Sorry so long but im trying to give a full description.The engine in my 84 Z i bought rebuilt/used.I started it on the ground and it sounded fine.I changed the intake and carb over to a 71 vette alum highrise and a 600 holley carb i rebuilt myself.Put it in the car and it popped constantly through the carb.Changed carb to a new edel 600 and it still had the same pop.Dropped the exhaust and it still popped.Pulled the timing cover and it was set straight up and in time.Gave full tuneup and new distibuter vacuum advance and that didnt help.Ive readjusted valve lash atleast 8 times.I could advance the timing up to where it would stop popping but it would crank so hard and slow it would barely start but it ran decent.And when cranking you could here the sound of air pushing up through the intake from 1 cylinder everytime it came around to that cylinder.It finally fried the starter so i decided to pull the pass side head since it seemed to come from that side.When i pulled it i found number 6 plug was coated with carbon and could in no way fire-the rest looked fine.Down in the cylinder bore(6) was a patch of loose carbon that looked like the shape and size of a exhaust to head gasket but it crumbled up like it was pure carbon.Head gasket looked pretty new as you can see in the pic below.Cylinder number 4's exhaust pushrod was wore twice as long as the rest of rods like the cam was lifting way way too much on the one lobe and the spot it sat in the rocker looked dry compared to the rest.I eye sighted the cam lift while spinning the engine by hand and a couple look wore but no where near bad enough to cause a pop i dont think and they werent 4 or 6.I layed the head face down on the ground and sprayed carb cleaner in all ports both intake and exhaust to see if it seeped by the valves.I got a tiny bit of seepage on number 2 and 8's exhaust valve but no seepage on 4 or 6.Im stumped at where to start looking.Any idea where??

Also if you wouldnt mind could you give me idea of the dish on this piston.The shiny stuff is where i coated it with bearing grease to keep it from rusting till i figure out the prob.Also could you estimate the compression on a 64cc and a 76cc head with a standard thickness gasket??Btw ewngine is bored 4.040.Thanks

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worn cam lobe maybe?

 

What a mess. It could be a lot of things from simple to complex. I'd have the heads checked out to make sure there are no burnt/leaking valves or trashed valve seats. Then put it back together (making sure the valves are properly adjusted) and cross your fingers. If you put ANY more work into it than that you're nuts- just do a complete rebuild (or get a fresh long block) and KNOW that it's right. There are SO many unknowns going on here.

BTW- piston dish guess: 12cc of total dish. Using my calibrated eye.

No way to know the compression unless we know how big a motor this is- 305, 350, 383, ????

 

Sorry its a 350.Right now im just trying to get it up to par and running decent for a emergency car incase the idiots across the road hit my iroc again.They do try very hard.If things work out i'll be moving next year and the whole car will go through a complete tear down and rebuild when i get situated there plus having it running will help in moving it during the move.The heads on it are 624's,i have another 624 head from a running engine but it has bigger valves.Think it will hurt anything by putting a bigger valve head on it??I also found out i have a set of 416's and a set of 450's both from running engines.They are 305 heads so im unsure if they will feed the 357 and will the compression be too high with them??The 416's have 58cc chambers and the 450's have 60cc chambers.

If these heads will work i'll throw in a new cam and lifters and see if that brings it alive.Thanks

 

I've heard some of the high milage 80's sbc's were known to have a little pop due to a worn down cam lobe. Are your pushrods all in tact? Before you pull the cam, check for worn valve guides and burnt valves. Definately sounds like the problem is somewhere in the valvetrain.

 

I show the 624 heads (#462624) being 76cc smogger-era castings. If my guess of 12cc is close for the pistons you'll be WAY down on compression- 8.2:1!

I used the following specs (guesses) to arrive at that number: 12cc of piston dish, pistons sitting at a stock deck height of .025" in the hole at TDC, .039" Fel Pro "rebuilder" head gaskets (which is what it looks like is on there now) and a 76cc combustion chamber.

If you put on a 58cc head, making no other changes, your compression would come up to a much more reasonably 9.6:1. A 64cc head would be at 9.0:1.

If you have to swap the cam due to worn lobes I'd definitely just yank it and rebuild it. If not, slap on a set of those 305 heads and cross your fingers. I'm assuming the cam in the motor is stock or a very mild aftermarket unit. The 305 heads probably don't flow much less than the 624 heads, stock vs. stock. But even if they did you're going to feel a lot more power increase by going up a point and a half on the compression than you'll feel by losing some flow from the heads.

I agree that SOMETHING is probably wrong in the valvetrain. You really need to figure out if you've got cam problems. You can yank the lifters one at a time and see if the face of the lifter is worn. It should be slightly convex on the face (bubbled out towards the center). Not so much that you could see it by eye but if you put it on a piece of plate glass you could rock it side to side ever so slightly.

 

Next chance me gets me will pull the lifters and check the cam.I know a couple/few are worn some so i might as well just skip the checking part and replace it.Id hate to take it to a shop for cleaning to start a rebuild because they always scream bore it bore it bore it and id never bore one past .040.Its a prob i'll have to think on a while.Thanks