Briggs & Stratton 21 HP platinum exhaust valve barley moving

M

Moonbud

Forum Newbie
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
 
If the valves are not opening the same amount that basically points to the exhaust lobe on the camshaft being worn to the point it no longer opens the valve fully. Need to open it up and change the camshaft.
 
Does the valve to rocker arm ever get really loose while you hand turn the engine with the spark plug(s) out? If yes then something is holding it open - it is either stuck or the valve seat has fallen out of the head.

If you could look at the valve inside the combustion chamber, there should be no gap under the valve between the seat and valve when it is closed. To eliminate the possibility that carbon has built up and stuck the valve, you might take a propane torch and heat the head around the area where the valve is. Don't over heat it - just make it really warm to the touch, not so hot it would burn you. Spray a little penetrating oil on the stem - like Liquid Wrench, Kroil, Knock er Loose, PB Plaster etc., and tap the valve/rocker arm with a lead, brass or piece of hard wood. Do not hit it with a steel hammer because you don't want to bruise anything. That may unstick it if it is just stuck open. If there is huge play between the rocker arm and the valve during part of the engine rotation then another possiblitiy is that the valve seat has dropped out and the valve is stuck on it. If heating the head a little does not loosen it, get a small borescope, like this, and use it to look thru the spark plug hole to see what is going on in the head. If however there is never any excess play between the valve and rocker arm, and they move just a tiny bit, (as ILEngine wrote) the exhaust lobe on the cam may be wiped out.

borescope examples:
Get the dual lens one if you can

This one may or may not work. The one above is better
 
The rocker arm never gets loose. I have a Depstech bore scope so I will still look inside just for the learning experience.

Thank you
 
perfect. There should be no gap between the seat and valve when it is closed. If the valve to rocker arm clearance is consistent during the engine rotation, and never really loose, then the camshaft is just not lifting it. It works by a lifter riding on the cam as it rotates. The cam lobe moves the lifter. If it is a L-head engine then the lifter pushes directly on the valve. If it is an overhead valve engine, then the lifter pushes a push rod which pushes the rocker arm, which in turn pushes the valve. There are no places I can think of that movement could be absorbed. The cam either pushes the valve open or it does not.

L-head. Operation is at about 8:20 into the presentation


overhead valve engine
final-valve-spring-aasembly-with-callouts-2.jpg
 
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
Click to expand...
This engine design has a history of cam lobe failures.

Did you rotate the engine 2 full revolutions while watching the valve movement?

Make sure it does not have a bent pushrod, otherwise it appears that the exhaust cam lobe is wiped out.
 
Auto Doc's said:
This engine design has a history of cam lobe failures.

Did you rotate the engine 2 full revolutions while watching the valve movement?

Make sure it does not have a bent pushrod, otherwise it appears that the exhaust cam lobe is wiped out.
Click to expand...
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.
 
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.
Click to expand...
I think I have seen 3 or 4 in 28 years in both the single and twin Intek engines. Never on the vanguard. And maybe a few years ago on a couple L head push mower engines with the plastic cam lobes.
 
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