engine dies with load or high RPM

i recently had my carb rebuilt, and it starts and idles fine. when going to high RPM it gags and sputters. if i am towing my trailer or going up hill it dies. i have been all over the range with the air and fuel adjustment. i got it to run for a few minutes at full RPM, but then it seems like it loads up and dies. i am frustrated! it is an LME 16 on a 66 S12

When you say it loads up, do you mean that it stalls out while black smoke is coming out of the exhaust? That would indicate an extremely rich mixture condition.

What happens when you try to start it again? How do you get it going again?

On the opposite end of the spectrum, if it seems to be running out of gas, then that may be easier to track down: Float level, gas tank vent, main jet passages restricted, etc.

Check the easy things first. Unscrew the gas cap when it's getting ready to die to see if it makes a difference. After it dies, check to see if there is fuel in the bowl. Look at the plug after it dies to see if it's black, or covered in gas. Post your observations and the members here will have answers.

Some of the older carbs won't run right again if you remove the main nozzle to clean it. You have to replace it with an updated part.

Hopefully you don't have to move onto the igition system, but I would make sure there is a fresh plug in the engine before even fooling with the carb.

i will post my findings for sure. i will try gas cap first. i am really starting to think that the fuel delivery cannot keep up with the engine. i am also going to remove the fuel petcock too and check it b/c it seems the tractor runs better with an extremely full tank. i wouldn't be surprised if it was dirty or almost clogged. i think it is letting just enough fuel through for idle and low RPM but when i ask for all 12hp it starves.

so, i cleaned the tank, replaced the fuel line and cleaned the petcock, thinking my engine was starving for fuel due to poor fuel supply. when cleaning the petcock i broke the part on the bottom that you use to turn it on or off. so, having bought a new plastic one that didn't work b/c the hole in the tank mount was too small, I modified the old one by removing the filter and stem so the fuel would run straight through w/o any restriction to eliminate that. Sunday i plowed snow with the tractor for a few hours and it still won't run at full rpm (still plowed well though!) so i figured it's the float or air. it does blow black smoke and tries to die at full rpm or with load. so it is not a fuel delivery problem for sure.
Today i needed to bring wood in and planed to use the tractor, only this time w/o an air filter to see if that was causing the air restriction even though it is a new filter. Instead what happened was the stupid new S/G belt i put on slips so darn much on the pulley (regardless of adjustment) it killed the battery so out comes the Jeep to jump it. While jumping it, the wire going from the S/G to the bottom of the V/R shorted it self to the frame and smoked like crazy all the wile engaging the S/G regardless of key position. so i unhooked the positive battery terminal to fix that and oh-by-the-way my home made/jerry rigged fuel petcock decided to start leaking. luckily i avoided a serious fire while my 4yr old anxiously waited to go for a tractor ride. Damn.
so,i have few questions:
1. does anyone now where i can get a NOS petcock that is the same as what i have?
2. has anyone ever bought a plastic tank to replace the metal one? (that fits the same as my current original one)
3. do those "gripper" belts make much of a difference?
4. what are the chances i seriously screwed up up my rebuilt S/G or my new V/R when i fried that wire?
5. Why is it that a fairly competent person like me is being bested by this simple old machine? hope i am not the only one!

josh , sorry to hear you are having a run of bad luck. starter belt is a 3/8"x33". i have in the past used a piece of sandpaper 100 grit or so and ruffed up and deglaze the pulley on the generator to help it have a better grip may give that a try
carb main adjustment should be set at 1 1/2 turns off seat
carb idle adjustment should be set at 1 1/4 turns off seat
begin with these settings then start engine and let it warm up to a normal temp. with the throttle at idle position turn the idle adj. about 1/8 turn at a time until it idles smooth then with no load open the throttle wide open turn the main adj. the same 1/8 turn at a time until you get it running smooth throttle back to idle and hear how its running if needed repeat again with adjustments if nothing changes and still blowing black smoke with throttle wide open check out the choke to make sure it is not partially closed or not opening all the way again like georgia ss stated if the main nozzle was removed during the carb rebuild it may not run right.
i would think you will have to get it started to be able to test for the electrical end of it. keep trying and good luck / tim

I guess my first question would be what was the reason for the initial carb rebuild and was there a running issue before then ? Also witch ignition system are you working with on your engine ?