B&S twin 20hp, one cylinder runs hotter

[StarTech]

Most if these carbs have single throat carbs, thus common jet for both cylinders. May be intake leak on gaskets causing that cyl to run lean.

You should take the time to look up the engine's IPL before posting. A post like yours is unrelated to the OP problem.

 

[flyhalhl@gmail.co.]

Trying to help my neighbor. John Deere L120A B&S 20hp twin,406777 0139 E1. Has some popping and loss of power. Checked with infra red thermometer and the right side exhaust is about 100 degrees cooler than the left. 1. swapped plugs, no change. 2. checked compression, both around 140. 3. Checked valve lash on both, all set correctly. 4. Checked valve travel, both exhausts were same and both intakes were the same. 5. Will run on each cylinder by itself. 6. Checked diodes in coil ground, both good. 7. Checked flywheel key, good. 8. Checked for intake leak by spraying carb cleaner around all mating surfaces, with no change in running. Not sure what to do next, any help appreciated.

Cracked/broken cylinder ring. But need good check on intake to that cylinder.

Honestly I wouldn't lose sleep over it. On a typical v-8 for example, every cylinder will not have the same air fuel ratio and temp as well. Many a factor causing this. Cylinder head torque and sealing condition. Valves and so on. Could be down to one coil firing hotter or a weak coil for example.

Backfire and popping sounds like lean running. You might run engine with some choke and see what happens. Bad cyl temp increase while good cyl decreases?
Also check head temps both cylinders after warmed up.
Checked spark plug color when first removing spark plugs. Lean plug would be white. Normal is slightly tan? This is your first indicator on engine problem.
.......Have you replaced carburator base gasket?
What is exhaust temp? Should be like 1200°F measured 3" from cylinder but normally with thermocouple in small hole in tubing. So tube on outside just slightly lower. Cylinder head under 400° F.
Does your carb have mixture jets as well as idle mixture? Parts diagram shows 2 ports - each for cylinder

 

[flyhalhl@gmail.co.]

Trying to help my neighbor. John Deere L120A B&S 20hp twin,406777 0139 E1. Has some popping and loss of power. Checked with infra red thermometer and the right side exhaust is about 100 degrees cooler than the left. 1. swapped plugs, no change. 2. checked compression, both around 140. 3. Checked valve lash on both, all set correctly. 4. Checked valve travel, both exhausts were same and both intakes were the same. 5. Will run on each cylinder by itself. 6. Checked diodes in coil ground, both good. 7. Checked flywheel key, good. 8. Checked for intake leak by spraying carb cleaner around all mating surfaces, with no change in running. Not sure what to do next, any help appreciated.

Where are you measuring exhaust temp? Normally measured near cylinder. Normal would be small hole about 3" from cylinder with thermalcouple probe about near 1200°F. But if on outside tubing little less. Heads less than 400° F (200 maybe idle). What adjustments on carburetor, maybe only 1 idle mixture and fixed high speed jets? Carb dies have a port for each cylinder. Replace carb base gasket?
We are assuming hot engine running lean. Does popping only occur at high rpm? If so, may be lack if fuel. I have seen plastic gas tank with output nipple whole shrunk causing problems at high rpm, but OK on morning startup but then problems after 5 minutes mowing. Last 3 Cub Cadets; one 5yrs old with 24hp, I fixed had this problem. Easy check by removing hose at fuel pump and a hose below tank level should run small stream, gravity flow - as long as gas in tank, or a few minutes; not slow to dribble. Should easily insert 1/8" drill bit in tank. I fixed by cutting off nipple, enlarge hole and threading with 1/8" pipe tap. Then get 1/8" pipe to hose adapter fitting. Do not overtighten fitting in plastic tank.
(Also, hose can develop low flow.)

Lot of weird issues. Internal engine problems are tough.

 

[bertsmobile1]

And to add to Stars comment back in post # 16 Slow030 replied that he fitted the rebuild kit and his mower is running like new
It is good that you took the time to respond but please read the entire thread.
I will guess that you are using a phone not a computer so it is hard to go back to the start .

 

[flyhalhl@gmail.co.]

Forgot to mention guy said it was running fine til he ran out of gas. When it started to run bad he replaced fuel filter and fuel pump. Maybe dirty carb?

 

[flyhalhl@gmail.co.]

Check gas flow.from tank