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compression check 360 good or bad ?

Racecar32

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I have a miss when I get on it from 1st to 2nd and 3rd to 4th . Timming was good and set the Elderbrock Carb with a vacuum gauge . All was good . Ran a compression check

No 1 120
3 118
5 122
The rest of the cylinders were
148 to 150 range Could this be my miss ?

Thanks
 

moparleo

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What was your procedure checking the compression ? Wet or dry ? How many crank revolutions ? Throttle blades wired wide open ?
Not likely on the miss. Recheck all primary ignition wiring and connections, grounds.
 

Racecar32

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What was your procedure checking the compression ? Wet or dry ? How many crank revolutions ? Throttle blades wired wide open ?
Not likely on the miss. Recheck all primary ignition wiring and connections, grounds.
I have new plugs ,wires , new dist. Throttle blades were open . could it be the fuel pump ? its a man. pump could it be starving on gas ?
 

moparleo

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Fuel pump not likely and nothing to do with compression. All cylinders should be within 10% of the others.
We know nothing about your engine. Just that it is a 360.
You say all is "good". What exactly does that mean ? Good is not a specification it is a judgement.

12 ° btdc is a spec. Vacuum 20 lbs Steady needle at 700 rpm is a spec. Edelbrock makes many different carburetors.
Engine stock ? Al factory manifolds ?
More info always helps. Just like when a Dr. ask you how you feel. If you just say "good" and not tell him about fevers, pains, etc... he can't diagnose you.
 

Racecar32

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Fuel pump not likely and nothing to do with compression. All cylinders should be within 10% of the others.
We know nothing about your engine. Just that it is a 360.
You say all is "good". What exactly does that mean ? Good is not a specification it is a judgement.

12 ° btdc is a spec. Vacuum 20 lbs Steady needle at 700 rpm is a spec. Edelbrock makes many different carburetors.
Engine stock ? Al factory manifolds ?
More info always helps. Just like when a Dr. ask you how you feel. If you just say "good" and not tell him about fevers, pains, etc... he can't diagnose you.
its missing on Excelleration , thats why I said fuel pump . All the plugs looks the same they are new about 50 miles on them no carbon , no oil on them . my 360 is built with a slight cam . headers Elderrbrock 600 ,stock heads electronic ing. 4 speed 355 gear . I,ve changed everything but the fuel pump and the filter .Its a glass filter with no particals that I can see. I ran vaccum on the carb in the green with needle steady . set timming to 16 and 32 degrees .
 

Challenger RTA

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That's kind of what I thought. I going to say drive it gearing up and down. Slowing down in a lower gear than need be. and accelerating. Not like you stole it. just moderately up and down. 2k - 3k rpm range 30 min or a little more.I don't think it was broke in right. It needs torque on the cylinder walls.
 
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Racecar32

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That's kind of what I thought. I going to say drive it gearing up and down. Slowing down in a lower gear than need be. and accelerating. Not like you stole it. just moderately up and down..I don't think it was broke in right. It needs torque on thhe cylinder walls.
thanks
 

Challenger RTA

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How To Do A Compression Test (Dry)


  1. Start with a warm engine, the ignition and fuel disabled, and all spark plugs removed.
  2. Above all, wear protective clothing and gloves, while working around a hot engine.
  3. While doing the test, the throttle and choke plates, must be fully open, for an accurate test.
  4. Have your helper, fully depress the accelerator, while he is cranking the engine.
  5. Avoid any flames around the engine, during the test.
  6. First of all, connect the tester to a cylinder and turn the engine over, 6 to 8 compression strokes.
  7. You will be able to hear the cranking speed slow, as the tested cylinder, comes up on it’s compression stroke.
  8. Note how fast the compression increases and write down the highest reading.
  9. Test all cylinders the same way, with the same number, of compression strokes.

What Do The Results Tell You​


There is more than one school of thought on, what the compression, of the average engine should be. So, as a general rule a compression of 135 PSI or better, is excellent. On the other hand, a compression of 85 PSI or lower, is extremely bad. Firstly, the most desirable situation is that all cylinders, give the same or close to the same reading.


In addition, that reading should be, above 135 PSI. Uneven readings are not uncommon on high mileage or worn engines. Also, the difference between, the highest and lowest readings, should be no more than 20%. So, as long as the lowest reading is, 100 PSI or better, then the engine is acceptable.


Consequently, greater differences between cylinders, indicate worn or broken rings, leaky or sticky valves or a combination of all.


While observing the gauge, during the cranking of the engine, you should have noticed, the way each cylinder pumped up. Usually, a cylinder will produce, about 40 PSI on the first stroke and about 35 PSI, on each additional stroke. click on the link Compression Test - How to do it, wet or dry - What do the results tell you.

Problem cylinders, may have trouble pumping up and may increase, by only something like, 10 PSI per stroke.


It may be possible to crank these cylinders enough times, to come close to, the other cylinder’s total PSI. This is why we try to crank, all cylinders the same amount. As a result, poor rings, usually cause this condition. Be aware that a cylinder suffering from excessive oiling, even from bad rings; can yield high compression test results. Other symptoms, may give you a clue to the problem (a smoking engine).


There are some variables that affect the readings, obtained from compression testing:


  • Cranking speed.
  • Altitude.
  • Temperature.
  • Worn camshaft lobes.
  • High performance long duration profile camshafts.

How To Do A Compression Test (Wet)


Many automotive books describe, a dry and a wet compression test together. Usually these tests, must be interpreted together to isolate the trouble, in cylinders or valves. To perform the wet test, a tablespoon of engine oil, is poured into the cylinder, through the spark plug hole.


So, you squirt the tablespoon of engine oil, into a cylinder, that is reading low. Crank the engine, two revolutions or so to spread the oil, then retest the cylinder. If the compression comes up markedly, 40 PSI or more, the trouble is, poor ring to bore sealing.

Adding Oil To Spark Plug Hole For Wet Compression Test
Adding Oil To Spark Plug Hole For Wet Compression Test
If compression, doesn’t increase much, about 5 PSI, then the problem, is probably with the valves. Furthermore, it could also be, pulled head studs or a, warped cylinder head.

What Does All This Info Tell You​

Normal

  • Compression builds up quickly and evenly, to the specified compression for each cylinder.

Piston Rings Leaking

  • Compression is low, on the first stroke. Consequently, compression builds up with the following strokes, but does not reach normal. And, your compression improves considerably, when you add oil.

Valves Leaking

Sticking Valves From Carbon
Sticking Valves From Carbon
Valve Leaking
Valve Leaking
Your compression, is low on the first stroke. And, compression usually does not build up, on the following strokes. Furthermore, your compression does not improve much, when you add oil.
Compression Test Showing Leak Between Cylinders
Compression Test Showing Leak Between Cylinders

Two adjacent cylinders, have lower than normal compression and injecting oil into the cylinders, does not increase the compression. Most often, the cause may be, the head gasket leaking between the cylinders.

Conclusion

Testing compression is really nothing more than, a way to figure out, which cylinder has a problem. If you do find any issues, the next step would be doing a cylinder leak down test. A cylinder leak down test, is similar to a compression test. It also tells you, how well your engine’s cylinders are sealing. But, instead of measuring pressure, it measures pressure loss.
TIP: You can also do a, A running (dynamic) compression test.
 
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Racecar32

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How To Do A Compression Test (Dry)


  1. Start with a warm engine, the ignition and fuel disabled, and all spark plugs removed.
  2. Above all, wear protective clothing and gloves, while working around a hot engine.
  3. While doing the test, the throttle and choke plates, must be fully open, for an accurate test.
  4. Have your helper, fully depress the accelerator, while he is cranking the engine.
  5. Avoid any flames around the engine, during the test.
  6. First of all, connect the tester to a cylinder and turn the engine over, 6 to 8 compression strokes.
  7. You will be able to hear the cranking speed slow, as the tested cylinder, comes up on it’s compression stroke.
  8. Note how fast the compression increases and write down the highest reading.
  9. Test all cylinders the same way, with the same number, of compression strokes.

What Do The Results Tell You​


There is more than one school of thought on, what the compression, of the average engine should be. So, as a general rule a compression of 135 PSI or better, is excellent. On the other hand, a compression of 85 PSI or lower, is extremely bad. Firstly, the most desirable situation is that all cylinders, give the same or close to the same reading.


In addition, that reading should be, above 135 PSI. Uneven readings are not uncommon on high mileage or worn engines. Also, the difference between, the highest and lowest readings, should be no more than 20%. So, as long as the lowest reading is, 100 PSI or better, then the engine is acceptable.




While observing the gauge, during the cranking of the engine, you should have noticed, the way each cylinder pumped up. Usually, a cylinder will produce, about 40 PSI on the first stroke and about 35 PSI, on each additional stroke. click on the link Compression Test - How to do it, wet or dry - What do the results tell you.
Thanks for the info
 

Racecar32

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Just ran a fuel pressure check on the fuel pump ... 4 psi ,so its not the fuel pump . anything else I could check ?
 

Ernies

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Check your cam lobes had the same problem on my buddys 360 in cop car turned out some of the valves were barely opening at low rpms when reving up rpms increase more oil pressure would clear up the miss pumping up the lifters enough to open valves more this is common on hydralick worn out lifters.
 

DaveBob

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I have a miss when I get on it from 1st to 2nd and 3rd to 4th . Timming was good and set the Elderbrock Carb with a vacuum gauge . All was good . Ran a compression check

No 1 120
3 118
5 122
The rest of the cylinders were
148 to 150 range Could this be my miss ?

Thanks
What spring rate, in inches of mercury (Hg) are your metering rod springs? Too weak of a spring will allow the metering rods to drop and restrict the primary jets when the throttle closes and vacuum momentarily increases. For example, if you have metering rod springs rated at 4 in Hg and vacuum momentarily increases to 6 in Hg during a shift, the rods will drop (be sucked down) and restrict fuel, causing a hesitation, which feels like a miss. I noticed a similar problem from a standing start, as initial acceleration did not cause enough vacuum drop to allow the metering rods to rise. The car hesitated until the throttle was opened far enough to cause the vacuum to drop below the rating of the springs. I solved the problem by installing metering rod springs rated at slightly lower vacuum than idle vacuum. Then, as soon as the throttle was opened, the rods popped up and allowed more fuel through the primary jets. The Edelbrock strip kits have a selection of metering rod springs, color-coded by spring rate, as rated by in of Hg. My experience is that the Edelbrock carbs come with metering rod springs that are on the weak side.
 
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