Compression numbers ?

Joker2

Joker2

Junior Member
I took some numbers today with a known good tester. My engine was rebuilt as some of you have seen my story in the past. Running good just wanted to know what you thought of these #'s

Cold with all plugs out.
1,103 3, 100 5, 100

2, 115 4, 115 6, 112-115

Hot with all plugs out.
1, 97-99 3, 97-99 5, 97-99

2, 105 4, 105 6, 105

They seem to be within the limits even though they all are a little low, from what I have read they should be more around 120, wondering if this is because of the rebuild ?
 
What would concern me is the drop of 10 PSI when hot on the 3. That sounds to me like to much ring end gap. They don't look that bad as it looks like they are even on each bank. It looks like it was decked on one side. Not to sure what to expect out of that 60 degree looper. I wonder what tsubaki gets out of his as he has a simliar engine?
 
I don't remember testing mine so I'm no help.
From what I remember wide open throttle is the only correct way to get a reliable reading.
Remember my Father telling me he picked up an engine for parts. The guy he bought it from said it ran like crap and had low compression, basically burned up. A good while later Daddy cleaned the carbs, done a compression test and it ran as good as any he had ever owned. He later ran across the guy he got the motor from and questioned him on the low compression. The guy did the test in the idle position.

Is it possible the side with the lower compression might be head gasket related or the like?
 
Just bury the shifter forward, that would be putting it in gear, is this OK. My shifter is not the kind you can pull out and rev in nuetral. I can't rev my engine in nuetral, only a little with the fast idle lever.
 
Yes or while you are at the rear of the engine, you can just open all the butterflies to do the same thing.
I assume you are jumping the starter solenoid to do the test. As the starter won't engage with the shifter in forward by using the key.
 
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I was just using the key with the emergency lanyard disengaged, that will not work with the shifter forward right?
 
Let me see if I can simplify the control box operation.
Lanyard removed is a kill switch, just like turning the key off.
When the shifter is not in neutral it disconnects the ability to operate the starter by way of the key.
Jumping the starter solenoid at the engine will start the engine if the key is on or if the wire harness is unplugged.
 
No need to.
Jumping the solenoid turns the engine over. It bypasses the control box ability to engage the starter.
Key on or off it don't matter.
It's probably advisable to leave the key off, push the throttle full forward, jump the solenoid and do the test. This way you don't have to worry about the ignition igniting atomized gas from the cylinders if it happens to spark to ground from one of the plug boots.
I can't remember if this solenoid has 3 or 4 terminals. A 3 terminal has a hot, starter and key switch lead. The 4 wire also has a ground.
Basically you determine the key switch lead and hot, then connect between the two. Best done with a remote starter switch.
Careful not to confuse the ground (if applicable), it should blow the fuse at the engine and could do stator damage.
 
joker, I'm pretty sure your comp. #'s should be 95-100 lbs. I have been told/read not to expect #'s like 125-140 on that motor. I've done it by warm engine, having all plugs out, throttle wide open. Not sayin that's the ONLY way to do it....just how I did it....right or wrong.
 
I've not been able to find published compression numbers on any of the engines I've fooled with. I would like to know though.
Hearsay is the loopers do have a higher compression value and a less than %10 difference between cylinders is preferable on all engines.
Joker, I'd keep running the engine like it is. I'm not tired of this thread but really there's not much to worry about with what you've got. At least from what I see.
 
just took a quick peek over at iboats, no factory specs, just dialog amongst members there, those #'s seem to be anywere from 90+/- to 105+/-....the 10% rule being most important. Ask Skools out.....he knows just about everything on the omcs
 
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