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Mikuni carb.


Tonybl
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Hi. I’ve a rev3 250. 05

can anyone help out with the mikuni carb. I’m looking to buy another float valve. as it’s worn a grove on the tip. Are they all the same size. I’ve found some on eBay for £9. 

But trials and tribulations web site are selling them at £36?

anyhelp would be good as I’m trying to stop my carb from p****** out fuel and think this is the problem. 

Edited by Tonybl
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ok here’s the condensed version; The Mikuni VM26-208 fitted to the Beta isn’t a bad carb. The fuel valve needle and seat are easy to replace and redily available. There are three hoses that come out of the carb. One is attached to the bottom of the float bowl. This goes to a brass tube inside the float bowl and is the overflow for the float bowl. If gas is coming from this hose you may have a valve or float problem.

The most common fuel piddling problem with Betas equipped with Mikunis comes from the other two plastic hoses which come from the sides of the carb body. These are atmospheric vents for the float bowl. They are never intended to see fuel however the bouncing and tipping that a trials bike is subjected too can splash fuel up into these plastic tubes. Because these tubes are long and end below the float bowl they can act siphons if the level of fuel is high in the float bowl and the bike bike is tipped enough for fuel to cover a vent hole in the top of the fuel chamber. Typically you see this when the bike is leaning on a side stand. The cure per Billy T’s instruction is to nip a small hole in both vent tubes at a point above the float bowl about halfway up the vent tubes. This prevents the siphon effect. The other piece is the properly set float level where the valve shuts off and float travel to limit the effect of the bike bouncing around on the fuel level. There are notes on how to do this on here but I’m on my phone so a bit limited in my ability to link to them. Do a search for Beta setting Mikuni should turn up either Billy’s excellent explaination or something similar from one of the SSDT prep documents. 

There are other documents that talk about fitting brass tubes and other vent trickery. Overkill I think. Just nip the tube, set the floats and be done. 

If you do decide you need a new valve needle and seat it’s worth the money to make sure they are genuine Mikuni from a reputable source.

Edited by dan williams
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Thanks for all the info. Everything posted above I knew by reading all the threads on here. 

My float valve is badly groved hence looking for a new one. 

What I was wanting to know is there a certain size or are they all the same in mikuni carbs. 

Or the best cheapest place to buy one thanks. 

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