reelfishin
Captain
- Joined
- Mar 19, 2007
- Messages
- 3,050
I can't help but notice how many here are checking compression on various engines. A few years ago I bought a motor from a local repair shop that was sold to me as needing an overhaul or new power head. I was told that it had very low compression. It looked like a new engine in appearance.
I wasn't going to bother checking the compression and was simply going to part it out, but for some reason I was messing around with an old piece of rope and spinning the motor over while talking to a neighbor one day. Thinking that it felt pretty good for supposedly having low compression, I got out my compression gage. I got a reading of 130 psi on both cylinders. To me that sounded fine, so I got out another gage, that one read 145 psi! I then got out my old gage which was one of those ones that you buy in Sears or the local pepboys, and it read only 88 psi per cylinder. The reading of 130 psi was with a brand new Snap-On motorcycle compression test set, and the second reading was with a gage from Harbour Freight.
I figured that I needed a good comparison gage, so I borrowed a few buddie's gages, one a newer Snap-On gage and another one which was made by Matco, both of them read within one or two pounds of my Snap-On gage.
After not seeing anything obvious that would make me condem this motor I hooked up a set of ears and hose, a battery and a set of controls and gave it a try, it ran fine, no problem other than a dirty carburator. I ran it for about a half hour waiting to find some strange problem to surface, but nothing ever did. I gave it a carb cleaning and some new gaskets and all was fine.
I think what happened was that the shop or a mechanic in the shop had a cheap or defective compression gage that read low, saw the low readings and wrote the motor off as junk. They may have never even tried to run it.
There are variations in different gages, if you don't have a gage that you trust or one that's been proven, and you get poor test results, I would definitely compare it to another known good gage.
I have have several Snap-On gages now, all are very close but all read a few pounds off from each other, which is no big deal, but the one that read 88psi is junk, and to make it worse, its near new. I bought it while heading out to check a used motor I was looking at, I had forgotten my good gage and stopped and bought that one so I didn't have to go back and get one. I never used it that day since the seller had a good gage already set up when I go there, it's a good thing I didn't, because I would have walked away from it thinking it was a tired motor if I had used that gage.
The motor in question was a 2 cylinder 15 hp Mariner which has been in use now for nearly 8 years without any problems. The gage in question was one of those chrome gages with a black face and a permanently attached hose and dual threaded ends. It came with several adapters packaged on a card. I have since run into other similar gages that read either real high or real low. I am not saying that all cheaper gages are bad, but make sure the one your using is accurate. It may save you lots of money in the long run. I am glad I didn't part out that motor or spend a lot of money to overhaul it. I don't know for sure what readings the shop had gotten, all they said was that it had 'little compression left'. What got me is that it felt good when you spun it over, I guess they weren't that throrough. The impression I got after checking the compression on that motor wasn't of a worn out motor, but one that most likely never got used much and had spent most of it's life in someones garage.
When you check compression, make sure all plugs are out, open the throttle so the motor can breathe, and be sure that the battery is good or that you give it a good strong pull too, a slow cranking engine won't build good compression either. I also always do a leak down test on any suspect motors, that gives me a little better idea of what's going on inside as well.
I wasn't going to bother checking the compression and was simply going to part it out, but for some reason I was messing around with an old piece of rope and spinning the motor over while talking to a neighbor one day. Thinking that it felt pretty good for supposedly having low compression, I got out my compression gage. I got a reading of 130 psi on both cylinders. To me that sounded fine, so I got out another gage, that one read 145 psi! I then got out my old gage which was one of those ones that you buy in Sears or the local pepboys, and it read only 88 psi per cylinder. The reading of 130 psi was with a brand new Snap-On motorcycle compression test set, and the second reading was with a gage from Harbour Freight.
I figured that I needed a good comparison gage, so I borrowed a few buddie's gages, one a newer Snap-On gage and another one which was made by Matco, both of them read within one or two pounds of my Snap-On gage.
After not seeing anything obvious that would make me condem this motor I hooked up a set of ears and hose, a battery and a set of controls and gave it a try, it ran fine, no problem other than a dirty carburator. I ran it for about a half hour waiting to find some strange problem to surface, but nothing ever did. I gave it a carb cleaning and some new gaskets and all was fine.
I think what happened was that the shop or a mechanic in the shop had a cheap or defective compression gage that read low, saw the low readings and wrote the motor off as junk. They may have never even tried to run it.
There are variations in different gages, if you don't have a gage that you trust or one that's been proven, and you get poor test results, I would definitely compare it to another known good gage.
I have have several Snap-On gages now, all are very close but all read a few pounds off from each other, which is no big deal, but the one that read 88psi is junk, and to make it worse, its near new. I bought it while heading out to check a used motor I was looking at, I had forgotten my good gage and stopped and bought that one so I didn't have to go back and get one. I never used it that day since the seller had a good gage already set up when I go there, it's a good thing I didn't, because I would have walked away from it thinking it was a tired motor if I had used that gage.
The motor in question was a 2 cylinder 15 hp Mariner which has been in use now for nearly 8 years without any problems. The gage in question was one of those chrome gages with a black face and a permanently attached hose and dual threaded ends. It came with several adapters packaged on a card. I have since run into other similar gages that read either real high or real low. I am not saying that all cheaper gages are bad, but make sure the one your using is accurate. It may save you lots of money in the long run. I am glad I didn't part out that motor or spend a lot of money to overhaul it. I don't know for sure what readings the shop had gotten, all they said was that it had 'little compression left'. What got me is that it felt good when you spun it over, I guess they weren't that throrough. The impression I got after checking the compression on that motor wasn't of a worn out motor, but one that most likely never got used much and had spent most of it's life in someones garage.
When you check compression, make sure all plugs are out, open the throttle so the motor can breathe, and be sure that the battery is good or that you give it a good strong pull too, a slow cranking engine won't build good compression either. I also always do a leak down test on any suspect motors, that gives me a little better idea of what's going on inside as well.
