High RPM Wobble in Timing?

[colnago]

Okay, so I decided to get more serious with timing the beast, so I started by finding TDC by using a piston stop. Very pleased to learn that my 50-year-old harmonic balancer is right on the money. For anyone who wants to do this, I got the Comp Cams 18mm version from amazon (under $19, delivered), but the 3/8-16 stud was too short, so I had to get a bolt at the local hardware store (4" long, $1.19 plus tax, worked great).

So, the next step was to use a dial-back timing light, and check the advance every 500RPM, starting at idle. I got up to 4500RPM, and everything was nice and smooth. It looked like all the mechanical advance was in, but I figured I'd check the advance at 5,000RPM, just in case. But as soon as I got above 4,500, my timing started jumping around. Am I getting cross-talk between the plug wires? I'm using aluminum stand-offs, so none of them are touching, but they're all lying on top of the valve covers. Should I put a spacer between the wires and the covers? Is this from within the distro? Is there a way to check?

I always get the occasional bounce at the lower RPMs, but this was almost like a switch was thrown, and I was bouncing +/- 5 degrees, which I don't get at slower speeds (slower, I get the occasional blip down). It has an RV cam, but that shouldn't affect the ignition. Any ideas?

Joseph

[sargentrs]

I also tune to max timing myself. I see that when I try to go past the point when it's around 38*+. I'd say you're maxed out at 4500 rpm.

[WarEagle]

How old are the plug wires? Is there any sign of wear?

I think that cross chatter only occurs with solid core wires. If you are using suppression wires, and there is not an issue at lower rpm, I wouldn't think they would be at a higher rpm. If they want to cross chatter, they will at any rpm. But yes, you should use spacers anyway.

Are you running a stock coil? I am not certain, but maybe a coil with a high voltage rating (40-45,000) would help raise your max rpm's?

Are you running points or electronic ignition? A cheap set of points may be causing an issue at higher rpms. Its called point float or point bounce.

[colnago]

If I had to guess, I'd say that the wires are probably three years old. MSD Street Fire, 8mm. So, nothing too wild, but nothing too cheap, either. The box doesn't say whether they're solid core or not, just "low-resistance conductor" and "high EMI suppression."

As far as ignition/coil, I converted to Duraspark, so distro/coil/box are all stock.

I have a dial-back timing light, and some people have said that they can be questionable. Maybe this goes non-op at higher advance settings. Maybe I need to try it on a different car, and see if it's the light or the truck. Never figured it was the light, though, since it's probably 45 years younger than the truck.

Joseph

[WarEagle]

OK, the wires should be fine, same with ignition, although a higher voltage coil never hurts. Just match the correct Ohm to your application.

One possible issue could be timing chain slack at a certain rpm. Is it a V8 with a chain or straight 6 with just gears?

Do you routinely rev the motor over 4500 rpm? Does the performance noticeably suffer over that rpm? If no to both, why worry about max rpm tuning? Unless you are into drag racing, it sounds like a moot point.

[colnago]

I have a 352 V8, but the chain and gears only have ~1200 miles on them (Cloyes, I believe).

With that engine, I'm certainly not out on the drag strip. It spends 99% of its time bumping through town. Even at highway speeds, I'm probably nowhere near 4500RPM. I like your answer: a moot point, indeed. It's probably just my OCD kicking in.

Joseph

By the way, extra kudos for using the right phrase ("moot point," not "mute point").

[WarEagle]

Well, my thought process was like, if it is not really broken, why fix it? We have enough to worry about with our old trucks!

Thanks for the kudos, btw