Sick L-79, Need Help!
04-29-2005, 06:50 PM
#1
Drifting
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Sick L-79, Need Help!
I've posted before regarding my 67 327/350 being under powered. Today I did a compression test.
I ran the engine for about 5 minutes then pulled the plugs. The plugs have less than 1000 miles on them. All had blackened electrode ceramics, and the plugs were wet with what appears to be oil on them.
I held the carb open while doing the test. The compression test revealed six cylinders with a compression of 150. Cylinder #6 was 140 and cylinder #8 was 110. When I tuned the car up last spring, the plug for #8 was fowled.
Any thoughts or suggestions would be greatly appreciated.
Joe
I ran the engine for about 5 minutes then pulled the plugs. The plugs have less than 1000 miles on them. All had blackened electrode ceramics, and the plugs were wet with what appears to be oil on them.
I held the carb open while doing the test. The compression test revealed six cylinders with a compression of 150. Cylinder #6 was 140 and cylinder #8 was 110. When I tuned the car up last spring, the plug for #8 was fowled.
Any thoughts or suggestions would be greatly appreciated.
Joe
Last edited by Joemac8; 04-29-2005 at 06:55 PM.
04-29-2005, 07:15 PM
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Joe,
It would appear your engine needs rings or valve and guide work. Low compression is piston rings or leaky valves. The oil fouling says it is probably rings, but could be valve guides. Recommend a leak down test (that will tell you if it is rings or valves).
Bring it on down and we can do the test in a couple of hours.
Plasticman
It would appear your engine needs rings or valve and guide work. Low compression is piston rings or leaky valves. The oil fouling says it is probably rings, but could be valve guides. Recommend a leak down test (that will tell you if it is rings or valves).
Bring it on down and we can do the test in a couple of hours.
Plasticman
04-29-2005, 07:22 PM
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St. Jude Donor '13-'14-'15-'16
Originally Posted by Joemac8
I've posted before regarding my 67 327/350 being under powered. Today I did a compression test.
I ran the engine for about 5 minutes then pulled the plugs. The plugs have less than 1000 miles on them. All had blackened electrode ceramics, and the plugs were wet with what appears to be oil on them.
I held the carb open while doing the test. The compression test revealed six cylinders with a compression of 150. Cylinder #6 was 140 and cylinder #8 was 110. When I tuned the car up last spring, the plug for #8 was fowled.
Any thoughts or suggestions would be greatly appreciated.
Joe
I ran the engine for about 5 minutes then pulled the plugs. The plugs have less than 1000 miles on them. All had blackened electrode ceramics, and the plugs were wet with what appears to be oil on them.
I held the carb open while doing the test. The compression test revealed six cylinders with a compression of 150. Cylinder #6 was 140 and cylinder #8 was 110. When I tuned the car up last spring, the plug for #8 was fowled.
Any thoughts or suggestions would be greatly appreciated.
Joe
Don
04-30-2005, 01:34 AM
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Joe,
Your compression test and fouled plug(s) indicate cylinder seal issues (piston rings / valves). If they are not in good condition, then cam also is probably "gone", and time for a total rebuild.
Note that one of the main failings of the SBC engine is the cast iron valve guides and marginal valve stem seals. When the stems / guides wear (and they can be pretty well gone under 100K miles depending on oil maintenance), it allows a lot of oil to flow down the valve stems. Upon start up, if you will see a cloud of blue smoke out the exhaust, then the cloud disapates, this is oil flowing down the stems overnight. This oil down the stems also causes a coked up mass of carbon on the backside of the intake valves which can severly inhibit intake air/fuel flow.
I originally thought your "lack of power" was cam related (either worn lobes or the cam timing off). But suggested the leakdown test to verify that the cylinder seal was OK. It appears that the basic cylinder seal is partially at fault. I still don't think it to be the whole answer to your lack of power (I have seen some pretty worn and loose engines perform a lot better than yours!). So it seems that it is probably time for a rebuild and go through the whole engine piece by piece (but do the leakdown test first, as a 2nd opinion). It "could be" just a head job that is needed (in other words, previous owner may of had a quicky rebuild done with new rings, but old "original condition" heads. He may of installed the valves minus the seals, etc. Bubba does some strange things! Only way to tell is tear the parts apart and check everything (after doing the up front testing to determine how far to go).
As far as what cam to use, I would stick with the original L79 cam. It is a great performing cam. It is GM p/n 3863151. Crane (among others) makes dupilcates of this cam. Cranes p/n is the same as GM's , but calls it the "Blueprinted 3863151". Don't forget to replace the lifters at the same time.
Good luck Joe!
Plasticman
Your compression test and fouled plug(s) indicate cylinder seal issues (piston rings / valves). If they are not in good condition, then cam also is probably "gone", and time for a total rebuild.
Note that one of the main failings of the SBC engine is the cast iron valve guides and marginal valve stem seals. When the stems / guides wear (and they can be pretty well gone under 100K miles depending on oil maintenance), it allows a lot of oil to flow down the valve stems. Upon start up, if you will see a cloud of blue smoke out the exhaust, then the cloud disapates, this is oil flowing down the stems overnight. This oil down the stems also causes a coked up mass of carbon on the backside of the intake valves which can severly inhibit intake air/fuel flow.
I originally thought your "lack of power" was cam related (either worn lobes or the cam timing off). But suggested the leakdown test to verify that the cylinder seal was OK. It appears that the basic cylinder seal is partially at fault. I still don't think it to be the whole answer to your lack of power (I have seen some pretty worn and loose engines perform a lot better than yours!). So it seems that it is probably time for a rebuild and go through the whole engine piece by piece (but do the leakdown test first, as a 2nd opinion). It "could be" just a head job that is needed (in other words, previous owner may of had a quicky rebuild done with new rings, but old "original condition" heads. He may of installed the valves minus the seals, etc. Bubba does some strange things! Only way to tell is tear the parts apart and check everything (after doing the up front testing to determine how far to go).
As far as what cam to use, I would stick with the original L79 cam. It is a great performing cam. It is GM p/n 3863151. Crane (among others) makes dupilcates of this cam. Cranes p/n is the same as GM's , but calls it the "Blueprinted 3863151". Don't forget to replace the lifters at the same time.
Good luck Joe!
Plasticman
Last edited by Plasticman; 04-30-2005 at 01:44 AM.