OS MAX 120 No Compression
09-26-2018, 02:26 AM
#1
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OS MAX 120 No Compression
Hi
I've got an OS MAX 120 AX that ran in total 40 minutes since new out of box on 20% oil and 10% nitro. It ran perfectly - no issues at all.
It was then standing for a year or so in my plane and when trying to start it, there is no compression.
Replacing the glow plug with a new one made no difference. The head 5 bolts are still factory tight - have checked it. The engine was not in any crash.
I've taken it off the plane - looking through the exhaust port I can see the piston go up and down as I turn the prop - cannot see any damage to the piston or ring, the engine turns freely, except there is no compression to start it.
It appears to have some compression turning the crank clockwise than turning it counter clockwise.
I have never came across this in any of my other 13 OS engines - I thought I knew engines well after 45 years of flying, but this one got me clueless.
Have somebody experienced this as well, or can give me some advice before stripping the engine down?
I'm really surprised as my other OS engines never gave me any trouble whatsover.
Cheers
I've got an OS MAX 120 AX that ran in total 40 minutes since new out of box on 20% oil and 10% nitro. It ran perfectly - no issues at all.
It was then standing for a year or so in my plane and when trying to start it, there is no compression.
Replacing the glow plug with a new one made no difference. The head 5 bolts are still factory tight - have checked it. The engine was not in any crash.
I've taken it off the plane - looking through the exhaust port I can see the piston go up and down as I turn the prop - cannot see any damage to the piston or ring, the engine turns freely, except there is no compression to start it.
It appears to have some compression turning the crank clockwise than turning it counter clockwise.
I have never came across this in any of my other 13 OS engines - I thought I knew engines well after 45 years of flying, but this one got me clueless.
Have somebody experienced this as well, or can give me some advice before stripping the engine down?
I'm really surprised as my other OS engines never gave me any trouble whatsover.
Cheers
Last edited by Propslapper; 09-26-2018 at 03:10 AM.
09-26-2018, 04:30 AM
#3
You can soak the innards in acetone for 10 minutes and that will do a pretty good job of dissolving old oil and free up the ring a little bit. Once it frees up enough to get a little compression pop, fuel it up and hit it with a starter and it should run and fight itself with some running.
09-26-2018, 11:51 PM
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You can soak the innards in acetone for 10 minutes and that will do a pretty good job of dissolving old oil and free up the ring a little bit. Once it frees up enough to get a little compression pop, fuel it up and hit it with a starter and it should run and fight itself with some running.
09-26-2018, 11:52 PM
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09-27-2018, 09:41 AM
#6
A little added heat, along with that shot of solvent. Let the solvent soak a bit and heat moderately with a hair dryer while rotating the crank can also help free things up a bit. Even non-castor oiled engines get sticky at times. In that case, it is usually from the moisture that synthetics readily absorb.
10-04-2018, 07:00 PM
#7
The thing many people forget is to open the carb on a 4 cycle engine to feel compression. If the carb is not open the engine can get no air to compress. If there is compression turning the engine backwards, it gets air through the exhaust giving the engine something to compress even with the carb closed.
