I have a 1990chevy c1500 4.3. five speed on

It may also be the catalytic converter. If that gets plugged up it puts a lot of back pressure on the motor. This will make it hard to get up to speed. When not pressing much on the petal it will seem to run fine. Then when you press more to increase speed you will feel it.

clutch needs rebuildn or pressure plates are worn--bring to specialist

possible slave cylinder failing

Have you ever had the fuel filter replaced? also have a fuel pressure test done on your car, you could either have a fuel pump or pressure regulater problem..
Hope this helps! Good Luck!

sadly the early version of the 4 speed is prone to this type of failure. I have owed 4 that had this exact problem. the rear end of the main transmission case wears down at the transfer case and this causes an internal leak. The loss of fluid pressure causes the loss of function. I'm confident that a good transmission shop can fix this by using a case and parts from a newer version of the 4 speed auto

Hi the problem you are having is going to be with the clutch plate you will find that it has worn down to the metal pins and has no actual clutch plate left.

What is happening is that when you put the car in gear and let the clutch up the clutch is slipping on the plate instead of gripping and this is why you loose the drive.

To solve this problem your going to need to buy a clutch repair kit if your doing the sork yourself or get someone else to fit the parts for you.

Your going to need to replace the following:
pressure plate
clutch plate
thrust bearing.

dont try driving the car as it is as you could cause damage to the gearbox if the clutch plate and pressure plate shatter so its not worth trying to move the car until you have had the clutch fixed.

getting the clutch replaced involves removing the gearbox from the engine and this is best done with the car on ramps as the gear box will come out from the bottom of the car so it needs to be high enough off the floor to allow you to remove the gear box, its not a hard job if you have done it before but it is heavy work and if you have not done this before then your best getting someone else to replace the clutch for you.

getting the clutch changed will solve your problem and get you up and running again in no time.

let me know how you get on or if you need further assistance ok

Could be clogged cat converter or it could be timing out of adj.

check fuel pressure 1st

Your clutch is slipping, doing this might ruin the flywheel. I cannot tell if this is wear or more likely an oil leak (slipping that much is normaly accompanied by lots of smell and smoke if it is a slip, not with oil leak though)

The trick to driving a manual transmission is giving gas very gently while letting the clutch up. The clutch only starts to engage when the pedal is about an inch and a half from the floor, so any gas you give before that will cause unecessary revving.
To get a feel for when the clutch engages, try the following excercise. Put the car in first gear and bring the clutch up ever so slowly without giving gas. At a certain point you will feel the clutch starting to engage and hear a difference in the sound of the engine. If you're bringing the clutch up too fast the car will stall. If you bring it up slowly enough the clutc will start to engage and you will begin to advance.
This excercise will develop your motor skill to manipulate the clutch effectively and will allow you to know exactly when to give gas when letting the clutch up.