Bike: Evo 4T 300, 2012

I took the carb off the other day and didn't initially notice this rubber "cap" had fallen off from somewhere. It was lying on the top of the crankcase when I went to put the carb back. It was lying just under where the carb sits.

It has a hollow interior, just as if it fits over a spigot of some sort.

I cannot see where it has come from. The only possible place I can see is that it fits over where the mixture screw is, just under the engine side of the carb venturi.

Can anyone help with its proper location?

Incidentally the rubber has already perished, hence the split which can be seen in the photo.

Thanks for your collective wisdom.

99% sure it blanks a pipe off on the top part of the carb poss on right side think its a shinny right angled pipe

99% sure it blanks a pipe off on the top part of the carb poss on right side think its a shinny right angled pipe

Thanks Zero.

I thought about that but the inner diameter of the rubber cap was too small meaning it was really stretched to go over the shiny right angled pipe (called oil breather joint in the parts manual, but I'm certain there was no oil breather pipe connected to it). Perhaps its split because the diameters are wrong.

At the moment the pipe is open to atmosphere and I did think that strange.

Couldn't see anything listed in the parts manual which looked close.

i had to change mine as was perished just like yours

i had to change mine as was perished just like yours

Thanks again Zero, I think you've solved it!

Is this a Beta 4T. If so this part is an important little part.

Is this a Beta 4T. If so this part is an important little part.

Yes, it's a 4T BillyT.

You say important so do you know what are the consequences of it not being fitted? Just that the bike was recently intermittently backfiring on overrun when it had not done so before. That was why I took the carb off to check it over.

This little rubber part goes over the atmospheric equaliazation tube for the slide diaphragm on the carb. Otherwise known as the angled shiny part at the tops side of the carb.

This angled spigot tubes job is to let the slide breath/move freely as it adjusts to the pressure differential as the slide moves up and down.

If one where to block this angled spigot's hole up completely (epoxying it) the slide would not move up and down correctly.

With that being said, leaving it completely open lets it breath properly, Caveat: If it was left completely open because it points straight up it is VERY prone to water getting into (washing the bike) and also dirt. It is best to take a rubber hose put it on the spigot and let it arc or bend over gently facing downwards. The little rubber cap installed from Beta just lets it breath enough for it to function. In the normal application of this CV carb i.e. dirt bikes, quads etc it would have hose attached to it.

Cheers

Edited October 9, 201213 yr by billyt

This little rubber part goes over the atmospheric equaliazation tube for the slide diaphragm on the carb. Otherwise known as the angled shiny part at the tops side of the carb.

This angled spigot tubes job is to let the slide breath/move freely as it adjusts to the pressure differential as the slide moves up and down.

If one where to block this angled spigot's hole up completely (epoxying it) the slide would not move up and down correctly.

With that being said, leaving it completely open lets it breath properly, Caveat: If it was left completely open because it points straight up it is VERY prone to water getting into (washing the bike) and also dirt. It is best to take a rubber hose put it on the spigot and let it arc or bend over gently facing downwards. The little rubber cap installed from Beta just lets it breath enough for it to function. In the normal application of this CV carb i.e. dirt bikes, quads etc it would have hose attached to it.

Cheers

Billy T

Just trying to clarify what you are saying.

You say the angled tube lets the slide diaphragm breathe enough, but there's no hole in the cap, and it is a tight fit. If the tube should be left open to allow for the equalisation of pressures then why do Beta fit a cap which seals off the tube?

Hey Portman

The little black cap is flexible enough at the tip (round part) to give a little plus air does get past the edges of it.

Any serious application of this CV carb on any other bike, quad would see this spigot have a little rubber hose attached and pointing downwards to avoid water, crap from getting into it.

Try this test, Take the carb off and lift the slide up and down with your finger, feel the resistance, and how it lifts up and down quite smoothly, now block up the spigot good and tight with another finger and see how many times you can lift the slide up and down before the slide gets sticky and sluggish. You can even try this exercise with the little black cap installed and not installed.

Or stick your mouth over the spigot and lift the slide up and down and feel the air going in and out.

I am guessing that the movement of the slide going up and down due to pressure changing makes the little black cap eventually move and come off that is why they always come off or split.

The issues with this little black cap is amplified if you live/ride in high in elevations.

I guess Beta felt if was good enough with the little black dust cap ??????

Cheers

Edited October 10, 201213 yr by billyt

OK, thanks Billy T

Portman