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Posted By Discussion Topic: Compression drop -- page: 1 2     sarahcecelia 03-16-2019 @ 3:40 AM reply profile send p.m. Senior Posts: 1183 Joined: Mar 2013 Did a major tune up and checked compression on my 1950 V 8 - all were 120 or 115. Got the problem solved with it quitting but now it shakes too much at idle. Rechecked idle mixtures, 1/2 turn out from RPM drop. Pulled plugs, rechecked compression. All had 120 or 115, except #3 had 105? What can cause that to happen in about 3 days, and only 4 or 5 test drives of about about a 6 block run? I ran it up to about 35 in second gear a few times because my flat buddy said it could be carbon on the valve seat, thinking that would clear it out. This is a Putt Smith rebuilt with maybe 200 or 300 miles on it! Regards, Steve Lee 37RAGTOPMAN 03-16-2019 @ 12:51 PM reply profile send p.m. Senior Posts: 1958 Joined: Oct 2009 HI In #3 cylinder, did you try a wet compression test ? or just dry ? let us know, 1937 RAGTOPMAN sarahcecelia 03-16-2019 @ 4:03 PM reply profile send p.m. Senior Posts: 1183 Joined: Mar 2013 What do you mean by wet or dry compression test. Worked for dealers for ten years and never heard that one. Please explain. I just cranked her w/my hand held remote start gizmo, with compression tester in each cylinder, about 4 revolutions, as I have always done. They were all 120 or 115; now, 3 days later #3 has 105?? never had that happen in 60 years of this! Regards, Steve Lee cliftford 03-16-2019 @ 9:54 PM reply profile send p.m. Senior Posts: 845 Joined: Jan 2014 Wet: with a couple of squirts of oil in the cylinder. Dry: without oil. 37RAGTOPMAN 03-17-2019 @ 9:33 AM reply profile send p.m. Senior Posts: 1958 Joined: Oct 2009 HI if when testing wet, the compression goes up , it might be rings, that little difference in compression should not effect the rough running,that much I would think that it might be a fouled spark plug, try removing one spark plug wire at a time,with a insulated pliers, while running, when it makes no difference you found the bad cylinder, wire , plug ' you can also do a leak down test to find out what may be the problem, you can also use a vacuum gauge to see if it is a vacuum leak, in the intake manifold carefully spraying carb cleaner on the intake gasket, if it makes a difference it might be the intake manifold gasket leaking, if the engine has not run it years, it might have stuck rings, it might be a good idea to remove all the plugs and use a rust buster, in the cylinders, soaking for a few days, turn over engine before reinstalling the plugs, hope this helps 1937 RAGTOPMAN sarahcecelia 03-17-2019 @ 9:54 AM reply profile send p.m. Senior Posts: 1183 Joined: Mar 2013 Never have done a wet test for compression, but either way that cylinder would be lower than the others. Can't understand it, just 3 days before that the compression was good on all 8, and the vacuum was in the Green area (good) and steady! Then that #3 cylinder was low!! How could that happen when I was only driving the car around the block 5 or 6 times in between to find out why the motor was dyeing, and found out it was a "fuel starvation problem," and I fixed that problem. Regards, Steve Lee sarahcecelia 03-17-2019 @ 9:59 AM reply profile send p.m. Senior Posts: 1183 Joined: Mar 2013 The motor was rebuilt about 100-150 miles ago by Harold "Putt" Smith. What "rust buster" are you referring to?? maybe a sticking valve??? "over night??" Regards, Steve Lee 37RAGTOPMAN 03-17-2019 @ 11:21 AM reply profile send p.m. Senior Posts: 1958 Joined: Oct 2009 Hi , it is most likely I am thinking you have a sticking valve in the guide, if you locate which one you can remove the spark plug and push down the valve by hand using a bent rod of some kind and use light oil and make sure the sticking is not happening any more, it might take a few tries, I have done this in the past, use TOP OIL in it, this will lube the valves and guides, and a lot more,,,!!! MARVEL MYSTERY OIL !!! to the RESCUE , if this fixes the problem < I would use it for at least 1000 Miles, I always use this , in my 37, if it needs it not, since 1980 valves have a habit of sticking if the engine sits for long periods of time, on rebuilt engines, that have been sitting for period of time, with out turning over, even if by hand, or just hitting the starter button once in a while, dry compression test, wet compression test, seals the rings,if have issues, goes up LEAK down test, by putting compressed air into cylinder at top dead center, air coming out of exhaust, leaking exhaust valve, too tight or bad ? air coming out of carburetor leaking intake valve, too tight or bad ? or bad valve seats. air coming out of oil filler leaking rings or bad piston, Bubbles coming out of radiator bad head or gasket, crack in cylinder to water jacket, this is a quite common procedure to do a diagnostic on a engine with problems, no power, missing, smoking,noisy, problems etc RUST BUSTER, LIQUID WRENCH,,,, is a great product , using for years hope this helps 37RAGTOPMAN, Maine hope to hear what the problem was, GOOD LUCK,,,, This message was edited by 37RAGTOPMAN on 3-17-19 @ 12:14 PM 37RAGTOPMAN 03-19-2019 @ 7:58 AM reply profile send p.m. Senior Posts: 1958 Joined: Oct 2009 wet compression test vs dry compression test, wet , adding oil to the cylinder, then take compression test it will seal the rings temporally and will increase the compression, if it does not there might be something else going on, in that cylinder, like valve, bad head gasket ? my 2 cents, 1937 RAGTOPMAN 37RAGTOPMAN 03-19-2019 @ 7:58 AM reply profile send p.m. Senior Posts: 1958 Joined: Oct 2009 wet compression test vs dry compression test, wet , adding oil to the cylinder, then take compression test it will seal the rings temporally and will increase the compression, if it does not there might be something else going on, in that cylinder, like valve, bad head gasket ? my 2 cents, 1937 RAGTOPMAN

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