Vacuum advance unhooked, no difference?

[Number21]

Another day, another problem. Tonight I was trying to measure my timing, so I unhooked the distributor vacuum advance. In the process the vac hose fell apart on me, and I had to drive home with it unhooked. Shouldn't there be a big difference between being plugged in or not? The truck didn't feel any different on my way home, no pinging, lots of power.

Would it be safe to say that my vacuum advance isn't/hasn't been working? How do I test that?


Why do I keep a 40 year old truck for a daily driver?

[scottofksu]

I'm by no way an expert, but could it be that the hose was so decrepit before that it was leaking just as bad as if it was disconnected altogether (hence no difference in how it drives)?

[jjzepplin]

Yeah-should be a big difference! You may have a bad vacuum diaphragm and you could also have a locked mech advance. Take the cap off and see if our vacuum is moving the timing plate. Also take your rotor off and make sure your mech has the springs still there and is moving freely. These vacuum diaphragms go bad all the time and tons of people out there have no idea their distributors are locked. Imagine the potential of your motor when working correctly.. If the things are wrong then go buy an advance "curve" kit from Accel or MSD and follow the directions carefully and watch that beast come to life!

[68gmsee]

Actually, if it was hooked to ported vacuum on the carb it wouldn't make much difference at idle. You may notice some difference at slightly higher rpms but the more you open the throttle the less the vacuum advance comes into play. In reality, when the butterflies are open all the way, there's very little vacuum, if any, to the vacuum advance. The mechanical advance comes into play.

Having said that, if it is hooked up to manifold vacuum, you would have noticed some difference in the idle rpms but once you took off you probably wouldn't notice it. I once drove with a busted hose on a vacuum line for several days before noticing it. Actually, I happened to hear it while adjusting the idle rpms.

[storm9c1]

You'll probably notice the biggest difference in your wallet. Vacuum advance does most of the work at cruising speed (butterflies mostly closed, high vacuum). This is when your engine can benefit from massive amounts of advance, which will significantly increase MPGs. With it unhooked, you will get poorer economy at cruise. There are a few other areas where vacuum advance helps (like off idle transition, base idle speed if hooked to full vacuum, or emissions control if hooked to ported vacuum, etc), but it shouldn't be a night and day difference by feel. Mechanical advance does all of the work under hard acceleration and is what contributes to "seat of the pants" feel.

[86 K-10]

Originally Posted by Number21

Why do I keep a 40 year old truck for a daily driver?

because theyre a heck of a lot more fun to drive ...and you probably dont have a payment and a high insurance premium.

[Number21]

Originally Posted by storm9c1

You'll probably notice the biggest difference in your wallet.

That's good to hear! Been getting terrible gas mileage and I can't figure out why. I think I'll just replace the vacuum advance, they seem cheap enough.

[68gmsee]

Originally Posted by 86 K-10

because theyre a heck of a lot more fun to drive ...and you probably dont have a payment and a high insurance premium.

Agree. And easier to fix than the new ones, parts are a lot cheaper and can be found in the local auto parts stores. If you're young you look cool in one, if you're old like me, you look like someone from another era (what my nephew said.)

[Dunenutt]

Originally Posted by Number21

That's good to hear! Been getting terrible gas mileage and I can't figure out why. I think I'll just replace the vacuum advance, they seem cheap enough.

If the mech advance is junk too, you may want to check on a rebuilt dist. The last one I replaced was dirt cheap, and a whole lot less of a pain.

[Number21]

Alright, I got the advance hooked back up and now it's pinging. Why? Was running great before I "fixed" it. Timing is only set at about 8 degrees.

[68gmsee]

Originally Posted by Number21

Alright, I got the advance hooked back up and now it's pinging. Why? Was running great before I "fixed" it. Timing is only set at about 8 degrees.

Probably wasn't running great... Maybe good enough..

Before you re-adjust the timing, check to make sure the vacuum advance is in fact working. I have a small hand vacuum pump but mouth suction will work. Just hook up a short piece of hose and see if it holds vacuum and also moves smoothly without any binds. Doesn't take much vacuum to move it.

If it moves without binds check the mechanical advance to make sure they're not binding either, and last, I would use manifold vacuum rather than ported vacuum. Any vacuum port below the throttle butterfly shaft is considered manifold vacuum.

[storm9c1]

If you are pinging now at cruise and you weren't before, this means your vacuum advance is working! It just needs to be tuned. You have a few options:

1) Back off initial timing 2 degrees. See if still pinging. Trial and error. There is a butter zone here.
2) Get an adjustable vacuum advance or add a stop to your existing one.
3) Run higher octane fuel (I know, not desired, so leave that for last).

Note on #2 -- this is a common problem. Many distributors come with a vacuum advance that is too aggressive. I have been told that you want 10-14 degrees of vac advance and that's it. I run 10 degrees on my vehicles. Some pull in as much as 16-18, which is too much. Some can be adjusted by way of a small hidden allen screw under the vacuum nipple (you stick the allen wrench through the nipple). Others have a stop plate. And others need to be modified with a stop plate.

It's worth it to get this dialed in.

[Number21]

That's good to know. There's a real fine line on this engine between pinging and not, since I've got old "powerpack" heads with 64cc combustion chambers. (and I refuse to run premium) Seems like I can get it to stop pinging at about 0 degrees timing but that obviously isn't right. Never met a distributor that didn't give me problems though, still lots of things to play with.

[cableguy0]

Refusing to run premium with north of 10-1 compression on iron heads is asking for a melted piston. Its penny wise and dollar stupid. You save 20 cents a gallon but waste an engine that will cost you thousands to replace not to mention you will never have it running efficiently without sufficient timing advance. Your blowing fuel out the tail pipe with the timing retarded. Its costing you more to try to save pennies. You will forever have issues and never actually have the engine running right without the proper fuel.

[Number21]

Hmmm...would I get higher enough MPG with premium to pay for itself? Right now I'm not all that concerned with performance, I just want it to run as cheaply as possible...

[Gumby]

you have a hi comp V8 and a carb, neither = good MPG or cheap fuel prices.


get a mini truck or car with a V6 and fuel injection for some MPG if your not using the truck in a way it pays for its own fuel.

or if you hate computers gotta go for a stick 4cy carb for MPG


People ask all the time why nearly 13 years later my Firebird still has that tiny V6/T5 in it, cause even with today nuts-oh prices, I can put $20 in it and drive all over.

Put $20 in my V8 car and the next day it want's another $20

[Number21]

Yeah, I know this isn't a vehicle designed to get any sort of gas mileage, but right now I need to do the best I can with what I have. 10mpg is killing me. It would not make sense for me to get another vehicle with the amount of driving I do. Even if I did it would be another square body.

[fleetsidelarry]

Originally Posted by Number21

....Seems like I can get it to stop pinging at about 0 degrees timing but that obviously isn't right.

and you've verified that 0° is, in fact, true TDC or you're assuming that the damper/tab is correct?

[storm9c1]

At 10:1 compression on a truck, he will probably need to change timing curve and add a stop to the vacuum advance. I've been to that rodeo before. And even then, might still need to run 89 Octane and leave some power on the table.

Or throw a tank of premium in there now and see what you get. You might already get 13 MPG on premium with an agressive setup.

Either way, make sure your timing mark is right, and don't go any less than 4 degrees advanced. Honestly, that engine probably needs at least 8 degrees. Zero degrees is plain wrong.

My 383 is running 9.8:1 Vortecs. I had pinging at cruise too. I changed the timing curve on mine with lighter weights and heavy springs. I run 14 degrees initial (what my cam likes), mechanical pulls in about 16 more, and my vacuum advance pulls in 10. No pinging. But I had to buy a digital dialback timing light to get all of that dialed in properly. Otherwise it was a big guessing game fiasco that took months to get nothing right until I broke down and spent the money on the good light. And then everything came together in an hour.

[fleetsidelarry]

Originally Posted by storm9c1

At 10:1 compression on a truck, he will probably need to change timing curve and add a stop to the vacuum advance. I've been to that rodeo before. And even then, might still need to run 89 Octane and leave some power on the table.

Or throw a tank of premium in there now and see what you get. You might already get 13 MPG on premium with an agressive setup.

Either way, make sure your timing mark is right, and don't go any less than 4 degrees advanced. Honestly, that engine probably needs at least 8 degrees. Zero degrees is plain wrong.

I agree.

Number21 doesn't say what cam he has, but if it's a stock truck cam with that kind of static compression, then that would make matters worse.

also, it would help to know under what driving conditions (level cruise, light acceleration, etc) the pinging is occurring.

[Number21]

Originally Posted by fleetsidelarry

and you've verified that 0° is, in fact, true TDC or you're assuming that the damper/tab is correct?

Yes, I have checked the timing tab.

Originally Posted by fleetsidelarry

Number21 doesn't say what cam he has, but if it's a stock truck cam with that kind of static compression, then that would make matters worse.

also, it would help to know under what driving conditions (level cruise, light acceleration, etc) the pinging is occurring.

I have no idea what cam I have, if I had to guess I would say it's stock but I can't say that for sure.

The pinging happens mostly under light acceleration and slow cruising. If I stomp on the gas it usually stops. Lots of pinging with city driving, not much on the highway.

I guess I'll have to try some premium and see how it runs....

[fleetsidelarry]

Number21, just so I don't mis-understand: you used a piston stop to verify that the piston is exactly at TDC when the damper and timing tab line up on zero?

[Number21]

Yep, when the piston is at TDC everything lines up at zero degrees.

[fleetsidelarry]

sorry, '21, I had to clarify. a slipped damper was one "easy" explanation I could think of for having to run at 0° to stop the pinging.

if premium solves the pinging, are you going to let it be or are you interested in exploring further?

[Number21]

I don't know, I'll have to wait and see what kind of mileage I get. Still haven't had a chance to put premium in it. (gotta rob a bank first!)

Ultimately I'd like to replace the heads but for now I don't want to tear apart a working engine.