Problem solved!
Turned out to be a vacuum leak going to the fuel pump. Engine now performs as it should under load. Now I can cut grass and cultivate my garden! Woohoo

(Doing a happy dance, not filming for amusement though!)

This solution came from reading posts here. Thank you to everyone who pulls together to solve problems and support each other here.

The engine ran good for one run. Then back to chugging and not running under load properly. Had to go back to choke to keep engine going with deck blades engaged. Happy dance is over. Back to contemplating dynomite.

Sound like carb problems. When you had the top off, did you run tag wire into the open orifices facing up in the upper body of the carb and blow out, especially the one that is brass looking? Did you remove the hex nut on lower right hand side and blow out the main jet under it? That adjusting screw that looks straight back at the top of the carb is the low speed mixture. Initial setting is 1-1/2 turns out.

Might try running some SeaFoam in fuel mix.

Walt Conner

Agree with Walt (as usual)
Maybe with the pump working better, it managed to pumps some "loose" debris into the carb

Also look for vacuum leaks on the carb/intake system.

Rechecking everything. Fuel flow through line is good to carb. Going to pull carb again and go through it one more time I guess. I hate this vacuum fed system. It should be working. This will be the third time working on carb.

IMO would start at the fuel tank, clean it out by removing it bottle brush the inside and rinse several times. good time to replace the hose from the tank to the fuel pump also. now hold the fuel line down let some gas run into a plastic bottle to get all the air out. connect it at the fuel pump. remove the hose from the carb. or while you have it off for the 4th time. turn the engine over see if the fuel will surge into a plastic bottle. Now that you know you have proper fuel delivery you can work on cleaning out the passages of the carb.

I have learned this several time, just yesterday matter of fact with the used Kolher engine I put on LX266. You wouldn't believe the junk I got out of the carb.

Update:
Checked fuel flow of pump and it's pumping as it should. Lots of fuel coming through to float bowl.

Also removed the new spark plugs and they are black with carbon from unburned fuel. (Due to having to run engine partially choked.) Re-tested spark and it's still weak and dim. Not a nice bright bright blue spark as it should be. Also removed kill wire off coil to make sure it wasn't shorting. Same dim spark. I'm 99% convinced this is an ignition problem related to the coil.

New coil should be here today or tomorrow.

Is it possible that somehow the magnet on the flywheel has weakened it's polarity? The flywheel key is undamaged. Either way even a sheared key wouldn't affect the strength of spark, only the timing, so key is really a mute point at this stage.

"Is it possible that somehow the magnet on the flywheel has weakened it's polarity?"

Possible but highly unlikely.

Walt Conner

I am having a very similar problem with the same engine. It idles ok but chokes when you load it down. Rebuilt carb and it did not help and checked the spark and it is weak and it skips. I would love to know if the replacement coil fixed it

Stephen

I have went back through the carburetor, recleared all passages. However another coil is on the way at no charge to me. So far no change in engine symptoms. I still have to apply choke to keep engine from dying when I engage the mower blades.

I replaced the coil with a new one, BUT the coil I got is defective! Only one lead has spark. However another coil is on the way at no charge to me.

What other possible issues could cause engine to do this? Would a bad breather cause this?

Sounds like the carburetor is way out of adjustment! Those adjustable screws on the carb really do need to be fine-tuned, in the adjustment problem you seem to be having.
Try screwing them all the way in, then 1 1/4 to 1 1/2 turns out, start it, then fine tune them. (This is assuming you have that type of carb.

"Code starts with 96 (year)" "(This is assuming you have that type of carb"

I THINK that year will only have the low speed mixture screw looking straight back at top of carb.

Did you check for vacuum leaks around the manifold gaskets and carb mounting gaskets?

Walt Conner

Walt. Yes the engine is a 96. There is only one mixture screw and the schematic I have for the engine from briggs website says its an idle mixture screw. I have tried adjusting that. Screw all the way in, then back out 1 1/2 turns. Engine will run, but then dies after choke is opened. All intake and carb gaskets are clean, new, undamaged except for being compressed. The head gaskets were new as well. Thinking I might need to pull heads again and check those, and valve seats one more time.

Another thing....when the engine is running at idle (almost fully choked) it sounds almost EXACTLY like a airplane engine idling (cessna 152, I used to fly those). Propeller sound and all. Like a lot of air moving. Strange.

There is only ONE adjustment screw and that is supposed to be for IDLE only. I do not engage mower blades at IDLE, only at higher RPM. The new coil is junk. Will have another new coil Mon or Tues. Now have intermitent spark on one side, weak spark on other side. Old coil is corroded through in one area. I'm still trying to figure out how most are saying it's a carb mixture problem when there is nothing to adjust, and idle speed isn't the problem.

I still say it's the coil...we shall see.

All this trouble over this engine and not getting much gardening done! Invested a lot in this machine. New blades, belts, engine parts, plus a new cultivator and hitch system. I need it to be working....just only so much I can do with little money. Very exasperated now. Neighbor down the road has 2 MTD riders for $50. Or 1 1/2 rather. Engines are different but still Briggs. No idea how bad those are...or if one of those would even mount since they are single cylinder and mine is a twin. 16 hp on the one I believe.

Thanks for bearing with me on this and your suggestions.

"Thinking I might need to pull heads again and check those, and valve seats one more time"

Your symptoms do not fit a valve problem. Did you check the float level when you had the carb apart? When the upper carb body is turned upside down, the float should be parallel with the gasket surface of the upper body. Also, gas tank vent, have you tried removing gas cap. Doubt this is it. Did you have the fuel pump apart? IF so, hope you didn't loose one of the little springs. How about the condition of the vacuum hose from engine to fuel pump? Someone recently had one that was so soft that it would collapse not letting full vacuum thru or your might have a leak.

Walt Conner

Walt.
Yes I had the carb apart to put in the rebuild kit.
Yes the gaskets, springs, diaphram, everything is as it should be in the carb.
The float is level with the carb body when inverted.

The spark plugs are black from unburned gas. Spark plugs are "new". Not clogged , just black soot, not oil fouled.

The vaccum line is in good shape, no leaks, no collapse.
I get plenty of fuel into carb, already checked that.
Also tested float to make sure it had no leaks when I had it out and cleaned it.

Oh, and yes, check the gas tank vent, even tried loosening cap while engine was running, no change or improvement there. Plus I blew through the cap vent and it's clear.

You have mentioned the "black smoke, especially at idle" and the black coated spark plugs.........This is definitely an overfuel or rich condition. My guess is there is something "wrong" about the needle and seat that might not be holding the fuel to the correct level in the bowl. Do you have an "inline fuel shutoff valve" in your fuel hose between the tank and fuel filter? If so, turn it to the closed position (off) and try running the engine like that. If indeed the problem is caused by an overfull bowl, there will be a brief interval where the engine seems to "clear up and run right" before it runs the fuel out of the bowl.

Put on second new coil. Refit vacuum line to fuel pump. Engine ran as before but a little better, then went to running real rough and SPEWING black black. Carburetor is being pulled this morning.

So it is a fuel problem...just not sure where. Going through the carb again. I have missed something.

The thing that has continued to throw me is, I have to pull the choke out to keep engine running when I engage the mower blades. Then ease the choke back in a little to keep it running while blades are spinning. The blades are new, the bearings are free and freshly greased.

Keep this thought in mind: Carburation is not just about the fuel. You must also consider the induction air as well. The black smoke indicates a rich mixture but the fact you have to use the choke (which reduces the intake passage size) would seem to indicate too much air. What I think might be going on with that is when the choke is not used, the induction air flow is pulling too much fuel out of the carburetor bowl (due to some "as yet undetermined defect"). When you reduce the air flow by applying some choke, the air flow velocity is reduced enough to somewhat "even out" the mixture. But as you have observed, this "manual" control of the mixture ratio is not very good. Another thing in the carb that can result in overfueling would be if a pilot jet was missing or loose. If this was the case you would have a hard time starting the engine and it would not want to run at normal idle RPM.

could this be a governor issue ?

Success finally. Got it running like it should.

All Problem areas were as follows:
Bad coil.
Bad spark plugs.
Carb upper body gasket not sealed. (Seeping fuel into intake)
Main jet plugged.
Vacuum line leaks at block and on carb.

Anyone want to buy this now? $750 and it's yours WITH a new cultivator and hitch system! (Well...maybe I'll use it awhile first)

Thanks for all the input.