compression for single cylinder oil engine.

[Frank Boyle]

Having completed the centaur gas engine I have tried to start it but to no avail.I stripped the engine and blanked off the cylinder and with a pressure gauge attached turned the piston into the cylinder,there was resistance and the gauge reached 1.5 bar and could hear air forcing it's way slowly past the rings.When spinning the flywheel as if starting the engine the gauge reaches 2.5 bar and back to zero.Is this enough compression and if not are different rings the answer.The inlet valve was tested with parrafin and it leaked past the valve stem so there is grinding or polishing of the seat to do.Fuel was drawn into the carburetor when I tried to start it but ran out of the air intake on the next cycle am I right in assuming that the leak on the input valve will stop the full suction into the ignition chamber.How open dose the needle valve need to be when starting.I am a complete novice where ic engines are concerned and I need all the help I can get,
Frank

[Jasonb]

I've never really measured it on my engines. What I like to see is some bounce as the flywheel is flicked and the compression will push the piston back in teh opposite direction.

Leaky inlet will let some pressure out when compressing the mixture which is more of a problem than allowing more mixture to be sucked in.

I find most needles just need opening a crack, maybe 1/8th of a turn. They are more on and off than a fine adjustment!

Bounce here

[youtube1]https://www.youtube.com/watch?v=o6675ykoGLE[/youtube1]

[Brian Rupnow]

Static compression ratio (read that as calculated compression ratio) should be from 3.5:1 up to 6:1. I'm not sure about dynamic compression as read with a gauge, but if your valves or rings are leaking badly, it's not going to start.  Have a look at this link.---Brian
http://www.modelenginemaker.com/index.php/topic,6238.0.html

[stevehuckss396]

Having completed the centaur gas engine I have tried to start it but to no avail.I stripped the engine and blanked off the cylinder and with a pressure gauge attached turned the piston into the cylinder,there was resistance and the gauge reached 1.5 bar and could hear air forcing it's way slowly past the rings.When spinning the flywheel as if starting the engine the gauge reaches 2.5 bar and back to zero.Is this enough compression and if not are different rings the answer.The inlet valve was tested with parrafin and it leaked past the valve stem so there is grinding or polishing of the seat to do.Fuel was drawn into the carburetor when I tried to start it but ran out of the air intake on the next cycle am I right in assuming that the leak on the input valve will stop the full suction into the ignition chamber.How open dose the needle valve need to be when starting.I am a complete novice where ic engines are concerned and I need all the help I can get,
Frank

couple things i would check. Either the engine is pushing raw fuel past the intake valve via bad seating or valve timing is a little out of wack and the engine takes a breath and the piston is returning to TDC before the intake valve is closed. Maybe the carb screw is out so far that fuel is dumping everywhere.

If the fuel is running out the intake screw the carb or mixer screw all the way in and see if it will pop on the next 4 or 5 tries.

[Roger B]

Lack of leaks is more important than actual pressure. You could try putting some very thick oil in the cylinder and see if the pressure increases, in that case the rings would be leaking. Some more questions:

How are you turning the engine over to start it, by hand or with an electric drill or similar?

Is the sparkplug getting wet with fuel?

Have you done a basic check of the timing in case something has slipped? Turn the engine slowly by hand and check the sequence of events:
Top dead centre, exhaust valve just closing and inlet valve just opening.
Bottom dead centre, inlet valve closing.
Top dead centre, Spark plug should spark.
Bottom dead centre, exhaust valve exhaust valve opening.

If you are turning the engine with a drill or similar close the needle valve completely, start turning the engine and then slowly open the valve.

[Frank Boyle]

Thanks chaps for the responses. I tried your method Jason,with the blanking plate over the end of the cylinder which has a 1/4" hole in the centrewith my finger covering the hole I had difficulty turning over the crank and with flicking the crank over I got the bounce that you recommended,such a relief.I am now concentrating on the valves and am using brasso to polish them.I have tested them with parrafin sitting on top of the valves,I still need to grind a little more and then I will try sucking with a tube through the valve ports as suggested by Brian.I will keep you posted of progress .
Frank