Topic: CB400 Idle Issue - Post Carb Rebuild

[texastriple]

Guys & Gals,

Looking for some wisdom.  My buddy beat me out for a steal of a deal on a CB400F. I passed because it was "locked up" - which was just the front rotor, for a measly $250. 

So in his efforts to get it running, he rebuilt the carbs, and now has an issue he can't figure out. 

The bike for the most part, will not idle, at all. The only way to keep it running is to keep the throttle open, and it will hold RPM's well, runs smooth. If you close the throttle, it maintains that engine speed for about 15 seconds, then slowly falls back to idle to eventually stall out.

I'm thinking there are 2 issues going on:
1) idle gas flow is plugged/impeded
2) float leaking into the airbox giving it the extra fuel to keep running after holding open the throttle

I'm going to go help him Friday to figure this out, and I'm going to start with a complete tear down and clean.

Anyone seen any issues like this before that would point to something else causing these issues?  I'm hoping just a tedious rebuild will solve the problem, but I don't know what else he has done on his own that may be causing this.

Appreciate any insight!

[flybox1]

What (aftermarket brand kid) did he rebuild the carbs with? 
Anything but OEM Keihin Jets will cause issues.  This is a known issue.
You can check for adequate gas flow by placing a pan under the carbs, and opening the bowl drain screw.
You can check for proper fuel level in the bowls by performing a clear tube test.   
http://forums.sohc4.net/index.php?topic=149299.0
Did a bench sync get done?
Timing and advance?

[texastriple]

What (aftermarket brand kid) did he rebuild the carbs with? 
Anything but OEM Keihin Jets will cause issues.  This is a known issue.
You can check for adequate gas flow by placing a pan under the carbs, and opening the bowl drain screw.
You can check for proper fuel level in the bowls by performing a clear tube test.   
http://forums.sohc4.net/index.php?topic=149299.0
Did a bench sync get done?
Timing and advance?

Don't know the kits, claims to not have replaced any brass parts, only rubber.

He has fuel flow, that doesn't seem to be an issue as all 4 pipes get hot. I think it's getting too much.

I'll use that trick to test fuel level in the bowls.

Yes, bench sync done.

No idea about timing and advance. That's an angle I didn't consider. I'll toss my timing light into my tool bag.

[flybox1]

Hopefully it still has the original jets in it.
Did they get pulled and cleaned? 
Even the emulsion tube hiding under the main jet.  Did it get pushed out and checked?
NGK D8EA plugs?  What color are the tips?
What is your air screw setting?

[texastriple]

Made some good progress tonight. Got it to idle and throttle up right for about 3 minutes before things went south again.

I kept finding a bowl leaking. So I made him take the carbs back off, and found a mangled seal on the float seat seal. We replaced the worst, but found damage on another, so he is replacing all 3. Once we replaced the worst is when we had it running mostly right.

The plugs were soot covered, which is incredible since it only has about an hours worth of run time.

I think we're still chasing excess fuel, but I'll provide an update next week, once the new parts come in.