I'm working on a friend's 90cc Apex mini-quad. When he dropped it off, it wasn't running, and hadn't ran in years. I put a coil on it, as well as a new spark plug. I also cleaned the carb, emptied the old gas from the tank, as well as replaced all the fuel lines.

I finally got it to fire up after hours or troubleshooting and kicking. Now it runs, but will die when I let my thumb off the throttle. I've tried adjusting the idle screw on the carb, as well as the fuel/air screw. The idle screw doesn't seem to do anything when turned in or out.

I took a video of it to post on here. I'm not sure what the issue is. The video is with the idle screw all the way in, and the fuel/air screw 2 turns out. I'm not sure what to try next. Maybe the piston rings are fried? No idea!

Any suggestions or ideas would be greatly appreciated.
Youtube Video Link -->

 

I'd check the compression.

 

...and then look for air leaks if compression is good...It's clearly not getting enough fuel...is there an in-line fuel filter? If yes, is it clean or remove it and see if idle improves...might want to re-check the carb & jets...the idle circuit is usually tiny and clogs easily...as you hit the throttle the needle moves up allowing the much larger main to feed fuel...it sounds decent when you give it throttle...

 

Sorry about the delay, I was out of town.
I did a compression test. It is sitting between 95 and 100psi. I'm not sure what it's supposed to be at, due to the limited info about these quads online. I would assume that's low though.

 

Cleaned the carb and jets again, same issue. There is also no in-line fuel filter. Could the clip on the needle possibly need to move up a notch?

 

I just found a link saying 150psi is good compression. Guess I'll be tearing into the head. Thanks for you time.

 

95 is ok. less than 90 and it needs freshening up.
If the idle screw is in all the way, move the needle up a notch.

 

I don't think compression would effect idle...if it's running, then compression is ok. This almost sounds like a cable/slide issue...meaning I'd check how the cable is installed onto the carb slide. Screwing in the idle adjuster should move the slide up and the needle with it increasing gas/airflow...perhaps the needle is not in the slide correctly...not moving when the screw is turned, but moves when more throttle is given...it does seem to run fine with more throttle...you should be able to see all this visually...something else to try before tearing a motor apart...


Moving the clip down on the needle may work...should start at the middle notch...but if you have to move the clip all the way down to get it to idle then I think something else is amiss...

 

I did look at the carb needle and it wasn't installed right, so the needle wasn't lifting. Now I am having a hard time starting it. When it does start, it is doing the same thing (bogging and backfiring). It is also dying when I left off the throttle. I'm not used to 2 strokes, but it seems like it's flooding out. The clip is on the middle slot on the needle, and the plug looks wet when removed. The problem is, with the clip moved up or down, I can't get it to fire at all. So lost here.

 

So...couple things I can think of:


1) Since this is not your quad I'd highly suggest you verify the jets that are in the carb and determine if you are dealing with stock or not...that includes the needle...I'd also think about looking closely at the choke circuit since you mentioned the wet plug (may not be seating when closed allowing too much fuel)...mini's can be finicky with starting...so not too concerned with that at the moment.


2) I'm not familiar with the Apex carb, but does it have an fuel/air mixture adj/screw? Some carbs have it...some don't. If it does...turn it in until it lightly seats...then turn 2.5 turns out...this is the "general" starting point and could be the issue...


3) float level (starting issue) - on the LT80s that I work on...they always flood and cause hard starting...I usually turn the fuel on for just a few seconds, then turn off - then start...once running I turn the fuel back on. I've replace the float & seats and nothing works...almost like the floats don't have the ballast to completely close the fuel line...not a running issue however (at least not on the LT80 - but may be an issue with that quad)...but rather just a thought to get it fired up easier...


I'm leaning towards carb tuning because you mentioned the wet plug and it does run with more throttle...that tells me some jets are kinda ok...others may not be...


keep us posted...