If it makes a noticeable difference when you remove the air filter cover, the filter is badly obstructed and needs to be serviced, which may well mean replacement in the case of a chonda. However most properly serviced chondas run as well as Hondas, which means start first pull and never miss a shot, ever. (They do sound rather inferior to Hondas due to oddball muffler design, but they don't misfire.) Hence you still have problems to find and fix.

I think the chondas called 40, 400 and 4000 are all copies of the GXV120, which is advertised as 4 hp. You can learn quite a lot about them from these threads:

https://www.outdoorking-forum.com.au/forum/u...p;Words=gxv120&Search=true#Post53596

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=58437&page=2

With regard to tuning the engine, the best procedure for most adjustments is to follow the Honda owner manual, here:
http://engines.honda.com/parts-and-support/owners-manuals/gxv120

However valve clearance is not covered in the owner manual. Here is the page from the workshop manual:

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