Briggs & Stratton 21 HP platinum exhaust valve barley moving

Tiger Small Engine said:
I have never seen, verified, and replaced a cam that had intake and exhaust lobes that were so badly worn they were affecting the engine enough that cam needed to be replaced. How often does this happen? Apparently less than the compression release mechanism breaking on the camshaft.
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Since I didn’t go in deeper to inspect the camshaft for worn lobes, this makes sense to me. Sounds like it rarely happens, but it does. Happy 4th of July to all!
 
Hello, I have an update.
When I took the cam out I could see that it was worn a lot. The intake lobe was worn down to 1.20 inches and the exhaust lobe is down to 1.05 inches. Almost 1/4" undersize compared to another one I pulled.

I bought a used mower a couple weeks ago that I was told had not run in years and they didn't know if it could be repaired? I bought it because I wanted the transaxle from it. I decided to pull the motor and take the camshaft out of that one. The lobes look very good and they measure 1.24 inches. I’m thinking they started at 1.25. The decompression mechanism looks good also so I am going to put this one in the mower that I know runs.

Thank you for the input!
 
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If the tips of the lobes are discolored from wear, the case hardening is failing which means this used cam will not run for long before the problem repeats itself.

Case hardening is a microscopic thickness and does wear off if the cam was poorly heat treated and hardened.
 
Moonbud said:
Thank you. That's the answer I was hoping for. I wasn't sure if the valve could have been stuck?
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Moonbud said:
Hello,
After watching a video I thought the decompression mechanism on my 21 HP Briggs & Stratton engine was faulty. However, after removing the valve cover, I found that the decompression mechanism appears to be working properly. While turning the flywheel by hand, I noticed that the exhaust valve barely moves, whereas the intake valve appears to have a normal amount of travel. The rocker arm is gapped properly.
Do you have any suggestions as to what might be causing this? I have not removed the sump cover yet. The mower backfires when I start it, but then it idles normally. As soon as I put it under load, it begins to sputter and backfire again. I would appreciate any ideas or suggestions you may have.
Thank you!
Dave
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Differential pressure test it to check valve leakage.....you don't have the gauge manifold so find someone who does.....or, place piston top dead center, valves closed, on one side and blow compressed air into the spark plug hole. Don't put your fingers anywhere near the fan....100 psi is enough to get hurt. Being able to lock the engine at TDC and listen to where the air is....or isn't...going will inform the next step.
 
Auto Doc's said:
If the tips of the lobes are discolored from wear, the case hardening is failing which means this used cam will not run for long before the problem repeats itself.

Case hardening is a microscopic thickness and does wear off if the cam was poorly heat treated and hardened.
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If you call 1.5mm a microscopic coating....which it's not....the steel is infused with carbon or nitride to about that depth but not further. Turn a piece of case hardened steel on your lathe and you'll know when you can switch from carbide to high speed steel tooling.....

www.paulo.com

Understanding effective vs. total case depth

Discover the difference between effective case hardening depth and total case hardening depth in this technical heat treating article.
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RevB said:
Differential pressure test it to check valve leakage.....you don't have the gauge manifold so find someone who does.....or, place piston top dead center, valves closed, on one side and blow compressed air into the spark plug hole. Don't put your fingers anywhere near the fan....100 psi is enough to get hurt. Being able to lock the engine at TDC and listen to where the air is....or isn't...going will inform the next step.
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Hello R.,

You missed the part in the updates about the cam being worn down. Just the same though, there is no harm in doing a leak down test once it is reassembled.
 
In my case, I have seen a valve seat being raised and most recently, a rocker arm cracked and not raising correctly.
 
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