Skipper
Member
I don’t have much compression, about 25 psi/cylinder. How much is needed to run?
skipper
skipper
Compression Snow engine
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If your engine is free of binding and compression is relatively uniform, spin it over by hand and note if there is any flywheel bounce back when it stops on compression. If it does, and everything else is good, the engine should run.
Skipper
Member
Have very little compression. Don’t know if it is leaking valves or piston rings. How can I check? Skipper
- Joined
- Aug 18, 2008
- Messages
- 343
- Reaction score
- 134
Skipper,
You indicated that you had 25 psi compression. Is it uniform at all four combustion chambers? If so you should be able to see a little bounce back when the flywheel stops provided there is no binding present. Possibilities for compression leaks are valves, piston rings, or damaged piston rod seal.
I would check the valve cages first as they are the easiest to check. The best method is with a hand operated vacuum pump with gauge as used for bleeding brakes (Harbor Freight). Remove the cage and valve assemblies. Disassemble the valve and clean the valve and seat. Reassemble with the valve spring and retainer with a coating of grease or SAE 90 gear oil on the valve stem. Draw a vacuum on the inlet (exhaust) port. A good valve will allow you to pump down to 25” of Hg. Then if it bleeds down from 25” to 20” in more than about 6-7 seconds it is a good sealing valve, not excellent sealing, but probably good enough.
If the valves are sealing well you will have to check the piston rings or piston rod seals, but the seals rarely leak on a new engine unless they were torn during engine assembly. Ring sealing can only be checked by inserting a piston ring squarely into the cylinder and holding the cylinder up to a light and looking for any light peaking through at the outside circumference of the ring where it contacts with the cylinder wall. Leaks usually are on either side of the ring gap or opposite the gap, but can be anywhere. If all rings are good, check the side clearance between the ring and the ring groove. Should be .0005” to .001” max. Ring gap should be .003” -.004”.
Jeff
You indicated that you had 25 psi compression. Is it uniform at all four combustion chambers? If so you should be able to see a little bounce back when the flywheel stops provided there is no binding present. Possibilities for compression leaks are valves, piston rings, or damaged piston rod seal.
I would check the valve cages first as they are the easiest to check. The best method is with a hand operated vacuum pump with gauge as used for bleeding brakes (Harbor Freight). Remove the cage and valve assemblies. Disassemble the valve and clean the valve and seat. Reassemble with the valve spring and retainer with a coating of grease or SAE 90 gear oil on the valve stem. Draw a vacuum on the inlet (exhaust) port. A good valve will allow you to pump down to 25” of Hg. Then if it bleeds down from 25” to 20” in more than about 6-7 seconds it is a good sealing valve, not excellent sealing, but probably good enough.
If the valves are sealing well you will have to check the piston rings or piston rod seals, but the seals rarely leak on a new engine unless they were torn during engine assembly. Ring sealing can only be checked by inserting a piston ring squarely into the cylinder and holding the cylinder up to a light and looking for any light peaking through at the outside circumference of the ring where it contacts with the cylinder wall. Leaks usually are on either side of the ring gap or opposite the gap, but can be anywhere. If all rings are good, check the side clearance between the ring and the ring groove. Should be .0005” to .001” max. Ring gap should be .003” -.004”.
Jeff
On an assembled engine, I would add a teaspoon of oil to each cylinder in turn and check each cylinder individually to see if that makes a difference. A significant improvement suggests valves are OK but piston rings are not.
Use an endoscope (£20 - uses you phone as a screen) to view inside to see if bores are badly scored.
Fill intake tracts individually with petrol, paraffin, turps substitute or fuel oil, and see if it runs into the cylinders.
Repeat with exhaust ports.
On a small engine, with a tube inlet, you can simply remove the carburettor, close the inlet valve, use a rubber balloon (inflate first) ans tick it on the inlet stub and release the air to see how quickly it deflates. If the exhaust pipe is "gas-tight" you can do the same there.
You don't need much pressure - lung pressure in a balloon will show a leak as well as anything if significant. And you can easily see something deflating quickly - or slowly/not at all.
Enjoy!
K2
Use an endoscope (£20 - uses you phone as a screen) to view inside to see if bores are badly scored.
Fill intake tracts individually with petrol, paraffin, turps substitute or fuel oil, and see if it runs into the cylinders.
Repeat with exhaust ports.
On a small engine, with a tube inlet, you can simply remove the carburettor, close the inlet valve, use a rubber balloon (inflate first) ans tick it on the inlet stub and release the air to see how quickly it deflates. If the exhaust pipe is "gas-tight" you can do the same there.
You don't need much pressure - lung pressure in a balloon will show a leak as well as anything if significant. And you can easily see something deflating quickly - or slowly/not at all.
Enjoy!
K2
