08 Beta Running

[westy270]

Hi people i need your help!!

I have just got a 270 2008 beta, i know theres things been posted about float height on the Keihin carbs but an not sure if its what i need to do.

I went on a trial today and was dropping down a farily steep hill got to the bottom to turn and my bike cut out, it did this each time on this hill section, also if i lifted the back end up in a section it did the same, the bike starts fine when hot e.t.c its just this

If anyone can help would be greatful, i have the instructions about float height thats been posted befor just need to know if it it that.

[westy270]

Thanks m8 just didnt want to take it out and it not be that m8

[dan williams]

Yeah if the floats are too high the engine will cut out on downhills. Lower the floats and all should be good. I also find a little higher idle on the '08 helps a bit too.

[tinde]

I had same kind of problem with my '08 270. Riding downhill and getting top of some bigger steps caused the bike to run lean and even stalling, like the fuel was running out. First I checked the float height but is was ok. Then after being pretty p****d out with the whole machine I took the carb out (again) and took a proper look at the carb.

The Beta Keihin has only one air intake to the float chamber, as the exhaust pipe so close on the other side. The chamber air intake channel in the carb body is very near to the gas surface even when the bike near level, so when going down enough or after bigger steps when the front of the bike goes down again, the gas can begin to flow out through the chamber air intake channel and its hose, by the "siphon" effect (is that the correct word?). And having only one air channel, this causes vacuum to floatchamber and the bike will be running lean.

This was also clearly visible, after knowing what to watch, I went to test it and always when the bike began to run lean, there was some fuel flow in the chamber air intake pipe.

To solve the problem, I suppose opening the other air intake could help (as it seems that all other bikes with Keihin have both of them in use).

I simply fitted longer hose to the air intake and routed it to go loop high enough and down again where it originally goes. "High enough" was about the height of the gas tap.

It has been perfect since.

Edited September 1, 2008 by Tinde

[jse]

The Beta Keihin has only one air intake to the float chamber, as the exhaust pipe so close on the other side. The chamber air intake channel in the carb body is very near to the gas surface even when the bike near level, so when going down enough or after bigger steps when the front of the bike goes down again, the gas can begin to flow out through the chamber air intake channel and its hose, by the "siphon" effect (is that the correct word?). And having only one air channel, this causes vacuum to floatchamber and the bike will be running lean.

This was also clearly visible, after knowing what to watch, I went to test it and always when the bike began to run lean, there was some fuel flow in the chamber air intake pipe.

To solve the problem, I suppose opening the other air intake could help (as it seems that all other bikes with Keihin have both of them in use).

I simply fitted longer hose to the air intake and routed it to go loop high enough and down again where it originally goes. "High enough" was about the height of the gas tap.

It has been perfect since.

Years ago, we ran into a similiar problem with siphon effect in the SuperCross bike's carbs, before the carb manufactuers started double-venting (actually four lines and an overflow line for the bowl) the float chambers (they saw the same problem). When the carbs had single vent tubes on either side of the carb body (like the Trials carbs now) and the lines were run straight down, the vent lines would have fuel in them from the bike being bounced around on the track and when the rider would hit the on-ramp to a big triplejump, gravity would pull the fuel out quickly and create a vacuum in the floatbowl, causing the engine to miss or cut out (NOT a good thing on a steep ramp to clear a big triplejump). We figured out the cause of the very dangerous problem and all the carbs now, like the PWK39, have a "T" at the vent fitting, one line is run down and the other is run up over the carb body. This way the fuel can pull down the lower tube and the upper tube is able to draw air so a vacuum is not created, therefore no more problems.

Not sure if this will help, but if siphon effect is the cause, you can get "T"'s at aquarium supply stores. The center barb can be cut back a little so it does not stick out so far and use a short piece of tubing to attach to the vent outlet on the carb body, run the bottom line down and the top line up and over the front of the carb.

Jon

Edited September 1, 2008 by JSE

[dan williams]

As always Jon, YOU DA MAN! Hadn't thought this through to this detail but it makes perfect sense now.

[jse]

As always Jon, YOU DA MAN! Hadn't thought this through to this detail but it makes perfect sense now.

Thanks.

This was a really serious problem at the time and several riders had careers ended when they dropped, in effect, three stories and the bike's skidplate landed on the peak of the last jump (known as "casing a jump", the engine cutting out killed the momentum and speed up the onramp and the bike landed short of the downward landing ramp). This meant no suspension to absorb the shock and ankle bones were often shattered by the violent impact (spinal problems occured also).

Jon