Help! I am totally lost!

[Titanosaurus]

I have a Manco Helix 150. When I got it it had been sitting for a year, and ran o.k. Right away I installed a new NGK plug, plug wire, and fuel filter. Soon after, the fuel petcock leaked so drilled it out so it had flow without the vacuum portion. About the same time it started crapping out at the top and stalling when stopping. Then it would only go half throttle. Where I live it is very limited in repair shops. I found a saw shop that said they could fix it. They said it was starved for fuel( no kidding). I asked to have the carb cleaned and inspected, and because I had installed a uni, I wanted the main jet drilled at 125(114 before drill). I live at 955 feet altitude. When I got it back it ran worse. They said they couldn't do any better(confused here). I brought it home and removed the carb. It had not been taken apart. I inspected everything and it looked good. The main was not drilled out. I read up on it and found the proper size bit and drilled it out to 125. After re-assembly, I jacked up the rear end and fired it up. After adjusting the mixture screw on the bottom left (near the intake boot) (from the rear of cart) side of the carb and the spring loaded idle screw(correct?), I fired it up and it ran perfect all the way through the throttle range. It actually startled me as it has more umph than before. I set it down and backed out of the garage with no problem. But the moment i give it more than 1/6 throttle it starts coughing and gives the appearance of running rich, but the fuel filter has hardly any fuel in it. The filter is new and the spark plug has two hours on it. the plug was white before jet was drilled. I have not checked it since attempting to drive it. I chose not to work on it myself in the beginning as I am an auto mechanic, and although similar, I don't have to tell anyone here that it is NOT a car and I am not the best person for the job. Since there is no one around here except Tractor Supply which doesn't seem to do repairs, I'm not sure what to do. I am so frustrated. I have drove it for a whopping two hours, and even then it wasn't totally 100%. I don't know what to do. Please, if anyone is willing to talk, I am willing to listen. I am to the point of maybe just selling it like it is if I can't get it going right.

[metalstudman1]

You may try a couple things.Since you say the fuel filter isn't full I'd go back to the tank and start checking for blockages.Check the tank too for loose debris that may be floating around.Try unscrewing the gas cap a little to verify you don't have vapor lock slowing the flow of fuel since you're not getting vacuum from the engine.Are the fuel lines clean and no kinks?,are they large enough?(cheap to just replace with clear lines and you can SEE if there's an issue.I think it ran good in the garage and ran out of fuel by the time you backed out or verify the intake isn't leaking or have a crack in it-will run fine with no load with a cracked intake but as soon as you put a load on it'll suck air.This is a start-see if it helps

[ckau]

Originally Posted by Titanosaurus

), I am an auto mechanic, and although similar, I don't have to tell anyone here that it is NOT a car

True it is not a car but you are a mechanic so you understand the basic principles of the internal combustion motor. The mechanics of this motor are a little different but the same principles apply, air, fuel, spark.
From the symptoms you descibe I'll go through a few items.
you altered the fuel petcock so look at this first. the petcock is activated open/shut by engine vacuum. when vacuum is applied the valve opens to supply fuel, closes when motor is shut off. A good safety feature but prone to malfunction. check for vacuum leaks. The slightest leak causes eratic carb behavior.
Check the tank and fuel lines for trash. replaceing with clear line helps to observe flow. ethenol gas will eat up those cheap chinese rubber lines so watch for this or replace with a ethonol friendly line and filter.
The main jet at 114 is about right for a motor with stock air and exhaust systems so go back to a 114 to get a base line to tune. The carb is a Kehein clone 24 mm cv type. The main jet is a Kehein # 99101-393. The pilot jet is a N424-21. Any cycle shop can get them for you.
Disassimble the carb and clean it well! A lot of low to midrange problems are caused by almost invisible trash in the pilot jet. A new carb can be had from e-bay for aprox. 30 bucks if you wanna go that route but it will have to be jetted for your buggy so save your jets.
The choke or actually a enrichiner is another cause of problems. it is not a choke as such it closes off the air supply but rather gives an extra amount of fuel instead. it is a extra circut in the carb that is controled by a needle and seat that opens for more fuel when cold then closes as motor heats up. this enrichiner need electric power to stay closed. There all kinds of info here to show the proper procedure to check it .
Check All electrical connections! A poor connection can cause malfunctions in the CDI contol unit or the coil that can give the same symptoms as a bad carb adjustment. the china wiring circuts on these buggys is screwed up at best, A bad ground at the switch can cause ignition problems. Most igniton problems show up at higer rpms but you haven't got there yet. you got spark, so focus on the carb systems first
there's tons of info on the web about these carbs and motors in general so read up a little ,get familair and ask us a ton of questions

[SYCARMS]

Also check the fuel cap by removing and inspecting the underside the ethanol fuel will react with the plastic cap and deform it causing a no vent situation. If the cap looks like someone put a blow torch to it replace it. I am not a betting man but I have a nickle that says it is your cap. Common problem.

TOM

[Titanosaurus]

Well, first of all thank you to everyone who has contributed information.
I finally found a shop near me who would work on it, and the problem was
something very odd. They found that the throttle stop on the carb
had been cut off so when it was going wide open, the butterfly actually
went beyond 180 degrees. They adjusted the cable so that it can't go that far now. They said it runs perfect now. They are ordering the shifter cable cam(?) that goes on the transmission as it was broken and it was stuck in forward gear. It was that way when I took it to them, and they are getting a metal one so it will be ready in a week. I can't wait to get it back! It hasn't ran 100% since I bought it. Running perfect and having the main jet drilled and a uni is great start. I see a long off road trip in my near future! Yeah!

Thank you again to this awesome community!

[Titanosaurus]

Oh and by the way Tom you were right about that cap...

[SYCARMS]

Great to hear your up and running. Now the fun starts.