Hi,

I’ve just removed the carb from a new to me Factory 4t as I was having problems with it occasionally popping and stalling off idle. 

When I’ve removed it I've noticed a rubber bung on a breather pipe at the top of the carb had perished and split. 

Two questions:

1- would that split cause the problems I’m having?

2- is this bung normal (it seems too smaller diameter for the pipe?)

Thanks for any help, it’s a bit more complex than the 2t carb I’ve been used to! 

Yes, that decomposed rubber cap will make your bike run crappy and its a common part to fail. That plugged pipe is for equalizing pressure on street bikes with multiple carbs and not used in your application. Should be able to find one in a automotive parts supply store...hopefully better quality. Good luck 

11 minutes ago, pauls320 said:

Yes, that decomposed rubber cap will make your bike run crappy and its a common part to fail. That plugged pipe is for equalizing pressure on street bikes with multiple carbs and not used in your application. Should be able to find one in a automotive parts supply store...hopefully better quality. Good luck 

Brilliant, thanks very much. 

51mon, also check the Beta manual.

They came out of the Factory lean, the Manual shows what jets are required for competitions use, its well documented on here.

Ive just changed mine, a 2016 Beta Evo 300 4 T for a new standard one and my old one use to backfire occasionally, it scared the ******* out of observers and fellow competitors. But what a superb bike, especially with a 10 tooth front sprocket and an alloy Amal 80/200 slow action throttle, enjoy it. ??????

29 minutes ago, chappo said:

51mon, also check the Beta manual.

They came out of the Factory lean, the Manual shows what jets are required for competitions use, its well documented on here.

Ive just changed mine, a 2016 Beta Evo 300 4 T for a new standard one and my old one use to backfire occasionally, it scared the ******* out of observers and fellow competitors. But what a superb bike, especially with a 10 tooth front sprocket and an alloy Amal 80/200 slow action throttle, enjoy it. ??????

Currently got a 120 Main and 27.5 pilot. Seems to go ok but read about people going for a 132.5 main? May give it a go after I’ve got used to the bike a bit more and figured out what’s what. 

10 tooth just been fitted - much better!

Spoke to a chap from Acklams about a mate`s bike that he was having problems with. He said they fit a 130 main jet as a matter of course in all the Beta 4t`s, replacing the 120.

Does the 130 produce any black smoke when full reved up in neutral?  I have a 125 in mine and get just a hint of black smoke at full throttle in neutral.  I don't know if it does that under load.

I'm at sea level, and mine had serious flameout/decel popping (PO rode at 10,000+ feet with stock jetting).

I went with a 132.5 main and 25 pilot, dropped the clip one slot (second slot from top) and changed to the silver spring.  Solved all of my issues.  Starts easy, purrs like a kitten, no decel pop or flameouts, good low end grunt.

My poor running same as yours was fixed with half a turn on the fuel screw. Anti clock to ritchen.

Edited September 16, 20187 yr by 4stroke
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