Flooding Issues on Vintage Hondas

Flooding is a common problem in vintage Hondas – especially on bikes that don’t get run much.  Flooding can be just a minor aggravation …like hard starting or poor fuel mileage. But, it can also lead to catastrophic problems such as severe engine damage or even a fire!  Flooding issues should not be ignored!

Elsewhere, I wrote:

“Fuel regulation is accomplished by the rise and fall of each float. As the float rises to the appropriate level, the float pushes against a rubber-tipped float “needle” which operates against a precision machined “seat” to stop the flow of fuel. Likewise, when the float falls, the float needle opens the fuel inlet and allows fuel to flow into the bowl. In actual operation, the float needle oscillates through a relatively small range of motion. Sometimes theorists describe this motion as “vibration.” A running engine has a constant need for fuel, so the float needle is rarely 100% closed. Rather, it’s mostly regulating the flow rate of fuel into the carb bowl.

Only when the engine is shut down, does 100% closure need to occur.

Here are the usual causes of flooding.  This material is written for the Honda GL1000, but much of this applies to other models:

  1. Debris under float needle from contaminated fuel
  2. “Dirty” carbs that need internal cleaning
  3. Misadjusted, damaged, saturated  or “binding” float(s)
  4. Worn out float valves (needle and/or seat)
  5. Defective aftermarket float needle / seat assemblies (regardless of age…even new ones leak)
  6. Damaged oring/washer at float seat
  7. Float seat not installed all the way “home” into carb body
  8. Defective float bowl gaskets
  9. Float bowl gaskets damaged by carb cleaning agent abuse
  10. Inferior, “re-active”  float bowl gaskets swollen to the point of float operation interference
  11. Damaged plenum gasket
  12. Damaged carb-to-plenum FUEL seals
  13. Damaged carb body (rare)
  14. Carb body casting defect (rare)
  15. Float(s) installed upside down – more common than you would expect!
  16. Excessive fuel pressure from incorrectly chosen electric fuel pump conversion
Using a vacuum pump, perform the fuel circuit tests shown in the Carb Rebuild DVD and also hereIf the rack passes this testing, that eliminates the central plenum gasket, the 4 carb-to-plenum useals, carb body casting and central plenum casting.
If the fuel circuit testing “fails,” that usually  points to one or more faulty float valve assemblies and/or binding or misadjusted floats.  If the rack fails this testing, you will need to completely disassemble and investigate.
(While you are doing this testing, also check the “air” channels that support the air cut-off valve system.)
I use and recommend only genuine Honda float valve assemblies. Aftermarket float valve sets are notoriously unreliable.  Used float valves are difficult to evaluate and test.  That’s why renewing during a rebuild with new Honda parts is advised.
…and according to noted expert Mike Nixon:
“With new float valves, flooding should be a very rare event. However, it does not mean the carbs are at fault if the float bowls occasionally overflow. Float valves are designed just like toilet valves with the same kind of stopper valve. You can expect them to “burp” once in a while due to debris, water or just plain orneriness.” 
Be aware also that due to the design of the GL1000 plenum, just because fuel appears to leaking in the vicinity of carb #x, that does not mean that carb #x is the one that is actually leaking.  This is especially true if the bike is resting on the sidestand.
Float bowl volume tests are not reliable.  The available volume above “normal full” level is trivial and then overflow commences.
These carb don’t have external overflow circuits!  Excess fuel overflows and spills into the central plenum then disappears (usually) into the engine …washing down the protective oil film on the cylinders then diluting the oil in the oil pan! This makes tracing the actual source of the flooding more difficult.
Finally, you should always turn off the fuel using the petcock whenever you shut off the engine.  This is especially critical until you resolve any flooding issue. Otherwise, you risk hyrdo-lock engine damage.  This is reason Honda engineers provided the petcock. It’s a best practice to close the petcock every time you shut off the engine.  Do it!

14 thoughts on “Flooding Issues on Vintage Hondas

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  1. Hi, I bought a GL1000 in as new condition except for a few small issues.
    The chief one being that the engine wont run over 1000 rpm and if the throttle is opened up there is no change and no difference in the sound,
    I presume that flooding could do this? What external checks can I do before pulling the carbs apart? Thanks

  2. 77 gl1000 after completing all randakks recommended checks,and carb rebuild it runs great, 3 gas tank average mpg is 28mpg tailpipes look black inside smell of exhaust even while riding is the full vetter fairing and luggage the cause? I’ve added turndown tailpipe extensions,still smells.

  3. I have restored a 1970 CB350 and rebuilt the carbs, but did not replace diaphragms. It starts and runs good until 4500 rpm and it acts like it id bogging down when I accellerate. The plugs are black like it is flooding when I pulled them. And suggestions?

  4. 1979 CB250 struggles to start cold but runs fine when warmed up. Need to use electric start for wife so need to start first up but full or half choke not helping. Any recomendations?

  5. When my fuel petcock is off, engine runs fine with fuel left in system, however as I’m working on the bike this week, when I turn the gas on, it appears the #4 cycliner fills up with gas and kills the engine. It’s been a while since I rebuilt the carbs, don’t drive the bike very often. It sounds as if the float on that carb may be stuck in the open position?? Any suggestions?
    1978 Honda GoldWing GL1000, 34Kmiles

  6. Mine is leaking from the same place, front left carb. However, mine is spraying out from somewhere between the carb and plenum, spaying so hard it is shooting out and getting on the back carb………would this be a plenum gasket???

  7. Mine appears to be leaking bottom left of carb 1 , its seem to leak after pressure is built looks like its coming between flange and plenum

    1. Where the leak “appears” is not a definitive clue since these carbs do not have external overflows. Any overflow (for any reason) will eventually end up inside the plenum and eventually find the ground. If the bike is parked on the side stand, the leak will appear on the left side …but that does not mean the leak is actually on the left side!

      The usual remedy is complete disassembly, inspection and repair.

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