B25 mikuni

Here you can talk about all kind of TECH for the B50 B44 B25 And Other BSA unit Singles

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Sprint955
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B25 mikuni

Post by Sprint955 »

Hi there, has anyone fitted a mikuni to the 250 engine, if so what type and settings?
eebtr7
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Re: B25 mikuni

Post by eebtr7 »

Yes, first you have to change the head to a H*nda 250. Then, change the barrel to a H*nda 250, along with the entire lower end of the engine to a H*nda 250. Since the shift lever is now on the wrong side, you have to change the frame and all the sheet metal and plastic to, you guessed it, a H*nda 250.

I find the AMAL 28mm concentric works just fine in all kinds of weather. Besides, I don't like H*nda 250's.
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Sprint955
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Re: B25 mikuni

Post by Sprint955 »

Nothing like a sense of humour. I run a ducati 160 on a mikuni Carb and it is fine. The dell'orto was gubbed. So with my 250 bsa I am looking for an alternative just to try out, think plenty of folk have tried these options
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FGF
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Re: B25 mikuni

Post by FGF »

Hello sprint955 ,

I 've choosen a mikuni VM 34 for my B50SS because the Amal 34 is bigger and they are very close in the B50 frame.
It 's a shame , sorry...

Motocarb in Uk sells mikuni for british bikes and could advise you.
Francois
Jeff K
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Re: B25 mikuni

Post by Jeff K »

I have run them on Nortons before. Not a simple bolt on even with the "pre jetted kits" They can work, but be prepared for some time to get them dialed in. Add to the cost any extra slides, jets etc that you may /will have to purchase to get them dialed in. Here in Michigan, they also tend to be very cold blooded when starting and warming the bike up in cold weather. I find the Amal is a simpler and more forgiving Carburetor to fit and adjust. I would also strongly recommend buying or borrowing a exhaust gas /O2 meter for setting it up. The Amal seems to be able to have more overlap and transitions from one circuit to the next easier then the Mikuni. The Mikuni is more precise and very sensitive, this will lead to flat spots in the circuit transitions until you get it dialed in.
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Re: B25 mikuni

Post by lathejack »

I have fitted and run a Mikuni carb on my B25T Victor for about four years now, it works so well that I also fitted one to my B25 Starfire and another to a C15 I am working on.
The type I use is a VM26 but it's a rather old style looking Mikuni with a 30mm outlet and a 26mm ventura. The ventura is a sort of oval shape so the throttle slide is a smaller diameter, and so the length of the carb body front to back is similar to a MK1 Amal. This carb will then fit an OIF B25 with the standard airbox and inlet rubber.
The carb has a flange but the holes are smaller and 48mm centres, so the holes need enlarging and elongating to fit the standard mounting studs. But I made new studs which are reduced to 6mm on one end to reduce the amount of metal that needs to be removed from the flange.
This type of Mikuni VM26 carb costs around £25, there are copies available but despite the price tag this is a Genuine Mikuni. They are exquisitely made inside and out, have a hard coated throttle slide that is a very close fit in the body and are carefully assembled by the manufacturer.
20210610_151614.jpg
I don't know if top end power is a bit restricted because I don't blast along at full throttle, but the bikes are still rapid enough. The very low mileage B25 Victor still has its original factory fitted points ignition, and behaves so much better with the Mikuni that I have never bothered to fit the new electronic ignition kit that I bought for it.
I think I went two sizes up on the main jet and one size up on the pilot jet that they are supplied with. Starting and running is faultless with no hesitation, no more bloody infuriating cutting out when the bike is brought to a stop and a rock steady tick over before, during and after a ride without any further fiddling required.....er....a bit like a H*nda 250!
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Sprint955
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Re: B25 mikuni

Post by Sprint955 »

Lathejack, thanks for that, very helpful
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Sprint955
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Re: B25 mikuni

Post by Sprint955 »

Jeff, thanks for the reply
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Re: B25 mikuni

Post by Jeff K »

The Triumph 500 twin, the T100c came for many years with a single Amal 26mm on it it ran great. When Triumph dropped that motor in the BSA Single OIF TR5T model they bumped it up to the Single Amal 28mm. I met a guy decades ago who had two of the 1973 TR5T's and he went back to the 26mm. He said they had better low end and got better gas milage with the 26mm's.
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Re: B25 mikuni

Post by Momus »

Sprint955 wrote: Wed Jun 09, 2021 5:57 pm Hi there, has anyone fitted a mikuni to the 250 engine, if so what type and settings?
[/qwuote]

MikuniOz sell a prejetted VM30 that works very well on the 250 Beezers.

26 mm is way too small: the 30 will give you more everywhere.

The Mikuni is the superior spigot mount so insulated from vibration, heat and distorted flanges.
If you love it, lube it.
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Re: B25 mikuni

Post by Jeff K »

When I was younger, my bikes were never fast enough and I did everything I could to make them faster. As I get older I find that my bikes will now go faster then I will.
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Re: B25 mikuni

Post by Johnq1 »

Michael waller on YouTube fitted one and listed the correct jetting etc
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