• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Carb leaking when fuel is on??

FrostDesigns

CB750 New member
Messages
3
Reaction score
0
Points
1
Location
Indiana
I have a 78 750k and for some reason when the bike is running and fuel is on the first carb pours gas out of the overflow tube. I pulled the bowl and inspected the float, needle valve, and seat. Float works properly when manually closed. I set the float level to just under 3mm below the gasket line. Is there anything that could be causing this? Maybe another carbs float not working properly and causing this one to flood and run over?? Any help is very appreciated!!
 
It happening to me too, Honda CB750F1 1977. I own it for nearly 30 years and recently replaced all of the carb interior. That was needed but still have one single outer carb to overflow. With engine off, fuel open and floater closed by hand it remains dripping and fast. There doesn't seem to be a crack in house cast housing of the carb. Mysterious. I will let you know when I find out. I do have a spare CB.
 
Does this happen every time the fuel is turned on or sporadically? It happened to me just one time, the day I bought my bike out of a lengthy sit in someone's garage. To be perfectly honest, the only thing I could think to do, and I wasn't ready for a rebuild, was to run carb jet cleaner through the tank for about a month. It may be coincidental but I've never had the problem again. I figured if fuel is leaking, there's corruption somewhere in the fuel path and if I'm lucky it will break down. Seems to have worked.
 
It was happening to me too until I changed my gas cap. Yes, my gas cap. The one that was on there got plugged up with the tank coating stuff. It wasn't breathing, I noticed it when I went out to the garage and heard a bubbling coming from the bike. Check it out, take your cap off while your carbs are overflowing and see if it stops. If not, then its just a matter of wet testing your carbs for correct float level.
 
That's not a normal cause of carb overflow, rather a freak one. Cap vent closed means all fuel expansion due to heat then must go into carb but petcock shut off stops that unless it leaks.

The norm is fine trash hanging the fill valve partially open to leak. If they use ethanol now around you in your gasoline the common cause of it, the superfine rust it makes in these old steel fuel tanks wrecks havoc with the best fuel filters which commonly can pass much of the smaller rust particles to give fits with carbs overflowing.
 
Back
Top