1975 Honda CB400F Carburetor Overflow
April 24, 2014
If older bikes are your thing you will face overflowing carburetors from time to time.
The symptoms are self evident, a petrol pool appearing under your bike whenever the fuel petcock is switched to on.
The first check is always to pop your head underneath your bike and observe where exactly the fuel is coming from. In this case I could clearly see the exit point, the overflow pipe from carburetor #1.
Remedy #1: tap the fuel bowl with a rubber mallet, plastic end of screwdriver, piece of softwood etc (i.e. anything that won't damage your alloy bowl). The idea is to dislodge stuck floats. If that doesn't work...
Remedy #2: a quick ride up and down your street, preferably with some bumps. Remember you are dropping fuel so peak hour and traffic aren't in this mix. Again, the idea is to dislodge stuck floats. If that doesn't work...
Remedy #3: drain the fuel bowl. This is to clean the fuel valve, found in every carburetor, by flushing with fresh fuel while emptying potential particulates. Seeing as remedies 1 and 2 didn't work in this case, here is #3 in action...
In this case that didn't work either so remedy #4 is to remove the fuel bowl on the offending carburetor.
Normally I'm not keen on gimmicky type tools, the standard suite of screwdrivers, sockets and spanners tends to manage whatever is required. However...when you have limited clearance to access a small screw head (or bolt head) these knurled knob drivers can be a good solution.
This example came in a set of three so I keep one hex head, one blade driver and one Phillips ready to go. A quarter inch drive ratchet can be fitted to the underside for more torque, however if space is at a premium this may not be feasible.
In any case, no other screwdrivers I had would fit in the confines presented here.
Here is the removed fuel bowl with some carburetor cleaner sloshing around. Note the colour, that is dissolved hydrocarbon and particulate residue. Best to be removed...
An old toothbrush helps to dislodge the hardened varnish and dirt.
And the finished product, ready for service.
After a quick spray with carb cleaner, you can test the effectiveness of the fuel valve by raising the floats with an old piece of rubber tube. After turning the petcock to 'On' the fuel valve was ''fuel tight' so job complete here.
A quick spray up the main jet boss to clean internally. This isn't really part of the problem resolution process but when additional maintenance opportunities present it is hard not to resist.
Here is the main jet (cleaned and blown out internally with carb cleaner) and it's retaining leaf spring ready for re-installation. Again, these components weren't strictly related to this task but they were quick to clean (soft wire brush for the spring, carb cleaner and rag for main jet).
Reassembled, awaiting only the fuel bowl to be re-attached.
Once the reassembly is complete, turn of the fuel petcock, wait a few seconds and if no fuel overflows you are likely successful. Take the bike for a quick ride to ensure introducing heat and vibration don't cause a repeat.
No more fuel on the shed floor or drive way here so onto the next task.