I have a '77 L82 4 speed with 74K miles. It has always had a rough idle but runs great after 1500 rpms. I have changed the AC Delco plugs twice after a couple of thousand miles each time and had mixed results, Black heavy deposits on some but not all. I recently installed a 1/2" non metallic riser plate between the AVS-2 Edelbrock carb and performer intake because it would stumble from a take off after a long spirited drive. Well it solved the problem. Today I pulled all of the plugs and they were all very clean and showed no signs of burned areas. Next thing was the compression test. the left side was 150 PSI on all 4 cylinders except for one 155. unfortunately the right side was a disaster , #2 135 #4 150 #6 125 #8 130. I ran these positions several times and got the same numbers. As bad as it looks this motor has plenty of power all the way past 90MPH and then some. I know the heads on these old vettes are not the best so rebuilding them is out of the question. If I replace with after market heads I am just opening up Pandora's Box. I have had the intake off a couple of times because the previous owner did a poor job sealing the gaskets. I have rebuilt a couple of small blocks in my early years and its a wonder that they actually ran pretty good not having all the info that is available today, Should I do a leak down test or just drive it for a while and hope something doesn't break.

 

I'd pull the valve covers and check the valve adjustment. Compare both sides and if the cam or valves are out of spec it should be pretty obvious. Loose push rods would be easy to spot.

 

x2
also, run test on pass side and do it both Wet and dry. If a couple cc of oil raises it dramatically, it's likely a ring issue.

 

Fairly common to see rust pits in the bores. with that low of mileage on the car it obviously sat for a period of time. Probably more than once. Moisture gets into the cylinders and light rust forms. Once the engine is run again the rust gets cleaned off quickly, but the pitting in the bores remain. This also affects the rings themselves. And this could cause this slight drop in compression.
If this is the case. It'll run like that for years. Or you can pull it, bore it and rebuild it. The L82 is a good block with a good crank and rods. Well worth freshening up.
And of course, replace those heads. even 150 isn't exactly high compression. And those heads are prone to cracking.

 

Just as important are compression numbers, is the location relative to other cylinders.
But first. On average, you are allowed about 10% difference between cylinder comparisons before anything of major repair is needed.

For example: cyl #4 is at 150 lbs. Ten percent of 150 is 15. Therefore 150 minus 15 is 135 cyl #2. Bingo. That's acceptable.

Now let's look at some locations of cyl relative to the block. Note that cyl #6 and #8 are not only below acceptable levels but are also side-by-side.
Coincidence? Maybe.
Or is it a head gasket between the two?

So, having noted that, are there any extra bubbles in the radiator? A failure of a head gasket at that location usually introduces exhaust gases / air into the coolant.
Seldom does that location introduce coolant into the oil pan

There is a special tool / tester that fits into the rad neck and "sniffs out" those gases that might show up.

 

check everything and run it. If it isnt burning oil its fine. If its got rusty bores you may have a sticking ring or two. They may break free eventually.

 

I am going to check the valve lash first and if all checks out I will keep a look out on oil usage. There is some rear main drips but no evidence of burning excessive oil.
Maybe next year a new set of heads. Eventually build a nice 355 motor with 350 HP.

Thanks

 

Put a squirt of oil in the cylinders that are low for compression test. If they come up you know it's the rings.

 

x2 per p#3

 

Yes check the valve lash and if it isn’t smoking or consuming a ton of oil, I’d just drive it regularly, keep watching the plugs and tweak the carb a bit more if it’s rich at off idle or stumbling on throttle tip in. See what develops and keep us in the loop. Nothing you’ve described indicates that anything severe is afoot. Enjoy it and keep notes of your tweaks and performance/drivability observations for a while. It’s a great learning experience.