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Timing advance issues...

5K views 27 replies 8 participants last post by  onovakind67 
#1 ·
Okay... Heres the deal, I have a SBC 305 .030 over low compression, World Torquer 305 heads, Weiand 144 Supercharger with a 625CFM Carter AFB, Crower supercharger hyd. cam designed for up to 10lbs boost, Crane roller rockers with guide plates, fresh double roller timing chain, MSD 6AL, MSD Boost Timing Master, HEI w/ Vac advance...

I have two main issues:

1)My truck is a pain to start on cold mornings...when its cold sometimes it backfires through the carb, doesn't do it warm. Tried messing with the electric choke, its still the same. The distributor was dropped in correctly set in the No. 1 Position with the No.1 position at TDC, so that tells me its not my rotor phasing and I double checked the timing set to make sure its not a tooth off. Timing is set at 8 BTDC. I recently purchased an Innovate LM-1 Wideband Air fuel meter and the backfire is not caused by a lean condition. The distributor has some miles on it, so I ordered a new one which will be here in a couple of days... Im thinking the distributors mileage might be a contributing factor. What can be some other possible causes to the backfire through the carb?

2) I haven't been able to find anything online pertaining to running vacuum advance with a blower. I had it hooked up as usual to a full vacuum port on the carb, but when the motor sees boost, it seems like the motor is working against itself and the top end suffers. I figured out that I would have to run the vac advance off the manifold because the carb is constantly seeing a vacuum signal so it constantly advances the timing when hooked to the carb. When I have it hooked on the manifold, its even harder to start but seems to run better when decelerating and as a plus,the top end is brutal, however the bottom end is really weak. Im thinking the bottom end is weak because the vac advance is seeing boost and not advancing, so I thought about getting a lockout plate for the Vac Advance...But that in itself presents more shorterm problems being, how can I limit my centrifugal advance so I can increase my initial timing to make up for the lack of timing w/o the vac advance? I guess Im just really interested in what others with a similar set up are running for a distributor or what they did to fix these issues.

I eventually plan on getting a Holley TBI projection setup and using that to dictate my timing and fuel issues, but until then I would like to get this set up dialed.


Any ideas or help would be appreciated.
 
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#3 ·
Why would you not want vacuum advance at idle? VA at idle certainly makes my car run smoother.
When is idle not idle? If I raise my idle from 600 to 800 rpm and the ported slot is uncovered, am I not idling anymore? Is there damage being done?
Do modern computer controlled ignition systems reduce the timing at idle? Is this a smog thing?
 
#4 ·
IIRC, a friend of mine has told me about some factory type applications (think Superchargerd T-Bird of the 80's) that used a vacuum advance that was a little different. It could BOTH advance and retard the timing based on the pressure that it saw.

So, at cruise, if under vacuum, it gave more advance than base, then under part throttle, some boost, it would retard the timing a little from base, and at WOT, it would retard the timing even more, to avoid detonation as the cylinder pressures and temp. got higher while under max. boost.

I think there are factory Ford vac. advance cans that do this. I have no idea how to figure out what ones do and don't. I'm sure someone around here will.

If not, look for some T-bird sites or something and ask them.

If it was me, I would run a vacuum advance for part throttle conditions, and use my Mallory Hy-fire ignition with the 3 bar MAP sensor and set the boost retard up - and its easy to program. Then you would have control (separately) of both the advance side and retard side independently. FYI, that Mallory box (I think it's called the Hyfire VI) has a list of features that far surpasses the comp. and costs the same as the ol' faithful 6AL. And it works great. Digital controls, start retard, nitrous retard, boost retard, two stage rev. limit. It's awesome for the money.

Course when you go to Projection or Commander 950 you won't need that anymore.

But hey, that's just my $.02.

Good luck.
 
#5 ·
When you set the base timing do you not disconnect the vacuum can ?
At idle, I use 750 rpm, there is no need for spark advance, but when you open the throttle and the revs go up the spark has to keep up with the piston.
Revs go up, timing advances.
Revs come down, timing retards.

On computer controlled vehicles to set timing you disconnect the distributor from the ECM.
Example, my Camaro, was then set to 0º BTDC.
The computer when re connected took over the advance of the spark.
 
#7 ·
malc said:
When you set the base timing do you not disconnect the vacuum can ?
At idle, I use 750 rpm, there is no need for spark advance, but when you open the throttle and the revs go up the spark has to keep up with the piston.
Revs go up, timing advances.
Revs come down, timing retards.

On computer controlled vehicles to set timing you disconnect the distributor from the ECM.
Example, my Camaro, was then set to 0º BTDC.
The computer when re connected took over the advance of the spark.
Whether or not you disconnect the VA when you set the base timing has nothing to do with whether or not you should use it at idle. You disconnect it to get a true reading of the mechanical advance curve.
If there is no need for spark advance at idle, why do you set the initial timing to anything other than 0°?
Why would you use vacuum advance at all if your timing is solely dependent on piston speed? Does your timing curve continue to advance the timing all the way to the maximum rpm of the motor?
When the computer takes over, what does it do? Does the timing stay at 0° at idle?
 
#8 ·
When you set initial timing there is no mechanical or vacuum advance.
You may set you initial timing at 8/10/12º or whatever to give the spark time to initiate the burn process before the piston gets to TDC, then as it hits TDC
the full force of the burn is at the right moment, not before not after.
Off idle the vacuum gives advance until the mechanical weight system takes over, then when all the timing is in around 36º there is no more need to advance.

The computer does the same as the vacuum can and the distributor has weights in it as well.
When you time the engine with the computer disconnected to 0º and re connect the ECM then sets the timing to the right degrees BTDC, and advances it as needed as the engine revs climb.

I separated out vacuum and computer advance systems but you seem to be confusing them.

Some more on blower timing etc.
 
#9 ·
malc said:
When you set initial timing there is no mechanical or vacuum advance.
You may set you initial timing at 8/10/12º or whatever to give the spark time to initiate the burn process before the piston gets to TDC, then as it hits TDC
the full force of the burn is at the right moment, not before not after.
Off idle the vacuum gives advance until the mechanical weight system takes over, then when all the timing is in around 36º there is no more need to advance.

The computer does the same as the vacuum can and the distributor has weights in it as well.
When you time the engine with the computer disconnected to 0º and re connect the ECM then sets the timing to the right degrees BTDC, and advances it as needed as the engine revs climb.

I separated out vacuum and computer advance systems but you seem to be confusing them.

Some more on blower timing etc.
There's no confusion between computer generated timing curves and mechanically generated ones. The engine doesn't know who decided to light the spark.
When you set the initial mechanical timing, you are setting the lower limit of the WOT timing curve. Too much initial and the engine will detonate at low rpm WOT conditions. This is not to be confused with throttled conditions, when the fuel/air mix is much less dense. Timing under these conditions needs to be advanced considerably to get the peak cylinder pressure at the right point in the cycle.
 
#10 ·
Vac Adv...

First off...Thanks for all the responses!

When I set the timing, yes I disconnect the Vac Advance and the idle is at 800rpms but even so there isn't enough vacuum to pull on the vac adv. The problem is even with an adjustable vac advance can, there isn't enough vacuum when first starting it, after firing it up and it fully warms up, builds vacuum, I cant get it to idle at any less than 1000rpm with the vacuum advance hooked to the carb . Another problem with the Vac Adv hooked to the carb is deceleration, it wants to shut down especially around corners because the throttle drops out and the vac advance retards too quickly almost like a "retard spike."

My timing is not yet dictated by a computer, its just your standard HEI distributor wired to run the MSD 6AL and MSD BTM. I have recurved my distributor to advance as fast as possible using the lightest springs since I have an automatic. In N/A form I ran the Vac Adv. from the carb and it ran great, but since its boosted the blower is constantly pulling air through the carb and because of that the carb never sees a "no vacuum" condition and it constantly pulls on the vac adv in turn advancing the timing from idle to wide open throttle instead of functioning correctly as in hit the throttle vac adv comes in and near the mid to top end vac adv disappears and only the initial and mech advanced are used. So because the blower constantly pulls a greater volume of air through the carb, Im constantly under Vac Adv which isnt good, that is why I ran the vac adv to the manifold. The manifold sees boost and vacuum, when it sees vacuum under a cruise condition, its good to go, when it sees boost, it kills my low end but functions correctly on the top end and just my mech/centrifugal and initial are used. It seems like Im damned if I do and damned if I don't as far as vacuum advance goes.
I hope my explanations are somewhat okay to understand, its my theory of whats going on with the vac advance...I could be wrong.

I am going to look into the vac adv that ford makes for supercharged applications as I might be able to adapt it to my set up.

If I increase my initial timing to make up for the lack of vac adv. how can I limit my mech advance? I was thinking about increasing the initial, keeping the vac adv. to the manifold and adjusting the retard limit per pound of boost with the BTM...is this logical am I on the right track? I have heard that retarded timing can burn valves so it worries me to use the BTM too much.

Please forgive me if I forgot someones comments/advice, Im not very experienced using forums.

Thanks again, I look forward to more comments.
 
#13 ·
Another thought and question...

Another thought and question...When an engine is naturally aspirated and the vacuum advance is connected to the carb and you floor it from a stand still, the vacuum advance doesnt advance the timing because there is no vacuum to pull it correct?

So...that means the engine is relying just on the mech and initial timing, maybe my vacuum advance isnt really the issue and its more of an initial timing issue?
 
#15 ·
malc said:
If you spin the distributor fast the weights will fly apart and advance the timing. This vacuum thing only functions from off idle as a transition into mechanical advance.The springs retaining the weights have to be overcome.
I thought the vacuum advance shifted the whole plate the weights are anchored to, thus adding to the timing when not under wot.

ie - 3000rpm = 36 dg of cent + 20dg of vacuum = 56 total under part throttle or just the 36dg when wide open. (just an example)

Anyway dizzies are so yesterday - all the cool kids are now plugging in laptops :thumbup:
 
#18 ·
"Let’s take an example to show you how it works. Let’s say we have a mild, 9:1 compression 350 Camaro with a basic point distributor. The initial timing is set at 8-degrees BTDC. By testing the distributor, we discover we have 12 degrees of vacuum advance and an additional 20 degrees of mechanical advance that’s all in by 3,000 rpm. By adding the initial advance to the mechanical advance, we find we have a total of 28 degrees of total advance. This is the total we will have with the engine at wide-open throttle at or above 3,000 rpm. Below that speed, there is less timing.

The only time vacuum advance comes into play is at part-throttle when manifold vacuum is present. Let’s say we have 8-degrees initial and 20 degrees of mechanical at a 3,000-rpm cruise with 14 inches of manifold vacuum that gives us another 10 degrees of advance. Now we have 38 degrees of total advance, but only at part-throttle. As soon as you mash the throttle, that 10 degrees of vacuum advance disappears. Vacuum for this advance originates from a ported vacuum-advance outlet on the carburetor. “Ported” manifold vacuum means that at idle with the throttle blades closed, there is no vacuum present at the port. But as soon as the throttle is opened slightly, a port is uncovered in the carburetor venturi and vacuum is routed to the vacuum can. You could connect straight manifold vacuum to the vacuum can, but this tends to add more ignition timing at idle than the engine can use."


This is what I´ve been trying to say, written by Jeff Smith in
THIS ARTICLE.
 
#19 ·
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph)....
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.


From a former GM engineer..
 
#20 ·
Actually it doesn't shift the whole plate the weights are mounted to, it shifts the thing underneath by the pickup.

Cool kids... I dunno about that one. I will however go to the dark side by this time next year and have the Holley Projection TBI, but mostly for the coldstart feature. My truck is slowly becoming too "electronic" as is. I love carbs, I just don't like running out on a cold morning, pumping the peddle twice firing it just to sit there for 5 minutes to make sure it doesn't die all while shivering. I have to side with Jessie James on carbs..." You need a carb so the cops cant shut you down with a satellite." Plus theres the cool factor of tuning a carb so well that it runs like its injected, starts like its injected and out performs the "cool kid," not to mention when people ask if its injected saying,"Nope its carbed." :thumbup:
 
#21 ·
Popping through the carb on cold only starts would generally be caused by a choke not closed enough. I would take a hard look at the choke when the engine is cold to be sure it is closed tight and stays closed upon startup. It should open about 1/8 of an inch or so on initial startup and slowly open over a 2 minute period or so. Electric chokes are not as good as a mechanical heat operated choke as the opening rate has noting to do with engine temperature. Timing would not gererally be temperature specific.
I have a similar setup to yours and I just pump it 5 or 6 times, turn the key and it starts. It occasionally stalls, and I repeat the 5 pumps and re-start it and it is good to go.
 
#23 ·
autonuerotic said:
Thanks, T-bucket. Ill look into it more and who knows it might save me $1500 for that TBI. What exactly are you running for timing and a distributor? How do you have your vacuum lines?
I run about 12 Initial 38 total with an Accel billet distributor. I really dont think your issue is ignition related.
When its cold, stab teh gas once and go check the choke, it should be closed tight. have someone else start it and you should see it open 1/8 or so. If either or both of these are not happening, you need to fix the issue.
 
#24 ·
...

Your right, timing probably isnt part of my cold start issue, however I think it could be linked to why my truck hesitates off the line... which with boost and a light weight T-bucket wouldnt be an issue and more along the lines be one heck of a ride! I first thought my hesitation was because of a lean conditon in my carb so I bought an Innovate LM-1 wideband air fuel meter...Not the case and quite a few people have led me towards timing which makes sense.
 
#25 ·
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph)....
He's good up to this point.... but,
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream.
This is where his OPINION gets in the way of fact. First of all, catalytic converters were used long before most cars went to ported vacuum, and it wasn't for emissions reasons (especially since less lead at idle makes more HC emissions), it was for mileage reasons. Adding more advance at mid throttles was capable with ported advance, and therefore better (and cleaner) cruise was possible.
One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC).
Uhh... no. No engine will operate at 2 degrees ATDC unless its a diesel
This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system;
This has been a misconception for years. When the charge burns is irrellevant to HC emissions. Why would they ensure that some of the charge burns in the exhaust? Why not burn it earlier when it can be used to help smooth the idle and make power? The real reason this was the case was for NOx emissions. Making the spark late reduced cylinder pressures to reduce NOx. trust me, there is enough heat in the exhaust that the AIR system does just fine without extra hydrocarbons there to burn up the cat and destroy exhaust valves.
as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
Only at idle... the rest of the time it increased cylinder pressures due to more part-throttle lead.
"ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
And he sums up his meager grasp on ignition theory right here. Just because he likes the old way, doesn't make it a better way. Isn't it possible that newer technology and discoveries found a better way?
From a former GM engineer..
And, ladies and gentlemen of Toyota incorporated, this concludes my lecture on "why GM has been almost bankrupt for 20 years and why we should buy them."

Just kidding :mwink:

Onovakind67, I respect your contributions, but some of the information that someone else has given you is just incorrect on this one. I've read this guy's ramblings before and he is just off the mark of fact. He's so close, but just slightly off the mark with this one. No argument intended with you, just the guy who wrote this.
 
#26 ·
autonuerotic said:
I have to side with Jessie James on carbs..." You need a carb so the cops cant shut you down with a satellite." Plus theres the cool factor of tuning a carb so well that it runs like its injected, starts like its injected and out performs the "cool kid," not to mention when people ask if its injected saying,"Nope its carbed." :thumbup:


That is the funniest thing I've read all day

Mikey
 
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