C2Dude

Working on assessing the health of the L79 in my 66. A check of the plugs show them to be black, several wet with gas. No clumpy deposits. On a cold start the drivers side bank smokes a bit. For a time the exhaust exiting the drivers side is warm where the exhaust on the passengers side is cold. As she warms up the smoke on the drivers side resolves but the passengers side begins to smoke a bit. There is no crossover pipe in the exhaust system. Hard to give you a color although I would not call it black. Engine runs smooth, idles well. Vacuum indicates 17.5" steady. Goose the throttle and it jumps to 0 in one motion and returns to 17.5 in one motion. I believe the vacuum indications are normal. Tomorrow I will do a cranking compression test. The book essentially says to rig the carb to WOT with the choke butterfly open, install the tester, crank to highest reading, and to do the remaining cylinders cranking for the same amount of time. It makes no mention as to the engine being cold or at operating temp. Checking bleed down compression on an aircraft you always use operating temperature. So my questions regarding cranking compression are:
Perform the test cold or at operating temp?
Is there a target acceptable psi range I am looking for?
What% psi variance, high to low cylinder, places me in worry mode? (I have heard 5%)
Thanks in advance all.

SDVette

I've always done it cold.

The numbers you get will vary widely depending on you CR, cam (overlap), etc..
What's important is to find any outliers.....

All being within 5-10% is generally what you would like to see.

edit: If you force WOT (I have never done this), be sure not to let a bunch of fuel get into the intake while you perform the tests.

GTOguy

I've always done it warm, as most manuals specify this. It's not that critical. What is critical is that the compression is roughly 5-10%, no more than 25 psi difference between cylinders. Throttle doesn't have to be wide open, half to 3/4 is fine also. Remove ALL the plugs when doing this test, and disable the ignition. Keep a fire extinguisher at hand. You may want to perform a cylinder leakdown test as well, which can tell you a LOT about an engine's shape. Example: the 383 in my '61 has 195-200psi in every cylinder. Runs strong. But it also blows smoke out the breather tube at WOT and uses some oil. Cylinder Leakdown testing showed that although my compression was great, my leakdown was 30% in every cylinder. Time for a freshen-up.

Powershift

I have done it both cold and warm...........but never hot. Start with cold.

I have typically recorded psi at 6 cranks/revolutions and then around 10 cranks. I generally see the psi in each cylinder reach it highest value at 6 revs.........but I also go a few more just to be sure. Generally only 2-3 revs should bring the pressure up to very close to the final value.

Carb wired open with wire or ty-wrap, ignition voltage disabled/disconnected or coil to dist. wire pulled, and all spark plugs out.

Chevrolet Service Manual says 150 psig for the L-79 engine, with max variation between cylinders of 20 psi. I could live with 25 psi (GTO guy)

A leak down test should also be done if you are concerned.

Larry

Pop Chevy

I'm curious, do you still have a heat riser in the manifold?

C2Dude

Yes the spring loaded mechanism on the passenger side exhaust manifold is there and actuates freely (I confess to not knowing what it does yet). All good info, Wednesday I will pull all plugs, ensure the ignition is disabled, wire the throttle open a bit (why does that matter?) and attempt to keep the cranking time consistent to reach the highest psi reading for all cylinders. 5-10% differential acceptable. Following this I will rent a leak down tester from auto parts joint and perform that test. I will report back, thanks all.

DansYellow66

If you have a battery charge I would put it on to keep the battery charged up. I would run it warm - doesn't have to be hot. Cold is probably OK but if the engine has forged pistons and large piston clearances it can make some difference. The psi you achieve can vary from day to day, test to test for reasons I don't completely understand. Don't get too hung up on the psi, just pay attention to the variance between cylinders. And yes, keep the throttle wired back and don't cycle it between tests which can put gas down the intake.

Duck916

The heat riser mechanism is there to send hot exhaust into a crossover through the intake manifold under the carb. It's to help with engine warm-up. The smoke symptoms you see are most likely just a bit of moisture in the exhausts turning to steam, first on the driver's side when it's getting all of the exhaust stream (because the passenger side is blocked by the riser valve), then on the passenger side after the riser valve opens and allows exhaust gases to travel down that pipe. It's perfectly normal (and my car does it if it has been sitting for a while).

The reason for wiring open the throttle is twofold. First, you want a consistent throttle opening for each test. If the throttle opening varies, it will make the cylinder pressure readings inconsistent, reducing the value of the test. Second, even if you had a precisely calibrated foot that could open the throttle the same amount during each test, doing so would push gas through the accelerator pump each time, causing problems associated with all that gas running down the manifold and cylinder walls.

65tripleblack

An engine can be "using oil" with strong compression numbers if there is a problem with one or more of the oil scrapers, or if it waas built using low tension oil scraper rings. Light oil smoke on startup usually points to dry/cracked valve seals.

Another reason for WOT during testing is to ensure that there is no restriction to airflow into the cylinders being checked.

The condition of the battery (cranking speed) has an effect on pressure readings.

The heat riser is not needed unless you drive your car in cold weather. They are usually problematic and generally become carboned/rusted shut in time. Most of us have long ago either eliminated them or wired them wide open. A side benefit to doing this is better longevity to the passenger side muffler. The heat riser prevents exhaust gases from exiting the right side until the engine warms up, keeping more condensation trapped in the passenger side pipe/muffler.

SWCDuke

Compression/leak down tests should normally be done hot. So with the engine fully warmed up, shut it down, pull all the plugs as quickly as possible and begin the test. Crank each cylinder until the gage achieves maximum reading - usually 4 to 6 revolutions.

You didn't specify idle speed that you measured vacuum, so your reading doesn't have full context. Idle vacuum must always be accompanied by the idle speed it was measured at to have any meaning.

Normal L-79 idle behavior, in neutral, is 14-15" @ 750, so I doubt if it has a real L-79 cam or anything close.

Do you have a service manual? Cranking compression nominal values are listed for all engines and maximum acceptable variation is 20 psi, or the engine is not healthy.

Compression gages can give very different readings than what is specified in service manuals, even if all components are "stock", so the more important parameter is variation between cylinders. If the readings are well within 20 psi, regardless of the absolute readings then the engine is reasonably healthy, but that doesn't mean it won't consume oil. I had a car that had "perfect" compression, but consumed a quart of oil every 300 miles or less. It turned out the valve seals were disintegrating and valve guides clearances were WAAAAY over the limit, so if was off with the head for new guides and seals. Since I don't drive this car very much between oil changes I now can't even measure oil consumption.

Duke

Powershift

An excellent summary.........

Larry

MikeM

What are you trying to fix?

What heat range spark plugs are you running?

A steady vacuum reading, idles well and runs smooth are good signs but your vacuum reading is a little too high for L 79.

C2Dude

Well that explains the exhaust temp and pulse strength variation question, thanks. I am new to the hobby, just fiddled around as a much younger guy. Never saw or heard of that kind of exhaust riser arrangement, so that issues is explained. The Holley is also leaking fuel from most of her gaskets, likely due to long term exposure to Ethanol based fuels. I am picking up a rebuild kit today from an auto parts joint so I plan to just pull the carb/seal off the fuel line for the compression test today. Picked up a leak down tester from HF yesterday, plan to modify it with more accurate gauges I have lying around. Should have those numbers tomorrow. Anyone have a trick for determining TDC? I am thinking of placing a strand of 14 gauge insulated romex wire in the spark plug hole and hand cranking the motor at the crank pulley but I am assuming there is a better way. I'll post the results as I obtain them.

Frankie the Fink

That's the way I've always done it too...

C2Dude

Just responding to a couple of responses. I believe she was idling at about 900 rpm at the time of the vacuum test and that was during the first few minutes of start up. I hope I am not fixing anything but I am trying to determine if what I am seeing, black plugs that smell of fuel and some smoke emitted from the passenger side exhaust port once warm, is a sign of trouble I need to consider addressing. I have read that unhardened valve seals and unleaded fuel are incomparable for longevity (is there longevity in a 52 year old ride?) and that O rings used on the valves can dry and crack over time. Pulling the heads, if it came to that, would still be preferable to ring issues but no need to get ahead of myself, just in diagnostic mode at present. Seems the opinion on engine temp for compression runs the gamut from cold to just under hot so I'll shoot for warm. Sounds like the important issue is variance and I'll post what I come up with.
One other thing. I did just find cranking compression values in my shop manual, I have 3 reference books, still getting comfortable with whats written where in each. The plugs I pulled were AC Delco R45S plugs. I did pick up a set of NGK plugs with a heat rating of 6, the cooler end of the spectrum. I had no opinion on that, it was just what came up in the parts store computer.

C2Dude

I did just find the cranking compression figures in my shop manual, I have 3 reference books and am still learning what info is where. The spark plugs that came out of her were AC Delco R45S. I did pick up a new set of NGK's yesterday, that have not gone in, with a heat rating of 6 which is on the colder end of their spectrum. I had no preference on that, the plug was just what was listed on the part store computer.

MikeM

Metering block and float bowl gaskets shrink when the dry out causing the leaks like you describe. Ethanol has nothing to do with this. The original gaskets I believe were brown, rubberized cork and they all shrink. There are now blue gaskets and black gaskets that don't shrink. Make sure your kit has this type gasket.

If you fix the carburetor, your other perceived problems may go away.

PS. Sometimes Holley carb leaks are caused by a ham fisted owner cranking down on the bowl screws to stop the mentioned shrinking gasket leaks. This can warp the metering blocks and cause them to leak with new gaskets.

Critter1

With today's lead free gasoline, you should be able to use the original/correct AC 44 spark plugs. Using hotter than recommended heat range plugs to prevent fouling is a thing of the past and just a patch for the original problem. If your carburetor is running way too rich, that will foul spark plugs no matter what heat range you have. The plugs should never look black/sooty.

As MikeM mentioned above, check the metering body for warpage and gasket failure.

If original/correct heat range plugs wont last, something is wrong but it's not the plugs.

SWCDuke

Idle vacuum MUST be measured at normal idle speed (which must be specified with the vacuum reading) with the engine fully warmed up.

If you have to remove the heads for rebuild DO NOT INSTALL HARDENED VALVE SEATS.

AC R45S, which is an extended tip AC R45 are okay: "5" is one heat range hotter than OE (4), but are less likely to foul in typical vintage car service. NGK heat range 6 is too cold. Take them back and swap them for B4, which is a non-resistor plug that is equivalent to the AC 45. The B4 is one of the plugs I recommend for vintage small blocks.

You need to buy a service manual set for your car and the factory Assembly Instruction Manual (AIM) They are available from most Corvette parts vendors. Working on a car without the best available service documentation is a fool's errand.

Be aware that both GM and aftermarket parts books are FULL OF ERRORS, so start with what's installed and inquire here is it's okay to use or should be substituted with a different part number.

Duke

65tripleblack

Use a set of NGK XR4 spark plugs, WIRE THE HEAT RISER WIDE OPEN, check your float bowls and metering blocks for warping before re-assembling using BLUE gaskets. A slight amount of smoke on startup is normal for an older engine due to weak valve seals. The smoke should clear after about 30 seconds. Once you wire the heat riser open, the vapor burnoff and/or slight valve seal related oil smoke will emanate equally from both sides and should stop evenly across both sides.