Vacuum Advance & HEI Weights Replacement?

[savatreatabvr]

I finally replaced the vacuum advance and centrifugal weights in my HEI distributor on my bone stock mid 70's 454 truck! It's got good timing & compression, new plugs, wires, cap, rotor and Quadrajet kit but still don't have the response I believe it's supposed to have! The Quadrajet has issues with the secondaries, not sure why, but that won't affect the off idle response so any idea's on what to check next?

[Firebirdjones]

I set most engines up with about 16-18 degrees of initial timing and then limit the mechanical advance to 20 degrees. Easy to do on an HEI with a 10-32 tap and a small 10-32 screw.
I'd have to have you in front of me to show you how I modify the HEI's.

That will give you 36-38 total depending on where you want the initial set. 36 is pretty safe for a low compression engine on todays gas, even for towing so long as everything else is tip top.

From there I prefer to run an adjustable vacuum advance plugged into manifold vacuum only (not ported above the throttle blades) and I dial in about 10-12 degrees of extra vacuum advance to help light load cruising.

Set up this way the throttle response should be very crisp and I usually see 2-3 mpg gain as well.

From there it's all in the carb tuning.

[savatreatabvr]

Some of what you've said makes since to me but it definitely sounds like you know what you're talking about! My vacuum advance is plugged into manifold vacuum right now and I think I've got my valves adjusted correctly so can you describe your tap & screw procedure is layen's terms for me? I'd really appreciate it! The old vacuum advance didn't hold vacuum at all so I advanced the timing for a while until I got another one then retarded the timing! I put a Quadrajet kit in it but since I have secondary opening issues, I'm assuming not opening when floored, there could be other issues with the carb too but it always fires right up that's why I'm leaning towards the vacuum advance or weights to be my problem! Who know's, I could be bassackwards on all of it! I'm not trying to make it run like a raped ape, I just want it to run right!

P.S.

Originally Posted by Firebirdjones

I set most engines up with about 16-18 degrees of initial timing and then limit the mechanical advance to 20 degrees. Easy to do on an HEI with a 10-32 tap and a small 10-32 screw.
I'd have to have you in front of me to show you how I modify the HEI's.

That will give you 36-38 total depending on where you want the initial set. 36 is pretty safe for a low compression engine on todays gas, even for towing so long as everything else is tip top.

From there I prefer to run an adjustable vacuum advance plugged into manifold vacuum only (not ported above the throttle blades) and I dial in about 10-12 degrees of extra vacuum advance to help light load cruising.

Set up this way the throttle response should be very crisp and I usually see 2-3 mpg gain as well.

From there it's all in the carb tuning.

[Firebirdjones]

I'll have to take a picture to help explain. Give me a little time and I'll pull an HEI off the shelf and try to show what I'm saying....

[Firebirdjones]

Okay, I found one on the shelf that I've already done this modification to, so it might help explain.

Here you can see the screw installed (from the bottom side) on the inside hole where my finger is...



Here is a shot of me pulling the distributor to full mechanical advance, as you can see the contact it makes with the screw limiting the advance.



In these pictures you can see I've actually ground on the screw to increase the amount of mechanical advance. I don't remember what engine this was in so I don't remember the reason. But generally with more mechanical advance you would have to knock down your initial setting to keep your total in the 34-38 range.

I can tell you though, the screw trick without grinding, will give you nearly exactly 20 degrees of advance on a GM HEI so there shouldn't be any need to grind the screw. You can set your initial at 16 or what ever you can get away with, then checking your total with a dial back light should give you in the neighborhood of ~36 give or take.

GM originally used plastic bushings for this stuff, and they get old and brittle, crack apart and fall off. I can't tell you how many cars I've tuned with HEI's that came in for pinging problems due to over advancing (bushing was gone) This screw trick is a perminant solution. You'll have to remove the distributor gear and pull the shaft out to do it. Which is a good time to check for bushing wear on these things.
Installing the screw the other way would leave the head of the screw on top, which limits advance too much and would require way too much initial timing to get the total you need. I guess you could grind the head of the screw down if you wish to gain more mechanical advance, but you would never be able to remove it if the need arrose. Hope this helps....

Edit" just to add, there is no drilling required. A 10-32 tap works perfectly in that hole, you can do either end you wish, it doesn't matter, just use the inside hole only....

[savatreatabvr]

Wow, thank you very much, that is a great idea and a valuable "Trick of the Trade! My 454 isn't pinging so maybe it's not advancing enough? I've had BBC's and SBC's my whole life but I've never had one with such a sluggish throttle response, it doesn't hesitate, it just acts tired! LOL!

Originally Posted by Firebirdjones

Okay, I found one on the shelf that I've already done this modification to, so it might help explain.

Here you can see the screw installed (from the bottom side) on the inside hole where my finger is...



Here is a shot of me pulling the distributor to full mechanical advance, as you can see the contact it makes with the screw limiting the advance.



In these pictures you can see I've actually ground on the screw to increase the amount of mechanical advance. I don't remember what engine this was in so I don't remember the reason. But generally with more mechanical advance you would have to knock down your initial setting to keep your total in the 34-38 range.

I can tell you though, the screw trick without grinding, will give you nearly exactly 20 degrees of advance on a GM HEI so there shouldn't be any need to grind the screw. You can set your initial at 16 or what ever you can get away with, then checking your total with a dial back light should give you in the neighborhood of ~36 give or take.

GM originally used plastic bushings for this stuff, and they get old and brittle, crack apart and fall off. I can't tell you how many cars I've tuned with HEI's that came in for pinging problems due to over advancing (bushing was gone) This screw trick is a perminant solution. You'll have to remove the distributor gear and pull the shaft out to do it. Which is a good time to check for bushing wear on these things.
Installing the screw the other way would leave the head of the screw on top, which limits advance too much and would require way too much initial timing to get the total you need. I guess you could grind the head of the screw down if you wish to gain more mechanical advance, but you would never be able to remove it if the need arrose. Hope this helps....

Edit" just to add, there is no drilling required. A 10-32 tap works perfectly in that hole, you can do either end you wish, it doesn't matter, just use the inside hole only....

[Firebirdjones]

I can't answer that without having the truck here in front of me. To check it you'll need a dial back timing light and check your total advance.

You will basically wing the engine up as far as needed until you don't see any more advance happening. With stiff advance springs (stockers) that might not happen until 4,000 rpm or more. I like to have it all come in around 2500 and be done. With your low compression engine you can get pretty aggressive with how quick you have it come in. Alot of other minor variables but it would bore you.

36 total is a good number to start with on an older engine with old combustion chambers that aren't very efficient. You may find it would like more, if you really want to test that theory you'll need a dyno or some track time.

I found BBC's and SBC's with original "old style" cylinder heads will like more ignition lead (36-40 degrees) to be optimum, the question is will the octane gas you run support that...on a low compression mid 70's BBC (8.5:1 at best) that shouldn't be any problem at all.

It's the more modern cylinder heads with very efficient combustion chambers that don't need much ignition lead. I found they tend to like as little as 26-32 degrees depending on the engine.