I'm not usually a dramatic guy. But I'm losing my 💩

I'm a very capable mechanic. I farm for a living, and mechanic pretty well everyday.

This ATV is going to be the end of me

https://youtu.be/jmJMibZsP7o

^^link to what's it's doing

I have 3 brand new carbs and have put each on to make sure the carb wasn't the problem. The SOB will not start. It tries. But won't.

When I'm lucky it runs for 3 seconds then quits

Any idea what's going on? Definition of crazy is ringing a bell right now

Shoot me some ideas

I have spark

 

Didn't watch the vid but when you get it started will it run if you give it some throttle? Iv got one here that when i get it started and its a bear to start it will only stay running if i rev it up. Iv narrowed it down to valves or timing.

 

Did you solve the problem?
If not did you check for an air leak between the carb, manifold and engine? Sometimes the rubber seal goes?
After attempting to start unscrew the spark plug to see if it's wet(that shows fuel is going into the cylinder). If it is wet then the problem is the spark or compression(valves or rings).
If the plug is dry I usually add 2 or so teaspoon fulls of petrol into the cylinder through the spark plug hole and immediately I attempt to start it. If it fires and doesn't rev, I try half a choke to a full choke depending on the revs.
If all this fails then I check the valves in case they need cleaning or are stuck.

 

Did you test compression? It may feel like good compression, but sometimes good is not good enough...

 

Check valve lash. Haven't worked on a Chinese 250 but lots of 110's and most that had spark but hard to start the valves were just a bit tight