Hi there. 
I have a Beta Rev 3 2008 with KEIHIN carb. The bike starts great with the choke. The second I touch the throttle it bogs down and dies. As soon as it’s slightly warm I can turn the throttle fully open quickly and it’ll bog for a second and spring to life. No problem whatsoever when warm. 
Could this be a fuel issue? Maybe need a slight adjustment to the air screw? 

Thanks 

To me that sounds pretty normal

Normal starting from cold would be on any two stoke trials bike I know

Full choke - start engine- after a few seconds - close choke - until engine is warm enough - run carefully

Air screw has no function at mid throttle. Use it for idle mixture (throttle closed)

Keihins are prone to needing an occasional cleaning. The exit hole for the pilot circuit is tiny as in microscopic. Yes it looks bigger if you eyeball it (the little hole in the bottom of the carb bore behind the slide) but it necks down significantly. Even a tiny bubble of water in that hole will make it bog coming off idle.

Best to pull it apart and run some carb spray through the holes. Usually clears it up.

I think it likely to be a blocked pilot jet, it could be as simple as the fuel has evaporated leaving the oil behind. 

Best to find a diagram of the carb then dismantle that from your bike check all gaskets and O rings and blow it clean through all the drillings using compressed air, careful with the float and notice the needle position is back where you found it. Check and clean the air filter as well.

1 hour ago, dan williams said:

Keihins are prone to needing an occasional cleaning. The exit hole for the pilot circuit is tiny as in microscopic. Yes it looks bigger if you eyeball it (the little hole in the bottom of the carb bore behind the slide) but it necks down significantly. Even a tiny bubble of water in that hole will make it bog coming off idle.

Best to pull it apart and run some carb spray through the holes. Usually clears it up.

Thank you. I’ve recently taken the carb apart and cleaned it. I thought I blew enough air etc through everything to clear it properly but I might have missed a bit. I’ll take it off and give the pilot jet a good jet of air. What Ifind strange is why it doesn’t do it when I rev up from idlewhen warmed up? 

Edited April 6, 20206 yr by StP

1 hour ago, trialsrfun said:

I think it likely to be a blocked pilot jet, it could be as simple as the fuel has evaporated leaving the oil behind. 

Best to find a diagram of the carb then dismantle that from your bike check all gaskets and O rings and blow it clean through all the drillings using compressed air, careful with the float and notice the needle position is back where you found it. Check and clean the air filter as well.

Thanks! I cleaned the carb recently so I’ll take it apart and check it again tomorrow. I was planning on replacing the air filter and cleaning out the air box soon so I’ll do that tomorrow too, to make sure no crap could get into the carb. Makes sense that it would be the pilot jet since it idles perfectly but as soon as I touch the throttle it bogs. What I find strange is why it doesn’t do it when I rev up from idle when warmed up? 

Does it bog if the choke is on when cold starting? Could be you are right on the edge of lean when cold, no choke. It then bogs if you ask for throttle. However, once warmed up the motor doesn’t need as much fuel so you don’t get the bog. 
 

Don’t forget you have an air screw. You could try 1/8th to a 1/4 turn in (richer) to see if it helps?

All bikes will do that when cold, they run leaner, even when you take the choke off the bike is still warming up for the next minute or so. As pindie says, if this disappears when warm, its nothing to worry about, if theres still a slight hint, then its indicating you are too lean.

Edited April 7, 20206 yr by faussy

7 hours ago, pindie said:

Does it bog if the choke is on when cold starting? Could be you are right on the edge of lean when cold, no choke. It then bogs if you ask for throttle. However, once warmed up the motor doesn’t need as much fuel so you don’t get the bog. 
 

Don’t forget you have an air screw. You could try 1/8th to a 1/4 turn in (richer) to see if it helps?

Yes, only occurs when starting from cold. I’ll give it an 8th of a turn in when I take the carb off later. Almost impossible to adjust the air screw without spinning the carb around! Thank you 

8 hours ago, faussy said:

All bikes will do that when cold, they run leaner, even when you take the choke off the bike is still warming up for the next minute or so. As pindie says, if this disappears when warm, its nothing to worry about, if theres still a slight hint, then its indicating you are too lean.

Great, thank you. Cleaned the carb out and blew air through pilot jet today. Turned the screw 1/8th in also. The same thing happened again after I started it but disappeared a lot quicker so I’ll leave it at that. 

8 hours ago, faussy said:

All bikes will do that when cold, they run leaner, even when you take the choke off the bike is still warming up for the next minute or so. As pindie says, if this disappears when warm, its nothing to worry about, if theres still a slight hint, then its indicating you are too lean.

Thank you! Cleaned carb and pilot jet today as well as turning air screw in 1/8. Same problem occurred however it went away a lot quicker today. No issues whatsoever when warm so I’ll leave it as it is! 

5 hours ago, StP said:

Yes, only occurs when starting from cold. I’ll give it an 8th of a turn in when I take the carb off later. Almost impossible to adjust the air screw without spinning the carb around! Thank you 

Cleaned carb and turned air screw in 1/8. Same thing happened when starting however it wasn’t as severe and it went away a lot quicker so I’ll leave it as it is as it’s perfect when warm. Thanks 

Be careful with air pressure on the Keihin as there’s a gasket at the base of the float tower that is impossible to find a replacement for. 

One other thing that can happen is putting the needle jet in upside down. Sounds ridiculous but I’ve done it.? The tapered hole goes up.

You can access the air screw from the left of the bike easy enough. Just look through  above the case and pipe and you’ll see it. A long flat head screw driver helps. 
 

Sounds like you have gone in the right direction. As the weather warms up and air becomes less dense you’ll probably find it gets better as the setting you have now will be made to feel slightly richer by higher air ambient temperature even though you change nothing more with physical settings. Come winter you generally need more fuel going in as the air is denser. 

9 hours ago, pindie said:

You can access the air screw from the left of the bike easy enough. Just look through  above the case and pipe and you’ll see it. A long flat head screw driver helps. 
 

Sounds like you have gone in the right direction. As the weather warms up and air becomes less dense you’ll probably find it gets better as the setting you have now will be made to feel slightly richer by higher air ambient temperature even though you change nothing more with physical settings. Come winter you generally need more fuel going in as the air is denser. 

Thank you. I’ll keep an eye on it!