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Old 07-27-2003, 10:41 PM
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Angry low compression!! no good???

well after having a motor i baught a while back i decided to get a compression tester just to see how the internal condition is roughly.. well being told the motor is rebuilt i expected even pressure in all cylinders.. turns out i was wrong.. the passenger side of the motor was all around 120-130 psi.. except for one plug i couldn't get at so didn't bother.. but then i go to the drivers side and i get 1 reading of 120 then 90... then around 85-90 again.. think the last plug was 100.. so whats with that?? i put some oil in one of the clyinders to see if it was the rings that can be worn out..but its olny increased compression a couple PSI.. so im thinking it might not be the rings.. but im not sure at all.. does anyone know what the problem here?? i plan on porting and polishing the heads next year.. and rebuilding them.. so em i ok to drive around a reay with the motor like this? or will it just get worse and wear things out faster?? maybe my vavles are leaking?? if anyone know's how i can test more to pin point what the exact problem is plz let me no, bye the way its a 350 small block chev.. around 1969.. if that makes a diff? thanx alot.

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Old 07-27-2003, 10:51 PM
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Well if ya squirted oil in and the pressure didn't rise its not rings.It could be a bad valve or evan an over tightened valve or headgasket etc...etc..
But befor anything were you using one of those hold it over the plug hole testers??If so they have to be exactly perfectly centered in the hole to work right.I would try a screwin type befor you go tearin in or evan a leak down tester.That is unless yur sure its a tired motor then start fixin.
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Old 07-27-2003, 11:26 PM
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actually i did buy the top of the line gauge.. screws in and its a leak down tester.. holds the pressure in the gauge? isnt that leak down? cuz i don't think they were leaking down either.. i ddin't no how long to wait.. i gave it about 5 mintues.. so i might be safe from tearing out pistons?? i hope so.. i don't mind taking the heads off cuz gona some day.. but i don't wana do pistons and rods and crank..etc.. thanx for the reply
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Old 07-28-2003, 02:12 PM
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A leakdown tester squirts air into the cylinder forcing it down, then keeps it under pressure. There is a gauge that tells you what percent of the air going in, is going out and gives you a percentage of air leaking past the rings. I could be wrong, but normal should be about 10%?
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Old 07-28-2003, 03:24 PM
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If the motor has set for a while and not run the poor compression readings could be just a symptom of air corrosion causing the valves,etc. not to have a good seal. Most compression tests are done after warming the engine up and with the throttle opened all the way. If you could run it and recheck, the readings would most likely be better.
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Old 07-28-2003, 03:54 PM
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you have a very good point woodz428.. the motor was warm.. but not exaclty operatiing temp.. i read a book on small blocks last nite.. says the carb should be fuly closed?? r u sure its suppose to be open? and motor has to be operating range like u said. ok i'll give her another shot when its warmed up. thanx for the help.. i'll keep ya posted
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Old 07-28-2003, 06:09 PM
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You said you couldn't get one of the plugs to come out. I hope you're pulling all the plugs when doing the test and throttle wide open (not just one plug at a time). The readings getting progressively lower sure make it sound like this condition.

Oh yeah sounds like you have a good battery and starting system though.
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Old 07-28-2003, 06:37 PM
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I have always done it with the throttle all the way open. If you don't have any air to compress how are you going to get any pressure, at least that's my philosophy and it's the way I was taught in the '60's.
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Old 07-28-2003, 09:54 PM
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ya stupid me i did it one plug at a time.... so im gona try it again proobly tomorrow.. so my list will be
1. all plugs out
2. carb wide open
3. engine at operating temp.

thats it right? or did i miss somethin... just checkin wit u guys first.. i like to get it right the second time around
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Old 07-29-2003, 10:08 PM
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NEW TEST!!!

ok i did another test today.. just like u guys said.. plugs out and carb open etc... heres the new readings..
cylinder
1. 140
2. 150
3. 120
4. 150
5. 100
6. 145
7. 140
8. 145

if i were looking at the motor from the front the whole passenger side was the most consisten from 145-150.. its the drivers side thats all over the place. let meno what u guys think about these readings?? i read that its should be ok if the lowest cylinder is at least 75% of the highest... does that sound right? thanx for any replys.
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Old 07-29-2003, 11:08 PM
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All cylinders should be within 10% of each other. I would go off the highest reading, 150. That means they should be within 15 psi. Obviously they aren't. Try loosening all your rocker arms and rechecking them. Maybe a valve or two is hanging open slighty. Not very much chance of that being the case, but your only alternative other than that, or what everyone else has said would be a rebuild of the heads and bottom end.
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Old 07-30-2003, 12:09 AM
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o man.. ya i baught this motor for alot to.. lookes real clean on the outside.. and seems to run good.. the the #'s don't look good.. i did a vacum test also.. but everythings ok there.. so maybe that does mean bottom end is hooped.. k now this sux!!
would a leak down test be the awser to tell me for sure its the rings and pistons??
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Old 07-30-2003, 10:15 AM
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140 to 150 is fine. I would be most concerned with #3 and #5. They are adjacent to one another as well as low. Possible head gasket failure. Try loosening the rockers 1/2 turn first, then retest the cylinders. Try a leak down test to see where the leak is occurring. If you don't know how to do that, then do a search on hotrodders.
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Old 07-30-2003, 02:19 PM
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Quote:
Originally posted by Trav
o man.. ya i baught this motor for alot to.. lookes real clean on the outside.. and seems to run good.. the the #'s don't look good.. i did a vacum test also.. but everythings ok there.. so maybe that does mean bottom end is hooped.. k now this sux!!
would a leak down test be the awser to tell me for sure its the rings and pistons??
I wouldn't go that far. If you have checked the compression with a regular compression tester, and all the rocker arms loose enough so that they don't lift, and it the readings still aren't even, then I would track the guy down who sold it to you and get your money back.
I had someone sell me a dead engine like that once and I went back to them and asked for my money back. They gave it back right away with no questions asked. Maybe you'll be just as lucky. Just don't go getting into any brawls over it.
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Old 07-30-2003, 07:38 PM
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ya i'll have to try loosining the rockers and try again.. its kinda hard for me to gie the motor back now tho.. i already put carb, intake, gear drive.. manifold on it.. the guy i baught it off of is a friend of mine.. works at lordco.. witch is a big parts store.. so he gives me the best deals on everything as it is.. but ya im gona definatly bring this up with him!! maybe get at least some of my money back and keep the motor... this sux..
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