Wildly varying compression readings on an old 454

1977 chevy trucks

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Hi guys, been a while, need to know about wildly varying compression readings on an old 454.

Just bought a 75 chevy c20 with a 454 {farm truck its whole life,re-probably ignored to death}69000 miles on the clock .Today I thought I`d give it plugs wires compression check just see what I had before pulling the motor trans.I`m not a professional wrench by any means,butI`ve been a back yard wrench on these old trucks for about 20 years.Today is the first time I`ve seen this...upon doing the comp test ,I came up with 4 holes that were zero.Dead ass zero. however,igot it started with some fresh gas,a liitle break clean downthe carb,typical for an engine that is old,probably old carb etc.Once it was started,it would idle vey roughly at estimated 500 rpm.sure doesn't sound like 4 dead holes to me.Upon cold compression check on #6 cylinder,`it read90 psi.Ididnt believe that so, Ichecked it again,it read 0. These loqw and freaky readings went on through the whole motor.As I said, pnce started it would idle for 15 minutes at the aforementioned speed.ANY ADVICE WOULD BE GREATLY APPRECIATED,HUGH.I would also like any info on this motor,such as what cam to se etc,It will be going in a 77 GMC swb lowrider with hiway gears,it will be a steet cruiser,but I do like to burn rubber.Ttrans will probably be a th400.buddy jusy gave me aComp Cams 258h{new},can I use it with no other mods?Budget=TEENY.,Thanks in advance,Hugh.
 
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hurst01

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Hi Hugh,

Are you sure that you had the compression gauge seated good in the plug hole? It doesn't make sense that it wold have 90 PSI one time and Zero the next. 500 RPM is a bit slow for idling, should be around 700. Did you do the compression check wet or dry? In case you don't know, wet is with a couple shots of oil in the cylinder. Typically, the test is performed dry all the way through and wet the second time around. If it is low on the dry test and the compression comes up on the wet test, it can tell a lot of the condition of the piston rings. If it is still low on the wet test it indicates a valve leaking compression.
A long time ago I had a compression gauge that is one you hold to the spark plug hole. I could get erroneous readings such as you are describing. I got tired of that and purchased one that has a hose that screws into the spark plug hole. Much more accurate. Another ting that I ran into quite a few years back, I had a SBC 350 that I had purchased. Flipped it upside down to check the bearings and oil pump before I installed it. After I installed it, it fire right up. At the time I was working night work and i decided to drive it to work that night. Ran perfectly. After sitting for a bit over 8 hours I went out to go home in the morning and it would not start for love or money. Had to have it towed home. After getting some seep I checked the compression and it was down to 30-45 PSI in every cylinder. I squirted a shot of oil in every cylinder and it started fine and ran the whole day with no problems. Left it sit overnight again that the same thing happened. The oil was draining away from the rings and it was losing compression. After thinking about it, when I flipped the engine upside down the oil ran from the bottom of the engine to around the pistons and it had enough compression to start the first time. That was the first time I had ever experienced that.
Since I had already had done the bottom end I pulled the engine and did a top end job on it and never had another problem with it.
I love big blocks but I can do way more with a small block for less money, but the big blocks sure look good sitting in the hole.
 

1977 chevy trucks

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Thanks for the reply,havent tried the wet test yet,I`ll do that soon.Alocal mechanic told me that possibly Igot a chunk of crap inside my guage doing one of the first holes,and it was working its way around inside and jamming the Schrader valves open.Sounds possible,this engine is covered with years of crap.Typical farm truck.I`ll let you know how things go,Thanks again,Hugh.
 
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