We're here buddy! Me, I liked the older hemis, you're not alone...
now we need to move that vacuum advance how. As it is you are running full advance at idle and lose timing when throttled up. That why you need it ported, so it will advance when you mash it off the line... The end seals on the intake manifold can hold it up too high and cause a vacuum leak if the heads and block were ever milled. Not common but it happens.

 

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No no no, my friend. It doesn't work like that. Seen any top fuel cars? No vacuum advance on them! Their advance is strictly mechanical - advancing the timing as the motor turns over faster.

Vacuum advance is something used when the fuel/air in the motor is scarce (thus the vacuum is high) When you have little fuel/air you have to light it sooner. If there is lots of fuel/air advancing the timing (lighting the fuel/air sooner) will make it ping, even detonate! 'Ported' vacuum advance came along for one reason - pollution. It sure as hell does not make a motor run better at idle! As soon as you open the throttle butterfly past the port the advance works, just like it always did.

This is not the reason for ping, most of the time. Ping is caused by glowing deposits on the combustion chamber lighting the fuel/air too soon - before the spark plug fires. Same thing - too soon causes ping. The pinging sound is caused by the pistons being forced down, before they reach TDC.

'Octane' is a reverse number, representing the burn rate of the fuel. The higher the octane, the slower the burn. Tetraethyl (led) in the fuel works by making the burn go around molecules that do not burn - slowing the rate of expansion of the flame front in the cylinders.. Same thing with EGR - burnt fuel will not burn again, of course. But the crap they put in fuel to slow the burn results in less power because burning fuel, AND THAT ALONE, makes power.

So you might wonder why you can't buy a can of tetraethyl. The stuff is DEADLY. Just a little on your hand will kill you, from led poisoning!

An alcohol molecule is huge, when compared to a gasoline molecule. So it takes more time to burn. Motors that burn alcohol run at about 60 degrees full advance. Motors tuned for gasoline run at about 30 degrees, total advance. Alcohol acts like led, in that it slows the expansion of the flame front. Alcohol does not, and CAN not burn, inside a motor timed for gasoline. It burns in the exhaust manifolds and it completes the burn, thereby reducing pollution from unburned fuel.

Looking on the bright side, alcohol in the fuel is the reason most modern motors don't need air pumps. But you pay a price, for that - namely less power. Because burning fuel, AND THAT ALONE, makes power.

Somebody is going to tell me I'm full of ****. Fact is, 'they' don't want you know this. But I am old. I learned the basics of internal combustion engines before pollution was an issue.

I hope you're Ok with me saying this? I'm not here just to take. I'll give something back, when I can... OK?

 

The vacuum reservoir on the '73 and '74 was a heavy, ~4" steel tube running crossways behind the bumper and provided support for the 5 mph bumper shear bolts. I doubt that is leaking unless there is a lot of rust because it was fairly thick. You could check it if you have a little hand vacuum pump and gauge.

I always thought that the vacuum advance was for economy at idle and low speeds because it allows you to keep the throttle plates closer to closed than without it.

FWIW, I ran my vacuum advance directly off the manifold and 12° initial timing with about 32° all in at full mechanical advance. Mechanical advance starts in at about 1200 RPM. That was with the stock L-82 setup. I don't know what the rebuilt engine is going to take because I increased the CR to 10:1 and have a slightly hotter roller camshaft in it. It will never see gas with ethanol in it. I hate ethanol. It has less energy and therefore less MPG. It also takes more energy to produce ethanol with the process that we use with corn than you get back.

 

How do you avoid ethanol? In Indiana we can buy alcohol free gas at a very few places. It seems to have something even worse in it?

 

Here in the Kansas City area, the Hy-Vee grocery stores have both mid-range and premium alcohol-free gas now. I don't know if the Gas Buddy app on the smartphones still delineate which stations carry it anymore or not because I don't use the app anymore since I got rid of the Airstream Interstate diesel. Haven't heard of any detrimental entities being put in the alcohol free versions.

I also learned a number of years ago that ethanol gas doesn't work well in small engines and jet skis even though the manufacturers claim that they can use it. In small engines such as weed eaters and leaf blowers, it does something to the carburetors that carb cleaner will not fix. It does something in jet skis to the spark plugs and even though they look fine, they're not. Had to replace the spark plugs in my jet ski every season and the ones I put in looked just slightly better than the ones I took out.

 

Good Morning

I used to buy a harley and fix it up over the winter, some years. MY bike is a '12 Yamaha FJR. So you can deduce how much I like harleys - lol. Po boy gotta do what he gotta do... Maybe the worst thing about hogs is the people on them. They're rich snobs or 1%er wannabes... sometimes both - rofl

Anyhow, I LISTEN to my motors. When burning the 'straight' gas there was a difference - a big one. I could feel each cylinder firing. The motor (twinkie) smoothed out and ran cooler BUT it produced less power than with the ethanol-laced premium. That much is certain. But why is not.

If you could get pure gasoline (you can't) you would not like it. And you would have to drain the tank of a harley! It would be miserable, to say the least. ALL of it has additives. If not ethanol, something else. And, really, what's in it does not matter. In terms of producing power, the volume of the non-power producing additives is the key.

Removing led from gasoline ruined lots of motors. People found their valve seats pitted, and leaking badly. Led also 'lubricated' things in the combustion chambers. Changing the valve seats is expensive, and does not always work... A friend with a BITCHIN' '58 desoto sportsman found it cheaper to buy new heads!

Likewise, ethanol ate up the neoprene in their carbs - including the needle seats that control gas flow. Some classic bikes (I used to be a Norton guy) literally began leaking gas, overnight! The formula for the neoprene was changed, to accommodate the gas. So when you say 'years ago...' I assume it was around the time alcohol was introduced.

I recommend bosch, thin wire, double iridium plugs. Not available for the 350 chevy and some other vehicles, sorry. They literally last forever and the difference WILL be noticed, within 5 minutes of starting the motor! Maybe they will solve your plug problems, IF they make a plug that will fit. The FJR came alive! No kidding.

Maybe the strangest thing about SB GM motors is the way the plugs are 'hidden away' back in the heads. They don't protrude into the firing chambers, at all! It would seem that would cause the need to set the timing faster... Who knows? What do you think about that?

I 'live' in a hell hole called Terre Haute, just across the Indiana line on I-70. I've been here, ever so long. I always dreamed of leaving but this place is like fly paper - when you land here, you're stuck! I could sell my humble home, take the money somewhere else and not have enough to buy a vacant lot!

Anyway, do you know what DIM SUM is? Literally translated, it means 'weekend brunch.' It can be had only in places big enough to have a 'chinatown.' You know you're in the right place when you are the only 'white boy' (sic) there. It's like, SO FINE! It can be had in St Louis. I know a place with a huge parking lot - more than big enough to accommodate a swarm of corvettes. Ever hear of a 'flash mob?" lolol Or maybe just your car and mine?

I'm really enjoying our 'tech chats.' Meeting some more 'vette freaks' might be interesting....

Have a great day bud.