On Thu, 15 Jan 1998 06:45:21 -0800 Andy Ramm <aramm@concentric.net>
writes:
>Today driving to work in my '67 BGT, the car seemed to lose a lot of
power as it climbed a steep incline, as though one or two cylinders were
cutting out temporarily. Once the hill was crested, the power gradually
came back. Any suggestions of what this could be? Fuel pump? Floats?
Insufficient fuel flow. Under high load conditions (climbing hill with
heavy throttle), the engine needs more fuel than the fuel pump is
supplying. Incoming fuel gets to the rear carb first, filling the float
chamber. If there's not enough fuel coming in, there's none left over to
continue on to the front carb, so the front carb float chamber goes dry
and you're running on two cylinders.
Actually there's a little more to it. Because there's a balance tube in
the intake manifold, there is some mixing of the intake between the
carbs. The result is that the rear two cylinders go a little lean and
loose a lot of power, and the front two cylinders go extremely lean and
refuse to fire.
Look for faulty fuel supply. Pull off the fuel supply line where it
comes up to the rear carb. Put the hose into a container, turn on the
key, and keep an eye on your watch as the fuel comes out. You should get
at least one pint per minute. In simple terms, that would be
"substantial flow". If all you get is a trickle, you've found the
problem, then you need to find the cause.
If you have a fuel filter anywhere in the line between the tank and the
carbs, check the filter first. Remove it and try blowing through it, you
should be able to blow through it easily. Otherwise, install a new
filter.
If you have a fuel pressure gauge, check the static pressure. Stick the
gauge in the end of the hose at the carb, turn on the switch, read the
gauge. With an SU fuel pump you should get 2 to 4 psi static pressure
(less when the fuel is flowing). If you get adequate pressure, it's an
indication that the pump is OK and there's an obstruction somewhere in
the line.
Some SU pumps have a filter screen in the fitting where the line from the
tank enters the pump. Remove the incoming fuel line from the pump and
check or clean this screen. And while you're at it, consider the next
item.
You could have a clogged pickup pipe inside of the fuel tank. Powdered
rust sediment from the tank inner wall can settle in the bottom of the
tank and close off the pickup tube. And it doesn't take much rust to do
it. Open the fuel line between the tank and the pump, remove the gas
filler cap, and blow through the line back into the tank with a little
low pressure air. (People power is usually enough, but try not to
swallow gasoline while you're at it.) This should temporarily clear the
junk away from the pickup tube. The fuel flow would improve
dramatically, but it would likely get clogged again in a short time of
operation. In this case you would have to remove the fuel tank, slosh a
little fuel around inside and dump it out a few times to get the junk out
of the tank.
Check all the steel fuel lines for physical damage. A kinked or crushed
steel line will obstruct flow. Especially check the steel line that runs
along the frame rail on the right side. Some negligent quasi mechanic
may have smashed the line here while jacking the car. Any crushed steel
line would have to be replaced.
When you get the pump to supply a pint per minute at the carbs, you're
back on the road again.
Barney Gaylord
1958 MGA with an attitude
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