Carburetor uneven idle and engine seems to gasp when throttle is opened

When adjusting the throttle slide height on a 1977 GS750, the manual states that the slides should be set at .5-1mm above the top of the carburetor when they are opened wide open. The correct position to set the slide height is on the airbox side of the carburetor slide.
When the throttle slide is set too high on the engine side, it can cause the bike to idle high, as you mentioned. This is because the slide is not allowing enough air to enter the carburetor, which can cause the engine to run rich.
To adjust the throttle slide height, you will need to follow these steps:

1. Remove the air filter and carburetor top covers to access the slides.
2. Using a feeler gauge, measure the distance between the top of the slide and the top of the carburetor.
3. If the distance is greater than .5-1mm, adjust the slide height by turning the adjustment screw located on the airbox side of the carburetor slide.
4. Once the slide height is set, replace the air filter and carburetor top covers.
5. Start the engine and check the idle speed. If it is still high, repeat the above steps until the correct slide height is achieved.

It's important to note that when adjusting the slide height, you will also need to make sure that the throttle cable is adjusted correctly, as this can also affect the idle speed. Additionally, when making any adjustments to the carburetor, it's best to have a professional mechanic to check it, as it could cause other issues if not set correctly.

Your question isn't detail enough such as.

Q: It starts up wide open with the trigger depress fully but when I release the trigger it dies.

A: The carburetor idle jet (Picture A item #27) is dirty/plugged. the carburetor needs to be taken apart and cleaned or it just needs to be turned out a bit to flow fuel or the idle screw that sets the idle speed (Picture B) on the carburetor need to be screwed in to open the flap a bit more. Your carburetor may not be like either of these two carburetors but close enough. If you need to figure out which carburetor you have to locate the idle jet look search for picture of your carburetor and that tells all such as in picture A.

Another couple things. In picture B you see the spring on the throttle shaft that closes the flap to idle. If that area is really dirty the flap could be stuck wide open. Make sure the it is free to open and close. If the chainsaw has had a lot of use the engine could be wore to the point at idle the piston isn't making enough compression for the engine to stay running even if you turn the idle speed screw (Picture B) in as far as you can.

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.

Any turn in a CCW direction will increase fuel flow. The H jet screw should be adjusted so that the engine 4-strokes at speed unloaded, but immediately 2-strokes when cutting. The L jet screw should be adjusted so that the engine idles fairly well, but will allow the engine to 'follow the throttle'. If too lean (CW), the engine will 'gasp' when the throttle is opened. Hope this helps!

Operate the throttle trigger to see if it returns to the idle speed screw when the trigger is released. If it seems to 'float' somewhat, the trigger assembly may be full of dirt/sawdust, or the throttle return spring on the carburetor is damaged or disconnected. When taking the trigger assembly apart, be sure to make notes so that it can be put back together correctly. Make sure the throttle plate in the bore is tight to the shaft. When the carburetor was off of the engine, did the gaskets and heat insulator seem to be ok? Any air leak behind the carburetor will cause run-away speed. As for the engine quitting, check the fuel filter, air cleaner, and muffler for plugging. Check the condition of all of the fuel lines for decay or other damage. Did you check the condition of the diaphragm in the lower chamber of the carburetor--it should be quite flexible with no holes or cracks. With the jetting set where you have it, the engine should be 4-stroking heavily. Hope this helps!

With the engine off, inspect the throttle mechanism at the carburetor for complete movement of the throttle arm. Any restriction is likely sawdust in the handle near the throttle trigger. If the carburetor lever moves correctly, then turn the 'L' needle for the carburetor CCW about 1/2 turn. If adjusted correctly and there is no other problem, the engine should "follow the throttle" when accelerated. If it seems to 'gasp', turn the L needle a little further CCW until the engine responds. Be sure to inspect the air cleaner and muffler for plugging before adjusting 'L'. Check the condition of the spark plug--it should be tan colored on the internal insulator if operating properly (after running at high speed for a while). Hope this gets you running!

Make sure the muffler is not plugged. Sounds like the pump section of the carburetor is not working. Remove the carburetor and disassemble it. Keep track of where things go. Re-kit it if the rubber diaphragm is stiff or damaged. Same with the valves and diaphragm in the pumper section. Soak only the metal parts in spray carburetor cleaner, then blow out all passages with compressed air. Reinstall the H and L needles CW to stops (lightly), then turn both CCW 1-1/2 turns. The engine should start and run with these settings. Allow the engine to warm up, then apply full throttle and adjust H needle for fastest speed, but 4-stroking--which smooths out immediately when trying a cut. Next, adjust L needle for smooth idle, bur rich enough to allow the engine to follow the throttle without gasping. Then adjust the idle speed screw to just allow the chain to stop turning. Make sure the air cleaner is in good condition.

Remove the spark plug and look at the inside porcelain--if it is wet with fuel, put the wire back on, lay the base of the plug on engine metal, and pull the starter through while watching for spark. If no spark, try another plug. If still no spark, then something is wrong with the ignition system. Gain access to the ignition coil. Remove the small wire that connects to the coil and try the starter again. If now sparks, the stop switch is probably defective. If still no spark, replace the ignition module. Use card stock to set the clearance between the module poles and the flywheel magnets. Try the starter again--you should have spark by now. Put everything back together and try to start the engine.
If the plug was not wet in the first check and it does spark, then you have fuel problems. Make sure you have fresh fuel in the tank. Check the condition of the fuel line from tank to carburetor. Remove the carburetor and disassemble. If the rubber diaphragm is stiff and cracked, re-kit the carburetor. While it is apart, use spray carburetor cleaner and wash out everything that is metal--don't wash anything rubber or plastic. Blow out all passages with compressed air. Turn both jet screws (H and L) in CW to stops (lightly), then back out CCW 1-1/2 turns each. This should get the saw running. Start with H needle (after engine warms up)--open throttle full speed and turn needle CW until highest speed is reached, but 4-stroking. Engine should immediately smooth out when cutting lightly. Next, run at idle and adjust L needle for smooth idle, but rich enough to allow engine to follow the throttle without gasping. Set idle speed low enough that the chain stops turning.
Hope this gets you running.

Shut off fuel to carburetor, drain carburetor. Open fuel and count to three then shut off fuel. Crank engine then pull choke for 1 second then push in. If engine starts open fuel valve at tank. Also try lowering float level in carburetor. Good luck.

Hello

>>The Following is an Instruction File I made for Correcting this Problem. Any Input is Appreciated.
>>****All the Carburetor Adjustments are the Same for Lawn Mower and Trimmers.
>>There are Several Location for the Air Mixture Screws.
>>The Idle Air Mixture Screws are Usually Located at the Top of the Carburetor Bowl and the Top of the Carburetor Body.
>>If the Main Air and Idle Air Mixture Screws are Side by Side on the Side of the Carburetor, then the Idle Air is the Nearest to the Engine.
>>The Main Jet Air Mixture Screw is Located in the Bottom of the Carburetor Bowl or Beside the Idle Air Screw on the Side of the Carburetor Body.
>>The Engine Idle Screw is Usually Easy to Identify because you can See where it Makes Contact with the Carburetor Throttle Shaft Plate Stop Ear. The H and L Air Mixture Screws Go Into the Body of the Carburetor and the Tips Cannot be Seen.
>>Some of the Older Model Carburetors have the Main Jet Mixture Screw Straight in from the Top of the Carburetor Body (this is Rare anymore).
>>If you have Cleaned the Carburetor (Disassembled and Soaked Overnight in Carburetor Cleaner (I Use Gunk). Then Blow Out the Passages with Compressed Air and Install a New Kit if Required.
>>Now Setting the Air Mixture Screws:
>>****Turn the Idle Air and Main Air Mixture Screws In Until SNUG **Do Not Jam** Then Reverse Both Screws 1 1/2 Turns.****
>>Holding the Throttle Full Open, Start the Engine.
>>Turn the Main Air Mixture Screw Clockwise Until Proper Revs are Obtained. Occasionally you May have to Turn this Counter Clockwise to Achieve the Proper Revs. Now Allow the Engine to Idle.
>>Set the Engine Idle Screw (Not Idle Air) so the Engine will Stay Running if Required.
>>Now Set the Idle Air Mixture Screw so there is No Hesitation when Throttling from Idle to Full Throttle.
>>Reset the Engine Idle if Required. If This Carburetor has a Single Air Adjustment.
>>Use the Section Above that Pertains to Full Throttle RPM Air Mixture Screw and then Adjust the Screw if Required to Eliminate Any Hesitation when Rapidly Throttling from Idle to Full Throttle.**
>>1- http://www.repairfaq.org/samnew/lmfaq/lmclctc.htm
>>2- http://www.cpdonline.com/692509.pdf .
Please, Do Not Hesitate, If you Hit a Snag or this Does Not Correct the Problem, I am Here if You Require More Assistance.
Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Be Safe and Be Happy. Thanks.

Good Luck

Respectfully

jbridger (John)