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Author Topic: Compression loss ?  (Read 805 times)

Offline Brent_olbrych

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Compression loss ?
« on: April 04, 2010, 09:22:37 AM »
 any ideas why a brand new, properly broken in OS 25 las motor, will lose compression after only 2 flights? It was broken in using 10% nitro, 18% Castor, 3, 4 1/2 oz tanks , one full flight , next flight no compression at all. When we got it started we had to flood it, and when it did start there seemed to be a brown froth near the head( this could be gasket?) thank you,  Brent   
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Online Brett Buck

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Re: Compression loss ?
« Reply #1 on: April 04, 2010, 11:06:47 AM »
Quote from: Brent_olbrych on April 04, 2010, 09:22:37 AM
it did start there seemed to be a brown froth near the head( this could be gasket?) thank you,  Brent  

    There's not supposed to be any brown froth!  Pour fuel over the interface between the head and the liner, and turn the engine over. If there are any bubbles, you have a leak. Make sure the screws are tight (use a good quality screwdriver that actually fits the slots). Check again, if it still leaks, back off all the screws, and retighten them in a criss-cross pattern - first just touching, then sightly snug, then pretty tight. This ensures that the head is straight and flat. If it still leaks, then take the head off and replace the gaskets, and inspect the mating surfaces on the head and liner. They must be perfectly free of any grit, flakes, etc , perfectly flat, and perfectly clean. If there is a scratch on the liner, you may be able to lap the liner surface on a soapstone to flatten it. If there's anything wrong with the head, you probably need a new one.

    I haven't seen this problem on any OS engine but its possible.

    Brett
« Last Edit: April 04, 2010, 08:45:16 PM by Brett Buck »
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Offline Scott B. Riese

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Re: Compression loss ?
« Reply #2 on: April 04, 2010, 11:22:31 AM »
Quote from: Brent_olbrych on April 04, 2010, 09:22:37 AM
any ideas why a brand new, properly broken in OS 25 las motor, will lose compression after only 2 flights? It was broken in using 10% nitro, 18% Castor, 3, 4 1/2 oz tanks , one full flight , next flight no compression at all. When we got it started we had to flood it, and when it did start there seemed to be a brown froth near the head( this could be gasket?) thank you,  Brent   

Need some more info.
Make of fuel.
Prop.

Just from your info the engine is running WAY to hot. LA-25 like the 10x4 prop and fuel that's 50/50 blend with at least 18% total oil. NOW that it run hot and you have that plastic back plate YOU may have to reseal that gasket. I had an LA-25 that did the same thing for a customer. RAN hot and then lost compression. After several runs and taking the engine apart I found that I had a bad sleeve. The plating was coming off. Got a new Piston and sleeve...all is good. Hope this helps
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Scott Riese
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Offline Brian Hampton

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Re: Compression loss ?
« Reply #3 on: April 04, 2010, 08:34:51 PM »
Also check the glow plug post by pouring a little fuel around it and turning the engine over. If you see bubbles you've found the problem.
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