Seems odd that after changing carbs you have this symptom but a dirty air cleaner or restiction in the air intake ducting can cause this. I note you said you changed the air filter(s). I am assuming you changed both the paper and foam filters. My thoughts are maybe there is some gasket material partially blocking the intake under the carb so when under load it is acting like the Choke is on. (
Before you go lifting off the carb.. a partially engaged choke valve will do the same thing. It will run just fine at low speeds or little load but when you put a heavy load it will choke out / get too much fuel.
Last thing that can cause the engine to act really weird is that the gas cap is vented and if the vent is clogged in the gas cap it will cause very unstable operation of the engine at higher rpms.
Your engine service manual is here:
Carb informationis in section 2 - 8
I reommend you inspect
- the tank cap (no plastic bag in place of the rubber under the gas cap to prevent the tank from leaking)
- the choke linkage and position of choke valve (Off or partially on?)
- screen if any under the air filter (Thinking out of the box... what if the air filter is being drawn against the air cleaner / filter housing causing a choke situation under load)
Don't do this for very long but try a high load test with the air filters removed.
One last dumb thought is... the fuel pump is ported to the engine crank case breather. If the diaphragm fails it can put fuel in the oil which will cause the oil level to rise due to the added gas and then end up over pressurizing the crank case. (You said you changed the oil.. and I would think the oil level is still the same as it was post oil change.)
Those are my thoughts and recommendations for the moment. If anything else come to mind I will send you another post.
If you have questions feel free to ask. I wll tell you that I am using the service manual and my 40 + years of experience to help you.
Thanks for choosing FixYa,