1991 Subaru Justy
5 speed
3 banger, carbeurated
new transmission
This problem began when the weather started to get cold late last year. Regardless of engine temperature, the little fella will just start lurching back and forth and gradually lose speed in the higher gears. It will run fine for a few minutes before it starts lurching. 50mph in 5th soon becomes a downshift attempt to stay at 45mph in 4th but i have just had it trying to fight this thing and i have to pull over and drive home in 1st at 10mph (usually just 2 or 3 miles).
I don’t think it is something that is choking the engine b/c if i hold the clutch and rev the engine all is well until it goes under the load of the transmission. I guess it could be the tranny, but it’s “new” (came out of a Justy that only had about 4000 miles). But it only seems like cold weather makes it act like this b/c the same problem happened last year. What kinds of things should i be checking or doing differently?
Sounds like carburetor icing, where moisture in the air freezes to form ice in the carb’s venturi. There may be a malfunction of the heated air intake system contributing to this.
To check for icing, let the symptom come on strong, then pull over and quickly remove the air cleaner top so you can look into the carb. You should be able to see the ice buildup inside the venturi. I wouldn’t expect this to affect the engine’s revving in neutral, which requires only a small fraction of the air and gas needed to propel the car.
If it is icing, check the heated air intake system. [The following is generic, and may not exactly describe the system on a Justy.] There should be a flexible tube,about 2" diameter, from the exhaust manifold to the snorkel of the air cleaner. There’s a flap in the snorkel that selects between this heated air and cold outside air. The flap is moved by a vacuum motor on the snorkel. The vacuum for this goes through a temperature sensitive valve on the body of the the air cleaner. Possible problems include a clogged vacuum port, a faulty temp valve, missing or cracked vacuum hose, faulty vacuum motor, stuck flap, and missing flex tube.
Sounds like carburetor icing, where moisture in the air freezes to form ice in the carb’s venturi. There may be a malfunction of the heated air intake system contributing to this.
To check for icing, let the symptom come on strong, then pull over and quickly remove the air cleaner top so you can look into the carb. You should be able to see the ice buildup inside the venturi. I wouldn’t expect this to affect the engine’s revving in neutral, which requires only a small fraction of the air and gas needed to propel the car.
If it is icing, check the heated air intake system. [The following is generic, and may not exactly describe the system on a Justy.] There should be a flexible tube,about 2" diameter, from the exhaust manifold to the snorkel of the air cleaner. There’s a flap in the snorkel that selects between this heated air and cold outside air. The flap is moved by a vacuum motor on the snorkel. The vacuum for this goes through a temperature sensitive valve on the body of the the air cleaner. Possible problems include a clogged vacuum port, a faulty temp valve, missing or cracked vacuum hose, faulty vacuum motor, stuck flap, and missing flex tube.
The “holes” the carburetor sucks air through. You probably have a one-barrel carb so with the aircleaner removed you will see a single hole (about 1-1.5" diameter) going straight through the carb. In the bottom of the venturi you will see a plate which is operated by the throttle. Above the venturi (the top of the carb) you should see a second plate which is the choke plate.
Besides carb icing, you could also have a malfunctioning choke. If the choke is stuck closed the engine will get way too much fuel and have the symptoms you describe (no power, jerking). With the engine warmed up make sure the choke plate is fully open.
Let me try to ascii what i see (please don’t yell Lynn):
_____
/
|---------|
\O____/
Hope fully that turns out. This is the view from the top. The top of this ‘hole’ has a plate that is controlled by the throttle. When i step on the gas or pull the throttle cable myself the plate swings open/closed. On the bottom half of this hole is a metal tube that connects to a rubber hose outside the carb which runs underneath the carb into a massive mess of rubber hoses. Is this carb backwards by chance?
That description doesn’t make sense to me. I’ve looked at the diagram in my repair manual. See if this makes sense to you:
Underneath the air cleaner (the housing which holds the air filter) is the carburetor, at the top of which is a round hole which matches the hole in the bottom of the air cleaner – probably about 3-4" diameter. If you look down through this hole at the top of the carb, you’ll see that it is divided into two parts which become round passageways, at the bottom of each of which is a smaller (1-2") round hole with a flap or plate. One of those flaps opens with the throttle pedal/cable.
The passageways from the large hole at the top to the small holes at the bottom are the venturis ( = barrels = throats). The venturi whose throttle (flap) is controlled by the pedal and cable is the primary venturi. This is the one I suspect may be icing. Ice will form inside the passageway and restrict it.
