Hello everyone, first time post and first time looking for help on a quad. Ill begin with my purchase of the coleman scrambler at125ut from tractor supply. Worked great for about a month , then we had issues. most of them have been fixed how ever we have run into a catastrophic issue. The quad will start but sputter out without throttle. When we throttle the quad the moment we let off the throttle it spins down the RPM and chokes its self off. when riding it there is a noticeable difference in speed from when we got this to its present state by almost half the power. I have been unable to diagnose the issue but here is what I have tried so far.

1. Checked the bowl of the carb, (had to replace it the one that came on it didn't have screws for the bowl and it ended up breaking when we drilled out the retaining pins that were installed instead of screws) carb is new.
2. spark plug was fairly dirty and brownish , we changed that as well.
3. checked lines to make sure no kink / clog / cut.
4. replaced the air filter
5. checked the throttle cable (not sure how this is adjusted) seemed to have some slack at the carb but still the throttle functions
6. oil seems to be fairly dirty but thats to be expected apparently with no filter for these. oil level looked within limits

these are the things I have checked. I have no clue what to check next. we did take the piston cover off it looked clean inside no debris or loose parts we just put the cover back on it.
if you can think of anything that may be causing this let me know please I am at a total loss.

thank you for your time. any help is appreciated.

 

When you fit a new carb you have to set the idle speed stop, sounds like you have not done this. Assuming it is a slide choke carb, the stop screw will be in the side of the carb and stops the slide from going right down, screw in, so it stops it from going low enough to stop the engine when you take your finger off the throttle. Cables should have a bit of slack, otherwise you can't adjust this.

Trouble is, on a new carb, mixtures may not be set right for your engine either, the needle in the centre of the slide may be adjustable, if it is, up for richer and down for weaker, "brown and dirty" plug would indicate it needs to be weaker. If it isn't adjustable, try fitting the main jet from the old carb. Idle jet too may need adjusting, but sort the main jet first.