On Thu, 29 Sep 1994 WILSONHB@ctrvax.Vanderbilt.Edu wrote:
> Bad problem here. Just when the 100-6 is all warmed up and has been ripping
> along the boulevards for 15 minutes or so, sometimes it begins to hesitate.
> The carbs seem to "choke" as if they aren't getting enough gas momentarily.
> The most common time for this to happen is if I have been on the highway at
> 70-80 mph and try to accelerate; it just seems to bog down but often recovers
> and becomes "normal
Ummm. Do you mean this happens when you're doing 70-80 mph and try to
accelerate? Or do you mean this happens at slower speeds _after_ you've
been going 70-80 mph?
If it's the latter, it's a classic symptom of fuel percolation in the fuel
lines and/or float chambers. It's not at all uncommon in BritCars,
especially in hot weather. If pulling the choke out to richen the mixture
when the stumbling occurs stops (or lessens) the stumbling, then that
confirms it.
At high speeds the engine creates a lot of heat, but it's kept at an
acceptable level due to fast airflow through the radiator, fast airflow
through the engine compartment, and fast coolant flow through the engine
and radiator. This keeps under-hood temperatures down, and consequently
the fuel lines and float cambers stay relatively cool. In addition, fuel
is moving trough the lines and chambers quickly enough that it doesn't have
time to pick up a great deal of heat.
When you slow down there is still a lot of leftover heat in the engine,
but it isn't dissipated as quickly. Underhood temperature rises for a
while, and the slower-moving fuel heats up. If it heats up enough, it
vaporizes in the fuel lines and/or float chambers. This weakens the
mixture at the jets, resulting in fuel starvation.
The cure is to (1) keep the underhood temperature as low as possible, and
(2) make it as difficult as possible for the fuel to pick up heat on its
way to the jets. This doesn't necessarily mean you need to provide extra
radiator capacity, ducted airflow or extra eat shielding for the float
chambers, or anything heroic like that. Usually all it takes is to make
sure all existing systems are working up to factory specs.
Start with the fuel delivery system. If the car originally had a heat
shield between the carbs and the exaust manifold, make sure it's there and
in good shape. The factory wouldn't have put it there if it wasn't
necessary. Make sure the fuel lines (especially metal pipes) are not
routed too close to the engine block or exaust system.
Make sure the carbs aren't set too lean in the first place. A weak
mixture will make the engine run hotter than normal, so fuel percolatiuon
will occur earlier than normal, which weakens the mixture, which makes the
engine run hotter, which increases percolation, which weakens the mixture
even more, and so on.
Also make sure the ignition timing isn't retarded, which will also cause
overeating.
Make sure the cooling system is up to snuff. Clean and flush the cooling
system, and make sure nothing is blocking airflow through the radiator.
If you're using a 180 degree thermostat, switch to a 160 degree one
(unless the specs call for a 180 degree one). Check the operation of the
pressure release in the radiator cap, or install a new cap if you have no
way to check the old one.
If none of the above helps, then start looking for ways to improve on the
original design, concentrating on keeping the fuel delivery system as cool
as possible. Reroute the fuel lines as far from heat sources as possible.
Replace metal pipes with neoprene rubber hose if possible, as it doesn't
transfer heat as rapidly. Insulate the fuel lines (sometimes wrapping them
in aluminum foil is all it takes). Install a heat shield between the
exhaust manifold and the carbs, if there isn't already one. Install a
flexible duct to aim air from behind the grille at the float chambers.
None of this stuff should be necessary (after all the car wasn't designed
to vapor lock), but can elp in stubborn cases. Make sure the standard
equipment is up to snuff first, though.
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily transportation)
feold@umd5.umd.edu
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