Twin engines uneven compression Magnum
03-22-2012, 02:17 PM
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Twin engines uneven compression Magnum
Hey guys i have a question for twin 4 stroke guys. my magnum 160ft still breaking in, feels like it has a low compression on right side looking from back plate. Right side has less smoke coming out, almost same amount of oil as the left side. while cranking by hand I can feel a hard stroke and next stroke is definetely easier. Should I be worried or should I keep breaking it in and hope it will get better.
03-23-2012, 05:35 AM
#3
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RE: Twin engines uneven compression Magnum
ORIGINAL: pe reivers
Check valve lash first!
Check valve lash first!
That's what I was going to say.
1st rule of troubleshooting:
Check the simple, easy fix 1st.
It seems unlikely that ring problems (unless the ring was completely missing) would yield 0# compression, but a valve staying open would.
They'll run W/a bit too much valve lash, but not too little.
Run the offending cylinder up to TDC on the compression stroke & see if you can wiggle the rocker arms. If either wonn't wiggle, there's your problem. If they both have wiggle room, your problem is elsewhere.
Not sure what the minimum valve lash is for the Magnum, but .002" should be close enough.
03-23-2012, 10:27 AM
#4
RE: Twin engines uneven compression Magnum
I would agree, do check the valve gaps and make sure they haven't closed up. All of our model 4 stroke engines use solid lifters and need to have the valve gaps checked from time to time. With a new engine the gaps can close up fast as the engine is getting run in. So you might need to check the gaps more frequently at first.
03-23-2012, 03:36 PM
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RE: Twin engines uneven compression Magnum
G'day
I currently have two MAGNUM/ASP engines in my workshop with leaky valves. Have a careful listen to the engine as you turn it over. I can clearly hear the leaks. One is an exhaust leak, the other an inlet leak.
I tried to lap the leaking inlet valve to no avail so I have ordered a new head for this engine ($23 from Hong Kong). The other belongs to a friend and he is trying to find a head for it. If the new heads don't work, then the engines will be come "boat anchors" for very small boats.
Mike in Oz
I currently have two MAGNUM/ASP engines in my workshop with leaky valves. Have a careful listen to the engine as you turn it over. I can clearly hear the leaks. One is an exhaust leak, the other an inlet leak.
I tried to lap the leaking inlet valve to no avail so I have ordered a new head for this engine ($23 from Hong Kong). The other belongs to a friend and he is trying to find a head for it. If the new heads don't work, then the engines will be come "boat anchors" for very small boats.
Mike in Oz
03-23-2012, 06:09 PM
#6
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RE: Twin engines uneven compression Magnum
That engine has a twin throw crank, Ibelieve that both pistons are on compression at the same time. That would explain the difference in feel. Please correct me if I'm wrong.
Don
Don
03-23-2012, 06:55 PM
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RE: Twin engines uneven compression Magnum
ORIGINAL: Campgems
That engine has a twin throw crank, I believe that both pistons are on compression at the same time. That would explain the difference in feel. Please correct me if I'm wrong.
Don
That engine has a twin throw crank, I believe that both pistons are on compression at the same time. That would explain the difference in feel. Please correct me if I'm wrong.
Don
Twin throw crank, yes.
Both pistons on compression at the same time, no.
It's a four-stroke.
Each side fires alternately giving one power stroke for each full revolution of the engine.
03-24-2012, 06:18 AM
#9
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RE: Twin engines uneven compression Magnum
ORIGINAL: Silvaire
Not ''Spot on Don.''
Twin throw crank, yes.
Both pistons on compression at the same time, no.
It's a four-stroke.
Each side fires alternately giving one power stroke for each full revolution of the engine.
ORIGINAL: Campgems
That engine has a twin throw crank, I believe that both pistons are on compression at the same time. That would explain the difference in feel. Please correct me if I'm wrong.
Don
That engine has a twin throw crank, I believe that both pistons are on compression at the same time. That would explain the difference in feel. Please correct me if I'm wrong.
Don
Twin throw crank, yes.
Both pistons on compression at the same time, no.
It's a four-stroke.
Each side fires alternately giving one power stroke for each full revolution of the engine.
That engine is a true opposed twin. (twin throw crank) Both pistons move in opposed directions. You can identify a true opposed twin by the (substantially) off-set cylinders.
Each piston is @ TDC @ the same time, however, 1 is on the compression stroke while the other is on the exhaust stroke.
As Silvaire stated in his post above, one should feel compression every 360 degrees.
A single throw crank like a Saito 182 (both pistons move in the same direction) would have compression @ uneven intervals 0 degrees, then 180 degrees, another 540 to complete #1 cycle (0 again) 180/540 etc.
A single throw crank twin will have both cylinders directly opposite of each other or possible slightly offset.
03-24-2012, 11:54 AM
#10
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RE: Twin engines uneven compression Magnum
As I was writing my reply, I was thinking about the four stroke effect, and that's why I added the correct me. Iwasn't able to find an online manual or parts break down for that engine to get a look at the Cam(S) . The photo showing the pushrods cleared up my thoughts of two cams vs one.
A while back, Magnum had an issue with weak valve springs on a couple engines. I had purchased one and when I ask them about it, their instructions were to pull the rocker cover and on the compression stroke, push on the valves. If it hurt your finger to push it open, the spring was OK, If it was easy to push open it had a weak spring. It might be a good idea to do this test prior to adjustments and such.
By the way, thanks for clearing up my misunderstanding of the engine.
Don.
A while back, Magnum had an issue with weak valve springs on a couple engines. I had purchased one and when I ask them about it, their instructions were to pull the rocker cover and on the compression stroke, push on the valves. If it hurt your finger to push it open, the spring was OK, If it was easy to push open it had a weak spring. It might be a good idea to do this test prior to adjustments and such.
By the way, thanks for clearing up my misunderstanding of the engine.
Don.
03-24-2012, 12:52 PM
#11
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RE: Twin engines uneven compression Magnum
A friend of mine have one of these ASP twins and his too has stronger compression on one sylinder than the other, It is easily notice when turning it over by hand
03-24-2012, 01:55 PM
#12
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RE: Twin engines uneven compression Magnum
ORIGINAL: Campgems
As I was writing my reply, I was thinking about the four stroke effect, and that's why I added the correct me. I wasn't able to find an online manual or parts break down for that engine to get a look at the Cam(S) . The photo showing the pushrods cleared up my thoughts of two cams vs one.
By the way, thanks for clearing up my misunderstanding of the engine.
Don.
As I was writing my reply, I was thinking about the four stroke effect, and that's why I added the correct me. I wasn't able to find an online manual or parts break down for that engine to get a look at the Cam(S) . The photo showing the pushrods cleared up my thoughts of two cams vs one.
By the way, thanks for clearing up my misunderstanding of the engine.
Don.
Even some "two cam" engines are true apposed twins.
The Saito 300 TTDP is one such engine. One could time the cylinders to fire simultaniously though, but why do that?
My 300TTDD is as smooth as silk, even when spinning a 22X8 @ 7200 RPM. Love the dual carbs!
03-24-2012, 09:42 PM
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RE: Twin engines uneven compression Magnum
Thanks for all the replies and ideas. I was away for couple of days but now im back. Below is a list of what I did.
Relapped the valves with a toothpaste. put them back on same results.
moved the valves around, right side to left side. still low compression on right side.
moved the pistons, rings, liners around. Same results.
then removed the valves and replaced the order intake to exhaust, and then move the heads around. still the same results. Even moved the valve train around with same results. Moved all the suspected o rings and head gaskets nothing is helping.
I didnt see any change on the compression. Im thinking , my right side piston rod is maybe a mm or 2 short causing less compression.
Interesting thing is, when I suck on the exhaust and then place my tongue on it ( i know it sounds weird) the amount of suction can hang that engine from the tip of my tongue and it wont let go for at least 10 seconds, this is the same for all valves . this is how I checked all the valves to see if they seal properly.
When I picked this engine up last year from rc universe market, it was the same. a year later im still tackling the same problem. I think i would be better off spending that $400 on a used OS instead.
Relapped the valves with a toothpaste. put them back on same results.
moved the valves around, right side to left side. still low compression on right side.
moved the pistons, rings, liners around. Same results.
then removed the valves and replaced the order intake to exhaust, and then move the heads around. still the same results. Even moved the valve train around with same results. Moved all the suspected o rings and head gaskets nothing is helping.
I didnt see any change on the compression. Im thinking , my right side piston rod is maybe a mm or 2 short causing less compression.
Interesting thing is, when I suck on the exhaust and then place my tongue on it ( i know it sounds weird) the amount of suction can hang that engine from the tip of my tongue and it wont let go for at least 10 seconds, this is the same for all valves . this is how I checked all the valves to see if they seal properly.
When I picked this engine up last year from rc universe market, it was the same. a year later im still tackling the same problem. I think i would be better off spending that $400 on a used OS instead.
03-25-2012, 08:51 AM
#15
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RE: Twin engines uneven compression Magnum
ORIGINAL: stssa
Thanks for all the replies and ideas. I was away for couple of days but now im back. Below is a list of what I did.
Relapped the valves with a toothpaste. put them back on same results.
moved the valves around, right side to left side. still low compression on right side.
moved the pistons, rings, liners around. Same results.
then removed the valves and replaced the order intake to exhaust, and then move the heads around. still the same results. Even moved the valve train around with same results. Moved all the suspected o rings and head gaskets nothing is helping.
I didnt see any change on the compression. Im thinking , my right side piston rod is maybe a mm or 2 short causing less compression.
Interesting thing is, when I suck on the exhaust and then place my tongue on it ( i know it sounds weird) the amount of suction can hang that engine from the tip of my tongue and it wont let go for at least 10 seconds, this is the same for all valves . this is how I checked all the valves to see if they seal properly.
When I picked this engine up last year from rc universe market, it was the same. a year later im still tackling the same problem. I think i would be better off spending that $400 on a used OS instead.
Thanks for all the replies and ideas. I was away for couple of days but now im back. Below is a list of what I did.
Relapped the valves with a toothpaste. put them back on same results.
moved the valves around, right side to left side. still low compression on right side.
moved the pistons, rings, liners around. Same results.
then removed the valves and replaced the order intake to exhaust, and then move the heads around. still the same results. Even moved the valve train around with same results. Moved all the suspected o rings and head gaskets nothing is helping.
I didnt see any change on the compression. Im thinking , my right side piston rod is maybe a mm or 2 short causing less compression.
Interesting thing is, when I suck on the exhaust and then place my tongue on it ( i know it sounds weird) the amount of suction can hang that engine from the tip of my tongue and it wont let go for at least 10 seconds, this is the same for all valves . this is how I checked all the valves to see if they seal properly.
When I picked this engine up last year from rc universe market, it was the same. a year later im still tackling the same problem. I think i would be better off spending that $400 on a used OS instead.
You can check the rod length by measuring top of the cylinder to the piston crown @ a given crank position. They should be equal.
03-25-2012, 11:25 AM
#17
Senior Member
RE: Twin engines uneven compression Magnum
On full sized engines, a 'differential pressure test' is done to determine the leak rate in the cylinder. Engine manufacturers have a leak rate spec in their manuals to let you know if there is excessive leakage in the cylinder. If the leak rate exceeds the mfgs spec, you can put your ear up to the intake, the exhaust, or the crankcase to hear whether the leak is in the intake valve, the exhaust valve, or the piston rings.
Although I doubt that this sort of test is at all practical for small engines like we use, I do wonder if you could perhaps rig up a pressure fitting for the plug hole so that you could pressurize the chamber and at least get an idea about where the leak is. If you hear nothing conclusive, then perhaps one combustion chamber volume is much smaller than the other.
Stay with it and let us know what you finally discover. Good luck.
Oh, by the way - be sure to secure the propellor (crankshaft) when you pressurize the cylinder. That much pressure can sure spin a prop and cause considerable damage.
Although I doubt that this sort of test is at all practical for small engines like we use, I do wonder if you could perhaps rig up a pressure fitting for the plug hole so that you could pressurize the chamber and at least get an idea about where the leak is. If you hear nothing conclusive, then perhaps one combustion chamber volume is much smaller than the other.
Stay with it and let us know what you finally discover. Good luck.
Oh, by the way - be sure to secure the propellor (crankshaft) when you pressurize the cylinder. That much pressure can sure spin a prop and cause considerable damage.
03-25-2012, 01:32 PM
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RE: Twin engines uneven compression Magnum
I will try measuring while the piston is at TDC. (one is at TDC for combustion and the other is at TDC of exhaust or vice versa. I will try that this evening. if they measure the same , i don't know what else to do.
thinking about drilling a glow plug to make a custom leak down tester. My wife has a nice dermabrasion kit, which does have a vacuum pump with a gauge that goes up to 50psi, this should help me a little bit. Before I do this, I need to get rid of her for a day. if she sees her kit is all custom fitted to an rc engine project she will go nuts. I will let you know, how it goes.
thinking about drilling a glow plug to make a custom leak down tester. My wife has a nice dermabrasion kit, which does have a vacuum pump with a gauge that goes up to 50psi, this should help me a little bit. Before I do this, I need to get rid of her for a day. if she sees her kit is all custom fitted to an rc engine project she will go nuts. I will let you know, how it goes.
03-25-2012, 01:32 PM
#19
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RE: Twin engines uneven compression Magnum
Does your engine run fine? If so just let it be. My friends engine is just like yours, but it runs great nevertheless so he doesn`t worry about the difference in compression between the sylinders.
03-25-2012, 06:58 PM
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RE: Twin engines uneven compression Magnum
Thats the thing, I just mounted on a bench. it runs good. idles around 1500 and turns 18x6 prop at 6500rpm. what do you guys think? is 18x6 at 6500 is a low or a good value. high needle is about 2.5 turns out. should i just leave it the way it is?
03-25-2012, 10:17 PM
#21
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RE: Twin engines uneven compression Magnum
I think 18-6 is a bit too big for it, that`s really a .180 single prop. Try 16-8 it should rev about 9000 rpm or more, or you can use a 17-6 if you need bigger prop and less speed
03-26-2012, 04:05 AM
#22
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RE: Twin engines uneven compression Magnum
ORIGINAL: stssa
Thats the thing, I just mounted on a bench. it runs good. idles around 1500 and turns 18x6 prop at 6500rpm. what do you guys think? is 18x6 at 6500 is a low or a good value. high needle is about 2.5 turns out. should i just leave it the way it is?
Thats the thing, I just mounted on a bench. it runs good. idles around 1500 and turns 18x6 prop at 6500rpm. what do you guys think? is 18x6 at 6500 is a low or a good value. high needle is about 2.5 turns out. should i just leave it the way it is?
My Saito FA 150 single (2 1/2 HP) spins an 18X8 @ 7700 RPM so I would say that you are a bit low on power. I'm sure the 160 twin makes less HP than the FA 150, but 1200 RPM W/2" less pitch seems like a BIG difference.
Is the 1500 RPM idle W/or W/O a glow driver? A 1500 RPM idle W/O a glow driver does suggest that the engine is running firly well balanced.
03-26-2012, 08:28 AM
#23
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RE: Twin engines uneven compression Magnum
Just a thought -
You might try to use a lower pitched propellor and try to measure the max rpm with only the right cylinder firing and then compare that result to the same test with only the left cylinder firing. Seems to me like if the results are close, then there is little to worry about and you should go flying.
Maybe a little nutty, but it's the way my brain is working this morning. Good luck.
You might try to use a lower pitched propellor and try to measure the max rpm with only the right cylinder firing and then compare that result to the same test with only the left cylinder firing. Seems to me like if the results are close, then there is little to worry about and you should go flying.
Maybe a little nutty, but it's the way my brain is working this morning. Good luck.
03-26-2012, 10:00 AM
#24
RE: Twin engines uneven compression Magnum
The rings of most single piston engines take gallons before completely broken in. Till then the compression comes and goes as the rings travel around and back and forth, as the bore and rings wear smooth, as more or less oil coats the parts, etc. I don't know why you would think two cylinders of a twin should have the same compression during this. Even after break in there is going to be some varibility.
03-26-2012, 06:35 PM
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RE: Twin engines uneven compression Magnum
I've been flying one of these engines for at least 10 years on my 1/4 scale DR-1 and it has always had noticably more compression on one cylinder. No worries as long as it runs ok which yours sounds like it does. The 18x6 is a bit on the large size but actually that's what I've always run on mine because it looks more scale on the DR-1. You'll need to be careful that you don't get too lean because with that size of prop it is difficult to determine when you have hit the peak. There isn't a great deal of change in RPM when changing the high speed needle with a prop that size. I'd say go fly it!