cb450 doesn't stay running?

[clutch]

Not sure what to search for so I'li put this out there. Starting this thing up is pretty easy, but as it warms up, the throttle does nothing when you twist it, meaning if I crank on it the engine doesn't rev. After a few minutes the throttle seems to take and I can get going. Then it dies and just does not seem to want to run. I can start it half choked with the throttle wide open but it has about 15 seconds then dies. Vacuum? Fuel flow? I want to say that looking at the clear lines I feel there should be more fuel in there but I hear going by that can screw with you. New petcock, carbs rebuilt, tank cleaned and treated, etc.

 

[teazer]

What carbs, and other mods? Stock pipes or custom, regular Honda air filter or pods? Is this a bike that "used to run" or one that was running well before something changed?

 

[danmpls]

I would maybe check your float level.

 

[hillsy]

Sounds like the tank is not venting and the carbs are starving for fuel.

 

[clutch]

No mods. Stock Kehein carbs, open headers (just until I can get my mufflers welded). Just put this thing back together recently. Bike seemed fine on the first shakedown run except it would bog on wide open throttle...that I just attributed to the open pipes.


I tried getting it going with the cap off so I'm gonna rule out venting. I do agree it's starving for fuel but carb issues are confusing as hell to me.


Do floats get stuck like that (up, causing fuel cut-off)?
-Thanks for the replies!

 

[teazer]

Sometimes. It can happen. Turn off the fuel and either drain the float bowls or just pop them off and see if they both have fuel in them.

Half choke and throttle wide open is an unusual combination to try. Usually choke make it run richer. Opening the throttles when starting makes it run lean because the gas velocity in the throat is too low to pick up fuel.

You can also stick a container below a carb, remove the float bowl or drain screw and turn on the petcock to see how much flows out. Try it for at least 10 seconds. Repeat on the other side.

If it flows in reasonable amounts, it's not the petock or where fuel enters the carbs.

How confident are you that EVERY tiny hole and drilling in both carbs are spotless? One issue with CV carbs is the bleed hole arrangement just by the butterfly valve. On a CB450 IIRC, there are two outlets - one is through the "mixture control screw" and the other is via a small drilling above the aluminum screw on the lower face of the carb. Both must be spotless.