Compression Test Update

super99

Well-known Member
This afternoon, I made a new set of plug wires and put them on, then I put the gas tank on and about a gallon of gas in it. Hit the starter but it wouldn't fire. I check for fire at the plugs and that was good. So I pulled the plugs out and started checking the compression. #1 had 40#'s, #2 had 30#'s #3 had 40#'s and then the battery ran low, so I quit. Looked inside the cylinders with Bore scope. Cylinder walls are shiney where the pistons ride but full of crud above there. I took a long screwdriver with a hook on the end and reached in and could feel a ridge at the top of the cylinder wall on #1 cylinder. I didn't feel that on the others I tried . I have a spark plug with an air fitting welded into it, tomorrow I'll put it in and hook up the air hose and see where the air comes out. I took the valve cover off and all of the tappets move when cranking and the ones with no pressure on them move freely but it is really sludged up under the valve cover. I will end up pulling the head just to see how bad it really is. I have another motor that came out of a pulling tractor that will go into this one until I decide what to do with this motor, might be better off to part it out.
 
Running a compression test with a low battery is not going to be truthful.

If you are going to do the leak down test, since the valve cover is off, go ahead and remove the rocker assembly. That way all the valves will be closed and won't be opening when the engine turns.

It would still be a more valid test to get it running first. There could be surface rust on the valves and seats.

I suspect a squirt of oil in each cylinder and a hot battery would get it running.
 
It ran before. I would pull it around to get the junk flying out , do more good than just air. You want the valves opening and closing so they blow the junk out and seat them selves. Plus make sure the valves are set and not tight
 
I did squirt oil in the cylinder and it didn't change anything. All of the plugs were out and it was spinning good on the first 2 cylinders. I put the battery charger on it overnite, I'll try it again in the morning, but it won't run without compression.
 
Exactly. Thats why you want to pull it to blow the garbage out of the valve seats Experienced this. Be surprised at the carbon an junk that collects and flakes off.
 
Fill the cylinders with ATF put the plug back in then pour as much down the exhaust as you can. Then let it sit a week. Then pull the plugs and spin it over and check the compression. I'll almost bet the compression come up since good chance it has stuck rings
 
I have never heard it run, I bought it from a kid that bought it from a guy who said it ran. Total unknown.
 
If the battery died that fast with the plugs out probably not turning over fast enough for a good reading.
 
If it has a belt pulley give it Old's treatment and belt it up. If not pull it with the plugs out then put in new plugs. I would leave the muffler off after the treatment when starting filling a muffler with transmission fluid creates a lot of long term smoke. Don't ask.
 
i have better uses for atf oil than pouring it down the muffler, why is more better? makes no sense to me. once you have 1/2 a cup in each cyl. and turn it over the oil is where you want it to be. i highly dont think you do that either that is lot of money wasted and mess to clean up. i am sure that will take care of a pail of oil to fill 6 cyl's , fill the combusion chamber, and fill the manifold.
 
Makes perfect sense to pour it down the exhaust since the exhaust valve are the one most likely to stick open
 
In your earlier post (here) you ask -
Can you do a compression test on a motor that has set several years and get an accurate reading??
If you do the test properly, you will get an accurate reading of the exact current condition of the engine. With that being said, those numbers will have nothing to do with the engine once it is running.
So basically [b:c07fda2041][u:c07fda2041]no[/u:c07fda2041][/b:c07fda2041], that test won't really tell you much about the condition of a running engine. Having sat around 4 or 5 years, the rings are probably frozen and valves might be sticking.
If at all possible, getting the engine to run first is what you want.

After doing a compression test before running, you said in this post
#1 had 40#'s, #2 had 30#'s #3 had 40#'s and then the battery ran low, so I quit
Those numbers are not surprising with an engine sat around for 4 or 5 years

Looked inside the cylinders with Bore scope, Cylinder walls are shiny where the pistons ride but full of crud above there
Good, Don't worry about the crud at this point.

I took a long screwdriver with a hook on the end and reached in and could feel a ridge at the top of the cylinder wall on #1 cylinder. I didn't feel that on the others I tried
To be expected on a used engine. Don't worry about it at this point.

I have a spark plug with an air fitting welded into it, tomorrow I'll put it in and hook up the air hose and see where the air comes out
Kinda irrelevant at this point. Air will probably come from just about everywhere. Will most likely change after running

I took the valve cover off and all of the tappets move when cranking and the ones with no pressure on them move freely
Good. Are all the valves moving up AND down?

I will end up pulling the head just to see how bad it really is
If all your valves are indeed free, I'd hold off on this until you have exhausted all options of trying to get it to run first


In summary, ALWAYS BETTER to run an engine first, for diagnoses. Once you start opening them up, not much to do other than a rebuild at that point. Unfortunately you won't have a good idea of the condition, so usually end up with a complete rebuild.
 

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