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attn:350 chevy tuning gurus!!

mostwanted

mostwanted

1/2 ton status
Joined
Feb 5, 2006
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Location
Kansas City, MO
I am trying to get to the bottom of some of my 350s gremlins. I bought the truck with a rebuilt motor last winter and I have been trying to get it tuned up. I have no idea what is inside as far as the cam specs or wheather or not it has flat top pistons or what the compression ratio is. I borrowed a comppresion test kit today and tested each cylinder. I hope I done this correctly cause it took me overr an hour and one of my plugs came out of my new MSD helicores I bought last week:mad:. I already called MSD, and all I have to do is fax em a reciept and they will send out a new plug wire for #7. :bow:

Any how, heres how I tested:
1st I removed spark plug and wire and threaded the 14mm adapter into the head. Next I cranked the motor for about 5 seconds, which I figure is about 6 revolutions. I then noted the compression reading and watched it fall, noteing how long it took to fall to where. Finally, Iam posting my readings on here. Hopefully someone can tell me what is going on inside my motor.

cyl. # spark plug condition; max compr.; compres loss.
1 good; 155; 120 after 5min
2 excessive carbon; 155;120 after 2min
3 excessive carbon; 155; 120 after 3min
4 good; 110; 80 after 3min
5 good ; 155; 120 after 5min
6 good; 155; 120 after 1min
7 good; 150; 120 after 4min
8 good; 150; 120 after 2min

broken_msd.jpeg
 
#4

#4 seems to have somthing going on . do you hear air leaking in that cycl , like asmall rush of air ? if so the rings or piston is the problem .
 
BROKEN-ONE said:
#4 seems to have somthing going on . do you hear air leaking in that cycl , like asmall rush of air ? if so the rings or piston is the problem .
Click to expand...

nope can't hear anything. How should I listen for this?
 
BROKEN-ONE said:
#4 seems to have somthing going on . do you hear air leaking in that cycl , like asmall rush of air ? if so the rings or piston is the problem .
Click to expand...
you cant determine that with just the sound of air rushing out. could be rushing out the valves, through a head gasket, or through the rings. hell, for that matter, it could be the adapter in the spark plug hole.
 
leak down tester

there is a leak down test you can buy or maybe one of the local shops have one they can use to see where the air is escaping from if it is going pasted rings or piston you can hear it down low towards the oil pan . if it is up top like a vavle or vavle seal or guide the sound will be from the top .
 
I'm guessing that you have done the dry test. I'd recommend doing the wet test to narrow down the problem if you only have the compression tester. If you have a leak-down tester it will tell you much more info.
 
My question is are you using a compression tester or a leak-down tester. The reason I ask is that all my compression testers have a check valve in-line to let pressure in but not out. If your is set up this way and is leaking down then it's not the engine loosing the pressure it's your gauge. Also did you retest #4? If you are already using a leak-down tester then ignore premention items...


Oh yea, what is the problem you are looking for?
 
shoot a little oil in the #4 cylinder. if it does the same thing it is probably in the valve seals. if it goes higher or doesnt leak down the problem is in the rings or piston. old school method, but usually works.
 
there is definatly something wrong with cylinder 4, either piston rings or valve seals. youve got damn near a 30% pressure loss on that cylender, which means i would have to shake the hand of the guy who got that engine to run right, and then buy him a beer. cuz despite damn near 15 years experience working on sbc engines with a cylinder that dead, i dont think i could tune it up properly.

if its valves, then you can fix it pretty easy with a valve job, if you pull the heads yourself it might cost around 300 bucks.

if its the piston rings, its gonna be a more serious problem and will most likely cause oil consumption problems, pcv system problems like blowing oil out of your oil breather caps and such. and will require a more involved engine tear down procedure. best would be tear down the intake and heads and oil pan, then disconnect the piston in question and push it up CAREFULLY through the bore. so as not to gouge the walls, then redo the pistons and push them back together.

but again, more specific information would be handy in diagnosing the problem.
 
I am useing a MAC CT100 compession testor. It does have a bleed on the side, that I press before removing it from the cyl.
 
JEBSR said:
My question is are you using a compression tester or a leak-down tester. The reason I ask is that all my compression testers have a check valve in-line to let pressure in but not out. If your is set up this way and is leaking down then it's not the engine loosing the pressure it's your gauge. Also did you retest #4? If you are already using a leak-down tester then ignore premention items...


Oh yea, what is the problem you are looking for?
Click to expand...

mostly I am looking to diagnose what seems to me like maybe a miss and a few of my plugs are getting heavy carbon buidup.
A buddy of mine has a fresh set of head laying around that he would let me borrow, but I want to be sure it's not piston rings.
 
jp0863 said:
shoot a little oil in the #4 cylinder. if it does the same thing it is probably in the valve seals.
Click to expand...

negatory, valve seals have no bearing on compression. valve seats on the other hand, have alot to do with it.
 
nice catch on the valve seals vs valve seats. simple matter of semantics, im sure we were all talking about the same thing, but could have been a costly misnomer.
 
mostwanted said:
I am useing a MAC CT100 compession testor. It does have a bleed on the side, that I press before removing it from the cyl.
Click to expand...

If that's the case, something is leaking in your tester. It should hold the pressure until you release it. They usually come with a few extra valves. They look like the valves inside a tire valve stem. I'd fix it, then do a dry/wet test.
 
exactly.... Yes you probably still have a problem with #4 but it should not be leaking down.
 
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