Have you ever purchased a new engine and less than two dozen flights all of a sudden have very low compression. This is my second OS 25 LA and after 3 bench runs on a rich setting and flying a half dozen flights on a 9 x 4 prop now it has low compression. Its so low that trying to start the engine turns into a 15 minute affair, but after it starts it runs good. My set up is a needle valve from OS thru the venturi, an aluminum back plate on the crank, and head and crank bolts replaced with socket head cap screws. I ran maybe a dozen flights using the 9 x 4 apc prop and then shifted to a 10 x 4 apc  prop like I have on my first OS 25. What do you think I did wrong? My flights started out slow and rich with no horse play and I worked my way up to an idle speed that was just starting to break into a rich 2  cycle. I have changed crank case gaskets , checked for a head gasket ,replaced glow plug and gasket, and even pulled the sleeve to see if there was any scaring in the sleeve and on the piston and both look brand new. I have a new piston and sleeve from Tower Hobbies coming soon and will re-break in this engine again but was wondering if this happen to any one else or did I jump up in prop size to soon and ruined a good motor?

It is very easy to warp the sleeve in these engines when tightening the head screws. I like to remove the glo plug and loosen all of the head screws and retighten while checking for drag on the piston at tdc. I usually snug them down pretty good and then loosen them a little one at a time and again feel the drag. You want to get the least amount of drag with the plug out that you can get, and then put the plug back in to check for compression. Hope this helps.

I have a book on the care and feeding of model two strokes.  I can't lay my hands on it so I can't tell you the author (he was motor columnist for MAN or something).  But, in that book were two bits of wisdom that pertain here:

• According to this guy, you do not want to break in an ABC, AAC, ABN, etc., engine cold.  They are designed so that the running fit gets looser the hotter the engine gets, so if you run them for prolonged periods of time when they're cold you wear them out prematurely.  Iron piston engines are the opposite, so a lot of the tribal knowledge of how to break in an engine is based on them.  (This is consistent with the few engines that I have bought from new -- the one AAC engine was tighter than tight until I hit it with a heat gun; it's only gotten out of the "squeaks when cold" stage, but it's always loose just after its been run).
• You should always follow the manufacturers instructions for breaking in the engine -- that trumps anything that I say, or that you may have thought, or anything else.

So, if the above is true, then the only thing you should do in response to this post is to do whatever OS says to break in the engine, and completely blow off what I (or anyone else other than OS) said about break-in.

HI Phil,

Slightly off topic, but tell us how you get the LA series to 4-2......... I can't seem to get a reliable 4-2 run out of any of them.

Thanks!
Bill

Hello,
The only times I have suffered from the type of behavior that you are seeing is as follows. Cox 0.049, during run in it was fine, Did one flight and totally lost compression. Culprit here was a leak had developed in the glow head insulating seal. Second time this happened was with an LA46. Broke it in as per schedule and after 10 flights , no compression. This time it was a leaking backplate, a couple of the screws had come loose.
Look for the simple stuff like leaks before you start getting complicated.

Regards,

Andrew.

          I should've added this in my earlier post, this is not related to the low compression issue but could surely be a problem in general. I had posted this problem on this site as well as Stuka Stunt. You didn't mention though who made the backplate. I contacted Curtis Ship who was also making metal backplates. Curtis based his machined backplate from the stock OS plastic ones. The one change he did make which is all for the better was to add an internal o-ring. I happened to have a new FP.25 backplate which I installed on the engine as soon as I pulled it out of the box. It fits the LA case perfectly except they're not identical to the stock unit. The FP backplate doesn't protrude into the rear of the case as far as the plastic unit does. While the engine was running, the rod was starting to work off of the crankpin and thus the engine would start to bind and slow down. After exploring several items a club member took the engine apart to discover the actual problem. I haven't had this problem with other LA's .15,.40,.46, only on the .25. This would be something I would surely check. Ken

Scott, going back all the way to post #1...

No one has yet mentioned the fuel blend. I'll presume you used a typical 10-10-10 (Nitro - synth oil - castor) blend, which should be safe.

If you used an all-castor fuel, you may have never reached the intended warmed-up expansion temperatures, and may have started building a castor varnish to the piston. These are not good for an Aluminum-(Plated)Brass engine.

Other aside comments - I've run in a few dozen OS ABN engines, mostly for others, and typically found that the first start is not guaranteed to be easy. Exhaust residues show dark from burnishing the initial fit for the first, and sometimes as long as to the 4th or 5th, brief bench run. Things get better rapidly after this stage, and it probably takes 6 to 10 flights to reach a durable plateau after it.

Haven't heard others remark on this...  Starts at previous flight settings usually benefit from two things.
... First, If you use the stock muffler for muffler pressure, goose the fuel supply by tapping the outlet while the engine spools-up/warms-up during the first 3 or 4 seconds of running. 
... Second, keep the glow plug battery attached during this time, too. You should hear it 'get on the step' and lock in before pulling the battery.

Launching before full launch RPM and temperature are reached may explain some complaints I've heard/seen.

My engine set up is exactly the same as my two other LA 25, aluminum back plate that I made at work,Sig 10% fuel, OS needle valve thru the venturi, and socket head cap screws to replace the stock screws  in the head and back plate plus the muffler bolts.Also running stock muffler under pressure. My other two engines run great with easy starts and consistent runs in a wet 2 cycle this one is extremely hard to start, has no bump when starting but once started runs fairly decent. I have not had the time to check thoroughly for leaks because I changed engines and the last few days have been great for flying. I will look into this using the advise from the above links.

Save the old piston and liner -- they're very likely still perfectly good.

Scott,
The source of the problem is most likely the socket head bolts in the head. On some heads the holes are off by a thousants or two and causes rubbing of the bolt head against the side. You can either go back to the original OS bolts or have the head bolt holes (in the area of the fins and seat) opened. What happens is that the socket head bolts are slightly larger then the seat area and rub against the side as you pull them down. This causes the head to distort and not seat properly. This happens on Foxes when you use the socket head bolts and will cause the engine to bind. As you tighten down the bolts make sure you go in a cross head pattern, keep the plug out and feel the motor as you turn it over make sure it is smooth with a little pinch at the top.

Best,      DennisT