Questions about vacuum advance units

1) Advance dashpot is supposed to hold vacuum
2) CCW firms spring and limits travel and advance. CW increases both.
3) Second port in back is vac retard via a thermal vac switch on the Tstat housing for faster warm up of the cats.

Check the advance mechanical curve on the replacement distributor as they are all over the place.






Scotty J. "AKA" The "Mad Porter"
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Ported BBF iron head specialist & Aluminum heads from all sources.
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surfbox said:

Thanks that helps a lot. I will get a new advance unit and take it from there. I've been studying up on fine tuning the curve based on David Vizard's advice among others. I do have the lighter advance springs from Mr. Gasket installed.

Click to expand...

As talented as DV is he offers no advise relevant to the BBF's timing needs.

OEM 1978 460 with D3 heads will want about 36* total timing in by about 3200 rpm.










Scotty J. "AKA" The "Mad Porter"
"EMC 2006" 3rd place finisher
Ported BBF iron head specialist & Aluminum heads from all sources.
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The OEM advance curves of the 70's were primarily emissions and warranty oriented.

OEM curves for utilitarian 460's typically brought advance in in two stages starting off aggressively and saving the last 4 to 6* of timing for 3.5, 4.0 and 4.5k rpm just to generalize.
The high compression 429's and 460's of the era offered about 26 to 30* total timing initial + mechanical all in at about 4500.

When bringing in total advance at a lower RPM one must then adjust vacuum advance.
When removing EGR one must remove vacuum advance timing. EGR slows flame front speed.
The leaner the cruise AF's the more part throttle timing needed.

Suffice to say it is a balancing act.

We've been sending out curved DS & points OEM style distributors and custom curving MSD and aftermarket units for 2+ decades now.
The Sun Machines have shown their worth many times over.

Reman and new DS style distributors have inconsistent ignition curves.
Many aftermarket distributors come with heavy advance springs.

Fine tuning the mechanical and vacuum curves to your intended use offers better acceleration, better drive-ability by way of tip in response and transitory response and fuel economy increases.

The key is to attempt to find Max Brake Torque for both WOT and Part throttle operation at various manifold pressures and RPM points.

Basically the analog version of creating a timing map for an EFI system but less precise.

Driving with a vacuum gauge is a really good way to glean insight into a particular vehicles operational behavior.

I drove my 1997 F250 HD 460 EFI for 9 months to get familiar. I then started addressing the fueling and timing maps of the OEM ECM via Tweecer.
I've posted elsewhere about the improvements found even with an OEM engine with good exhaust.






Scotty J. "AKA" The "Mad Porter"
"EMC 2006" 3rd place finisher
Ported BBF iron head specialist & Aluminum heads from all sources.
Custom ground cams
See our products in the Vendor for sale section
Customized crate engines
ParklandAutoMachine.com
R-H-P.biz
"Parkland Performance Auto Machine" Formerly RHP
(253)-988-6648
Parkland Auto Machine

All excellent points and I have read many of your posts and have a good idea of the respect you command here. I come from a background of building single cylinder two stroke racing engines, so I have a very strong grasp of the common elements among all forms of ICE's (compression ratios, jetting basics, etc.), with some large gaps when it comes to multi-cylinder four strokes, which I'm try to close. ;-)

One of these gaps is variable ignition timing. The engines we built delivered power over a pretty narrow range, and there was no source of programmable ignition systems at the time. Therefor I learned to squeeze out every ounce of power and reliability by optimizing every other aspect of the engine, particularly carb choice, jetting, port design, intake tract length and configuration (mostly piston port reed valves) and hand make expansion chamber exhaust systems, all of which when working together could more than triple the power of a stock engine.

So yeah, I'm really geeking out on the topic of ignition timing on my truck and trying to become a self taught Sun Machine of my own... ;-)

One thing you said really stood out to me:

The Mad Porter said:

Many aftermarket distributors come with heavy advance springs.

Click to expand...

I still have my original dizzy and I'm going to compare its springs with what came in the aftermarket unit vs. what's in it now (the Mr. Gasket ones). Those original ones may just end up back in there, we shall see...

Vince in LoCal

surfbox said:

All excellent points and I have read many of your posts and have a good idea of the respect you command here. I come from a background of building single cylinder two stroke racing engines, so I have a very strong grasp of the common elements among all forms of ICE's (compression ratios, jetting basics, etc.), with some large gaps when it comes to multi-cylinder four strokes, which I'm try to close. ;-)

One of these gaps is variable ignition timing. The engines we built delivered power over a pretty narrow range, and there was no source of programmable ignition systems at the time. Therefor I learned to squeeze out every ounce of power and reliability by optimizing every other aspect of the engine, particularly carb choice, jetting, port design, intake tract length and configuration (mostly piston port reed valves) and hand make expansion chamber exhaust systems, all of which when working together could more than triple the power of a stock engine.

So yeah, I'm really geeking out on the topic of ignition timing on my truck and trying to become a self taught Sun Machine of my own... ;-)

One thing you said really stood out to me:



I still have my original dizzy and I'm going to compare its springs with what came in the aftermarket unit vs. what's in it now (the Mr. Gasket ones). Those original ones may just end up back in there, we shall see...

Vince in LoCal

Click to expand...

The OEM light spring and 1 Mr. Gasket spring typically works well after perch adjustments.
The proper slot width as noted in the link is how mechanical advance is limited.

I provided a link specific to the DS in case you missed it.





Scotty J. "AKA" The "Mad Porter"
"EMC 2006" 3rd place finisher
Ported BBF iron head specialist & Aluminum heads from all sources.
Custom ground cams
See our products in the Vendor for sale section
Customized crate engines
ParklandAutoMachine.com
R-H-P.biz
"Parkland Performance Auto Machine" Formerly RHP
(253)-988-6648
Parkland Auto Machine

The Mad Porter said:

The OEM light spring and 1 Mr. Gasket spring typically works well after perch adjustments.

Click to expand...

Now that you mention it I had that setup for a while and it seemed to work well with the 13L slot.

The Mad Porter said:

The proper slot width as noted in the link is how mechanical advance is limited.
I provided a link specific to the DS in case you missed it.

Click to expand...

Oh yes thanks for reminding me. I had seen that and when I ordered the new dizzy I believe I bought one for a '75 Tuna Boat of some flavor as mentioned elsewhere. Mine has the 10L and 13L slots and I'm currently on the 10L while I dial in the vacuum advance and sniff out any vacuum leaks I may have. I have another thread about re-bushing the throttle shafts on the 4350 carb for example. (Did I mention I'm in Cali and need to pass smog here?)

Also I just bought these and I'm curious to see what kind of readings I get once I've added O2 sensors into the mix:



Another question... I understand that stock manifold is dual plane, making each side of the carb basically independent jetting-wise. Can I expect to see any noticeable variation between the two banks? (Assuming no other issues on one bank vs the other)

surfbox said:

Now that you mention it I had that setup for a while and it seemed to work well with the 13L slot.

Oh yes thanks for reminding me. I had seen that and when I ordered the new dizzy I believe I bought one for a '75 Tuna Boat of some flavor as mentioned elsewhere. Mine has the 10L and 13L slots and I'm currently on the 10L while I dial in the vacuum advance and sniff out any vacuum leaks I may have. I have another thread about re-bushing the throttle shafts on the 4350 carb for example. (Did I mention I'm in Cali and need to pass smog here?)

Also I just bought these and I'm curious to see what kind of readings I get once I've added O2 sensors into the mix:

View attachment 95601

Another question... I understand that stock manifold is dual plane, making each side of the carb basically independent jetting-wise. Can I expect to see any noticeable variation between the two banks? (Assuming no other issues on one bank vs the other)

Click to expand...

Cali smog should be no issue so long as your initial timing is not too advanced and your AF's idle and transfer are in spec.

In some cases where 14 or 15* BTCD is too much initial for smog simply back it up.

We build 460 EFI crate engines for Cali and all are available as 50 states emissions compliant.






Scotty J. "AKA" The "Mad Porter"
"EMC 2006" 3rd place finisher
Ported BBF iron head specialist & Aluminum heads from all sources.
Custom ground cams
See our products in the Vendor for sale section
Customized crate engines
ParklandAutoMachine.com
R-H-P.biz
"Parkland Performance Auto Machine" Formerly RHP
(253)-988-6648
Parkland Auto Machine