Getting ready to swap a 750 Quadrabog on the old tow pig and im wondering, anyone got any experience with them evaporating the bowl out and vapor locking? Ive heard that it can be an issue at times and id prefer to not get stranded waiting for it cool off and re-prime
Like
@pressureangle, I've worked through this issue on Q-Jets, nearly always with a successful
resolution. (And for the remainder, there is a simple, inelegant work-around you can use. More in a bit.)
****
Let's go through the fixes that are available, starting with what the factory engineers came up
with back in the day, and then on to what folks like
@L31MaxExpress have done since then.
1) Don't bolt the Q-jet to the intake manifold with a thin paper gasket that's included with some of
the carb rebuild kits. Starting with the hotter temps due to leaner idle mixtures when emissions-controls
were mandated, you want to use the 1/4" thick temp-insulating gaskets with the 4 phenolic spacers built in.
(So that the carb sits squarely.) Chevy upgraded to these around '71 or so:
You must be registered for see images attach
(credit: CorvetteCentral website: (
LINK)
2) The Corvette engineers back then took it a step further, by adding
a heat shield to cut down on the
radiant heat from the intake through
the air and to the carb:
Chevrolet #3969837, NOS '70-'71 Corvette/Q-Jet metal heat shield
You must be registered for see images attach
NOTE: If it was me, I would use this heat shield & substitute the better 1/4" gasket in step #1 above. (Or fab up a similar heat shield yourself - pic above just to give you an idea of what the paid help came up with back in the day -- no doubt based on failed high-temp heat soak testing at GM's AZ proving grounds.)
****
OK, Steps #1 & #2 above are to migitate heat transfer into the carb itself. (ie: The carb is
the perpetrator of the too-hot to control fuel issue.)
Conversely, the following steps are to help solve too little/too hot to control fuel
to the carb.
(ie: The carb is the victim, not the perpetrator.)
3) NOT putting the gas in the line from the tank to the carb under a partial vacuum. Just
like every other red-blooded mechanic, I fully appreciate the affordable, simple beauty to the
mechanical fuel pump mounted down on the right front corner of the block.
The only problem is that by
pulling fuel from the tank, it lowers the pressure on the gasoline
in the line. And as we all know, the lower the pressure of a liquid, the lower the boiling point.
And a mechanical fuel pump can pump a liquid, but
not a vapor. No fuel, no go.
Instead,
pressurizing the fuel from the tank forward will raise the boiling point of the gas
in the line, and Voila! the vapor lock is cured!
@L31MaxExpress is on the money with his
recommended setup!
4) It is documented in 1 or more of the Q-Jet books that the needle & seat in the carb
can control fuel, but cannot control heated gas vaporization that occurs when the fuel
transitions from ~6 psi liquid in the line to the carb to atmospheric pressure in the
carb bowl. And since the bowl is vented to the inside of the air cleaner filter, this
is what gives you the 'too rich to start' issue.
To fix the issues in step #4, we have to control the gas temps. Running the
gas line close & in parallel to the exhaust system is an obvious no-no...especially
when you are running a 1 gas line setup. A much better solution is to use
all the gas in the tank as a large heat sink -- this is done by having a pressurized
line from the tank to a fuel regulator near the carb...and then the unneeded extra
fuel pumped up front is sent
back to the tank. Done correctly, you minimize the
heat gain during the trip up to the engine bay, and then cycle what isn't used back
to the tank in a continuous loop. Makes perfect sense, and the factories did this
for years.
5)
The Workaround. Let's say for a moment that your truck falls into the
category of infrequent heavy use in hot weather. It just works out that normally
the truck is just loafing along, and with using steps #1 & #2 you never have a
problem. But today you are towing something heavy, it's really hot out, and
the truck is starting to have trouble restarting after a quick heat-soak. (ie:
Stopping at a rest area for a few minutes.)
The solution (which I have 1st-hand experience with) is as simple as opening
the hood and allowing that tremendous amount of heat trapped in the
exhaust manifolds, block, radiator, etc., to escape. Simple as that. And
thanks to convection, there will be a pretty good flow of superheated air
out of the engine bay!
NOTE: A cooler solution would be to have your hood louvered (allowing
heat to be pulled out both driving down the road as well as parked) ...but done right
it's pretty spendy. And it's a dying art form. But if I saw that on your truck it
would blow my socks off! :0)
One last thing. The electric cooling fans that cycle on & off until the radiator
gets down to ~180 degrees have a nice side effect where the engine bay gets
some much-needed ventilation during those post engine shutdown heat-soak
situations.
****
Anyway, that pretty much sums up what can be done in order to feed a cool
carb with cool fuel, so no more worries about 'vapor lock'. And, if all else fails,
just opening the hood during a quick stop after making some real heat will help
more than most people realize. And if asked why I pop the hood, I feel no
shame telling them that my engine makes a lot of heat.
Enjoy that Q-Jet!
