Offroad > Mongoose

90cc atv won’t run when warm

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BCrancher:
2006 Arctic cat DvX 90. Has been getting progressively worse over that last few years.
Multiple carb cleanings, air filter cleanings still giving me issues.

Machine would start up and idle great.  After it’s ridden for 10 minutes and gets warm, it won’t hold an idle anymore, usually stalls. At this point the machine has lost all power as well. Will not pull a 9 YO up a small hill.

This fall I gave up on the electric choke, thinking that this was the reason it would not run when hot.
I bought a new replacement carb with a manual choke, installed new jets the same size as the oem carb.
I the adjusted the valves to spec. They were tight before, so I am suspecting compression was lost due to this.

Machine started up right away, and idled great until it was warm then it did the same thing.
Then I could not get it to start back up.

Put a new plug in, started right away. My son took it for a ride until it wouldn’t pull him up a hill. I towed him back to the shop. Checked the valves and spark plug. Valves were within spec when it was warm. Spark plug was black and sooty though.

I cleaned the plug and it started up, but would not hold a steady idle. Almost stall, then rise up for a few seconds then almost stall for 20 seconds

I’m stumped.. any help is greatly appreciated

CROSSBOLT:
Could be the piston ring end clearance is too small and when it gets to temp they start to drag on the cylinder slowing the engine. That should be easy to confirm or disprove.
The sooty plug is an indication of over rich mix and that it is not caused by the choke but what?

BCrancher:
Sooty plug is probably due to my elevation and not having corrected my jetting accordingly.
Probably need to resize the jets or play with the air mixture screw.

This thing is so finicky it is very frustrating

CROSSBOLT:
Not end gap since 14 years old! Float height off or is sinking making it rich. What is your elevation?

BCrancher:
2286’ is my elevation. 
Not a major change but might be enough to try stepping down a size.

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