PS:
I also tried swapping the spark plugs after I determined which cylinder wasn't firing. No change.

Nothing stated about the air filter. How does it run without the air filter? or with a new one?

"Replaced intake manifold to carb gasket"

How about the Intake Manifold to block gasket, is the Intake Manifold tight to block.

" Could my valves be out of adjustment?"

These engine very seldom require valve adjustment, more likely is the valve seat coming out. You need to see if it feels like it has normal, the same, compression on the dead cylinder when it quits.

Walt Conner

The air filters are new. Runs exactly the same with or without air filters. Yes, replaced the gaskets on both sides of the carb spacer block. I just finished mowing at half throttle, it doesn't miss as bad with less throttle even when the governor adds more power. Seems to be more RPM related than load related but adding load when it's missing makes it alot worse. As far as compression goes, I will check it ASAP. My battery has a dead cell so the compression strokes are more pronounced when I'm cranking on it. It seems to have a little more compression hot than cold, which I read as normal. To Walt: So I could be losing a valve seat? Nice. What is involved in fixing that after I pull the head off & remove half the valvetrain? Do they just fall out eventually? You say the valves rarely need adjustment but what if some ham-fisted imbecil set them wrong before I bought it? It has never ran very well at high RPM. I sure appreciate everyone's input on this matter. Thanks to all.

Also, I will definitely post a follow up if I fix it. It does no good to get everyone's input & then not let anybody know what the problem really was. Thanks again.

Valve clearance for that engine is .004" - .006" for the Intake Valve and .007" - .009" for the Exhaust. You may be able to remove the inspection plates behind the Intake Manifold and check, much easier with Intake Manifold removed and it was this gasket I was asking about. It calls for checking the clearance with the piston 1/4" down past TDC, not sure how you do this as spark plug comes in from side. When I do it, it is in connection with an overhaul and the heads are off.

You can remove the spark plugs, turn the engine by hand while holding finger over plug hole, you should be able to pretty well tell where TDC is, continue turning a little bit to guess 14" down past TDC.

I said to check compression on dead cylinder when it stop to guess about valve seat OR valve sticking.

Walt Conner

Figured it out finally. Blown head gasket, DUH. Don't have a compression gauge, gonna have to get one. The exhaust valve lash is .012" but I can live with that. Both valves look good & move good, head is square, ports are clear, spark is hot, carb is rebuilt. Just get the gaskets & off I go. Gonna change both head gaskets while I'm at it. Will let you know if it works. Thanks again.

UMMM, I DIDN'T FIGURE IT OUT!

OK, I pulled both cylinder heads, cleaned everything (piston, valves, comb. chamber), replaced both plugs (gapped them too), & installed new head gaskets on both cylinders. Torqued it all to 20-ft/lb. JUST LIKE BEFORE: IDLES GREAT & DROPS THE LH CYLINDER UNDER LOAD.
ONE THING I noticed: The valve lash on the LH cylinder exhaust valve was .140" when it should be .07" - .09". I can hear that valve ticking lighly at idle but I really dont think it is hurting anything at all. Surely not enough to completely kill that cylinder under load. The valves all move smoothly & the valve seat are clean.

What the hell do I do now???

Probably pull the carb back apart purely for lack of a better idea. I just want to mow my stupid yard man!
ANY INPUT IS GREATLY APPRECIATED.

I'm going to pick on your decimal placement habits as a decimal place has great significance.
Surely you meant the valve clearance for the left exhaust to be .014" (14 thousandths") and not .140" (140 thousandths") which would be greater than 1/8" (.125").
Similarly, .07" is 7 hundredths", .09" is 9 hundredths".
Now that we took care of the details on decimalling, on to the main topic.

When you had the heads off and were turning the engine while watching the valves, did you take note of how much each valve raised up from its seat when the cam opened each valve? Or did you pretty much just note that each valve was opening, and not measure the height it raised to?

Here is where I'm going with this:
You may have a worn out cam on the lobes serving one of the valves on the left cylinder.
Essentially, when an engine is idling, it requires so little air and fuel to operate at that RPM that even a worn cam lobe can still furnish the cylinder with enough air and fuel for it to seem OK and match the power output (which ain't much at idle) of the right cylinder.
But, when the engine is asked to run at a higher RPM, and to do some real work, the worn cam lobe can't open the valve high enough, or keep it open for long enough time, to either "charge the cylinder" on intake stroke (if it's an intake lobe) or to purge the exhaust gases (if it's an exhaust lobe).
Either lobe being worn down would drastically reduce power output on the cylinder at higher RPM.
At idle RPM, the affected cylinder might seem quite normal.

While your head gaskets are still new, it won't hurt to pull the heads back off and carefully measure how high each valve rises up when it is operated by that cam lobe.
You can re-use the gaskets without fear because they have not deteriorated.
And I can tell you right away that .014" when it should be .007" to .009" is a pretty significant difference and will reduce power output somewhat at higher RPM, but I doubt that's the "whole story", but it could be a subtle indicator that the cam has lots of wear.

Thanks! Yes, my astonishment at the fact that the head gaskets didn't fix it temporarily clouded my math skills.

So you think it has a worn out cam lobe? That would explain the lack of high RPM function. That valve is the only one with an out of spec valve lash. I didn't measure the valve life but they all appeared to be opening the same height. I will pull the heads & measure the valve lift with a digital micrometer. Jeez, I hope it's not a cam lobe. That means gutting the engine.

Two different people told me to tear down the carburator again because they have rebuilt those carbs & had them run like crap. They tore them down again & reassembled them & they ran fine. I'm not really buying that because if 1 cylinder runs strong & the other cylinder completely quits firing under load it's gotta be a cylinder specific problem. If it was the carb I would think both would run bad.

I was wondering about the float height on these carburators. I never changed mine but I'm wondering if it could be too low (fuel level in bowl too low). But again I would think that if the carb were running out of fuel under load it would affect both cylinders. Anyway, does anybody know the float height on the factory carb on a B&S 400707 14hp twin?

I will reassemble the carb & pull the heads again to measure the valve lift. Thanks mownie!