What Carby To Use??
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Cactus Jack
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How do you know it's a 26 mm carb ??? The Mikuni carbs you buy off ebay that they claim are "30 mm" are 26's and the ones that they claim are 25's and 26's are actually Mikuni 22's .......
Carbs are sized by the area through the venturi in the centre of them (the smallest point thru the slide) ...... NOT by the size of the outlet hole .....
So if a carb is really a 26 mm ........ you can make a cardboard roll or get a plastic hose 26 mm in diameter and fit it right thru until it hits the needle jet with it touching hard up against the walls all around .
With oval bore carbs - the 26 mm roll will have to be distorted into an oval shape to push it thru - but the area of air flow is STILL equal to 26 mm ......
Basically for max power ..... the area thru the slide / venturi needs to be the exact same area as the inlet manifold bore . You can run a smaller carb .... but the bike will just go a bit slower up top like it would with a bigger carb at less than full throttle . Usually .... a slightly smaller carb gives better bottom end to mid range power .......
Carbs are sized by the area through the venturi in the centre of them (the smallest point thru the slide) ...... NOT by the size of the outlet hole .....
So if a carb is really a 26 mm ........ you can make a cardboard roll or get a plastic hose 26 mm in diameter and fit it right thru until it hits the needle jet with it touching hard up against the walls all around .
With oval bore carbs - the 26 mm roll will have to be distorted into an oval shape to push it thru - but the area of air flow is STILL equal to 26 mm ......
Basically for max power ..... the area thru the slide / venturi needs to be the exact same area as the inlet manifold bore . You can run a smaller carb .... but the bike will just go a bit slower up top like it would with a bigger carb at less than full throttle . Usually .... a slightly smaller carb gives better bottom end to mid range power .......
cass38a
Well-Known Member
What carb have you got now and what is wrong with it?
How do you know it's a 26 mm carb ??? The Mikuni carbs you buy off ebay that they claim are "30 mm" are 26's and the ones that they claim are 25's and 26's are actually Mikuni 22's .......
Carbs are sized by the area through the venturi in the centre of them (the smallest point thru the slide) ...... NOT by the size of the outlet hole .....
So if a carb is really a 26 mm ........ you can make a cardboard roll or get a plastic hose 26 mm in diameter and fit it right thru until it hits the needle jet with it touching hard up against the walls all around .
With oval bore carbs - the 26 mm roll will have to be distorted into an oval shape to push it thru - but the area of air flow is STILL equal to 26 mm ......
Basically for max power ..... the area thru the slide / venturi needs to be the exact same area as the inlet manifold bore . You can run a smaller carb .... but the bike will just go a bit slower up top like it would with a bigger carb at less than full throttle . Usually .... a slightly smaller carb gives better bottom end to mid range power .......
Thats all interesting and true, but you didn't answer the question. LOL
Pro70, yes a 26mm carby will work on a 138cc motor. But check to see if it is a 'true' 26mm carby and not an undersized knock off. I would still suggest, for best power (as Cactus Jack said) to go slightly smaller for better mid range and go for a Mikuni 25mm and rejet it to suit your bike.
What Cactus Jack is saying is that a slightly smaller carby has a higher velocity volume of air passing thru it and atomises the fuel mixture better, giving better power. But too small a carby (say 22mm) and you'll only strangle the top end power. So make sure you have a true 26mm carby, and it'll work.
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They are'nt chinese knock off's they're just genuine mikunis made in china, the chinese ones will usually have a butterfly valve for the choke.
Cactus Jack
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Who knows for 100% certain ? They have Mikuni (Japan - T/A) cast on the side but I agree with tubes - there is NO 25 mm Mikuni carb . The ebay carbs are 22 mm and 26 mm . Incidentally if you get a hold of one of the "25's" you'll see that that they do have 22 marked on the bowl - the good ones have a smooth finish and the newer obvious 100 % knockoffs referred to as 26's have a crappy dull look and the casting is not as clean looking ........ I'd say the reason why sellers size them by the outlet bore is for the average Jo Blo to match them to their intake manifold bore .
I won one of the good "25" carbs today and I also have 2 of the 26's with the butterfly choke which a lot of people imagine are 30's based on the 30 mm outlet measurement at the rear of the carb ! To determine your throat size - you don't measure your coit .......
Carb that measures 26 mm thru the smallest point at the venturi which a lot of people think is a "30" mm carb . Don't get me wrong - these are still good carbs - top value for the price and are as big as you'd need to go on ANY stock engine unless your intake manifold and is bigger than 26 mm and your exhaust header pipe is at least 27 mm I/D :
Mikuni 30mm VM26 suit 125cc 200cc 250cc 138cc 140cc - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 22-Aug-07 20:30:00 AEST)
Carb that measures 22 mm but sold as a 25 mm carb - what the seller says in the listing is dead accurate - they WILL run cleaner due to the higher velocity thru the venturi plus the outlet is 1 mm smaller than the 26 mm intake manifold which is good for blocking reversion pulse waves . Reversion pulses cause the air to make several passes over the jets resulting in the engine running richer down low and losing bottom end snap and torque :
Mikuni carburetor 25mm mini dirtbike thumpstar 125 110 - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 22-Aug-07 11:32:57 AEST)
Here's the cheaper looking copy of the above 22 mm "Mikuni 25" which measures 26 mm at the rear ..... note the lower quality casting where the other carbs have much cleaner and sharper detail - especially the lack of where it's made and the brand name ....... but it'll still probably work really well on a stock motor .
25mm mikuni carbi 110cc 125cc atv quad trail dirt bike - eBay, Scooter Parts, Motorcycle Parts, Cars, Bikes Boats. (end time 24-Aug-07 20:00:00 AEST)
I won one of the good "25" carbs today and I also have 2 of the 26's with the butterfly choke which a lot of people imagine are 30's based on the 30 mm outlet measurement at the rear of the carb ! To determine your throat size - you don't measure your coit .......
Carb that measures 26 mm thru the smallest point at the venturi which a lot of people think is a "30" mm carb . Don't get me wrong - these are still good carbs - top value for the price and are as big as you'd need to go on ANY stock engine unless your intake manifold and is bigger than 26 mm and your exhaust header pipe is at least 27 mm I/D :
Mikuni 30mm VM26 suit 125cc 200cc 250cc 138cc 140cc - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 22-Aug-07 20:30:00 AEST)
Carb that measures 22 mm but sold as a 25 mm carb - what the seller says in the listing is dead accurate - they WILL run cleaner due to the higher velocity thru the venturi plus the outlet is 1 mm smaller than the 26 mm intake manifold which is good for blocking reversion pulse waves . Reversion pulses cause the air to make several passes over the jets resulting in the engine running richer down low and losing bottom end snap and torque :
Mikuni carburetor 25mm mini dirtbike thumpstar 125 110 - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 22-Aug-07 11:32:57 AEST)
Here's the cheaper looking copy of the above 22 mm "Mikuni 25" which measures 26 mm at the rear ..... note the lower quality casting where the other carbs have much cleaner and sharper detail - especially the lack of where it's made and the brand name ....... but it'll still probably work really well on a stock motor .
25mm mikuni carbi 110cc 125cc atv quad trail dirt bike - eBay, Scooter Parts, Motorcycle Parts, Cars, Bikes Boats. (end time 24-Aug-07 20:00:00 AEST)
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Cactus Jack
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Well the oval bore 26's are the equivalent of a round bore VM26 Mikuni - the only difference is that the slide has to be lifted higher in the oval bore carb .... meaning more twisting of your right hand to get full throttle .
Incidentally ......... here's a listing calling the 22's .....25's ........yet if you look on the bowl it clearly says 22 ........
125cc dirtbike fast 50 thumpstar 25mm Mikuni Carb NEW - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 07-Sep-07 11:22:04 AEST)
And a yank listing calling them 24's ........ Pitsters have them on their 125 GPX's as stock and at least they've always been honest and called them 22's ......... take a look on the bowl and note the 22 printed there .
eBay Motors: MIKUNI 24MM CARB CARBURETOR XR50 CRF50 KLX110 TTR90 125 (item 110162147172 end time Aug-28-07 19:18:23 PDT)
Here's another yank listing calling them 26's ....... LOL ...... if you take a good close look at the bottom left hand pic in the cluster of images of the carb ....... you'll notice a small flat pad on the manifold mounting flange and it has 22 stamped there .....Note how they avoid showing the left side of the float bowl ......
eBay Motors: 26MM MIKUNI CARBURETOR CRF50 XR50 CRF XR 50 70 KLX CARB (item 150153812111 end time Aug-26-07 18:33:46 PDT)
After noticing the numbers stamped on the pad ..... I just took another good close look at my 26 (30 Mikuni) and noticed that it has 26 stamped there ..... the stamps are very small and hard to see and I doubt very much if the chinese would go to all that trouble ...... plus putting varying coloured paint dots and marker pen marks which are put there during assembly as part of quality control . That indicates the the Japan- T/A marked carbs could very well actually be made in Japan ....... they are neat , clean and well made .....
The others could still possibly be made in the same factory ..... but as lower priced generics with ID marks left out to distance the manufacturer from the shoddy quality .........
Incidentally ......... here's a listing calling the 22's .....25's ........yet if you look on the bowl it clearly says 22 ........
125cc dirtbike fast 50 thumpstar 25mm Mikuni Carb NEW - eBay Other Makes, Bikes, Motorcycle Parts, Cars, Bikes Boats. (end time 07-Sep-07 11:22:04 AEST)
And a yank listing calling them 24's ........ Pitsters have them on their 125 GPX's as stock and at least they've always been honest and called them 22's ......... take a look on the bowl and note the 22 printed there .
eBay Motors: MIKUNI 24MM CARB CARBURETOR XR50 CRF50 KLX110 TTR90 125 (item 110162147172 end time Aug-28-07 19:18:23 PDT)
Here's another yank listing calling them 26's ....... LOL ...... if you take a good close look at the bottom left hand pic in the cluster of images of the carb ....... you'll notice a small flat pad on the manifold mounting flange and it has 22 stamped there .....Note how they avoid showing the left side of the float bowl ......
eBay Motors: 26MM MIKUNI CARBURETOR CRF50 XR50 CRF XR 50 70 KLX CARB (item 150153812111 end time Aug-26-07 18:33:46 PDT)
After noticing the numbers stamped on the pad ..... I just took another good close look at my 26 (30 Mikuni) and noticed that it has 26 stamped there ..... the stamps are very small and hard to see and I doubt very much if the chinese would go to all that trouble ...... plus putting varying coloured paint dots and marker pen marks which are put there during assembly as part of quality control . That indicates the the Japan- T/A marked carbs could very well actually be made in Japan ....... they are neat , clean and well made .....
The others could still possibly be made in the same factory ..... but as lower priced generics with ID marks left out to distance the manufacturer from the shoddy quality .........
sean01
Legend Spanner Spinner
a round one with some bumps on it bro...
seriously start your own thread mate, people might take you serious instead of dragging up a 4 year old thread
