked and stuck from the heat .... but there is only a slight bit of browning directly behind the valve stem and in a low spot that I missed on the side of the port floor ....
The port was hand done and finished by wet sanding with fine wet/dry sand paper .......
