It sounds like you're doing some good tests. I've heard some very practical suggestions
from list members.
Anyway, here's another likely item to check out:
It is not at all uncommon for a carburetor's float valve to stick. It can happen with
all-brass valves, but it is even MORE common with resilient-tip valves, especially after
they have gotten a bit worn, taken a set, or have been damaged by fuel additives. Most of
the carb kits made in the last two or three decades have resilient tip valves, because
they're less prone to leakage.
Consider the resilient-tip-type valve: After a while, instead of the original conical tip
seating only against a very narrow edge on the seat, the synthetic tip wears and/or sets
in such a way that it develops a cylindrical band around the cone. This allows the tip to
wedge itself into the seat. Even if it doesn't exactly "wedge" itself, whenever this
cylindrical area of the tip is pushed up into the seat, even the slightest misalignment of
the valve or a bit of debris can cause it to bind. By the way, in some cases, this can
also cause the valve to leak, but usually it causes it to stick closed.
Another factor is that the resilient material (neoprene/nitrile/whatever) can be affected
by additives in the fuel. For instance, gasolines with alcohol additives (which are pretty
common) are OK with newer vehicles, but some synthetic rubber parts in some older vehicles
can be softened or dissolved by the alcohol.
Here's a related thought: If the fuel pump is marginal and not producing much pressure, it
will be easier for the float valve to stick, since the pump is less able to build up
enough pressure to force it back open again.
Worn float valves are less of a problem with systems equipped with pressure regulator
valves and electric fuel pumps, because they don't produce the strong pulsations
associated with the older pumps. These pulsations can wear out a float valve faster, even
the all-brass type valves.
The other thing about sticking float valves is that they can be highly INTERMITTENT,
making it very hard to trace sometimes. The tiniest bit of varnish, sludge, rust, or other
debris can make all the difference when the valve begins to wear.
I recall an airworthiness directive for a Marvel-Schebler carburetor regarding sticking
float valves. To satisfy this particular AD, it was necessary to install a small metal
clip that wrapped around a groove at the bottom of the float valve and around the float
tang. When the float level dropped, the clip would physically PULL on the float valve to
dislodge it. And this was a problem with NEW or NEWLY OVERHAULED carbs!
Note that some automotive carburetor overhaul kits come with these clips, even though the
carb may NOT have had them originally as standard equipment!!! It's very possible that
someone installed a kit that should have used this clip but may have left it off when they
rebuilt the carb, and the problem is now showing up as the valve becomes worn and as the
fuel pump loses its ooomph. Sometimes these clips come loose during a careless reassembly
of the carb, so even if you see the clip when you take the carburetor apart, it doesn't
mean that it was assembled correctly.
>From what you've told me so far, I would do the following, in this order:
Replace the condensor/capacitor with a NEW one (not NOS). It's cheap and it can be easily
eliminated as a cause.
Rebuild the carburetor, including a new float valve. This is a good idea anyway.
Rebuild or replace the fuel pump. This is also a good idea just on general principles.
If you have an extra coil, you could try swapping it.
Rebuilding the carb really isn't very difficult. Just a hint or two:
Clean the outside of the carb before you begin. Use real "carb cleaner" (outside the
house, of course) because some solutions can destroy the protective coatings or alodine
surface treatments on the aluminum.
Take the carb apart in a large plastic bucket to catch any small parts, such as check
valve balls, if they happen to fall out.
Don't take the butterfly valve apart. If it's obviously damaged or badly worn, replace
that part of the carburetor or take it to a shop that can replace the shaft and re-bush
the carb body.
Clean all parts well before assembly and blow out the passages (don't blow away any parts
by accident).
Follow the instructions to the letter.
The single-throat carb on your unit is a good "first carb" to overhaul...not too many
parts and usually a fairly simple assembly procedure.
Have fun,
Alan
(Salt Lake City, Utah)
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