Why does my truck die when I put the clutch in?

What liter engine?

Check for vacuum leaks and low fuel pressure.

Did you get PM?

Yes.

With a fuel pressure gauge that you rent or borrow from a parts store. Hoses aren't the only sources of vacuum leaks. Intake manifold gaskets. Cracked intake manifold. Carburetor or throttle body gaskets. Cracked throttle body. Leaking seal on brake booster. Etc.

Does it idle without using the throttle? Spray carb cleaner anywhere a vacuum leak is likely. Unbolt the brake master cylinder from the booster and see if you hear a hissing sound.

Ahah! I'm guessing the carburetor is incorrectly adjusted. Either the bowl float isn't adjusted right, the idle mixture is way off, or the idle jets are clogged.

See my last post.

Pull the plugs and take a look at them. What color are they? YOu adjusted the needle valve(s) on the throttle plate? I don't mean the idle speed adjustment on the throttle linkage.

What do you mean by 'the plugs are good'? How old are they? What color are they?

On the very bottom of the carburetor, probably on the front, but maybe one of the sides, you'll see a small screw. That is the idle mixture screw. It adjusts the amount of fuel that is allowed into the throat of the carburetor at idle. Close it until the engine starts acting badly. Then back it off one quarter turn. You want to turn the idle speed down some before doing this step.

OK. HOLD THE PHONE! Look at the attached image. It'll help you locate the idle mixture screw and may explain why you can't locate it right not. It's covered by a plug.
TUNE-UP - 4-CYL -1985 Isuzu Pickup
Page 1 of 1
IDLE MIXTURE

NOTE:
Mixture adjustment is NOT a part of a normal tune-up procedure and should not be performed unless carburetor is overhauled, fuel injection
system components are replaced, or vehicle fails emissions testing.
1.
Set parking brake and block drive wheels. Place transmission in Neutral. Remove carburetor from vehicle.
On all other models, drill a hole in seal plug and remove. Remove shavings with compressed air. Install carburetor.
On all models, disconnect and plug distributor vacuum hose, canister purge line and EGR vacuum line. Disable idle compensator vacuum line by pinching rubber hose. With engine at normal
operating temperature, choke open and A/C off (if equipped), adjust idle speed.
Connect positive lead of dwell meter or duty meter to duty monitor. Ground negative terminal. Turn idle mixture screw in all the way. Back out 1 1/2 turns.
2.
3.
4.
Fig. 3: Removing Mixture Screw Plug
5.
Adjust throttle adjusting screw to specified RPM. See IDLE SPEED SPECIFICATION table in this article. Check meter and ensure that dwell is varying. Adjust setting of idle mixture screw to obtain
an average dwell or duty as specified.
Reset throttle adjusting screw. Put idle mixture screw plug back in place. If equipped with A/C, follow steps 3) and 4) of IDLE SPEED ADJUSTMENT.
6.
DWELL OR DUTY METER SETTING Application
Dwell Meter Duty Meter
Setting 36°
40%
Federal

1.
Set parking brake and block drive wheels. Place transmission in Neutral. Remove carburetor from vehicle. Drill a hole in plug and remove. Remove shavings with compressed air. Install carburetor.
Disconnect and plug distributor vacuum hose, canister purge line and EGR vacuum line. Disable idle compensator vacuum line by bending rubber hose. With engine at normal operating temperature, choke open and A/C off (if equipped).
Adjust idle speed screw to 750-850 RPM on man. Trans. Or 850-950 RPM on auto. Trans. Turn idle mixture screw in all the way and back out 3 turns. Adjust throttle screw to 800 RPM on man. Trans.
And 900 RPM on auto. Trans.
Adjust idle mixture screw to achieve maximum speed. Reset idle speed screw to 850 RPM on man. Trans. And 950 RPM on auto. Trans.
Turn idle mixture screw clockwise (lean) until engine speed is down to 750-850 RPM on man. Trans. And 850-950 RPM on auto. Trans. Replace idle mixture screw plug. If equipped with A/C, follow
steps 3) and 4) of IDLE SPEED ADJUSTMENT.

The book isn't very clear, but I believe the first section is for California-equipped vehicles and the second is for non-CA cars. You should be able to borrow a dwell meter, tachometer, or timing light with tach from somewhere like O'Reilly's, Autozone, Advanced, etc. I just set it by ear, personally.