Quote:
Originally Posted by Cliff R
One thing you are not doing with the mechanical advance is adding a positive stop for it.
They can, and will add timing at high RPM's. I found this out decades ago and have been putting a mechanical stop in all HEI's we build here.
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Those two silver screws in the picture with their sides shaved are a positive stop. I didn't modify the centre cam. I abandoned that idea after hearing how hard it is to get right. That's my 394 centre (numbers down) with my 045 weights (numbers down) and the two beefiest springs I could find. All old factory stuff from junk parts.
I should add, just not to be confusing, that this was not the final shape of those screws. I had to shave them more to get the rotor to mount squarely and I had to do some fine tuning to get the right amount shaved for 10 degrees. A little material more and you are limited to 4 degrees. A little less and you are back at 14 or 16 again..
Quote:
Originally Posted by Cliff R
I have never once found the need to modify the stock weights or center cam with Pontiac units. They did a darned good job of setting them up from the factory. 99 times out of 100 I use the stock springs as well, and find that if they are in good shape, the timing curve starts right off idle and is all-in around 2800-3200rpm's.
I will on occasion "work" the spring eyes a tad if/when I want the curve in a little quicker or to reach total timing a tad earlier. Aftermarket springs are nothing but pure garbage these days, as are the weight kits they come with. As for the vacuum advance I do NOT recommend a can that starts sooner than about 5" vacuum. The one we use here starts around 5-6" and all in around 10-12".
As mentioned previously, ALWAYS tune full throttle and total timing first. No need to get all the timing in quickly or a lot of it. The more efficient the engine combo the LESS timing (and fuel) is needed at every RPM to make best power.
For about 2 years now I've opened up my Saturday mornings for custom tuning troubled set-ups. Folks make appointments and bring cars here, often from great distances. I've had them brought here as far away as Georgia and even had one customer stop in on a road trip for Colorado.
About year and a half ago I had a PRESTINE Buick GN Stage 1 car brought here, and a few weeks later a 1968 Hurst Olds. Since then (the word got out) I've had half a dozen more of those cars brought here. Interesting part is that the cure for ALL of them was to remove the "goofy" advance kit in the distributor and put stock weights and springs back in them. I also replaced the rubber advance pin bushing with a stainless steel one. Once I got the timing under control (all of them were adding timing at idle and some when cranking) I went into the carburetors and put them back where they should have been.
Each time the owners return to the shop after a test drive grinning from ear to ear! I'll also add here that I also remove the Petronix and put points back in them. Yes, we go back to the original set-ups. I've found WAY too many defective Petronix units in recent years and refuse to run them.
Just had a customer (actually the dyno shop called me) a few weeks ago. I built a custom Q-jet for a "high end" restoration, 1970 Chevelle 454 SS car. The engine "builder" stroked it to 496 CID, ported the heads, bigger valves, higher compression and flat tappet solid cam (my contribution). The goal was 550hp or so and still using the flat intake, Q-jet, stock distributor, exhaust manifolds, etc.
Well, I get the call while they are on the dyno and this thing ain't making chit for power. Stuck around 420hp and EVERYONE right down to the guy who takes out the trash is blaming the Q-jet. The builder is telling my customer the carb isn't big enough for his engine and to get that POS off of there and put a Holley on it. He's also recommending to take the "flat" iron intake and make a door-stop out of it. I'm trying to convince them to keep the Q-jet/stock intake and try to help by telling them in try more fuel pressure, larger inlet seat, etc. I get all caught up in trying to fix the carb and didn't even think about the distributor. After going no place with the carb I asked them how the timing was set-up? They said it's "all-in" by about 2800rpms and that the engine does fine till about 4500rpms then just quits pulling. I asked them about the points, and they said they eliminated them and put a Petronix in it.
BINGO, the light goes off........I told them to remove the Petronix and put the points back in it..........................the next pull was 548HP!
Just a reminder that a lot of these parts we are getting these days aren't up to par. That's about the 3rd or 4th time I've ran into issues in recent years with those parts.......FWIW.......Cliff
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Just a reminder, Cliff. I am also a believer in stock parts wherever possible. My HEI is all factory except for the adjustable vac advance. And it's six turns out, so fairly stiff. Just not quite as stiff as a factory can (or at least any of the ones I have). None of my cans would start at low enough vac to be stable at idle.
No junk rubber bushings in the dist. All factory weights and springs and cam. I have a positive stop, it's those two screws. That's how I got the mechanical to limit at 10º. I've got nothing against welding, I just don't have the equipment or knowledge.
Carb, again, as you know is factory QuadraJet, good rebuild parts from your own inventory. No funny stuff. The heads are common in this hobby and the larger chamber size available (87cc). The cam is a pretty commonly used 041 clone (Crower 60919) well recommended for my compression ratio and heads. Like you said, you see a lot of combos like mine. And mine is set up pretty close to how you would. No funny X cam, no junk advance kit, no funny carb tricks, factory carb and intake..
Anyway, I appreciate any thoughts on my situation. I'd also appreciate your advice on the idle videos as to if this sounds like the right sort of idle quality for what the cam and combo is supposed to be. Words like "lumpy" and "smooth" sometimes don't convey what they should.
Sam