7040207 Carb ported or non-ported?

[Robert M.]

Hello,
This afternoon while out setting the timing on my 1970 L-46, i got the timing set, with no vacuum, then with the aid of my dial back timing light, also set the rpm's to 750 at idle. I noticed that my timing light shown rpms, was about 150-200 more than my in dash tachometer rpm's.

After making the adjustments, i reconnected the vac hose to the vac advance pot, and saw no real increase in the advance. I have by passed the TCS for now.

I put a vac guage on the port from the carb, and only saw 8" at idle, not the 15+" i usually see. This stumped me. I stopped work for the day to consider this problem, my lost vacuum.

Tonight I am thinking maybe i have a ported idle carb. My reasoning is that I previously set the idle with the in dash tachometer, which yielded a higher rpm than shown on the tach, thus exposing the vac slot in the carb.

Can anybody out there, (maybe the ever resourceful Joe Lucia) tell me for certain, if the 2040207 carb for a 1970 L-46 is ported or non-ported?

Tomorrow I will test it more, and possibly remove it to look myself.


Thanks much,
Bob

[Joe L.]

Re: 7040207 Carb ported or non-ported?

Hello,
This afternoon while out setting the timing on my 1970 L-46, i got the timing set, with no vacuum, then with the aid of my dial back timing light, also set the rpm's to 750 at idle. I noticed that my timing light shown rpms, was about 150-200 more than my in dash tachometer rpm's.

After making the adjustments, i reconnected the vac hose to the vac advance pot, and saw no real increase in the advance. I have by passed the TCS for now.

I put a vac guage on the port from the carb, and only saw 8" at idle, not the 15+" i usually see. This stumped me. I stopped work for the day to consider this problem, my lost vacuum.

Tonight I am thinking maybe i have a ported idle carb. My reasoning is that I previously set the idle with the in dash tachometer, which yielded a higher rpm than shown on the tach, thus exposing the vac slot in the carb.

Can anybody out there, (maybe the ever resourceful Joe Lucia) tell me for certain, if the 2040207 carb for a 1970 L-46 is ported or non-ported?

Tomorrow I will test it more, and possibly remove it to look myself.


Thanks much,
Bob

Bob------


The vacuum advance fitting on your carb is PORTED vacuum. I'm surprised you get even 8" of vacuum at idle.

Usually, the choke unloader vacuum port will provide a NON-PORTED vacuum source if you want one. I'm not 100% sure of that on this carb, but I'm fairly sure.

[Robert M.]

Re: 7040207 Carb ported or non-ported?

Thank you Joe !

Guys like you and Duke, are why I pay my yearly dues and watch this only this Corvette board.

[Duke W.]

Re: 7040207 Carb ported or non-ported?

With the start of federal tailpipe emission standards in 1968 virtually all engines had ported vacuum advance. I can't think of any exceptions.

1966-67 engines with AIR for California had ported vacuum advance along with all L-72s and L-71s.

One pre-emission engine with ported vacuum advance was 1963 L-84, but this was a one year deal as the 64-65 versions were full time.

Sometimes ported vacuum advance ports will show some vacuum, but not full manifold vacuum, and many engines that are converted to full time need a new VAC that complies with the "Two-Inch Rule".

There are three VACs that will meet the need of all vintage Corvette engines with single point or TI ignitions as follows, and all provide 16 deg. crank advance maximum at the listed vacuum in inchec Hg:

1. All base small blocks and optional SBs with the base engine cam with manual tranmssions: B22, 15".

2. All base small blocks and optional SBs with the base engine cam with auto. trans; L-79; L-46; all big blocks: B20 or B26 12". (Note: 65-67 L-79s originally were equipped with a 8" VAC, but it's more aggresive than necessary and the B20 meets the "Two-Inch Rule")

3. All SBs with mechanical lifter cams: B28, 8"

The VACs used on ported applications generally take at least 8 or more inches Hg to begin to pull and provide up to 22 degrees of advance at very high vacuum, so in this case the 8" measured at the source is not going to pull the plunger at idle. They usually do no meet the "Two Inch Rule", which is why they must be changed when converting to full time advance. The "Two Inch Rule" states that the VAC must provide full advance at no less than 2" less than typical idle vacuum with 20-32 degrees total idle advance, which is the sum of initial and the full 16 deg. vacuum advance.

If no full time port is on Holleys and Q-jets, tee into the choke vacuum break hose for a full time manifold vacuum source, and if you're handy in the shop the carb base can usually be modified to make a ported source full time by plugging and drilling.

Converting to full time vacuum advance will reduce the tendency to run hot/overheat at idle and low speed driving and increase around town fuel economy.

Duke

[Dennis D.]

Re: 7040207 Carb ported or non-ported?

I bypass the TCS on my "207 as well. I have initial at about 12 degrees,all in around 30 deg, and there is definately a rise in rpm when the advance is connected.

Plus, as Duke states, lower temp and better fuel ecomomy

[Paul L.]

Re: 7040207 Carb ported or non-ported?

I bypass the TCS on my "207 as well. I have initial at about 12 degrees,all in around 30 deg, and there is definately a rise in rpm when the advance is connected.

Plus, as Duke states, lower temp and better fuel ecomomy

But that is ported vacuum!!

[Duke W.]

Re: 7040207 Carb ported or non-ported?

Once you get the total idle timing up into the proper range I specified above, the engine is operating at higher thermal efficiency and needs less mixture to overcome engine friction. That's why the idle speed increases, so the last step in the conversion is resetting the idle speed/mixture to the correct idle speed. At the same idle speed as with ported vacuum advance, full time advance will usually increase manifold vacuum.

As valve overlap increases, more total idle timing is required to overcome the slower flame propagation speed with increasing exhaust gas dilution, so base engine cams usually like something in the low to mid area of the 20-32 degree range, and high overlap cams idle best in the upper end of the range.

Duke

[Robert M.]

Re: 7040207 Carb ported or non-ported?

A follow up to my original problem.
i went to the garage and looked over everything, and ran the engine. Today my manifold vacuum was reading 14", and the the Dist Vacuum advance source was also measuring 14" at 750 rpm as indicated with an Actron CP7529. Different than yesterday.

I removed the carb, and the Dist Vacuum advance source is below the throttle plate. Confirmed this by blowing air through the vacuum port. The throttle plate appears to be closed. I will also say that this 2040207 carb was purchased from a re builder at the 1993 Bloomington meet, so it may not be 100% correct.

After reinstalling the carb, I have the 14" of vacuum at both sources, at 750 rpm. I check my timing and now I have no Vacuum advance. Open up the dist. and check the movement of the Vacuum advance, and it looks good. Attach a hose and gauge to the vacuum advance and manually draw 15", and movement is good. Reassemble and run, no advance. I open the dist. again, and ask, what is different than the last time she ran well?
Yesterday I had replaced the wire that runs from the Breakerless Single Wire unit to the coil. The spade connector to the Breakerless unit, is very long, and was binding on the dist. housing. I bent the spade up out of the way, reassembled, and now have the full vacuum advance I expected to have.

Bottom line, the mechanic, me, changed a part and failed to see a problem before I assembled the dist. Every thing seems to work nicely now.
It pulls a good 15" at both sources at 750 rpm. To also note, 2 years ago I did switch to the B26 vacuum advance, as Duke has frequently advocated.

[Paul L.]

Re: 7040207 Carb ported or non-ported?

So where compared to Dennis are you pulling manifold vacuum?

[Robert M.]

Re: 7040207 Carb ported or non-ported?

Same port as Dennis

[Paul L.]

Re: 7040207 Carb ported or non-ported?

With the start of federal tailpipe emission standards in 1968 virtually all engines had ported vacuum advance. I can't think of any exceptions.

1966-67 engines with AIR for California had ported vacuum advance along with all L-72s and L-71s.

One pre-emission engine with ported vacuum advance was 1963 L-84, but this was a one year deal as the 64-65 versions were full time.

Sometimes ported vacuum advance ports will show some vacuum, but not full manifold vacuum, and many engines that are converted to full time need a new VAC that complies with the "Two-Inch Rule".

There are three VACs that will meet the need of all vintage Corvette engines with single point or TI ignitions as follows, and all provide 16 deg. crank advance maximum at the listed vacuum in inchec Hg:

1. All base small blocks and optional SBs with the base engine cam with manual tranmssions: B22, 15".

2. All base small blocks and optional SBs with the base engine cam with auto. trans; L-79; L-46; all big blocks: B20, 12". (Note: 65-67 L-79s originally were equipped with a 8" VAC, but it's more aggresive than necessary and the B20 meets the "Two-Inch Rule")

3. All SBs with mechanical lifter cams: B28, 8"

The VACs used on ported applications generally take at least 8 or more inches Hg to begin to pull and provide up to 22 degrees of advance at very high vacuum, so in this case the 8" measured at the source is not going to pull the plunger at idle. They usually do no meet the "Two Inch Rule", which is why they must be changed when converting to full time advance. The "Two Inch Rule" states that the VAC must provide full advance at no less than 2" less than typical idle vacuum with 20-32 degrees total idle advance, which is the sum of initial and the full 16 deg. vacuum advance.

If no full time port is on Holleys and Q-jets, tee into the choke vacuum break hose for a full time manifold vacuum source, and if you're handy in the shop the carb base can usually be modified to make a ported source full time by plugging and drilling.

Converting to full time vacuum advance will reduce the tendency to run hot/overheat at idle and low speed driving and increase around town fuel economy.

Duke

So a B26 (B20-equivalent) is OK for a 1974 L-48 with TH400?

[Paul L.]

Re: 7040207 Carb ported or non-ported?

Same port as Dennis

OK, I guess each Q-Jet differs. That is well above the flappers. Not even close to manifold vacuum.

What is the solution? This is John Z's version.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

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).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

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.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

--------------------
JohnZ
__________________

[Duke W.]

Re: 7040207 Carb ported or non-ported?

So a B26 (B20-equivalent) is OK for a 1974 L-48 with TH400?

Yes, B26 and B20 have the same basic specs 0 @ 8" 16 @ 12".

If you have a vacuum gage I'd like you to measure vacuum/RPM idling in Drive. You should be able to get it down to about 450 RPM on a base engine with a 12" full time VAC.

Duke

[Timothy B.]

Re: 7040207 Carb ported or non-ported?

Robert,

If you review the directions for installation of the breakerless unit there is a note about bending the wire you mention to a 45* angle so there is no interference in the distributor. From what you describe, the wire spade kept the contact plate in a fixed position because of the interference.

There is also a small wire tang that captures these wires to keep them from the shutter wheel.

[Paul L.]

Re: 7040207 Carb ported or non-ported?

Yes, B26 and B20 have the same basic specs 0 @ 8" 16 @ 12".

If you have a vacuum gage I'd like you to measure vacuum/RPM idling in Drive. You should be able to get it down to about 450 RPM on a base engine with a 12" full time VAC.

Duke

My car is in commercial storage until mid-April. I can say from 2009 experience that with manifold vacuum (long spout near choke assy) it idles nicely at ~650rpm in P and ~550rpm in D. There is no hint of stalling. I haven't tried as low as 450rpm.