Author Topic: IC Engine runs without "apparent" compression  (Read 3416 times)

Offline philjoe5

  • Full Member
  • ****
  • Posts: 1011
IC Engine runs without "apparent" compression
« on: June 06, 2014, 12:40:14 PM »
OK, the subject of this post is a bit misleading but let me explain.

After the last intake valve failure on my Bonzer I replaced the intake valve spring with a much lighter one.  The engine runs quite reliably now and I have about 2 hours of run time on it.  With my laser tach I've measured a stall speed of about 700 rpm and a maximum speed of close to 1000 rpm. 

Now the question...when the engine is stopped and I spin the flywheel smartly, as it cycles to the compression stroke it bounces back, so, yes I do have compression.  But if I rotate the flywheel slowly it goes right through the compression stroke with little or no resistance.  Is this a symptom of an intake valve not seating fully or a consequence of using a light spring on the valve?

Thanks for any advice

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline Lofty76

  • Full Member
  • ****
  • Posts: 47
Re: IC Engine runs without "apparent" compression
« Reply #1 on: June 06, 2014, 01:56:53 PM »
More likely piston to bore clearance.

Offline Roger B

  • Global Moderator
  • Full Member
  • *****
  • Posts: 4010
  • Switzerland
Re: IC Engine runs without "apparent" compression
« Reply #2 on: June 06, 2014, 03:05:19 PM »
On engines with desmodronic valve gear (the valve is opened and closed by cams which requires some close tolerence machining) as I understand it they will run without any valve springs as the compression hold the valve shut, but a spring is needed to give some compession at starting speeds.

https://www.highpowermedia.com/articles/1895/desmodromic-or-demonic/

Best regards

Roger

Offline gbritnell

  • Full Member
  • ****
  • Posts: 1988
Re: IC Engine runs without "apparent" compression
« Reply #3 on: June 06, 2014, 03:29:36 PM »
It could be leaky valves or poor piston to cylinder fit, meaning the piston rings if it has them. When you flip the engine over quickly it doesn't have time to leak the compression. This is usually a sign of piston/ring clearance but could be the valves. When you turn it over slowly it's leaking wherever. The only way you'd know for sure would be to test the valves (vacuum test) to be sure. If they are sealed well then it's the piston/ring sealing.
gbritnell
Talent unshared is talent wasted.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 5165
  • Barrie, Ontario Canada
Re: IC Engine runs without "apparent" compression
« Reply #4 on: June 06, 2014, 04:02:53 PM »
Try the old hotrodders trick. Squirt a bit of #30 oil down the sparkplug hole and wiggle the engine around so that you know the inside of the cylinder is fully coated with oil. Turn it over by hand once to pump out any extra oil, then reinstall the sparkplug. Now try your compression, turning it over slowly by hand. If the compression still fades away, thats a pretty good sign that your valves are leaking. If it doesn't fade away then its your rings that are leaking--the oil is temporarily helping to seal the rings. Oil will have little to no effect on sealing the valves.---Brian

Offline Fabrickator

  • Full Member
  • ****
  • Posts: 13
Re: IC Engine runs without "apparent" compression
« Reply #5 on: June 06, 2014, 04:58:39 PM »
Brian is exactly right.  That's what I was going to add.   :whoohoo:
You can walk with a wooden leg, but you can't see with a glass eye.  Always wear eye protection!

Offline philjoe5

  • Full Member
  • ****
  • Posts: 1011
Re: IC Engine runs without "apparent" compression
« Reply #6 on: June 06, 2014, 05:33:12 PM »
OK, thanks guys.  I just finished a 30 minute run and it runs very well with this vapor carb.  I'm going to keep running it this way as long as its happy. 

The previous intake valve spring was a lot stronger and with it I had good compression all through the compression cycle so I'm inclined to believe the intake valve is not completely seating. 

When it cools down I'll try the diagnostics recommended by Brian.

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline philjoe5

  • Full Member
  • ****
  • Posts: 1011
Re: IC Engine runs without "apparent" compression
« Reply #7 on: June 07, 2014, 11:23:16 PM »
The oil in the cylinder trick pointed me in the right direction.  The intake valve is not completely sealing.  Another hour of running has improved the situation a lot so I'm expecting it will eventually seal up.

Thanks for the help and advice.  I'm hoping to get this engine portable enough to take to a show next weekend where I'll put some hours on it.

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline collbee

  • Full Member
  • ****
  • Posts: 17
Re: IC Engine runs without "apparent" compression
« Reply #8 on: June 10, 2014, 09:51:17 PM »
Hi Phil,

Listening carefully as you turn the engine over, should give a good indication of where the leaks are - especially in the cylinder (rings not embedded properly or an ovalized bore are usually the main culprits in my case). ....... very soapy water on and around the inlet valve may also help give an indication of leakage in the area. 

I do not think a heavier spring is the answer at this point in time  ... don't forget that the pressure, when cycling through the compression stroke, plays a part in closing and sealing the intake valve.

I have a 1908 open crank horizontal Crossley that has the same symptoms due to an ovalized bore but year after year for hours at a time she lives up to expectation.


cheers......
« Last Edit: June 10, 2014, 10:02:49 PM by collbee »

Offline philjoe5

  • Full Member
  • ****
  • Posts: 1011
Re: IC Engine runs without "apparent" compression
« Reply #9 on: June 10, 2014, 10:48:25 PM »
Thanks Collbee.  Now after yet another hour of running I do have compression usually (but still not always).  I think this is due to the light spring.  This valve has only about 3 hours on it so I think it's starting to seat better and a few hours running at our club show this weekend should really help.

I now have the engine on a base I can take out of the basement so the engine can run as long as I want.  Even when the compression seemed to be "lost" the engine runs well.

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline gbritnell

  • Full Member
  • ****
  • Posts: 1988
Re: IC Engine runs without "apparent" compression
« Reply #10 on: June 11, 2014, 02:09:41 AM »
Hi Phil,
From what you're explaining I'm going to say it's not the ring seal but one of the valves. The reasoning behind this is that the valve seat (on the valve) isn't concentric with the stem but as it's running it's possibly rotating and at one point it has a good seal and at another it's not so good. If this is the case it won't get any better, or worse. Valves and seats seem to be one of the hardest areas to perfect when building small I.C. engines.
gbritnell
Talent unshared is talent wasted.