Why does throttle valve move erractily whengiving it gas

Under the hood, locate the throttle control (near or under the air filter housing, you can have someone step on the gas back and forth to see it move and identify it). Once you find it, you can manually move it back and forth (do all this with the engine NOT running, in the off mode). The idle can be adjusted usually with a screwdriver but you also might have a problem with the cable itself sticking. Run some oil or other suitable lubricator (like WD-40) back and forth, moving the cable to get it loose.
(there's a little screw to loosen or tighten the idle, it basically opens or closes the throttle more)

Your compressor is made by Hitachi. The pilot valve is what controls the engine and the pressure. Most likely the pilot is set to idle engine at 125 psi. The pilot valve can be adjusted higher or lower depending on needs but not higher than say 140. As the pressure drops to about 80 psi the pilot will cut in the engine to refill the tank. Pilot and throttle controls are finicky and require constant tinkering and or parts replacement. Your compressor needs the pilot to control pressure however does not need the throttle control. You can set the engine on constant speed. Less trouble but uses little more gas. Just remove the throttle control from the pilot and plug the hole. If the bad part is the pilot itself you will need to replace it. The new pilot will be adjusted to about 125 psi. Good luck with your repair

I need the exact year please.

Sometimes on them the throttle shaft which goes in to the carburetor gets rusty and turns hard. If you can't move the throttle butterfly by hand spray pb blaster around the shaft and move it back and forth to unstick the butter fly shaft.

Wide open as in wide open throttle?? You mean the engine is racing? Something is wrong with the throttle controls, or the throttle valve in the carburetor may be sticking open. Look down the carb throat. At its base will be the throttle valves-"butterfly" valves just like the choke valve on top of carb. The throttle valve should be closed at rest, or at idle, or when gas pedal is not moved. Rotate the throttle lever on the carburetor by hand (when gas pedal is pressed down, the throttle is rotated). The throttle valve at the bottom of the carb should open and close with the throttle lever.
If the valve is sticking, spray some carburetor cleaner down it, and spray the outside clean, too. Work the throttle back and forth to make sure it is free to move.
For a high idle, have the carburetor adjusted. Timing should be checked, too-that can cause a high idle, but wouldn't cause it to go wide open.
Let us know what you find. Good luck.

check vacuum and then spark plugs and wires

Take off the belt & feel for the water pump bearing

The problem lies in the sending unit in the tank. It could have a wiring problem or the sending unit is bad.

There should be a small air gap at the bottom between the slide and the venturi. A couple things to check are (we'll start easy and progress from there):

Remove the carb from the air box and the intake boot, but do not remove the top screw cap or cable from the carb. Once you are holding the carb in your hand, twist the throttle and see if the slide moves up and down freely. Does it return to the botton of the venturi on it's own when the throttle is released (or you twist the throttle forward)? If not, and the slide is sticking open, it is allowing more fuel/air in which is the problem. There is a spring under the top cap which should push the slide back down when you release the throttle. If it's sticking open, it's either a broken cable (you wouldn't see the slide go up and down when you twist the throttle if it's broken), or an unlubricated cable, a piece of debris in the carb between the slide and the carb body, or the spring is whacked. Unscrew the top cap, slowly pull the cap, spring, and slide up and check the spring. You can't miss it, it's a big one right under the cap. If it looks ok, check for debris on the side of the slide and in the carb body to see if there is sand/dirt causing the slide to stick open. If all of this checks out ok, then check the cable for free movement when you twist the throttle. If it's sticky, lube the cable. If it's ok, move to the next step.

On the bottom of the slide you will see the jet needle. It's long, skinny and tapers at the bottom. Try moving the needle up and down. It should be pretty snug. If the "c" clip that holds the needle came off, it will sometimes allow the needle to remain open which will cause this problem too. If the needle is ok (in place nice and solid and doesn't move up and down freely), then go to the next step:

Remove the float bowl and check to see if the float, float valve (under the back edge of the float where the float attaches), and seat (where the valve rests in the carb body) are all in good shape. If the float has a hole in it, or the tang holding the valve clip is bent, it will cause the float to drop down lower in the bowl, which opens the valve and allows more fuel in. Also check that the clip that connects the valve to the float did not come loose. If there is a problem with the float or float valve allowing too much fuel into the bowl, the excess fuel runs into the engine (via vacuum when the engine is running) and will cause the problem you are seeing. In this instance though, there is normally excess fuel coming out of the overflow hole on the side of the carb when the bike is not running and the fuel is on. If your problem just happened once and you shut the fuel off (and haven't turned it back on yet), put the float bowl back on, reinstall the carb and try opening the petcock (gas shut off valve). Let it sit for a few hours or overnight with the gas on. If the float or float valve is bad, you should see gas drip out of the overflow on the side of the carb. You may not have seen this if the problem just started happening while the engine was running. It was sucking the excess fuel into the engine causing it to rev wide open. When you shut off the gas flow, it prevented having the excess fuel drip out the overflow hole and letting you know it was a float problem. Hope this helps.

Idle stabilisation valve or TPS (Throttle Position Sensor)
You can reset the TPS - Turn the bike on, but dont start it.
Turn the throttle to full on - release, do this three times.
Turn the bike off.
Turn on and start.
The idle stabilisation valve can be cleaned, but a replacement is relatively cheap and easy to replace. They get gunged up and liberal so can't close fully - the ECU tries to open and close it to the required setting, hence the surging idle. Use carb cleaner around the butterfly and stepper motor areas and see if that sorts it.