Will crank with choke engaged and run rough. Applying throttle or removeing choke and it cuts off.gas pump ok new plugs and oil. Breather clean.??????

take out the spark plugs and check them also check the pistons for carbon build up, check the vacuum lines for leaks and being clog or choked espaly the top pipes as they can get clogged up on the breather line. also check the sump breather pipe as this can clog up over time. remove the belt auxury and test the movement on the power steering (if fitted) should move easy and do the same with anything else on there like alternator etc. anything that is stiff to move this can add more fuel use. even an oil change can help with mpg.

fuel filter clogged or fuel pump problem

Ok I assume what you are trying to say is that once it starts with the choke on and you turn the choke off it dies.
This behavior usually means that the engine is leaning out. This means that it is not getting enough gas. Often if you put it back to choke fast enough it will keep running though very roughly.
The problem is the main metering jets are plugged or there is some kind of obstruction in the carburetor. This is assuming of course that the tank is full of gas and the fuel filter is clean.

First, you need to dump all of the fuel out of the gas tank, Preferrably into a gas container. If you know how much oil you dumped in, you can calculate how much more gas to add. You could also just dump the gas, re-mix some new gas and oil, whichever you prefer.

If you are comfortable doing this, remove the fuel lines going to the carberator and let them drain into a pan. Pump the fuel primer to purge the gas out of the carburator, and then re-install the fuel lines. This might help you get it started a little faster, but its not necessary. You can burn the old gas in the engine at it will be fine.

Next, remove and replace the spark plug. Its probably fowled out with too much oil. Keep a can of carb cleaner and a small wire brush handy, you may need to clean the spark plug a couple times. If it gets too dirty, you can replace the spark plug with a new one. Do not burn Carb Cleaner in your engine for long periods of time, it will damage the engine.

After you have installed the spark plug and put some correctly mixed gas in the tank, Try to start the engine again. There will be some residual fuel in the fuel lines going to the carberator, so you will need to burn off that gas.

If you can't get it to start, try these steps:
Choke off, throttle wide open, crank several times. (or let it sit in the sun for a couple hours) (this corrects flooding)
Prime carberatur, Choke full on, throddle idle position, crank until it fires once but does not start.
Choke half, crank until it fires again or starts.
If it starts skip this step: (Choke off, crank until it starts).
Keep the choke on half for about 10 seconds and let it idle.
Put the choke off and let the engine warm up for a couple minutes.
After warmup, test the throttle to see if it will run at full throttle (it may take some fluttering until the engine is fully warmed up)

Let the engine run at full throttle for a few minutes. The exhaust may be a little more blue-smokey than normal...this is the excess oil burning...totally normal and harmless.

I find with my Poulan Pro that you need to let it warm-up abit before applying full throttle, I live in Upstate NY where the weather has bee great lately,however, it stalls easy ..I mix 1-pint of 3 stroke oil to 1-gallon of gas for my weed eater and have no problem

It should start easily, start with fresh fuel/oil, pump the primer as many times as it takes to 3/4 fill the bulb, pull the choke fully out, crank the engine until it fires once, push the choke fully in and crank again until it fires up, after re fueling a warm engine, pump the primer again until it 3/4 fills the bulb,before cranking the engine pull the choke fully out and in again( this will engage the part throttle ) then crank the engine until it fires up.

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:
http://www.smallenginewarehouse.com/images/manuals/FH641-Service.pdf
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,
Kelly

if you are running the engine at full CHOKE, then it is flooding the engine, but if you meant off full choke, then the carb is still stopped up. Try soaking in carb cleaner overnight after removing jet & all rubber goods. HOPE THIS HELPS.

If the choke has an adjustment point, then adjust the choke to a good overall working powerband. If it still doesn't act correctly, look for vacuum leaks. If it is an option, you may want to install a manually operated choke lever.

fuel filter or fuel pump problem or it could be the automatic choke is closed and choking motor make sure choke is plugged in on side of carb start car let it warm up with air breather off watch and see if choke opens up .also you can look down into carb when motoe running crack open throttle and see if it sprays gas into intake if it doesnt could be filter or pump. to check fuel pump remove fuel line from carb insert line into small can or bottle so fuel doesnt run ontop of motor and have someone just bump over starter and see if it pushes fuel out line with a descent amount of pressure check fuel filter.and if all that checks out try looking for vaccum line off to check get a can of either and squirt lightly around vaccum lines and listen for car to idle up if it does bad vaccum line. and also check distributer. but it sounds like it is getting to much gas so i would check choke first..... hope that helps .. and by the way you have a 351 modified 400 wich is 351 m 400.