Interesting compression test numbers

[JL8Jeff]

I ran a compression test on the 396 tonight thinking I would find a burnt or dropped valve. But the results were interesting. The passenger side readings were 184, 184, 186, 185. All nice and close together. The driver's side readings were 200, 196, 199, 195. A little more spread but not bad at all. It's the difference of 10-15 psi between each side that makes me wonder. I'm guessing one side either has a different head gasket or the head was shaved more resulting in higher compression. So now I'm still trying to guess at what has caused my slight miss. The engine shakes a little when you rev it and it starts to make the miss sound. I don't think it's an exhaust leak and all the spark plugs look very similar in color. More investigation is needed at this point.

[Ian]

is that thing still running a points dizzy?

[Rich189]

*cough* 572 bbc crate engine from gm performance*cough* u know u want it jeff :} then just keep all the original stuff packed away for when u sell it

[johnjzjz]

A little known fact --- at the compression pressure you have ( around 200 lbs ) not knowing the valve spring pressure but my guess is 120 lbs on the seat 275 / 300 open you could have a slightly waffled valve head caused buy a vacume leak or not enough timing or too much -- or just carbon build up the gas today is different -- ( you could have a bent or not seating valve and the compression test will not pick it up ) even a leak down might not as well we just finished a 69 350 HP 396 and that was the case with it -- compression pressure and spring pressure can seal off an other wise incompatible valve seat combo -- i would exhaust all other possibilities before taking it apart -- dist advance - sticking 1/2 way out . vacume booster if any check valve -- poor coil out put , plug wires having to much resistance , incorrect plug heat range for the gas we have now ( full of alky ) and bubbles -- maybe this will help -- jz

[JL8Jeff]

Well, it has and HEI distributor with all new cap, coil rotor, wires and plugs so I don't think any of that is a problem. The plugs all look pretty good. I'm not sure if this engine still has a solid lifter cam but maybe I wiped out a lobe. Time to do a vacuum check next.

[johnjzjz]

ok with the engine running if you have tail pipes you can get to place a piece of regular printer paper over the tail pipe holding it firm but not as to close off the exhaust - if the motor has a miss it will make a slapping sound if you are not sure about the sound i am talking about remove a plug wire re start with it missing and do it again you will hear the sound -- if its a solid the valve adjustment will show a lobe right away one will be way loose if its hyd lifters use a small T style steel rule and measure the compression of the springs from the side wheir the slapping noise is -- more work but before taking it apart you can see if it is a lobe going away -- 20 / 25 thousands diff is a worn out cam and a mushroomed lifter not much -- but be sure its not in the ignition especially since you changed some stuff and now are having a problem - the china **** they sell today is junk -- be sure jz

[JL8Jeff]

If the engine is still mostly stock then it's solid lifters. It definitely has some lope at idle with that slap you describe. First thing I did was put my hand over the tailpipes at idle and you can't tell any difference from one side to the other. You can't hear or tell there is a miss at idle. But start to rev it up slowly and you can hear the miss and see the engine start to shake a little like it's out of balance. I need to check the distributor just to make sure but it wasn't cheap chinese parts that went on it. Thanks for the help.

[JL8Jeff]

After work I decided to look into the distributor. Since the cap, coil, rotor, wires and plugs are new they all looked fine. But the HEI distributor itself is showing it's age poorly. The weights were rusty and had a bunch of paint overspray on them. The springs were even worse, rusty and paint too. So I cleaned up the weights and dug out some new springs from an old recurve kit I had sitting around. I made sure the weights and springs were moving freely and cleaned up the rotor a little as well. I want to get an new HEI but this is all I could do for now. I put it all back together and fired it up with a vacuum gauge on it. It seems to be running fine and the miss type noise isn't really there like it was. The vacuum reading was around 15 at idle with a slightly vibrating needle. It was 20-22 as you slowly revved it up and it was nice and smooth. The Holley 750 carb has been jerked around with so I have no idea what the real condition of it is and I don't care since I have an original 780 to put back on it. I think it could use some valve adjustment but overall the engine seems pretty tight. It does burn a half a quart of oil every couple hundred miles so it's not as tight as it could be. But it's a solid lifter 396 and it seems to be running fine!

[johnjzjz]

careful with the springs 2 firm ones are better than light ones with a big block --- the oil control is more than likely exhaust valve seals -- you said the plugs looked fine so one could say the seals on the valves if they even put them on are bad ( as in hard like a rock ) take a valve cover off and look to see if it has any and the type -- best type would be a rubber type cap with a tin band type clamp to hold it onto the valve guide -- not a floating seal like some have that looks like an umbrella --- jz

[JL8Jeff]

Yeah, this whole car is a mystery to me. The previous owner probably didn't even put 200 miles on it in the last 3 years. It wouldn't surprise me at all if the valve seals are dried up. I put the stiffest springs I could find in the kit in the distributor to try and match what was in the distributor.

[johnjzjz]

once you get the oil condition straightened out say 1000 miles a quart hopefully better but ??? -- than you can try one lighter spring in advance mech -- street BBC can stand 34 / 39 deg of timing -- i would have the dist re curved for full centrifugal -- most Chevy dist are set to run full port vacume but when you cam them the vacume becomes erratic and less at the motor speeds you need it high --- if you do it the guy will put 25 / 28 total in the HEI you than set the timing at say 10* ( 35* total @ 3200 rpm ) than with a dial back timing light you can bring the motor up to 3500 for a moment or so --- letting the advance swing all the way out and you have put 35* in the dial back timing light -- and at the TDC mark for zero --- will be your set point --- don't know if you know what i mean but you can always call me or e me --- and i have a guy i know who can do the dist in a speed shop buy me if you dont know someone -- i believe he gets 50 / 100 depends on shaft bushing and other stuff -- best deal is a new msd type --- we had in the last car we did BBC put the new digit timing bell and whistle unit in don't know if i like it ( YET ) jz