I have the above engine on a John Deere ZTR that would start surging after running for about 45 minutes to an hour. Solved this problem by mounting fuel tank above engine instead of below, leading me to determine something was causing a decrease in fuel flow after the engine ran for that amount of time. Tried multiple fuel pumps and also ruled out problems with fuel tank cap. I’ve also checked compression on both cylinders and ensured valve clearances are within spec. What else would anyone recommend checking? I would prefer not to have to run mower with an elevated tank. Would like to figure out the root cause of the fuel flow problem. Thanks in advance.

Jim

Surging is usually a sign of running lean. 
 
Make sure you have adequate flow to the carb. Pull the hose
crank up the engine (end of hose in a container) and check for flow.
We want to make sure the lines/filters are clear.
 
Did you pull the carb and check for debris in the bowl?
 
While there, I would also clean the carb and all jets.
Please note, if your carb has TWO MAIN JETS (likely), there are of DIFFERENT SIZES.
Keep track of which one came from which side...
 
 
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Thanks for the reply. Initial interventions were to pull, inspect and clean fuel tank. Blew compressed air through all the lines. Replaced air and fuel filters. Swapped fuel tank cap with known good. Ran with fuel cap loosened. Tried multiple fuel pumps. Checked spark. Replaced spark plugs. Disassembled and cleaned carb. Swapped with known good carb. Checked compression and valve clearances. Still with lean condition after running 45 to 60 minutes until I elevated fuel tank to add gravity to the equation. Runs flawlessly under that condition. I also tried ethanol free fuel from different source and tried adding stabil and seafoam.

Interesting, so if you test fuel to carb is it reduced flow at 45 minutes?

Back when this was occurring, last spring, I never took the opportunity to check flow between fuel pump and carb. 🙁

Ok, it's important to actually look and see flow when it's acting up (and also when its' not).
 
Just cause the parts new, doesn't mean it's good. Blowing pressurized air is good, however
that going thru a restricted hose is MUCH EASIER than liquid (fuel). 
 
I would start at the fuel tank, and check for flow (use a catch can).
From there, move on to the fuel line, then to the OEM fuel pump, etc. 
 
It sounds as there's a restriction, just enough, however gravity can overcome it. 
We have to find that...
 
Also, don't bother with spark plugs etc as raising the tank fixes the issue.
Spark is fine, focus on fueling.
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Sorry, I failed to mention that I also replaced fuel lines. A little clarity. I think what I’m actually trying to figure out is what may cause a decrease in negative pressure, or a decrease in the pressure gradient from the engine that travels through the vacuum line that drives the fuel pump only under the aforementioned condition.

Of your fuel pumps, did you use an OEM Briggs pump?
 
Did you check, very closely, the vacuum line going into the valve cover?
 
Worth repeating, check for good flow starting at the gas tank and then further down the line..
 
 
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Yes on both counts. Very aware of how easily those little vacuum lines split. In fact I keep at least 2 spares for all of my machines. 🤣