Burnt Valve?

[Bill D 1,926]

Trying to fix my neighbor's snowblower.  It's a Toro Powermax 828LXE.  It has a Tecumseh OHSK120 swapped on it.  I've rebuild the carb, new fuel lines, new primer lines, set the valve clearances, .004 intake /.010 exhaust.  I swapped coils and plugs with no change.

The engine runs fine until you load it.  Once I try to move heavy wet snow it dies in a minute or less.  After it dies it's impossible to restart unless you use the electric start with full choke, and even then it starts hard.  I can't figure out why it runs well with no load and not when loaded.  Thinking burnt exhaust valve.  Any advice is appreciated.  Bill

Edited March 15, 2023 by Bill D

[RED-Z06 2,221]

Usually these engines don't work hard enough to burn a valve, Usually its cold and they arent often pushed to the absolute limit for long periods.  Might try going up a bit on intake lash...this would stand to bump up starting compression slightly.  A leakdown test would tell you more.

[squonk 41,158]

Is the outlet of the gas tank clear?

[953 nut 55,324]

If you had a burnt valve the engine wouldn't run well even without a load. You didn't mention an ignition tune-up. Is the engine strictly a magneto ignition or does it also have points and a condenser?

If an engine idles well, but runs erratically when revved up, like it hits and misses, pops and backfires, then chances are, it needs a new condenser/capacitor. And always install a condenser/capacitor with the wire facing downward so rain water and/or when washing off engine, water will not enter inside condenser/capacitor, ruining it. With water inside the condenser, this will also allow it to idle well, but cause it to run erratically when revved up.

[ri702bill 8,346]

6 hours ago, RED-Z06 said:

A leakdown test would tell you more.

A good place to start without pulling the head...

I have a mid 1970's Ariens SnoThro what I have to richen the main mixture screw about a half a turn the first time I use it - been like this for 25+ years. Only way to get power out of it. About 5 minutes later, it starts to run very rich - readjust the screw back in a half a turn - good to go !! Go figure.... Yours does sound like it is running lean - not good.

That and / or a chunk of loose carbon caught on the seat will lead to a burnt exhaust valve... DO check to see if any part of the muffler is glowing red while in use - indicates a severely lean mixture, engine running way too hot...

 

 

 

 

 

Edited March 15, 2023 by ri702bill

[Bill D 1,926]

19 hours ago, RED-Z06 said:

Usually these engines don't work hard enough to burn a valve, Usually its cold and they arent often pushed to the absolute limit for long periods.  Might try going up a bit on intake lash...this would stand to bump up starting compression slightly.  A leakdown test would tell you more.

I can borrow a leak down tester if needed.  Many Tecumsehs I've seen over the years tend to run lean which is why I was thinking burnt valve.

Edited March 16, 2023 by Bill D

[Bill D 1,926]

1 hour ago, squonk said:

Is the outlet of the gas tank clear?

Yes it is.  I've tried different tanks and gas caps.  No change.

[Bill D 1,926]

51 minutes ago, 953 nut said:

If you had a burnt valve the engine wouldn't run well even without a load. You didn't mention an ignition tune-up. Is the engine strictly a magneto ignition or does it also have points and a condenser?

If an engine idles well, but runs erratically when revved up, like it hits and misses, pops and backfires, then chances are, it needs a new condenser/capacitor. And always install a condenser/capacitor with the wire facing downward so rain water and/or when washing off engine, water will not enter inside condenser/capacitor, ruining it. With water inside the condenser, this will also allow it to idle well, but cause it to run erratically when revved up.

Magneto ignition.  I've swapped mags. No change.  Plug looks good when pulled.

[lynnmor 7,308]

42 minutes ago, Bill D said:

 Plug looks good when pulled.

Replace it anyway, I've had plugs go bad in motorcycles just sitting over the winter.  My theory is that there is some vapor from the gasoline/ethanol that fouls the plugs.

[RED-Z06 2,221]

1 hour ago, Bill D said:

I can borrow a leak down tester if needed.  Many Tecumsehs if sern over the years tend to run lean which is why I was thinking burnt valve.

Even with adjustable carbs, Tecs like to run a bit lean..not sure why, they just hate a little extra fuel.  These were used on alot of generators as the OH318EA, they would run for days at a time at 75% power output..we never really saw any problems out of them other than vibration was high.

[Bill D 1,926]

Update: Installed a new plug, no change.  Turns out I had the wrong float in it.  Installed a brass float instead of a plastic one.  It was literally starving for fuel under load.  The plastic float along with the small dimple in the float bowl was preventing the float bowl from filling up enough.  Definitely glad it wasn't a valve issue.