Topic: leaking carbs after rebuild

[digger]

Hi All,

I have a CB400f 1975 and a good amount of experience with cleaning and rebuilding carburetors. This set was hydro cleaned and dried with compressed air. On this carb set are all new Honda factory seals and parts. The rubber intake boots, floats, O rings and float valves are new.  On the bench the floats are calibrated to spec 21mm from top of the float to the casting at the point of touching the tip of the valve per the manual and operate freely closing the float valve correctly. The float levels have been checked twice for accuracy. Bike was bounced vigorously and bowels tapped attempting to dislodge the floats.

Fuel is exiting two of overflow drain tubes which may be carb two and three, although at the writing of the post that has not been verified. 

I recall briefly inspecting the over flow tubes for obstructions but did not look for cracks. In this forum there are posts about cracked over flow tubes but the fuel appears to exit the drain tubes at a flow rate that is consistent with the fuel intake. Its like the valves are full open and not closed but its unlikely given the bench tests unless something is preventing the floats from rising to the level to close the valve.

Before removing them from the bike AGAIN can anyone offer any points to consider or things to inspect or test.  Thank you for reading my post.

[Duanob]

How about the tank? Is there possibility gunk from the tank, lines, or valve could've flowed into the new float needles and keeping them from shutting off completely? Happens a lot.

[Dimitri13]

Vacuum test the overflow tubes, either with a vacuum tester, or your mouth and thumb. That'll probably be the ticket.

[digger]

I am using a hanging fuel bottle made for this purpose, it is clean so no chance of dirt clogging the float valves.

[digger]

Does anyone have experience with setting the float heights on a CB400f? If yes is the manual correct specifying 21mm float height from the edge of the carb casting? 

[andrewk]

21mm should be just fine.  You'll want to go back and look for cracked overflow tubes first.

[cameron]

overflow problem?

Tubes, floats or intake valve.
Must be one of those, right?

All can be checked without removing carbs.... just pull the bowls. If you think you can get the o-rings seated again I would have replacement float bowl gaskets on hand first... because of the expansion problem... but thats just me.

Given TWO CARB involvement, I would think either contaminant, as mentioned. or age-related. (cracked tubes). Dual stuck floats would be a funny coincidence, no?

[crazypj]

I've had several carbs where floats are slightly distorted
Draining float bowl then allowing to re-fill sometimes works as does tapping float bowl with a screwdriver handle

[toytuff]

On my CB 350G I had the same problem.

Crack in the tube. I had to look close with a flashlight but it was there.

tt

[chewbacca5000]

Hi Digger,

I ultrasonically cleaned my carbs and ran into similar problem with leaking after.  I did not see any visible crack, but I know there was something not right with the overflow tubes as everyone else is suggesting.  All I did is fill them with water and they started leaking out the overflow tube.  My theory is the heat and ultrasonic action somehow cause the overflow tube to come loose where it connects to the float bowl.  If would listen to other suggestions and pull the float bowl of the leaking carbs first.  It's not fun trying to adjust float levels 26 times and still get leaks.

[Spanner 1]

Hold on a sec!.... it's 21 mm from the BOTTOM of the float to the carb body gasket surface, not the top
Their round floats, right ?

[digger]

Thanks all for the feedback. I will check for cracks in the over flow tubes inside the bowls. I am very suspicious the float valves are not closing. I have set the floats at the 21 mm top of float to casting called for in the Honda and Clymer manuals. Pretty basic. I beginning to second guess myself and the manual so having said that the first task will be to check for cracks. I will report back. Thanks again.