G75 compression loss
10-01-2009 | 10:50 AM
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G75 compression loss
Okay, so I have G75 that has descent compression when it sits overnight, but after it runs, even if it runs for 5 seconds, the compression is nearly non-existant. I've already replaced the ring, and that helped a bit, but still cannot start it by hand, and still the hot/warm compression is terrible.
Fuel is 10% omega
Head is shimmed an extra .008" higher than stock
Prop is mas 13-6
flying RPM is normally 10600.
Any suggestions about what to check would be appreciated.
Fuel is 10% omega
Head is shimmed an extra .008" higher than stock
Prop is mas 13-6
flying RPM is normally 10600.
Any suggestions about what to check would be appreciated.
10-01-2009 | 11:39 AM
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RE: G75 compression loss
Hmmm... Good theory. I'll see if the bearings feel rough. I don't think they do but I'll double check, but the engine is new.
Something I left off was that the head was shimmed becasue it kept overheating. I thought it was overheating due to the 10% nitro, hence the head shim, but even with the shim its still getting really hot.
If I pulled the cyclinder do you think I would see gouges?
Something I left off was that the head was shimmed becasue it kept overheating. I thought it was overheating due to the 10% nitro, hence the head shim, but even with the shim its still getting really hot.
If I pulled the cyclinder do you think I would see gouges?
10-01-2009 | 11:42 AM
#4
RE: G75 compression loss
Maybe the sleeve is worn out. Is there a significent ridge where the rings stop near the top of the cylinder? Or the detonation could have put a hairline crack in the piston, one that gets wider when hot.
10-01-2009 | 02:30 PM
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RE: G75 compression loss
Broke in in March, probably has about 2 gallons through it, although that really should not have anything to do with drastic changes in compression from hot to cold right?
10-01-2009 | 04:04 PM
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RE: G75 compression loss
Bad compression compared to what? Ringed engines have less felt compression than ABC engines. Is it supposed to be stronger than those 10600 rpms you have on that 13-6 or is it produsing typical power for that particular engine, if so it propably is fine.
10-01-2009 | 05:19 PM
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RE: G75 compression loss
well, right after break in, it would run the 13-6 at 11300 rich, with good compression no matter how long it had run, now it will PEAK at 10800. Now the compression is so bad that you can take one finger and rotate the prop through 360 degrees without even having to hold on to the plane to keep it from turning when it hits the compression part of the stroke.
In fact, when you try to hand start it, a lot of times you won't even be able to flip it through a complete revolution, it stos at TDC with giving that saistying "bounce" off the compression.
I have downloaded the paperwork to send it to hobby services, probably send it out this weekend.
In fact, when you try to hand start it, a lot of times you won't even be able to flip it through a complete revolution, it stos at TDC with giving that saistying "bounce" off the compression.
I have downloaded the paperwork to send it to hobby services, probably send it out this weekend.
10-02-2009 | 09:25 AM
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RE: G75 compression loss
Are you sure the glowplug is fully tight and seated?
You can check the head for leakages. Get a short glass, fill it up with water almost to the brim, then put the head of the engine in the water...almost up to the exhaust port...and turn the crank slowly. If you see air bubbles being produced, that is your compression.
You can tell is the piston is cracked by looking at it with a magnifying glass, while heating it up slowly with a heat gun, or torch or something.
You should be able to tell by looking at the piston if your leaking past the ring...there will be "blowby" You'll know it when you see it.
You can check the head for leakages. Get a short glass, fill it up with water almost to the brim, then put the head of the engine in the water...almost up to the exhaust port...and turn the crank slowly. If you see air bubbles being produced, that is your compression.
You can tell is the piston is cracked by looking at it with a magnifying glass, while heating it up slowly with a heat gun, or torch or something.
You should be able to tell by looking at the piston if your leaking past the ring...there will be "blowby" You'll know it when you see it.
10-02-2009 | 11:18 AM
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RE: G75 compression loss
Blow by? Does it look like dark blob that runs up and down only a portion of the cyclinder?
As you look down from the top with the engine carb facing you, there is a dark patch at 11 o'clock that runs from the inside top of the cyclinder down. Everywhere else its relatively clean.
the ring is also dirty right there, everywhere else is clean.
Is this the blow by?
As you look down from the top with the engine carb facing you, there is a dark patch at 11 o'clock that runs from the inside top of the cyclinder down. Everywhere else its relatively clean.
the ring is also dirty right there, everywhere else is clean.
Is this the blow by?
10-03-2009 | 11:34 AM
#14
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RE: G75 compression loss
From his post: Head is shimmed an extra .008" higher than stock
I would say the extra .008 head shim you have in there would tend to lower your compression a little.[sm=idea.gif] Are you a self proclaimed engine guro, that you felt you needed to add a head shim and then wonder why the compression is less.[sm=idea.gif]
I would say the extra .008 head shim you have in there would tend to lower your compression a little.[sm=idea.gif] Are you a self proclaimed engine guro, that you felt you needed to add a head shim and then wonder why the compression is less.[sm=idea.gif]
10-04-2009 | 10:00 AM
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RE: G75 compression loss
Wow , ease up motorman 
, You got some issues eh? I added head shims after working with the hobby shop and advise from people on this forum after having the problem of over heating. If by guru you mean smart enough to put a themrocouple on the head to measure tempreatures and determine that by adding head shims I could lower the enine temps and stop burning out glow plugs, then yes.
Compression compared to when it was new, and from when its cold to just run for 5 or 10 seconds, thats what I'm comparing it to. I don't think that 8 thousandths of an inch added in head height would change the compression ratio enought to feel, but if I'm wrong I'm sure someone will let me know. This problem showed up even before I added the shims.
Again: cold with the added head shim = good normal felt compression, run for 5 seconds = mush, no pop, even stops at TDC when hand flipped, impossible to start by hand.
I have checked the tighness of the glowplug when cold, and even sprayed some bubble solution aound it and flipped it a few times, no bubbles. But this was when cold not hot.
, You got some issues eh? I added head shims after working with the hobby shop and advise from people on this forum after having the problem of over heating. If by guru you mean smart enough to put a themrocouple on the head to measure tempreatures and determine that by adding head shims I could lower the enine temps and stop burning out glow plugs, then yes.Compression compared to when it was new, and from when its cold to just run for 5 or 10 seconds, thats what I'm comparing it to. I don't think that 8 thousandths of an inch added in head height would change the compression ratio enought to feel, but if I'm wrong I'm sure someone will let me know. This problem showed up even before I added the shims.
Again: cold with the added head shim = good normal felt compression, run for 5 seconds = mush, no pop, even stops at TDC when hand flipped, impossible to start by hand.
I have checked the tighness of the glowplug when cold, and even sprayed some bubble solution aound it and flipped it a few times, no bubbles. But this was when cold not hot.
10-04-2009 | 10:28 AM
#16
RE: G75 compression loss
flhry,
Motorman is a cool dude and helps anyone he can. not really fair to say he has issues.
i have the same opinion however, how did you justify needing to shim the head from stock?
was there detonation?........you can tell if there was by looking at the head and top of the piston and see pitting.
are you sure you don't have a head leak?
i have a G90 i got from someone who has had fits and overheating problems with the engine from day one..................it was a warped head that was causing it. new head and gasket and the 90 runs perfect now.
but the reasons you gave for shimming the head a whole .008" is a quite jump, and not jusitfied by your reasons.
overheating can be attributed to several things...........but head clearance is optimized in fine increments.
also since you have been overheating this engine, you may have totalled out the ring...........a mush and thush and stopping at TDC when flipping the prop means a head leak, or a fried ring.
Motorman is a cool dude and helps anyone he can. not really fair to say he has issues.
i have the same opinion however, how did you justify needing to shim the head from stock?
was there detonation?........you can tell if there was by looking at the head and top of the piston and see pitting.
are you sure you don't have a head leak?
i have a G90 i got from someone who has had fits and overheating problems with the engine from day one..................it was a warped head that was causing it. new head and gasket and the 90 runs perfect now.
but the reasons you gave for shimming the head a whole .008" is a quite jump, and not jusitfied by your reasons.
overheating can be attributed to several things...........but head clearance is optimized in fine increments.
also since you have been overheating this engine, you may have totalled out the ring...........a mush and thush and stopping at TDC when flipping the prop means a head leak, or a fried ring.
10-04-2009 | 10:44 AM
#17
My Feedback: (79)
RE: G75 compression loss
ORIGINAL: flhyr
Wow , ease up motorman, You got some issues eh? I added head shims after working with the hobby shop and advise from people on this forum after having the problem of over heating. If by guru you mean smart enough to put a themrocouple on the head to measure tempreatures and determine that by adding head shims I could lower the enine temps and stop burning out glow plugs, then yes.
Compression compared to when it was new, and from when its cold to just run for 5 or 10 seconds, thats what I'm comparing it to. I don't think that 8 thousandths of an inch added in head height would change the compression ratio enought to feel, but if I'm wrong I'm sure someone will let me know. This problem showed up even before I added the shims.
Again: cold with the added head shim = good normal felt compression, run for 5 seconds = mush, no pop, even stops at TDC when hand flipped, impossible to start by hand.
I have checked the tighness of the glowplug when cold, and even sprayed some bubble solution aound it and flipped it a few times, no bubbles. But this was when cold not hot.
Wow , ease up motorman
, You got some issues eh? I added head shims after working with the hobby shop and advise from people on this forum after having the problem of over heating. If by guru you mean smart enough to put a themrocouple on the head to measure tempreatures and determine that by adding head shims I could lower the enine temps and stop burning out glow plugs, then yes.Compression compared to when it was new, and from when its cold to just run for 5 or 10 seconds, thats what I'm comparing it to. I don't think that 8 thousandths of an inch added in head height would change the compression ratio enought to feel, but if I'm wrong I'm sure someone will let me know. This problem showed up even before I added the shims.
Again: cold with the added head shim = good normal felt compression, run for 5 seconds = mush, no pop, even stops at TDC when hand flipped, impossible to start by hand.
I have checked the tighness of the glowplug when cold, and even sprayed some bubble solution aound it and flipped it a few times, no bubbles. But this was when cold not hot.
Maybe I was a little rough on you, and no I do not have any issues that I am aware of.
The .008 th's shim that you added is enough that you would definately feel the difference in compression with your bare hand. There is no reason why you should need the shim to keep it from over heating running 10%. I would start by making sure you don have a air leak at the carb base or back cover first. Then I would make sure that the engine is getting enough cooling air if it is cowled. Just because someone told you on here to add a head shim, does not mean that is the correct thing to do. I believe about about half of what I read on hear when it comes to having to do a mod on an engine because it will not run correctly from the factory in its intended application.
10-04-2009 | 11:09 AM
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RE: G75 compression loss
Thanks motorman, I hope your arm is feeling better.
I actually only shimmed it .005 with a .003 and a .002 shim. Reason I shimmed is because of a measured head temp of 235 at steady state (peaked at over 315). The .003 brought it down to 200 or so, then I added the .002 to get it down to 180 or so (using 10% temps measured with uncowled engine).
I believe summer wind is right about the warped head, and that I've already ruined it, but live and learn. Pretty sure it was ruined when i blew the first glow plug cause it never ran the same after that.
It still seems that there is blowby since I can see a dark patch on the cyclinder wall in one area, and the ring that contacts that area is not shiny like the other parts of it.
I've been messing with this thing for a while, changed the ring the first time it lost compression and that seemed to help, but now its doing the same thing. so the head or another ring, but I think I've reached my limit on this one and will let hobby services handle it.
If you would like to see the plots of the thermocouple.... just let me know and I'll throw it up here or search for a thread titled "overheat or lean"
I wish I would have bought the tower .75
I actually only shimmed it .005 with a .003 and a .002 shim. Reason I shimmed is because of a measured head temp of 235 at steady state (peaked at over 315). The .003 brought it down to 200 or so, then I added the .002 to get it down to 180 or so (using 10% temps measured with uncowled engine).
I believe summer wind is right about the warped head, and that I've already ruined it, but live and learn. Pretty sure it was ruined when i blew the first glow plug cause it never ran the same after that.
It still seems that there is blowby since I can see a dark patch on the cyclinder wall in one area, and the ring that contacts that area is not shiny like the other parts of it.
I've been messing with this thing for a while, changed the ring the first time it lost compression and that seemed to help, but now its doing the same thing. so the head or another ring, but I think I've reached my limit on this one and will let hobby services handle it.
If you would like to see the plots of the thermocouple.... just let me know and I'll throw it up here or search for a thread titled "overheat or lean"
I wish I would have bought the tower .75
10-04-2009 | 11:10 AM
#19
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RE: G75 compression loss
ORIGINAL: summerwind
flhry,
Motorman is a cool dude and helps anyone he can. not really fair to say he has issues.
i have the same opinion however, how did you justify needing to shim the head from stock?
was there detonation?........you can tell if there was by looking at the head and top of the piston and see pitting.
are you sure you don't have a head leak?
i have a G90 i got from someone who has had fits and overheating problems with the engine from day one..................it was a warped head that was causing it. new head and gasket and the 90 runs perfect now.
but the reasons you gave for shimming the head a whole .008'' is a quite jump, and not jusitfied by your reasons.
overheating can be attributed to several things...........but head clearance is optimized in fine increments.
also since you have been overheating this engine, you may have totalled out the ring...........a mush and thush and stopping at TDC when flipping the prop means a head leak, or a fried ring.
flhry,
Motorman is a cool dude and helps anyone he can. not really fair to say he has issues.
i have the same opinion however, how did you justify needing to shim the head from stock?
was there detonation?........you can tell if there was by looking at the head and top of the piston and see pitting.
are you sure you don't have a head leak?
i have a G90 i got from someone who has had fits and overheating problems with the engine from day one..................it was a warped head that was causing it. new head and gasket and the 90 runs perfect now.
but the reasons you gave for shimming the head a whole .008'' is a quite jump, and not jusitfied by your reasons.
overheating can be attributed to several things...........but head clearance is optimized in fine increments.
also since you have been overheating this engine, you may have totalled out the ring...........a mush and thush and stopping at TDC when flipping the prop means a head leak, or a fried ring.
10-04-2009 | 11:28 AM
#20
My Feedback: (79)
RE: G75 compression loss
ORIGINAL: flhyr
Thanks motorman, I hope your arm is feeling better.
I actually only shimmed it .005 with a .003 and a .002 shim. Reason I shimmed is because of a measured head temp of 235 at steady state (peaked at over 315). The .003 brought it down to 200 or so, then I added the .002 to get it down to 180 or so (using 10% temps measured with uncowled engine).
I believe summer wind is right about the warped head, and that I've already ruined it, but live and learn. Pretty sure it was ruined when i blew the first glow plug cause it never ran the same after that.
It still seems that there is blowby since I can see a dark patch on the cyclinder wall in one area, and the ring that contacts that area is not shiny like the other parts of it.
I've been messing with this thing for a while, changed the ring the first time it lost compression and that seemed to help, but now its doing the same thing. so the head or another ring, but I think I've reached my limit on this one and will let hobby services handle it.
If you would like to see the plots of the thermocouple.... just let me know and I'll throw it up here or search for a thread titled ''overheat or lean''
I wish I would have bought the tower .75
Thanks motorman, I hope your arm is feeling better.
I actually only shimmed it .005 with a .003 and a .002 shim. Reason I shimmed is because of a measured head temp of 235 at steady state (peaked at over 315). The .003 brought it down to 200 or so, then I added the .002 to get it down to 180 or so (using 10% temps measured with uncowled engine).
I believe summer wind is right about the warped head, and that I've already ruined it, but live and learn. Pretty sure it was ruined when i blew the first glow plug cause it never ran the same after that.
It still seems that there is blowby since I can see a dark patch on the cyclinder wall in one area, and the ring that contacts that area is not shiny like the other parts of it.
I've been messing with this thing for a while, changed the ring the first time it lost compression and that seemed to help, but now its doing the same thing. so the head or another ring, but I think I've reached my limit on this one and will let hobby services handle it.
If you would like to see the plots of the thermocouple.... just let me know and I'll throw it up here or search for a thread titled ''overheat or lean''
I wish I would have bought the tower .75
Cylinder head temps are kinda overrated in my mind with glow motors. They will let you know where they are happy running, and you should not tune them by temperature. I sometimes check them for the heck of it, but would never tune them by temperature if they running good, Which the Towers allways are.It is pretty easy though with a ringed engine to fry the ring, and in somecases also warp the piston.
ST's take a along time to break in, and I am wondering if you are pushing them to hard to fast and wrecking the rings.
10-04-2009 | 11:41 AM
#21
RE: G75 compression loss
another thing you haven't mentioned...........what is the engine being used in?
scale, pattern, or 3D.............if 3D it is hard on an engine in that when doing hovering or similar stunts, there is very little air passing over the engine.
ST engines take a bit longer to Break-in as well and if not fully broken in and subjected to overheating, they puke right away.
scale, pattern, or 3D.............if 3D it is hard on an engine in that when doing hovering or similar stunts, there is very little air passing over the engine.
ST engines take a bit longer to Break-in as well and if not fully broken in and subjected to overheating, they puke right away.
10-04-2009 | 12:03 PM
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From: , TX
RE: G75 compression loss
Your right its in a 3d plane, but I'm not doing any 3d mostly because the engine doesn't seem to be powerfull enough. WHen it started overheating, it was about 104 outside here in TX, but I had the better part of a gallon through it for break in before flying it, and I know thats not enough to really break it in but for the first 5 or so flights there were no verticle maneuvers, but still it obviosly overheated and puked as you say. So.... warp the piston? I would've thought it would warp the cyclinder instead of the piston. Either way, the cost of either/both will have me just buying a new (and bigger) engine.
I don't suppose there is a sure fire way to tell if the pison or cyclinder is warped?
I don't suppose there is a sure fire way to tell if the pison or cyclinder is warped?
10-04-2009 | 12:14 PM
#23
RE: G75 compression loss
no i said warped head.........but that was on my ST90 that was given to me.
Hobbico won't fix it either as i was stupid and decided to be honest and told the whole story (truth and nothing but the truth)...........so they elected to say that i am not the original owner, so no warranty.
you could try asking them for an exchange though for the Tower 75.........now that engine will have power to spare in comparison to the ST.
BTW.........try sending an email to [email protected]
she has always gone the extra mile for me.
Hobbico won't fix it either as i was stupid and decided to be honest and told the whole story (truth and nothing but the truth)...........so they elected to say that i am not the original owner, so no warranty.
you could try asking them for an exchange though for the Tower 75.........now that engine will have power to spare in comparison to the ST.
BTW.........try sending an email to [email protected]
she has always gone the extra mile for me.
10-04-2009 | 01:10 PM
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RE: G75 compression loss
Honestly I was thinking of switching to a 91.... Motorman, I know you have some experience with magnum/ASP.... should I steer clear of the ASP series 3 91 2-stroke and go with an OS?
