Alright where do i start...
A guess for a lack of a better place i will start by saying every gas powered internal combustion engine requires 3 things to start
1) Compression
2) Fuel/ Air Mixture
3) Spark
Here is where it hits the fan....
1) Compression- Here is how i go about measuring compression.
The vintage vmax garage has a compression tester EXACTLY like this:
http://www.carid.com/images/actron/hand ... cp7821.jpg
It simply gets pushed into the spark plug hole and as you pull the recoil the needle moves up and holds the highest reading...
TOOLS: There are several, if not hundreds, of differnet compression tester designs but they all do the same thing. One thing you have to watch out for is they will not all read the same or at least take the same amount of pulls to get to the true reading. One such as this:
http://www.northerntool.com/images/prod ... 507_lg.jpg can be screwed into the spark plug hole and allows for 1 person operation. However the longer tube will result in a lower compression reading, because the air in the cylinder is filling more volume in the tube. Some versions with a hose may have the check valve at the end of the hose opposite the gauge and in these models it will only require more pulling to reach the highest reading. The particular model I linked to has the valve near the guage and will ALWAYS read lower than the one we have at vintagevmax. (not that, that's a problem, just be aware)
we also have a leak down tester, exactly like this one:
http://www.harborfreight.com/media/cata ... e_2899.jpg but we have honeslty never used it! haha (as of yet)
HOW TO CHECK FOR COMPRESSION:
One very simple way to check for compression, especially on the trail is to take a spark plug (or both) out and press you thumb over the hole while someone else pulls the rope. You will want to make sure the key if off for this, for 2 reasons. 1 being, a non-grounded (spark plug) coil is bad when you pull the rope, and 2 when that plug wire tries to find ground through you hand your will not like how it feels. I have yet to see a man that can keep the compressed air inside of the cylinder with just a thumb and a loud PSST or POP should be heard. If you can keep your thumb over the spark plug hole, you might as well call the hook, you have lost compression. (been there)
The above is the "redneck" way of doing things but for those of you who have no compression tester, no access to one, or are in the middle of the woods it will get you by, barely.
The following is the CORRECT way of testing compression....
To check compression, I first start by taking BOTH spark plugs out and pressing the compression tester into a spark plug hole. Have a someone pull the rope with the throttle WIDE OPEN for as many pulls as it takes to get the needle to quit moving higher, usually around 5 good pulls. Then read the gauge (we'll say it measures 130psi) then clear the gauge and check the other side the same way. Now the manual says that the pressures should be within 10 percent of each other. Therefore of the pressure in the first cylinder is 130 then as long as the second cylinder is within 13psi you are within limit. AKA 117-143psi. If it is not within the limits then you need to take the motor apart and rebuild it. As I stated before all gauges are going to read slightly different but for reference my 84 engine, using the gauge we have, reads 130 in the right and like 125 in the left.
Readings will also be higher if the motor is warm (providing it runs). One last thing, oil is very important to piston rings sealing. If the motor has been sitting for 20 years it may be pretty dry in there and (among many other things, covered later) i suggest putting some thin oil (marvel mystery oil) down the plug holes and turning the motor over SLOWLY a couple of times before allowing it to soak in for several hours or days.