Lots of talk on here about doing the mikuni modification on the carb but couldn't make head or tail of the actual mod. What do you actually need to do? 

Thanks in advance 

http://www.motomerlin.co.uk/moto/betacarb.pdf

Google     { Billy T Beta rev 3 carb mods } and you will get an excellent how to fix it and how to set up your carb. LOTS have been posted so you might find it on the links but this is the best.It  worked for me Brilliant.

Nip a hole in each of the plastic vent tubes halfway up the carb body. Done. The stock tubes are so long they end below the float bowl so if fuel gets splashed up into the tube it acts like a siphon which is why the thing will keep flowing. If you nip a hole up above the float bowl the fuel will flow back into the float bowl. 

Spoiler

 

 

57 minutes ago, dan williams said:

Nip a hole in each of the plastic vent tubes halfway up the carb body. Done. The stock tubes are so long they end below the float bowl so if fuel gets splashed up into the tube it acts like a siphon which is why the thing will keep flowing. If you nip a hole up above the float bowl the fuel will flow back into the float bowl. 

 

That's what I got

59 minutes ago, dan williams said:

Nip a hole in each of the plastic vent tubes halfway up the carb body. Done. The stock tubes are so long they end below the float bowl so if fuel gets splashed up into the tube it acts like a siphon which is why the thing will keep flowing. If you nip a hole up above the float bowl the fuel will flow back into the float bowl. 

Iv not got a tube on the overflow tho. 

It’s not the overflow on the bottom of the float bowl that’s usually the problem. It’s the vent tubes on the side of the carb.

Dan Ban

BillyT here.

A few words of observation based on your posted picture.

All the oily spots, dirt and corrosion on the engine speaks to the standard maintenance or lack of done on the bike.

I would be giving the engine a good clean before performing any surgery on the carb, who knows what foreign matter is in the carb blocking jets or get in there during carb surgery etc.

Suspecting that the oil spots are an enthusiastic application of chain lube? 

The dirt is just dirt.

This is NOT a negative comment on the state cleanliness of the bike but rather about performing carb surgery in those conditions?

The bottom of the float bowl looks like it is making contact with the top of the gear box, if so this is no-no even a little tiny gap is bad as the carb will sink down and touch the gear box after a big/hard landing causing frothing (through vibration of the engine and the sudden stop) of the fuel in the carb and playing havoc with the float levels ie leaking gas and sticking floats.

The carb is suspended by rubber boots to keep heat transferring from the engine through the aluminum and into the petrol, isolate float bowl from engine vibrations, affording miss alignment between engine and an air box.

The top fuel hose tube seems to be coming down from the petrol tank VERY close to the exhaust pipe, this is not a good idea apart from flammability issues it can also heat up the fuel.

Move the fuel supply hose to the front side of the carb down and away from the heat source.

I wrote the The 'Mikuni Gas Leaking" post a VERY long time ago and it has been proven to work for many riders over the years.

There is nothing magical about it, just basic carb physics.

Please let us know how we can help you?

Respectfully

BillyT 

 

 

 

Edited December 28, 20187 yr by billyt

The master has spoken. I’ve found it best to do as he suggests.