carburetion issue??

[Leif A.]

Re: carburetion issue??

Leif, Unless I am mistaken, you should be able to remove the sight plugs while the engine is running and see the float level. The fuel should be right up to the bottom of the inspection plugs as I recall. I used to adjust the floats while it was running and that was that. No disassembly needed. All my stuff is AFB WCFB Carter now, but I played with a bunch of Holley's back in the day. Take a look and report back.

Dan,

Just watched a Holley video on the subject...pretty straight forward procedure. Thanks, Dan

Leif

[Dennis O.]

Re: carburetion issue??

Unless '67 L79 air cars are different from non-air cars, they have a Holley Model 4160 that does not have externally adjustable floats, or float inspection plugs. The floats are set "dry".

[Gerard F.]

Re: carburetion issue??

Leif,I agree with Dennis, if your carb is an original list-3810, the fuel bowls have to come off to adjust the float level. The adjustment procedure is in the 67 Overhaul Manual, not the 67 Service Manual.In 67 they went to the non-externally adjustable fuel bowls.

[Leif A.]

Re: carburetion issue??

Dennis and Gerald,

The carb on my car is the original list 3810. I'll educate myself further before proceeding. Thanks for the heads-up.

Leif

[Leif A.]

Re: carburetion issue??

I just put the Mityvac on my new B26 (not yet installed, NAPA/Echlin VC1765) vacuum canister. First time doing this, so please bear with my ignorance. The plunger began to move at 4-5" Hg and was pulled to the limit by 14" Hg. Having never dealt in inches of Mercury before or testing a vacuum canister, do these numbers sound within spec for my '67 L79? I've read articles on vacuum until my head hurts and it's all "rocket science" to me. I know everyone talks about the 2" rule and I'm still not clear on that whole issue, either. Hopefully, this week I will be able to start taking baby steps on the fine tuning of my car. 45 years ago, I could have tackled this with my eyes closed but it's like starting to walk all over again.

[Timothy B.]

Re: carburetion issue??

Leif,

A good tuned L-79 should make approx 14" vacuum at 750idle RPM. The vacuum control (B-26) should be pulled full at 12" if my memory is correct.

I think you will be fine, after you install it check the idle vacuum with everything hooked up including the vacuum advance, should be close to 14".

[Leif A.]

Re: carburetion issue??

Leif,

A good tuned L-79 should make approx 14" vacuum at 750idle RPM. The vacuum control (B-26) should be pulled full at 12" if my memory is correct.

I think you will be fine, after you install it check the idle vacuum with everything hooked up including the vacuum advance, should be close to 14".

Thanks, Tim.

[Duke W.]

Re: carburetion issue??

Take that B26 back and show them it doesn't meet the 12" all in spec; 14" is too much. Ask for an exchange. and when you go back to pick it up, measure it before you take it out of the store.

Since your L-79 has A/C you have to measure idle speed/vacuum with the compressor engaged and then apply the Two-Inch Rule.

The OE VAC is a 8" 236 16. Is that what you have? Does it meet spec?

Duke

[Joe R.]

Re: carburetion issue??

Leif,

Duke covers the two inch rule on page 17 of his San Diego NCRS National convention here:

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Joe

[Leif A.]

Re: carburetion issue??

Take that B26 back and show them it doesn't meet the 12" all in spec; 14" is too much. Ask for an exchange. and when you go back to pick it up, measure it before you take it out of the store.

Since your L-79 has A/C you have to measure idle speed/vacuum with the compressor engaged and then apply the Two-Inch Rule.

The OE VAC is a 8" 236 16. Is that what you have? Does it meet spec?

Duke

When I am next by the car (later this week) I will check the canister currently on the car. Can you clarify what 8" 236 16 means? I learn quickly and don't forget but first I need to have a crystal clear understanding of what I am learning...thank you for your patience.

[Leif A.]

Re: carburetion issue??

Leif,

Duke covers the two inch rule on page 17 of his San Diego NCRS National convention here:

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Joe

Joe,

I have read and re-read Dukes article and I know he is the guru (flattery intended) on this subject. However, I am not grasping the 2" rule.

• The vacuum advance control (VAC) should be “locked” at full advance at idle speed/vacuum
– To ensure the above, the VAC should provide full advance at no less than 2” less than typical
idle vacuum
– This is the Two-Inch Rule

Line two above is where I get lost. When and where are we measuring for this 2" difference? I understand this is maybe the most important thing to get correct and that is why I need crystal clear understanding. Thanks.

[Duke W.]

Re: carburetion issue??

Typical L-79 non-A/C, non-AIR idle vacuum at 750 is 14-15". Let's call it 14". The Two-Inch Rule states that the VAC should provide full vacuum advance at not less than 2" less than 14".

14 - 2 = 12.

So a 12" VAC meets the rule, and a nominal B26 VAC provides maximum advance of 16 deg. at 12" vacuum. The signal line to the VAC from the carb nipple on a non-AIR L-79s transmitts the manifold vacuum signal.

The OE VAC is stamped "MS 236 16". It provides maximum advance (also 16 deg.) at only 8" manifold vacuum, and it starts adding advance at about 4", so it is much more aggressive than necessary to meet the Two-Inch Rule. (The vacuum and centrifugal specs are in your service manual. Look at them.)

AFAIK there is no "idle up" function on your L-79 with A/C, so if the compressor engages at 750 it will lose 50-100 revs and manifold vacuum will drop. How much? I don't know. You need to measure it and report back.

This much idle speed drop may destabilize the idle and cause the engine to lose speed and stall. It also reduces fan speed, so there is less cooling air at idle. You need to do is determine an acceptable speed that the engine will idle at with the compressor engaged, and let the idle speed with no compressor load fall where it wants. For example, if you find that the minimum stable idle speed is 750 with the compressor engaged, then the idle speed when the compressor is not engaged may be 800-850.

You also need to measure idle vacuum with the compressor engaged and it likely will be noticeably less than 14", so a B26 VAC will probably not pass the Two-Inch Rule, and you will need the original VAC (assuming it is original and actually works - another test for you to do) or the modern equivalent, which is a B28.

So here's the step-by-step procedure. All steps except 1 must be done with the A/C compressor engaged.

1. What is the ID number on the installed VAC? Does it meet spec?

2. With the AC compressor engaged, measure idle speed/vacuum.

3. Does the installed VAC meet the Two-Inch-Rule for this measured manifold vacuum.

4. Is the above idle speed sufficient to maintain a stable idle?

4. If not, increase in 25 RPM increments until you find a satisfactory idle speed with the compressor engined.

5. Once a satisfactory idle speed with the compressor engaged is determined, apply the Two-Inch-Rule to verify that the installed VAC is satisfactory. If not, install a VAC that does, and repeat all the above steps.

6. Go through the idle speed/mixture adjustment procedure using the above idle speed you determined with the compressor engaged.

7. Test drive for satisfactory performance, stop and go traffic with the A/C on.

Write down your measurements. Don't rely on memory. You will confuse and/or forget.

For reference, standard sea level pressure is 29.92" Hg, so the higher the manifold vacuum, the less the absolute manifold pressure; 15" on the vacuum gage means the pressure in the manifold is about 1/2 of atmospheric. A pure vacuum would measue about 30. The highest you will likely see when you let off the throttle at medium to high revs is about 22".

Manifold vacuum is a surrogate for engine load, but it's an inverse relationship. At maximum load (WOT) manifold vacuum is near zero. As load decreases at a given RPM manifold vacuum increases. For example, if you are cruising at a steady speed on a flat road and the road begins an upgrade, you will have to increase throttle to maintain speed and this will decrease manifold vacuum. Then when the road transitions back to level, you will have to decrease throttle to maintain constant speed and manifold vacuum will increase.

For the test drive, it will be educational for you to use a 3' or so piece of 1/8" tubing to tee the vacuum gage into the VAC signal line so you can keep the gage in the cockpit and observe the manifold vacuum behavior as you drive.


Duke

[Leif A.]

Re: carburetion issue??

Duke,

Now THAT is crystal clear!!! I can't thank you enough for taking the time to answer in such detail, making it understandable to even a neophyte like myself. I will be working on the car later this week and will most definitely report back with any and all info. I really like the idea of routing a vacuum line from a "T" to a vac gauge while test driving to record changes at different speeds and RPM. I may need a co-pilot to do the writing but I'll figure that out. Thank you, again, Duke. BTW, I took your advice from back on March 18th and installed a DeWitts restoration radiator, as well.

[Duke W.]

Re: carburetion issue??

You will find that the vacuum gage is all over the place in normal driving as you constantly change throttle positions and revs to keep up with traffic situations.

If you have a mile or so of level road or freeway. you can test/record manifold vacuum at various cruising speeds in top gear. That's interesting information to know.

Standard sea level atmospheric pressure is 29.92" Hg, which is about 14.7 psi, so it's easy to remember that 2" Hg is about 1 psi.

Large supercharged reciprocating aircraft engines - and there are still plenty around - are usually rated for takeoff power at standard sea level conditions with a certain specified manifold pressure in inches Hg and RPM.

The common (and wonderful) Pratt and Whitney R-2800 sea level takeoff power is typically 2000 HP @ 2700 RPM with 60" Hg manifold pressure. This is what is called absolute manifold pressure, which is the sum of atmospheric pressure plus boost pressure added by the supercharger. Since standard sea level pressure is about 30" the supercharger boost is 30" or 14.7 psi or one atmosphere.

Tire pressure we measure is what's called gage pressure, sometimes abbreviated psig to ensure it's not confused with absolute pressure that is abbreviated psia.

If you take a tire inflated to 30 psi on your gage and sent it into outer space your gage will read about 45 psi. Either way absolute tire pressure is 45 psi, but gage pressure is 15 psi less at sea level because atmospheric pressure is pushing back from the outside.

Similarly, manifold pressure on a naturally aspirated engine is less than atmospheric unless it's at WOT because the throttle valve restricts air flow unless wide open. Unlike supercharged aircraft engines where absolute manifold pressure and RPM is specified, naturally aspirated engine operating conditions are usually specified by manifold vacuum and RPM.

Absolute manifold pressure = atmospheric pressure plus supercharger pressure or minus manifold vacuum if a naturally aspirated engine. Terrestrial "vacuum conditions" are simply terrestrial conditions that are less than atmospheric pressure, like a naturally aspirated engine idling or operating as less than full load conditions.

Duke

[Leif A.]

Re: carburetion issue??

Duke,

In Dallas we're about 430' above sea level so within very minor parameters I'm going to use sea level readings plus/minus a bit. Going back to our last post regarding the "MS 236 16", that guy is long gone. The one currently in the car is a replacement and I will look for the numbering on the collar...I will, also, test it before doing anything else as far as changing it out or anything. To your point about "idle up" function (you are correct, the '67 L79 w A/C does not have this), I was talking to a fellow enthusiast today about installing and electric switch to accomplish when I turn on the A/C. However, after some thought, I don't believe I will do this as I want to keep this car as original as possible. Also, not wanting to cut into the new wiring harnesses. With all of your assistance (and, others) I feel very confident that I will solve all my issues. Some of my issues may never be solved i.e. driving in red light traffic here in Dallas in 103 degree weather with the A/C running and expecting my car to not heat to over 215 degrees while burning 10% ethanol fuel.