Ok a little background, Running a 570 on a fresh rebuilt motor, sank the machine and messed up the rings, so i replaced the piston, rings, and jug last night get it all back together and it still wont fire, so we put a compression gauge on it and it isnt even registering. so assuming it had to be a valve issue we pull the cams and check the compression with the valves stationary, got around 35psi, which is still low. Anyone have any ideas on what may be causing this?
Rltice
I agree! A compression test has to be performed with the engine spinning-over and the valve-train actuating normally (not static). Barring further info, I would say that the O/P's technique is in question. My two-bits......
Cheers
From the 1012 570 shop manual:
Compression Test Specification
NOTE: This engine does have decompression components.
A smooth idle generally indicates good compression. Low engine compression is rarely a factor in running condition problems above idle speed.
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Possibly the "decompression components" have a malfunction? Manual says the decompression mechanism is located on the exhaust camshaft.
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Troubleshooting for low compression:
Low Compression
•Cylinder head gasket leak
•No valve clearance (cam wear)
•Cylinder or piston worn
•Piston rings worn, leaking, broken, or sticking
•Bent valve or stuck valve
•Valve spring broken or weak
•Valve not seating properly (bent or carbon accumulated on sealing surface)
•Rocker arm sticking
Compression Test Specification
NOTE: This engine does have decompression components.
A smooth idle generally indicates good compression. Low engine compression is rarely a factor in running condition problems above idle speed.
-----------
Possibly the "decompression components" have a malfunction? Manual says the decompression mechanism is located on the exhaust camshaft.
-----------
Troubleshooting for low compression:
Low Compression
•Cylinder head gasket leak
•No valve clearance (cam wear)
•Cylinder or piston worn
•Piston rings worn, leaking, broken, or sticking
•Bent valve or stuck valve
•Valve spring broken or weak
•Valve not seating properly (bent or carbon accumulated on sealing surface)
•Rocker arm sticking
I have worked on small engines most of my life. I have never seen the 570 apart or a manual on one but I can say an engine must breath in and out just like you and I do. Put the cams back in and do another compression test with both valves operating. You can watch them move if you leave the rocker cover off.
If you still don't get a reading check the valve timing. If the 570 uses a timing belt then it could be broken or have jumped out of time. If the 570 uses a timing chain look for same. I don't think any engines use timing gears anymore except diesels.
Perhaps another member can jump in with info on the 570 and say if it is an interference motor meaning that when a timing belt/chain breaks, the valves can strike the piston?? This will bend a valve slightly and it no longer seals when closed. Sometimes a bent valve is hard to see but this would cause low/no compression.
If you still don't get a reading check the valve timing. If the 570 uses a timing belt then it could be broken or have jumped out of time. If the 570 uses a timing chain look for same. I don't think any engines use timing gears anymore except diesels.
Perhaps another member can jump in with info on the 570 and say if it is an interference motor meaning that when a timing belt/chain breaks, the valves can strike the piston?? This will bend a valve slightly and it no longer seals when closed. Sometimes a bent valve is hard to see but this would cause low/no compression.
Tomato, tomahto. Either one pressurizes the cylinder with up to 100 psi, one by spinning the crank, one using external source. Both are troubleshooting resources.
A compression test and a leak-down test are two different diagnostic tools.
Each one has a different purpose...
Zero Compression is a bad thing...
Zero Leak Down isn't...
Each one has a different purpose...
Zero Compression is a bad thing...
Zero Leak Down isn't...
I now see that we were on the wrong track trying to eliminate the valves by taking the cams out. Thanks for bringing me back. the rings were indexed properly. The valves appear to be straight to my eye but that doesn't mean they are perfect, the motor did ingest water so its a good possibility one could have gotten slightly bent. Since I am getting absolutely "no" compression with the cams installed I can only assume that with a leak down tester this shouldn't be hard to diagnose. Thanks for the help and I will keep yall posted!
If the head is off you can put valves and springs in it then turn it so the cylinder side is up (the area the piston sees). Fill the area with WD-40 or solvent. If a valve is leaking you will see the fluid run right thru the valve seat and into the port (intake or exhaust).
If the valves are out you can roll them on the stem along the edge of a flat table and look for wobble as they turn. (like you check a pool cue stick at the bar).
If the valves seem to be sealing then I would double, triple check the cam timing again.
If a piston is going up and down at 280rpm (most engines crank at least this speed) and the valves are sealing then you should see SOME kind of compression?? Might be low but you should measure something.
Keep posting your progress. I'm sure we will learn something!!
If the valves are out you can roll them on the stem along the edge of a flat table and look for wobble as they turn. (like you check a pool cue stick at the bar).
If the valves seem to be sealing then I would double, triple check the cam timing again.
If a piston is going up and down at 280rpm (most engines crank at least this speed) and the valves are sealing then you should see SOME kind of compression?? Might be low but you should measure something.
Keep posting your progress. I'm sure we will learn something!!
Actually, Mowdoc, if you have the head off, it's much more effective to pour some acetone in each port. Due to the low viscosity, high vapor-pressure, and extreme permeability of acetone, if a valve is leaking at the seat, you'll see it immediately, around the head of the valve! My two-bits...........
Cheers
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