Can i use the same compression gauge that i use on my truck to check the compression on a small engine or do i have to buy a compression gauge to check compression on small engine like weeedeater?

You have to buy or borrow a compression gauge tester. It threads into the spark plug hole; so you pull all the plugs out, then check each cylinder's compression. The proper way is to first disable gas and spark, put the tester into the first cylinder, then crank the engine over a set number of times- the same amount of cranking for each cylinder, so all get a balanced test. While cranking, hold the accelerator pedal all the way to the floor-this lets the engine breathe normally, getting all the air available. then compare your compression readings: a good healthy engine will have all cylinders pretty even, in the range of somewhere between 150 psi-200 psi. Low compression readings means compression is being lost somewhere, like worn piston rings, bad head gasket seal, or valves not sealing.

Wondering if you have any compression at all? Hold your finger over a spark plug hole while someone briefly cranks the engine over. You can tell if there's any compression at all-you will feel the pressure build up under your finger. Rent or buy the gauge tester to get a true reading of your compression.

Sounds like a fuel delivery problem. Have you checked fuel pressure with a gauge?

If you need further help, reach me via phone at https://www.6ya.com/expert/tom_1788908372a4836f

Timing may be off. Have you checked it? Back firing could be valve timing problem; could also be too much gas-check injector spray pattern-should see a cone shaped pattern of fine mist on the throttle plates when cranking engine-this is if you have throttle body injection.
For $20-$30, you can buy a compression gauge tester and check engine compression. On a warmed up engine, pull all spark plugs, disable ignition and fuel delivery (pull injector fuse or unplug injectors), hold throttle wide open, and check compression on each cylinder by cranking engine over about 7-10 revolutions-the same amount for each cylinder, about 4-5 seconds each. All cylinders should be nearly even and balanced. Lowest cylinder should be at least 75% of highest reading cylinder. For any low reading cylinder, put a squirt of oil (about a tablespoon) in that cylinder and retest compression: if compression rises, rings are worn. If compression does not improve, valves are the reason for low compression-valves may be burnt or not sealing good.
I would do the compression test-on an engine that old, before putting expensive parts into it, you should know if the engine's internal mechanical condition is good first.

A compression test will tell you if your engine has good compression. An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily.

As a rule, most engines should have 140 to 160 lbs. Of cranking compression with no more than 10% difference between any of the cylinders.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

HOW TO CHECK COMPRESSION
Compression can be checked two ways: manually with a compression gauge, or electronically with an engine analyzer the measures cranking compression. With electronic testing, a computer analyzer estimates compression in each of the engine's cylinders by measuring slight variations in engine cranking speed.

The results correlate well with actual gauge readings, and can be completed in a matter of minutes without having to remove any spark plugs. What's more, the analyzer prints out the results of the compression test making it easy to see and compare the actual numbers.

To check compression manually with a gauge, all the spark plugs must be removed. The ignition coil must then disabled or the high tension lead grounded. If the engine has a distributorless ignition, the ignition coils must be disabled to prevent them from firing. The throttle must also be held open.

The engine is then cranked for a few seconds using a remote starter switch or a helper while a compression gauge is held in a spark plug hole.

The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders.

The individual cylinder readings are then compared to see if the results are within specifications (always refer to a manual for the exact compression figures for your engine because they do vary from the ballpark figures quoted earlier).

IS IT THE RINGS OR THE VALVES?
If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve.

Hope this help (remember comment and rated this).

The fault may very well be with the tester itself: The compression tester should have a check or "HOLD" valve which causes the needle on the gauge to REMAIN at it's highest recorded point. For example, my 1993 Isuzu Rodeo had a compression of 160 psi. The gauge "holds" the needle at the measurement UNTIL THE VALVE IS RELEASED-(mine had a button that needed to be pressed to release the pressure) then there will be a slight discharge of the pressure contained within, the needle should drop back to zero, and THEN you should be able to remove the tester. What I am getting at is if the pressure drops that fast, I think the "HOLD" valve on it may be defective, OR the tester is not properly sealing on the combustion chamber (stripped threads?). Might be worth buying another one for 20$ just to be sure.

A four cylinder engine, you say? Very rarely will you have ALL of your cylinders exhibit the same pressure loss symptoms. How did the other three cylinders test?

Here are the components required for a running engine to fire:
1-good spark (adequate voltage, good plugs/wires)
2-dedicated fuel supply (fuel pump with adequate pressure)
3-timing (slack in timing chain/belt?)

If the timing belt broke or maybe even jumped a tooth or two, it COULD cause DAMAGE to your engine-pistons hitting valves-if it IS what they call an "INTERFERENCE" engine.

Hope this helps.

FIRST OF ALL YOU NEED TO RUN ENGINE FEW MINUTES DONT GET IT TOO HOT. TURN OFF ENGINE. THEN PUT ON SAFETY GLASSES. USE COMPRESS AIR BLOW DEBRI AWAY FROM SPARKS PLUGS.TO KEEP FROM GETTING INTO CYLINDERS.DISABLE YOUR SPARK IGNITION.AND DISABLE FUEL PUMP.BY REMOVING FUSE.NUMBER AND TAG SPARK PLUG WIRES.REMOVE ALL PLUG WIRES AND SPARK PLUGS.BESURE YOU NUMBER THEM AND TAG THEM SO THEY GO BACK IN THE ORIGINAL PLACE.THEN GET COMPRESSION GAUGE TESTER SCREW IT IN SPARK PLUG HOLE TO CYLINDER 1 FIRST.TAKE YOUR FOOT PRESS ACCELERATOR PEDAL TO FLOOR.PUT KEY IN IGNITION SWITCH CRANK ENGINE AROUND A FIVE TURNS.THEN CHECK COMPRESSION ON GAUGE.IT SHOULD BE NO LOWER THAN 100 PSI.IF SO SQUIRT SOME OIL IN THAT CYLINDER. IF COMPRESSION INCREASES.THAT CYLINDER HAS BAD RINGS IF OIL SQUIRT DONT INCREASE COMPRESSION. VALVES IS LEAKING. YOU DO ALL CYLINDER THE SAME.SCREW COMPRESSION IN PLUG HOLES.TURN ENGINE OVER 5 ROUNDS. WRITE DOWN ALL READINGS.

buy manual, check for debris buildup at radiator; do a pressure check on coolant system

Check to ensure that there is fuel to the injectors using a pressure gauge on the schrader valve on the injector rail or fuel filter (which ever has it).

Check engine compression with a proper compression tester.

Buy an inline spark checking tool and install it between each spark plug and wire to ensure that you're getting spark to each cylinder while cranking.

These three things cover the basics. Once you've tested all these things, you can further diagnose the problem.

How to Test Engine CompressionAn engine depends on an equal compression reading in each cylinder to run smoothly. If poor compression exits in one or more cylinders it can cause a rough idle condition and low power. A compression test can be performed to check wear or internal damage. To start a engine compression test gauge is needed. There are two types of gauge styles, one threads into the spark plug hole which is more accurate. The other style of gauge is constructed with a rubber plug that is meant to be pressed against the spark plug hole, this style of gauge is difficult to use. To perform a engine cylinder compression test follow the steps below.

• Remove ignition coil connector or ignition system fuse to disable power to the ignition system
  
• Test ignition system to ensure that the power to the system has been disabled
  
• Remove #1 cylinder spark plug and insert compression gauge, most gauges have a hose attachment that is installed into the spark plug hole with the gauge connecting to the gauge hose.

Engine Cylinder Compression Gauge

• After the compression gauge has been inserted, use the starter to crank the engine over for about five seconds. Use about the same five seconds to test the remaining cylinders.
  
• Record the compression reading as each cylinder is tested
  
• Remove the compression gauge and reinsert the spark plug
  
• Follow this procedure until all cylinders have been tested
  
• Compare cylinders compression reading, all readings should be within about 5% of each other

If low compression exists a cylinder malfunction exits and further inspection is required. Possible causes for a low compression condition are: burned intake or exhaust valve, broken piston or piston ring, broken valve spring or a blown head gasket.