Carb Problems

[grahambaskin]

Hi,

any know the correct setting for a rev3 2007 carb, my bike is pouring petrol out the over flow, i have rang a dealer and they told me the the mikuni carbs all leak. is this true? any body have the same problem?

[dan williams]

Yes the Mikunis all leak because the air vent tubes are too long and end below the float bowl. Billy Traynor did an excellent piece about setting the floats and nippering a small hole in the vent tubes near the top of the tube. It cures all for no money.

See here

http://www.r2wtrials.co.uk/html_files/betacarbsetup.html

[grahambaskin]

Cheers Dan, i took carb off today looked at the setting of the tangs, they were way out. the only way i could get them as they are in Billys Diagram was by bending the little tab between the tangs was this correct? i then put the carb back on and started the bike it rang fine for a few mins put when i lay it on its stand it leaked out the overflow on the bowl. Does the vent tubes make the difference?

that carb is a real awkward one to put back on would you agree.

[the addict]

Graham, swap the air box/carb boot around the other way, the outlet from the airbox is larger than the carb mouth so makes getting the carb back on easier. I also removed the spacer in the reed block inlet and although its quite thin it make putting the carb back real easy. I had the drilling mod done to my Mikuni which stopped the leaking altogether, think there is a post somewhere here about it with some pics I took of the moded carb.

I see the K's are also p***ing fuel out worse than the Mikuni (if thats possible) The Surgeon used about 17 litres of fuel today at the Yennards, **** i'm gald i aint got a carb anymore

Edited May 3, 2008 by The Addict

[grahambaskin]

i have heard of the drilling mod, but dont know where i can get it done, . Can this drill mod be done by myself. I only have the bike a few months, i thought by buying a fresh bike i would have no problems. my friend has a montesa no probs there at all.

[the addict]

where are you Graham? had mine done by BVM Moto, expect you could probably post them the carb if eaiser,

[the addict]

Heres the pics I took Clicky here

[grahambaskin]

in Northern Ireland but i wouldn't mind posting it away if it solved the problem. Is it a long term solution and how much is it? i was going to buy a new carb at the begining before i looked hear and saw thats it seems to be a problem with most beta's

[dan williams]

The vent tubes are THE reason for the leaking fuel. There is one overflow tube. That's the little brass tube that sticks out the bottom of the carb. I've only seen one carb with the problem of leaking out there and that was because the floats were rubbing on the bowl gasket. Easily fixed. Take the carb off again and get some cutting pliers and nipper a hole in the tube about halfway up the carb body. That will stop the leaking. What happens is as the carb is bouncing around the fuel sloshes up into the vent tubes. Since the vent tubes end below the float bowl they turn into siphons and will drain the whole tank in short order. If you nipper a hole in the tubes above the float level a little fuel will come out the tubes but stop as soon as the bike is upright as the gas would rather go back into the floatbowl. Basically reversing the siphon. As for getting the carb in and out I wrote a procedure here on that subject. Search my old posts and you should find it. A big help is lubing the rear airboot with WD40 or something similar. The other thing is to have a rounded tool to chase the airboot onto the carb bell. I use a pair of curved hemostats so I can chase the boot on and then grab the flange to pull it the rest of the way on. It seems like a real nightmare to get the Mikuni on and off but after the first few times it takes about 1 minute to get it off and 2-3 minutes to get it back on.

[dan williams]

Make that "tubes" although the pipe usually melts one shut.

[dan williams]

Oh yeah forgot to answer your original question. Sorry about that. Yeah the little brass tang in between the towers sets the float level. i.e. where the fuel level is in the float bowl when the valve shuts off. The other little brass tang that bumps up against the tower sets the float travel limiting how much the bike bouncing around can cause the floats to bounce around. The needle valve is really close to an on off switch. It's tapered so there is a lot of surface area for sealing and when it opens there's no benefit to having it open further then ~2mm. The extended float bowl is only intended to provide a deeper fuel reserve for situations where the float bowl may not fill properly because the floats are bouncing around with the throttle wide open for an extended period of time. Not really a scenario trials riders are likely to encounter on anything but a wide open loop trail full of whoops.

Edited May 3, 2008 by Dan Williams

[grahambaskin]

cheers dan will try that tube thing aswell, yea my leak was out the copper overflow at the bottom of the bowl. wil check to make sure the floats inside the bowl. thanks

[grooster]

had the same problem with the overflow tubes - used the link Dan provided to the Texhoma club's newsletter with EXCELLENT photos on how to make the change. Less than $4 to fix this problem using the proper type tubing and a vacuum T. I'll know for certain on Sunday when I head to the quarry for practice. So far it seems to be the proper fix!

Thanks!

Greg

[bob]

The vent tubes are THE reason for the leaking fuel. There is one overflow tube. That's the little brass tube that sticks out the bottom of the carb. I've only seen one carb with the problem of leaking out there and that was because the floats were rubbing on the bowl gasket. Easily fixed. Take the carb off again and get some cutting pliers and nipper a hole in the tube about halfway up the carb body. That will stop the leaking. What happens is as the carb is bouncing around the fuel sloshes up into the vent tubes. Since the vent tubes end below the float bowl they turn into siphons and will drain the whole tank in short order. If you nipper a hole in the tubes above the float level a little fuel will come out the tubes but stop as soon as the bike is upright as the gas would rather go back into the floatbowl. Basically reversing the siphon. As for getting the carb in and out I wrote a procedure here on that subject. Search my old posts and you should find it. A big help is lubing the rear airboot with WD40 or something similar. The other thing is to have a rounded tool to chase the airboot onto the carb bell. I use a pair of curved hemostats so I can chase the boot on and then grab the flange to pull it the rest of the way on. It seems like a real nightmare to get the Mikuni on and off but after the first few times it takes about 1 minute to get it off and 2-3 minutes to get it back on.

In my experience the carb overflow is the usual cause for loss of fuel, not the atmospheric tubes which can lose you some fuel if the bike is thrown about a bit, and in occasion may set up a siphon effect, more often than not though I have found that fuel just pees out of the overflow on the float chamber, what with the carb being tilted forward steeply already and the float heights being a bit too high from new, it is a fine balance to get the float level low enough to stop the overflowing but not so low as to cause fuel starvation. I have adjusted the float height alone on both my betas and solved the fuel overflow issues, I also find that removing the tube on the carb which leads away from the overflow tang means it can't siphon, haven't had any problems with muck getting in, although water can when the bike is washed I clean the float bowl out anyway.

Just my experiences,

Bob