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9/21/15 4:31pm - Original Message: 'Opinions please'
DarinM
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Caddo Mills, TX - USA

Vette(s):
1980 Black on Black 4speed 383 Stroker 17x9 rear, 17x8 front with 255-50R-17 Nitto tires OBX Stainless Steel sidepipes DeWitts radiator with dual electric fans

Joined: 9/15/2015
Posts: 38
Hi all, i recently bought a 1980 corvette which i had been searching for a while. My goal was to buy a black on black 4 speed car which was the same car i took my wife to our high school prom way back in 1983. I found one that was in fantastic condition cosmetically, but need some feedback on the motor condition.
it's an L-48 350 that is bone stock with exception of stainless steel headers and side pipes, Weiand street avenger aluminum intake, Holley 600cfm double pumper with mechanical secondaries and a DUI distributor (other dress up goodies, but they're just for looks). 

This past weekend, i did a compression check and leakdown test. I scoured the forums to ensure i was doing it correctly since i hadnt done these types of things since my days back in high school.
What i ended up with was:
Compression test - 5 cylinders with 125psi, 2 with 120psi and 1 at 135 psi.
Leak down test ranged between 12-20%. All test done dry, no wet test.

What drove me to this was a slight smoking from the exhaust followed by when I did an oil change and dumped just under 4 quarts of oil and refilled with 5, that made me think i was either leaking or burning. I've also attached pictures of the plugs i pulled while doing this. I replaced them with new ones since they were out.
What i'm thinking is since the compression is on the low side and it's the original motor with 105,000 miles on it, that i'm not far off before a new one will be needed. 
I would appreciate feedback. Let me know if i'm off base or what.




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9/21/15 9:42pm - Reply: 'Re: Opinions please'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
Smoking from the exhaust when?All the time? Only on acceleration? Only on deceleration?

#7 looks to have some oil fouling/carbon build-up. #s 3-5-2-6 look fairly reasonable for some mileage. #s 4 & 8 are really lean.

Compression readings don't look all that bad to me. What is the mileage on the car? When checking compression, did you have all of the plugs out, and the throttle held wide open?
The later engines like yours dint have a lot of compression to begin with, compared to the engines of the early '70s.
I'd venture a guess and say you have more of a carb/ignition issue that a worn engine problem. Also possible the intake is leaking, causing the lean condition on the two cylinders, and oil fouling on others. 600 double pumper is really too much carb for a smog engine, imho.
 Need a little more diagnostics to determine just what is going on. Put the new plugs in and see what happens. Big smile

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Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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9/21/15 10:05pm - Reply: 'Re: Opinions please'
F4Gary Gold Member
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Grapevine, TX - USA

Vette(s):
1972 LT-1 convertible with factory air.

Joined: 8/26/2006
Posts: 869
Don't know where you live, but if you can get some RXP, run a couple of tanks treated with it to see if you can burn off some of that carbon.

Don't Holley's run kinda rich?  I know mine does, but it has bigger jets for the ethanol in the gas.  Has to run richer.

http://www.rxp.com/


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9/22/15 7:37am - Reply: 'Re: Opinions please'
DarinM
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Caddo Mills, TX - USA

Vette(s):
1980 Black on Black 4speed 383 Stroker 17x9 rear, 17x8 front with 255-50R-17 Nitto tires OBX Stainless Steel sidepipes DeWitts radiator with dual electric fans

Joined: 9/15/2015
Posts: 38
Thanks Joel, the smoking seems to be mostly on startup. After it's warmed up i dont really see any in the exhaust. As for miles, it's currently at 105, 000. and for the compression that's what i wasnt sure about being a 1980 block and heads which as i'm learning are low compression. I'm also reading that a cam/head swap will do wonders for the L48. I did install a fresh set of Accel shorty plugs while i had them out so i'll run it for a bit and see what they look like as well as the oil level.

I greatly appreciate any and all feedback !


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9/22/15 9:18am - Reply: 'Re: Opinions please'
DarinM
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Caddo Mills, TX - USA

Vette(s):
1980 Black on Black 4speed 383 Stroker 17x9 rear, 17x8 front with 255-50R-17 Nitto tires OBX Stainless Steel sidepipes DeWitts radiator with dual electric fans

Joined: 9/15/2015
Posts: 38
F4Gary, i'm on the opposite side of the Metroplex from you. Northeast area just into Hunt county. I'll stop in at the local Oreilly and get some.

|UPDATED|9/22/2015 7:18:29 AM (AZT)|/UPDATED|


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9/22/15 12:19pm - Reply: 'Re: Opinions please'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
Smoke mostly at startup is usually valve seals. Oil runs down the valve stem when parked, into the intake/exhaust ports, then into the engine on startup, causing the smokey start. Valve guide wear will do it, too, but installing new guide seals is much easier than pulling the heads for guide repair.

The only real way to wake up a low compression engine is to raise the compression ratio. Changing to a bigger cam will hurt more than help, because there's just not enough compression to take advantage of the cam. Different heads can raise the compression ratio some, but not much. Flat top pistons, or even domed pistons will do the most good, compression-wise. You really need around a 9.5:1-10:1 ratio to make one really shine. Changing the heads alone won't do that....but it would help. Thumbs Up

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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)
             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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9/22/15 1:01pm - Reply: 'Re: Opinions please'
DarinM
Standard Member
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Caddo Mills, TX - USA

Vette(s):
1980 Black on Black 4speed 383 Stroker 17x9 rear, 17x8 front with 255-50R-17 Nitto tires OBX Stainless Steel sidepipes DeWitts radiator with dual electric fans

Joined: 9/15/2015
Posts: 38
Thanks Joel, great info !

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9/22/15 5:11pm - Reply: 'Re: Opinions please'
dwa175 Gold Member
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oakville, CT - USA

Vette(s):
1979 coupe and now 2007 c-6

Joined: 7/19/2014
Posts: 183
yes  replace valve seals.  my 79 lets out a litte blue when started  has 92000 miles like a lot of old timers I like to see a litte smoke just seems right but if my 07 does ,now we take a look for a problem, ilke litte smoke makes engine be alive not some new super car,just me everthing I have is older than 20 years.     injoy it   you may never see again. dwaBeer

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7/2/16 8:31am - Reply: 'Re: Opinions please'
77Driver
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Lake Mary, FL - USA

Vette(s):
77 Lemans Blue L48 stock all the way around except paint color.

Joined: 3/29/2016
Posts: 8
I agree with DWA. I had similar fouling on a few of my plugs. I haven't replaced the valve seals yet but will this fall. I did a compression check on my 77 L48 and my readings were in the 135 to 150 range. I believe it is stock but I know that it has been rebuild by someone before me.
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