MXU 375 Stalls when you give it gas

[BookmanWV]

Hi Everyone.  I am new to the forum but not new to Kymco machines.  I have 2 2009 MXU 375s.  I recently replaced both carburetors on the machines.  One of them runs perfectly, the other has this issue in the subject line.

I can not find anywhere on the internet what adjustments can be done to a carburetor, or what "factory" settings should be.   The new ones did not come with any manual.  Not sure if it is not getting enough air, or possibly too much fuel.  Perhaps the electric choke is not disengaging?  Checked compression, spark arrestor, air cleaner, spark, valve timing, etc.  It starts and idles great.  Give it gas and it stalls. 

I did search this forum but I can't find anything specific to this issue.

I appreciate any help you all can give.

[Bratman2]

If everything is hooked up identical, as best you can tell, you could swap the carbs between ATVs and see if the issue follows the suspect carb to the other ATV. This would confirm the carb is the problem.

Only other thing I can think of is if there is a screw adjustment you could count turns to bottom on the one that runs, then set it back to factory setting. Go to the other carb and screw the adjustment screw all the way in and then count out the same amount of turns. Do not apply too much pressure on the screw when it stops. Can damage the seat. At least then you would know that both carbs are set the same.

Just thought of one more thing to throw out there. I know on my granddaughter's little ATV I had to pull and clean the carb. Replaced some of the fuel lines too. Had a heck of a time with it not wanting to run right after putting it back together. Best I could come up with was installing the new fuel lines I created a small section that dipped coming from that gas tank and then ran upwards every so slightly to the carb. I think it made it harder to fill the fuel bowl on the carb. Rerouting the fuel line so it progressively ran down hill to the carb made it run great again. May not be your problem, just something else I thought of.

[airshot]

Are the replacement carbs factory or aftermarket?  Aftermarket carbs have a tendency to be not set up correctly or wrong size jets.
If factory carbs then consider what Bratman posted here...lots of good ideas in his post !!! 

[BookmanWV]

Thank you for the feedback.  We are taking one of the old carbs and putting it on to see if it rectifies the issue.  I am reticent to mess with the bike that is running well.  After a good clean up we'll install it (I have both old carbs so will try both if needed).  I'll update the results.

[Bratman2]

Can't say I blame you! It was just a thought. Good luck!

[BookmanWV]

So we cleaned the old carb up and put it on.  It ran awesome for about 5 minutes.  Took it down the road and back (3/4 mile) and then it started stuttering again.  We think it's not getting enough air.  We have confirmed the fuel line is good and flowing from the tank.  Previous question asks if these are OEM carbs and yes they are OEM.  Process of elimination.

[Bratman2]

Few years back I bought some NGK sparkplugs off Ebay for my grandchildren's 2 atvs. About 1-2 months after installing them my granddaughter's little atv started -missing and would not half run. Cooled off it would start back up. Run maybe 10 mins and start stumbling again. Trying to troubleshoot it I skipped the sparkplug knowing it had only been in for several hours of run time. Then the next time my grandson came over to ride his it started doing the same thing. I spent hours trying to troubleshoot those two atv's. After a couple of weeks and many hours I found the two old spark plugs in my little parts bin. I thought what the heck, I know it will not make a difference but installed one. Fired up and ran like new. Only thing I can figure is the two new NGKs were defective or fakes. They both only lasted single digits of running time. It was a very frustrating couple of weeks troubleshooting. May want to give that a shot, plugs are cheap! I tried so many things to fix them and in hindsight I didn't follow the most basic advice I give anyone. Start with the easy stuff, like a plug, lol!

[BookmanWV]

Bratman2 we put a new plug in it.  Bought it from local auto parts store.  Could be an easy check though to see if it is bad.  Thank you for the idea.

[airshot]

As mentioned spark plugs are cheap so well worth a try.  I highly recomend using NGK plugs, over the years I have had many toys and was an old champion fan... Found I had toys that would not run on champion plugs !!!  I was taljed into NGK plugs and have veen using them ever since.  With that said...plugs can foul out for various reasons and once they do, they typically do not work well after that. Replacing the spark plug is a quick, simple and cheap test, but be aware...I have had new plugs that were bad, so grab a couple .....good luck!