Won't rev up

bigger saw

try setting your carb, you will see three holes at the side of the machine nr to the fuel cap. the one at the bottom screw it in as far as it will go then unscrew it 4 turns, the one above it do the same again but unscrew it one and a half turns, if this dont work its possible draging air in from somwere ie, crank seals, inpluse pipe, gaskets. it will need to be presure tested.

You may need to wire a switch to the cut out. Since you "eliminated" it, its not going to rev. You need a closed circuit and should be able to create one via a switch, however I recommend consulting a local mechanic or other Fixya expert for a second opinion.

Most likely a fueling issue, Check fuel line for air leaks or it could be a carb problem.

Could be blocked spark arrestor or carboned up in the exhaust port.

Starving for fuel.
Make sure carbs are getting fuel.
Make sure fuel filters are clean,
make sure fuel pump is working.

If carbs are getting fuel but wont rev, you need to clean your carbs because your jets are clogged

cutting out and sputtering at 4000 rpms are normal, its the rev limiter kicking in, all of them do it, as far as hard starting, is it just sense its been cold?? might want to put some fuel stabilizer in it.

Check the temperature sensor under hood far a short. Common problem with this car.

Sounds like an air filter cloged or a vacume leak. Check your air filter, and look at all the small hoses for rot or a break in them. Pulling in too much air will cause this also, ie vacume leak. 

it could be that or a clogged catalytic converter the converter controls air flow from your engine, if its clogged the car cant run right