[vectorit]

Timing out, can cause a nasty spark knock. Combined with a poor performing carburetor, the engine could be lean or rich. Rich mixtures will cause a sooty exhaust, and a noxious exhaust that will make your eyes water and give you a head ache in short time.

Tight valves in my experience can cause back fires out the carb, and or a rough idle.

Engine cubes don't matter if the ignition timing is incorrect, and the intake/exhaust valves are miss adjusted. No matter what size, it's gonna run badly.

Like noted above, get all that tuned and adjusted correctly asap.