I have a '75 with a 427 in it. I was told it was an L-88, but I can only verify the block heads and flywheel. I was told by the prior owner that it would ping heavily without 100LL, indicating that it was high compression (I am assuming 12:1 as for the L-88). I haven't runt it too much, but it seems to run pretty strong, but I don't necessarily know what 450-500 HP is supposed to feel like.

Since getting it, I adjusted the valves and it ran awefull. I realzed that I had left the distributor cap loose while cranking the engine and cracked the rotor. I assumed that I knocked the timing out of whack, so I set in for a dist rebuild. I got rid of the end play with shims and turned the posts 180 degrees to get rid of grooves. I then added the Mr. Gasket springs and and have 36 all in by around 2500. I also got an adjustable vacuum canister. I have no pining and am running 100 octane.

My challenge is getting the vac adv adjusted right. without vacuum, my idle advance is about 18 degrees. Below 1000 RPM, it idles like crap with about 5-7" of vacuum. If I get the idle up to about 1100, it pulls 8-10" and will bring in the vacuum adv, improving the idle, but it is a touch line to maintain. I think that I can fix this with the Napa vac canister recommended by Lars and others. Also, the L-88 had a recommneded idle of 1000, so I guess the cam just doesn't like low RPM. It is very strong up high.

When I was adjusting the RPM, I heard a terrible knocking type sound coming from the #7 cylinder. It only happened below 1000 and went away above that. I later notice that the exh. gasket for #7 is leaking. Could that cause that type of sound?

Now for the advice. I pulled the plugs and did a compression test on all cylinders but #1 (this one takes a special socket and 1/2 hour to get out). Here are the results (I cranked through four cycles on each cylider.

#3 70/110/130/150
#5 60/90/120/140
#7 60/90/120/140

#2 60/90/120/150
#4 90/120/150/160
#6 90/120/140/160
#8 120/150/150/150

I replaced the plugs about 30 miles before pulling them for this compression test. Here is a picture of the 7 plugs (they are in order and the missing plug is #1):



I would have guessed that 12:1 compression would generate greater than 160 PSI. Could the cam be causing this low of a reading? Also, I am looking for advise on why some cylinders are slow to devlope the compression. Do I need new rings? Could this be from running too rich? What do you think of the plugs? They seem pretty dark, indicating a rich condition. I have a Holley 850 Mech and have adjusted the float and idle circuit to best idle speed, but don't know what jets are in place. there is no bogs or stumbles.

Thanks in advance for your thoughts and ideas.

 

knock is hopefully piston slap.
my 61 350 does that due to forged pistons.
my 72 had half the pistons in backwards!
i'd take it apart and see what bubba has done.
read books on bb; build what u want

 

Sounds like you bought someones elses problems???

 

As far as the mixture goes your plugs don't look fouled. AC plugs are black-oxided so it's easy to get the impression that they're 'running rich' when they're not. Also, the best way to get a true plug reading is to shut off the engine during WOT, which can be a sketchy proposition in most residential environments. If it were me I'd just keep leaning it down until I started noticing a temp increase and then go one step richer. Also, if you really are running 100LL (airplane gas) it would have to be jetted significanly richer than it would to run correctly on pump gas, which has a greater specific gravity-if you re-jet to run correctly on, say, racing gas you'll have to go richer to return to using airplane gas or bad things could happen (overheating, burned valves and/or pistons) As far as your compression reading, cam overlap will retard that somewhat- as long as you're seeing a consistent reading across all eight cylinders I wouldn't be concerned. That knocking sound would have me worried though- you'll want to make sure it's just an exhaust leak (which can sound like 'knocking' to an untrained ear). If you fix the exhaust leak and the noise is still there drop the pan and pull the rod caps- if you see a problem you'll know what you need to do next....

 

I have to drop the header to rebuild the steering box anyways, so I will replace the gasket and see if the sound disappears. I have been running 100 octane gas not 100LL, which the prior owner ran. I will try leaning out a bit and see how it does. Should I change the power valve or just the jets?

I intend to buy the TetraEthyl lead in a can to mix my own 100 octane. I can experiment with lower octane levels and see when pinging appears.

Should I be conncerned that some cylinders take took long to develope their compression?

 

I failed to mention that I believe this engine spent quite a bit of time sitting. The previous owner saide he ran it every couple of months, but I am not sure I believe it.

Could this account for the compression readings? I am still concerned that some come right up to pressure and others take 3-4 cycles to get there. What typically causes this?

 

Did you have the carb at wide open throttle? At cranking RPM's it takes a few revolutions to get the air moving to fill the cylinders, & the carb must be w.o.t. If it is an L88 cam, it has a lot of overlap, which lowers cranking compression. What did you set the valves at? If they are adjusted tighter than the cam specs that will cause more overlap & less cranking compression. The power valve has to be lower than your idle vacuum, 7" to 8" needs a 6.5 power valve etc.

 

By the way, you should check all the casting numbers on that engine & the VIN#'s on the front pad, if it is a real 1969 L88, it is quite rare & valuable.

 

Thanks for the advise. I did not have the carb at WOT while doing the test. I will pull a plug or two and try this again. I adjusted the valves to .014 cold, but then had the rough running (before realizing the timing issue) so I reset them to about .024 hot. I wasn't super careful here, as I was in a hurry to see if the change would fix the rough running. This was still a bit less than what they were at previously. A different poster informed me that .020 hot was the spec for the L-88.

Does it really matter much that some cylinders get to max pressure faster or is that not really important over the actual max. pressure?

The block, flywheel and heads are the correct part numbers, however it is a CE block with no VIN. I don't know how to check any other engine numbers without disassembly. The intake is the one off an LS-6 to accomodate the standard hood. Based upon the history I got, I believe that the internals are original L-88 parts, but cannot confirm. I have been told by others here that the CE blocks don't have much value.

Is it worth tearing it down and honing the cylinders this winter? I also would like to verify the cam is in good shape. Can I do this with a dial indicator without a tear down?

 

How do I determine which Power Valve I have?

 

The rating is stamped into the face of the PV. If it's a 6.5, there will be a number "6", and a little bit clockwise from the "6" there will be a "5". Sometimes the numbers are not stamped well, and are hard to interpret.