What should the compression be for a 1983 GS650GT, I have readings of 160 psi on three cylinders and 180 psi the other, I'm going to redo the check again with a different compression tester.

petrol compression pressure should be around 160psi average ( low around 140 and high around 180
the difference between any cylinders should not be more that 10% so if the first and last readings are 160 and 145 that would be average and if the adjacent cylinders read low then that would indicate a gasket problem
The result should be that they are all very close in readings.

Info here:

http://www.landroversonly.com/forums/f9/how-compression-test-29158/

It could be a problem in the valve train. For a car that old, before putting new parts in, you could buy a compression tester for about $30, and do your own compression test. It should tell you if the engine is good enough internally to warrant proceeding. If compression is low on any cylinders, that problem must be addressed first. If a valve is not sealing and causing the popping, that cylinder will show low compression.
For the test, the engine should be at normal operating temperature. Then pull all the plugs out, connect tester to #1 spark plug hole. Have a helper hold the throttle wide open while cranking the engine over about 5 seconds. Watch the gauge-the first revolution or two will cause the gauge to jump high at first, and successive revolutions during the 5 second span will cause the needle on gauge to climb a little each time. Record or note the compression on #1, then do each cylinder. Try to do the same amount of cranking on each cylinder, and keep the throttle wide open. What you want are compression readings on each cylinder that is well above 100 psi, and all should be close. The lowest reading should be within 75-85% of the highest reading cylinder. Example: say your highest reading is 135 psi. 85% of 135 is 115-so your lowest reading should be at least 115 psi. Some general guidelines: any cylinder with compression below 100 psi is definitely a problem-that cylinder will not have hardly any power output, even if it is firing. Worn piston rings or poor valve sealing is causing compression loss. A newer engine should have compression readings above 150 psi. As the engine wears, compression will drop. In general, for high mileage engines, you would want to see compression in the range of 125-150 psi, the higher the better, but remember, it should be balanced-the lowest should be at least 75% of the highest reading cylinder.
Post back if you have questions-good luck.

A new engine would be about 180 psi for each cylinder. As the miles add up and wear occurs, compression will get lower. The important thing is that compression is even and balanced-lowest compression reading should be within 75% of the highest cylinder's compression. Some Toyota engines had a minimum allowable compression of about 135 psi, but if compression is balanced, you could have average compression as low as 125 psi and engine would still run good, some loss of power but still acceptable, and oil consumption would not be that bad.
If cylinder compression is below 120 psi as an average, you should be looking into a rebuild event-rings and valves need attention. If any cylinder is below 100 psi, it may not even fire, certainly will produce no power, and could be a head gasket failure, very worn rings, or valve problems.
Remember the important thing is to find compression readings that are very close to each other. I have an '87 Sentra with 75,000 miles(true!)-lowest comp. is 160, highest is 165 psi-runs great, good power. Also a '79 Datsun pickup (with god only knows how many miles-about 200,000 maybe). Compression is about 125-135 psi, fairly well balanced-noticeable loss of power, very little oil burning, still runs fair to good-dependable, at least.
That should give you an idea of what to look for. Good luck.

Unfortunately, you're NOT going to get an accurate reading with a cold engine, it needs to be up to full operating temperature!



Also, as much air as possible needs to be allowed into engine, so remove aircleaner/filter, and hold throttle WIDE open while cranking engine to get reading.

compression should be about 150-180 psi with maximum deviation of about 20 psi between all cylinders. A compression test is not a very reliable way to test for headgasket leaks. It should be tested for headgasket leaks by using a chemical test caled a block test or checked with a device that can check for hydrocarbons in the coolant.

bad rings you can add a teaspoon of oil the the cylinder in question and redo the compression test if the compression goes up then the rings are bad if the compression stays the same i would be looking at a burned or bent valve or bad head gasket.

To test the compression of the engine, use a good screw in type compression gauge. Hold the throttle wide open and rotate the engine through six or seven compression strokes.

The manual says, "If the final readings are 90 psi or more and if the final readings do not indicate more than a 10% variance between cylinders, compression is considered normal".

Do a "wet" compression test as well by squirting 1/2 ounce of oil into each cylinder and redo the test. If the compression comes up considerably, a top end rebuild is probably in order.

At 8.5:1 compression ratio, 120 psi max.

Good Luck
Steve

Normal compression will be between 185 to 205.
Minimum compression is 180
Lowest compression reading should not be less than 70 percent of your highest compression reading. None of the compression readings should be less than 100 psi.

Minimum compression is 100 psi @ 200 rpm. The lowest cylinder reading should not be less than 80% of the highest reading. Perform the compression test with engine at normal operating temperature, spark plugs removed and throttle wide open. Normally I look for about 170 psi to 180 psi.