I can only start my ATV when I remove the air filter and cover the carb intake by hand. Fires right up then. Any suggestions on what I should check? Should I pull out the carb again and check the pilot jet? What else can I check?

Note: I can feel a lot of suction on my hand, and can smell gas on my hand after the start. Not sure if this is expected.

 

Your obviously not getting enough fuel to mix with the incoming air. I would start by doing a thorough rebuild of the carb, not just a take-apart and put back together, but a complete rebuild with all new gaskets and proper settings according to the rebuild kit. Factory built carbs are designed to match the airflow of a factory airfilter setup. If you have modified the factory airbox to allow more air inside then your carb is going to be lean. Whenever an airbox modification is made, the carb must be rejetted to match. Also make sure you don't have any air leaks around the base of the carb or anywhere else the intake may attach.

 

Some machines may run too lean in colder weather. All depends on altitude and temperature and how it runs after it warms up. Any noticeable hesitation might require jetting change,but the main thing is too look at the plug color. Should be a tan to medium brown color.If whitish,then main jet may need to be jumped up,plus needle clip position can be moved down to slightly richen idle to mid range throttle. If you happen to have the old round vm series carb you can turn in the air screw a bit also to cut down on air.Cv carbs are blocked off on the pilot(fuel) screw and you shouldn't have to touch those anyway.

 

I originally did a carb rebuild - had no idea what I was doing, was going off some YouTube videos. I drilled out the plug for the pilot jet. The first time I didn't seat the bladder in properly at the top. Luckily that wasn't damaged. Then I noticed I missed some shavings in the pilot jet hole. I didn't replace the needle, but I can go back and look at the needle clip position again (and replace the needle from the carb kit). The spark plug is new, the old one was black and oily. I'll look at it again after the carb rebuild.

Assuming I can finally get the carb right - fully cleaned and adjusted properly. What could I look at next?

I might have some air leaks - there is exhaust leaking I need to check by removing the exhaust and examining, and I might have a coolant leak somewhere (steam coming from somewhere with coolant smell). Could this affect the start?

 

Sounds like you have some projects to finish.. Yes fix any exhaust leak and any coolant leak.Normal pilot jet screw is 2-2.5 turns out. This pdf fuel section from the manual can help help you on tear down and assembly along with jetting,needle clip positions and float setting whether on the BST34 or BST40 carb. https://www.google.com/url?sa=t&rct=...U2gGlaHJWSIlbA

 

looks like you might have multiple issues, but hard to tell. #1, why was original plug oily and black? #2, drilling out a main jet is not proper jetting of a carb, a proper size jet needs to be purchased and installed according the the rebuild kit you have. #3, allowing chips in the main jet hole, may have allowed them inside the passages so an extreme and thorough cleaning is needed, including a small dia wire to push thru all passages to be absolutely positive they are open and clean!! Even the smallest tiny particle can mess up the fuel flow. #4. if you have a coolant leak is it getting into the cylinder? Water inside the cylinder will cause major problems. #5. each time you take a carb apart it needs new gaskets to be reassembled or you take a chance of the old ones not sealing properly. My guess is you have some serious work here to get done properly..

 

Consider your self very fortunate to have OPT on board to steer you in the right direction. He is the man with all the details!!

 

Should I fix the exhaust and coolant leaks first? Or work on the carb first?

 

Exhaust and coolant leak first. The carb can wait..

 

Got it. I'll start separate threads for the leak issues.
Thank you.