My 70 LS5 has always ran ok, but I felt there more to be had form 10.25 and a 390 horse rating. It's had a slight miss since I got it. Over the years I have tried to dial it in but just have not hit the right combo to get it right. It has an after market chrome timing cover that I painted, I'm pretty sure the balancer is not the original. The timing mark does not line up with the pointer, so timing has been a guess by ear. Last year I changed out the braid covered wires for plain black thinking that one of them might be shorting out. That did not solve the problem. I have all the plugs out and ran some compression tests, they run from 160 to 180. While the plugs are out I was able to get a piece of aluminum rod into #1 and bring the piston up by hand get ballpark mark on the damper for timing. I'm not sure where the mark that's on the damper is when I'm in this position, it can't be seen from the top. This engine is not easy to turn by hand, and not as easy to find TDC with a stop as a small block.

So now I have something to shoot for and want to put the plugs back in as they have been out for a few weeks, I get to work on this every so often. What I took out were AC CR43TS, I am changing them just to eliminate something else that might be wrong. The guy at NAPA gave me Champion RV17YC, I wanted just a basic plug. I think the "17" is on the hot side? I have had some ping with the AC plugs but that could have been a timing issue.
So what plugs should I be looking for?
I don't really know what has been done to this engine over the years, the PO said it was rebuilt put could not produce any info to prove that out. It is the borne with engine, it has a real nice stamp and broach marks.

I will retire next year and I then might have more time to tweak the little things on this car that I just put up with now.

Thanks Eljay
Oh yah, every one likes pictures.

 

In my opinion, you really need to get that balancer sorted out so you can reliably set the timing. When I replaced my '72 LS-5 balancer I went with: Pioneer Harmonic Balancer DA-4541 ($75)

For spark plugs NGK is highly recommended. I use them in both my big blocks. How do the R43s look as to carbon build-up? That's a "cold" plug that may not burn off deposits. I use the NGK equivalent to R45/R46.

 

You may want to copy/paste your post on the below thread as well....

CF 70 LS5 454 owner thread - CorvetteForum - Chevrolet Corvette Forum Discussion

 

You may have been on TDC for #6 compression instead of #1 compression and the timing mark was on the bottom.

I would not run champion plugs in my lawnmower. Those things are junk. NGK is the way to go for sure.

If it's been missing for years I would say a bad lobe on the cam is a possibility.

 

When I was trying to find #1 TDC I bumped the engine around with my compression tester in #1 hole and the cap off. I knew where the rotor should point. As I came around pressure built as the rotor got close. I removed the tester and put the rod in the spark plug hole and cranked the motor around with a ratchet by hand until the rod was captured, then painted a mark on the damper. I repeated the procedure and got back to the same spot. I assume the damper is keyed so I'm not sure how it could be so far off, even if it slipped. Is there another big block engine that this damper could be from that is that much different from my engine?
As far as the removed plugs, they don't look too bad. And the cam condition is unknown. When I first got the car I removed the intake and reinstalled with fresh gaskets and that made an improvement.
The plugs

 

beautiful good luck

 

You are half way there!!!

If you put that mark on the damper lined up with the zero on the timing tab you are close to finding true TDC. Do the "rod" thing again with the engine again rotating in the opposite direction. Make another mark on the balancer using the same point on the timing tab (that you did the first time). The EXACT true top dead center point is in the middle of your 2 marks.

The key ways on dampers vary based on year and application. The timing tab locations vary by chassis. Ideally they would match but people put mixed combinations together all the time.

Damper might have slipped, ruining it but verifying TDC is the first step.

 

Is #8 the one that has the miss?

Those plugs look eh, ok. rich running perhaps but nothing glaring. They also look relatively new or with not a lot of miles on them. #8 look the richest or most contaminated vs the others.

Hard to read those dark colored plugs.
However they all look to have a pretty good covering of soot on them. Some appear to have a little oil contamination maybe as well. #1 perhaps oil on the threads or that could be just antiseize. #5 looks to have slightly more carbon build up as well.