Wednesday, December 14, 2022

We All Float Down Here, Georgie

In my last post, I mentioned that the front carb was flooding out and dumping fuel into the overflow / vent port.  I suspected the needle, but I was wrong...

There are a number of reasons why an SU carburetor might flood:

  • Damaged or worn needle/seat(s)
  • Incorrectly set or damaged float(s)
  • Excessive fuel pressure
  • Damaged fuel line (into or between carburetors)
  • Debris in the fuel supply (from lines or tank crud)
  • Damaged float chamber lid (cracked)

The usual suspect is indeed the needle and seat.  Those move and interact as fuel is consumed and pumped into the chamber.  Debris from the tank can cause a failure to seal, or the seat may develop a wear line.  Once the needle doesn't seat properly, fuel continues to get pumped into the chamber and out the vent.  Debris is easily managed with a filter in front of entry into the carburetor , and many cars have a filter at the exit of the pump.  (A filter before the pump isn't a good idea for our cars, as the filter may eventually impede flow into the pump and burn it out.  The pump doesn't really care if there's debris, unless maybe if you're trying to pump rocks.)

The second most common issue is an incorrectly set or sticking float.  The 'float level' determines how much fuel is maintained in the chamber.  The effect is to also set the level of fuel in the jet, since liquids in a connected chamber will equalize their level (and the jet with its needle is a very small chamber).  A level that's too high will dribble fuel out the jet and flood the carburetor throat, causing excessively rich mixture and poor running.  Too low a level won't draw enough fuel up into the jet and will cause excessively lean running.  There are differing opinions on how to do this - some swear by setting the level of the float by seeing how it sits with the float lid inverted, and others insist the best way is to measure the level in the jet with the piston and needle removed.  I find both work and ideally in conjunction; the first gets you in the ballpark but the second gets you a precise setting, especially for more inclement (hot) weather.  The first is easy; the second is less so.

Then there are more uncommon failures like a float that has a pinhole and 'sinks', meaning it won't rise to block the fuel inlet, or a cracked lid where fuel can leak past the base of the seat.  Old fuel lines or those that do not handle ethanol fuel can cause problems by flaking particles into the fuel flow even after the fuel filter.  This causes frustratingly inconsistent behavior.  Too much fuel pressure can overwhelm the needle's ability to block fuel input and cause a similar problem.  With an SU pump, this is generally not likely as the pump only puts out a maximum of 3.8psi.

In my case, it was the floats - but in a way I still can't fathom completely.

I went through everything else first, of course.  I mean, the floats were new and the 'guaranteed to float' expensive ones.  The needles were new, but the brass tip types.  And the car would run fine forever if I didn't drive it... but even going down the block would cause the problem to occur.

I checked the float levels repeatedly.  I changed the needle and seat twice: once for Viton-tipped needles, and once for Mini Spares' updated design that is similar to, but not a Grose-Jet.  I even verified the float chamber angle was correct and changed the rubber isolators that hold the chamber to the carb body.  I disassembled and reassembled the carburetors.  I even changed the fuel lines.  After all of this, the problem moved from the front carb to the rear carb.

But what didn't I test?  Yup, you guessed it - the floats.  So I took the floats off and tested them.

You're expecting me to say that I found one that sank or showed a pinhole.  But you'd be wrong!  Both floats floated fine even after extended immersion.  I was sure that was gong to be the culprit.  In a way, I was correct, but I am not exactly sure why.

I decided to change the expensive, guaranteed-to-float floats for a set of cheap plastic non-adjustable floats I had a a bag-o-carb parts.  I set the float height with shims under the seat.

And the problem disappeared.  I've put over 50 miles on Alice since without a hint of a problem.

My theory is that one or both of the floats was somehow sticking in the chamber.  I don't see how that's possible.  Perhaps the needle was binding up on the adjustable float arm, and that is my most likely theory.  The non-adjustable float has a flat surface and limited movement.  However, I use these same floats in Gidget without problem... so maybe one float is "bad" in a way I can't see visually.

I guess I can accept a good result here.  It means that I can get back to finishing up the other items on my punch list to get Alice fully roadworthy for my daughter!  I did rewire the stereo and replace the window seal, though that one is less than 100% complete in my mind.  It still rolls under and I suspect the door is somehow distorted.  I tightened the hoses into the heater box and the coolant smell is diminished, but still present.  I replaced the speedometer, but I get to do it again as the one I installed doesn't read properly (again).  And I did replace the dash lights with LEDs, so night driving is possible.  I still have to get to the hood (top) and replace the speedometer one more time.

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