I picked up a compression guage. I know that the best way to use it is to take the reading when the engine is new and then take it periodically and if the pressure starts to drop, you need a rebuild.
But, what about if you bought the engine used and there is no base reading. What psi would be the norm? I just bought a used engine and the reading was 30psi. Does that sound normal?

30psi? I dont know for sure, but I think that might be going towards the low side. I remember reading an article in which Ed Bridges (a pofessional engine modder) did a compression test on a brand new engine that was just properly broken in. His compression reading (if I remember correctly) was about 75psi.

How do you use t6his thiong?
I know you screw it where the plug goes but how do you turn the engine over? Starter box, by hand.....

I'm not sure, I never used one before. But I think you install it with the piston at BDC (or at least where the piston surface is below the trapped compression ratio volume) and you turn the flywheel to TBC while reading the gauge. When the needle (I think it is analog) hits the highest pressure reading just before the pressure pushes the piston back down, that its the pressure you record.

If you can hold the piston at TDC with the trapped air in the cylinder, I guess you can also check for compression leak (i.e. the needle begins to drop while the piston remains held at TDC).

You will need to spin the engine over with the gauge installed and the throttle held open.

Right now 30 PSI means nothing. Was the engine dry? Was the engine warm? Was it spun over on a starter box or did you just slowly pull the starter rope? What position was the throttle? All of these things can change the compression readings.

The main thing you will find over time in using a compression gauge on one of these little engines is that what ever you do, you have to do it consistantly.

When I was doing a bunch of testing several years back I liked a cold engine, two or three drops of oil on the piston and then spin in over for a few seconds before installing the gauge. Then I spin it for a second to get my reading. Throttle held open.

Ed M.

You spun it with the starter box I guess?

I spun it cold, with no oil. I'll try it again lubricated. I tried it on my nt15 engine which is brand new, never tested and it showed 35psi.

Those are both pull starts, you're supposed to give it 6 or 7 pulls and it records the psi. If you have a starter box, you're supposed to crank it for 4-5 seconds.

for $35, I thought it was a useful little device.

Once that nt15 gets broken in I think you will find the numbers come up. Not saying this is the case, but never confuse pinch with compression.

I did most of my testing on a starter box............took the easy way out

No need to spin longer than it takes for the reading to come up. It levels off quickly and any more spinning does nothing.

Ed M.

I have one forget what brand but on the package it says there is no set rule for what compressions is good and what is bad as engines as vary too much. It goes on to say check it to see what you have after the engine is broke in and write it down. Check it from time to time and compare it to what it had right after break in.