64 FI Vacuum Measurement

**Steve G.** · July 19, 2015, 11:59 PM

Re: 64 FI Vacuum Measurement

The caveat: I am not a ramjet fi owner and have had no fender time with one. I am a journeyman mechanic of almost 40 years with extensive experience in carbs, modern low pressure common rail fuel systems, high pressure common rail fuel systems and distributed high pressure fuel systems. I believe I have a good understanding of the principles and requirements of an induction system. That said, take the rest for what it's worth.

My first peak at the principles of operations of one of these systems was just a few days ago, brought about by a discussion of fuel percing in the spiders. The explanation that fuel boiling in the lines was causing a lean idle condition because the vapours were escaping through a vent hole in the nozzle box puzzled and intrigued me. That's for another thread.

The vents are in the nozzle blocks because the fuel is coming out of the nozzles with very little pressure behind it, less than .5 psi. Not enough to atomize the fuel sufficiently for proper vaporization. The engineers opted to deal with that by having the nozzles drip into the nozzle box where there is a steady stream of air being being drawn into the box by the engine vacuum on the intake side of the box. This stream of air picks up the fuel in the chamber and breaks it down further for proper vapourization. 40% of the idle air has to go through those vents to make that happen. If the fuel doesn't vapourize properly it won't burn completely. What doesn't burn may as well not be there. You have a lean fuel condition.

At idle, the throttle plate is supposed to be completely closed. If it isn't, whatever air goes past that plate is air that should have gone through the nozzle boxes. I would start by closing that off as per the manual.

When you turn your idle fuel needle in you are leaning the mixture. You are reducing the vacuum signal to the diaphram that meters out the fuel. Turning the screw out, the opposite of course. It sounds like your idle is not optimum. Turning it in makes it worse. That tells you that your poor idle is due to a lean condition. When it stops making a difference when you turn it out you know that the maximum signal is being sent to the diaphram, yet still not enough fuel is flowing for the amount of air the engine is receiving.

I don't know the reasons for the 1100 rpm idle, but I would start with returning that to spec. It is quite likely that the idle systems fuel circuitry, signal intensity to the diaphram, is not intended to supply as much fuel for the amount of air being consumed at 1100 rpm. This system differs from carbs in that the main metering system can't take over where the idle falls short because the air flow is separate between the two circuits. The main fuel metering doesn't appear to take a signal until air is flowing through the open butterfly which remains closed at idle.

Once you have turned your idle speed down (restricts idle air intake) reset your fuel mixture. 1.5 turns is an arbitrary setting to get you started. back out the screw slowly while watching a sensitive tach. As your rpm starts to pick up with the more accurate fuel metering, slow it down again with the speed screw. Turn the screw out until the speed no longer increases, adjusting idle speed as you go. If you go too far the speed should start to drop as the mixture gets too rich.

Restricting the air intake in the idle circuit by correcting the idle speed and closing the throttle plates as they're supposed to be will increase the vacuum at idle. This in turn will improve the vaporization of the fuel at idle. Lowering the pressure (increasing vacuum) in the chamber where the fuel discharges will improve it's vaporization. Higher vacuum will speed the flow of air through the nozzle box, again improving atomization and vapourization.

One of the benefits of fuel injection over carbs is their ability to atomize fuel without having to heat up the charge air with exhaust gas heat. This system doesn't do that at idle and has to rely on air flow, just like a carb, but without the benefit of an exhaust heated air charge. It will be sensitive to anything that impedes it's ability to do that.

Steve

**Jim L.** · July 20, 2015, 07:09 AM

Re: 64 FI Vacuum Measurement

Larry,

A little more information is needed before anyone can provide accurate comments to your posting:

1. Are you unable to adjust the engine idle speed to anything lower than 1100 RPM or did you choose that speed because of Duke's recommendation?

2. When you turn the Idle Fuel in and the engine surges.... are you saying that engine speed increases?

FYI: That plug on the air meter closes off a hole which can be used to provide ported vacuum to the distributor vacuum advance can. You don't want to use that port as a vacuum source.

Jim

**Duke W.** · July 20, 2015, 08:39 AM

Re: 64 FI Vacuum Measurement

Originally posted by Larry Meyer (37196)

Idle was about 1500 (I think high due to the improved vacuum) which I reduced to 1100 (I know the book calls for 850 or so but I seem to remember an article from Duke Williams which recommended the 1100). So with engine hot and at 1100 the vacuum was 14 in hg. I thought it would be lower. Does this seen to be right for a FI unit?? What should I expect??

Secondly, there is a screw on the top of the air meter which is described as idle fuel in the shop manual but looking at the diagram is really a control of supplemental vacuum from the plenum to the main fuel meter diaphram at idle since the throttle is mostly closed. The book says start it at 1.5 turns out at 850 rpm and then adjust for best idle. Mine is out 2.5 turns. Turning it in caused the idle to surge. Turning it out does nothing which I think means it is already out of way (no restriction). If this makes sense, does the fact that I need full plenum vacuum at idle indicate an issue?? The only one I can come up with is that the main diaphram is "stiff" and I need a little more help to flex it. I have a new one but have never changed it.

Chevrolet is vague about recommended idle speed. Most of the time it is only "implied" via the initial timing spec, such as X deg. @ xxx RPM. For example the initial timing spec for my Duntov-cammed 340 HP is specified at 700, but there's no way the engine will idle stably at that speed even after I replaced the 15.5" VAC (that didn't pass the Two-Inch Rule) with the 8" VAC.

My idle speed recommendation has always been "as low as possible commensurate with acceptable idle quality". That's a subjective criterion, so for any given engine configuration it will vary from owner to owner. Some like a real lumpy lopey idle, but others prefer a higher idle speed, which is a little smoother, and with the high overlap cams, you reach a point as you reduce idle speed where it destabilizes, and the engine can stall.

My baseline idle speed recommendation for SB mechanical lifter cams is 900 and at this idle speed the 30-30 cam should pull about 10" Hg. It can be tougher to get acceptable idle quality with Fuel Injection than with the OE Holley carb on the 365 HP engine, and I've stated that it may take 1000-1200 to get acceptable idle quality with FI.

My recommendation is that you set the idle mixture screw at the recommended initial setting of 1.5 turns out from the seat, and set the idle speed at 900. Then adjust the idle mixture screw in and out in quarter turn increments until you get the best idle quality and better idle quality will be indicated by slightly increased idle speed and manifold vacuum. Then you can reset the idle to 900 and try again. If you can't find acceptable idle quality at 900, try 1000 and go through the procedure again.

Use an accurate test tach as the vehicle mechanical tach may not be that accurate in the idle speed range.

Like typical carburetors of the era, turning the idle mixture screw in leans the mixture an out richens it.

It's an interative process although it doesn't really take much time, and the end result will vary somewhat from owner to owner and car to car.

Duke

P. S. Before you work on the idle speed, check the VAC. An OE VAC is stamped 236 16 on the mounting bracket, and the proper spec current replacement is stamped B28. Use a Mity Vac or equivalent to pump it down. The link should begin to move at about 4" Hg and be fully deployed at about 8". Pump it down to 18-20" and make sure it holds this vacuum level. The needle on the gage should not move.

An incorrect VAC that doesn't meet the Two-Inch Rule can cause idle stability problems. The initial timing should be set in 12-16 degree range, however, the best way to set it assuming the engine still has the OE centrifugal curve, which maxes out at 2350 is to rev the engine to over 2500, verify that the advance does not increase with increasing revs and set the advance at about 38 with the VAC disconnected and the signal line plugged.

Since the OE centrifugal starts at about 700 a 900 idle speed with add a couple of degrees centrfigal advance, so your total idle advance will be in the low thirties, which is good because high overlap cams need a lot of total idle advance due to slowing of the combustion propagation rate from high exhaust gas dilution.

A couple of years ago I did a minor tune-up on a local chapter members newly acquired low miles unrestored '65 FI Coupe. Idle quality was poor and it was running hot in low speed traffec. My initial check of total idle advance indicated that the VAC, which appeared to be the OE 236 16 was dead, but when I pumped it down, it meet spec and held vacuum.

I then removed the short piece of rubber tubing and tried to blow through it. Yup, bubba plugged it with a small ball bearing that I pushed out with a straight piece of coat hangar wire. With the VAC now functioning I checked total idle advance, which was in the ballpark, tweaked the idle speed and mixture screws and got a decent idle at about 900.

It ran cooler, but I told the owner than the original nearly 50-year old OE Harrison radiator was probably clogged up with deposits and would like need to be replaced, which he did a few months later with a properly datd coded DeWitts. Engine temps are now well within acceptable range even in 100 degree weather.

I think there's an old adage that goes something like: "90 percent of all FI problems are the ignition system".

Duke

**Steve G.** · July 20, 2015, 10:17 AM

Re: 64 FI Vacuum Measurement

Originally posted by Duke Williams (22045)

Chevrolet is vague about recommended idle speed. Most of the time it is only "implied" via the initial timing spec, such as X deg. @ xxx RPM. For example the initial timing spec for my Duntov-cammed 340 HP is specified at 700, but there's no way the engine will idle stably at that speed even after I replaced the 15.5" VAC (that didn't pass the Two-Inch Rule) with the 8" VAC.

My idle speed recommendation has always been "as low as possible commensurate with acceptable idle quality". That's a subjective criterion, so for any given engine configuration it will vary from owner to owner. Some like a real lumpy lopey idle, but others prefer a higher idle speed, which is a little smoother, and with the high overlap cams, you reach a point as you reduce idle speed where it destabilizes, and the engine can stall.

My baseline idle speed recommendation for SB mechanical lifter cams is 900 and at this idle speed the 30-30 cam should pull about 10" Hg. It can be tougher to get acceptable idle quality with Fuel Injection than with the OE Holley carb on the 365 HP engine, and I've stated that it may take 1000-1200 to get acceptable idle quality with FI.

My recommendation is that you set the idle mixture screw at the recommended initial setting of 1.5 turns out from the seat, and set the idle speed at 900. Then adjust the idle mixture screw in and out in quarter turn increments until you get the best idle quality and better idle quality will be indicated by slightly increased idle speed and manifold vacuum. Then you can reset the idle to 900 and try again. If you can't find acceptable idle quality at 900, try 1000 and go through the procedure again.

Use an accurate test tach as the vehicle mechanical tach may not be that accurate in the idle speed range.

Like typical carburetors of the era, turning the idle mixture screw in leans the mixture an out richens it.

It's an interative process although it doesn't really take much time, and the end result will vary somewhat from owner to owner and car to car.

Duke

P. S. Before you work on the idle speed, check the VAC. An OE VAC is stamped 236 16 on the mounting bracket, and the proper spec current replacement is stamped B28. Use a Mity Vac or equivalent to pump it down. The link should begin to move at about 4" Hg and be fully deployed at about 8". Pump it down to 18-20" and make sure it holds this vacuum level. The needle on the gage should not move.

An incorrect VAC that doesn't meet the Two-Inch Rule can cause idle stability problems. The initial timing should be set in 12-16 degree range, however, the best way to set it assuming the engine still has the OE centrifugal curve, which maxes out at 2350 is to rev the engine to over 2500, verify that the advance does not increase with increasing revs and set the advance at about 38 with the VAC disconnected and the signal line plugged.

Since the OE centrifugal starts at about 700 a 900 idle speed with add a couple of degrees centrfigal advance, so your total idle advance will be in the low thirties, which is good because high overlap cams need a lot of total idle advance due to slowing of the combustion propagation rate from high exhaust gas dilution.

A couple of years ago I did a minor tune-up on a local chapter members newly acquired low miles unrestored '65 FI Coupe. Idle quality was poor and it was running hot in low speed traffec. My initial check of total idle advance indicated that the VAC, which appeared to be the OE 236 16 was dead, but when I pumped it down, it meet spec and held vacuum.

I then removed the short piece of rubber tubing and tried to blow through it. Yup, bubba plugged it with a small ball bearing that I pushed out with a straight piece of coat hangar wire. With the VAC now functioning I checked total idle advance, which was in the ballpark, tweaked the idle speed and mixture screws and got a decent idle at about 900.

It ran cooler, but I told the owner than the original nearly 50-year old OE Harrison radiator was probably clogged up with deposits and would like need to be replaced, which he did a few months later with a properly datd coded DeWitts. Engine temps are now well within acceptable range even in 100 degree weather.

I think there's an old adage that goes something like: "90 percent of all FI problems are the ignition system".

Duke

Duke,

As I'm sure you are aware of, as you increase the idle speed on a carb you are opening the throttle plate further and further. When you get past a certain point and the idle circuit can no longer provide an adequate amount of fuel for the amount of air ingested the main metering system starts to take over and supply fuel. The idle mixture screws become largely ineffective. You can set the idle stop screw anywhere you like and you should get an appropriate amount of fuel. It's no different than if your foot is holding the throttle there driving down the road.

From what I'm reading about this FI system, it is totally different. At idle, which is any time the foot feed is not opening the throttle plate, the air is entering through a completely different pathway, going around the throttle plate. The idle speed screw is actually an air valve, controlling the amount of air flow at idle, not unlike the butterfly does at idle with a carb. The fuel screw adjust the amount of fuel flow for a given signal strength, which is dependant on that air flow. But once you've maxed the idle circuit fuel flow, ie you have the screw turned out as far as you can and you still have a lean condition, the main fuel feed system does not take over when you raise the idle speed. It needs the butterfly to open in order for the main metering system to come into play. So with this animal, if you have a lean idle condition, the idle fuel flow is maxed, raising the idle speed is simply going to make it leaner.

While raising the speed may create enough inertia to reduce stalling, it is not dealing with the problem. If you can't get a decent idle at 850-900 rpm due to lean fuel I would want to solve that problem first.

Steve

**John D.** · July 20, 2015, 01:01 PM

Re: 64 FI Vacuum Measurement

HI Larry, Nice to see that your '64 with a 30-30 cam has so much vacuum. Not typical though but consider yourself lucky.
I just refinished restored a very early '64 FI unit. It has the slotted plug on top of the air meter. It's usage was accurately described by Jim Lockwood.

Some of the early '64's have this slotted plug and some don't. RP was using left over '63 air meter body castings. But inconsistently on the early '64's. Don't try and remove that slotted plug as they get frozen in there. Nothing to clean out.
Although the hex shaped idle fuel screw on an FI unit looks identical(similiar) to one from a carburetor it is all one it's own. Taper on the end if different. You didn't ask this but I threw it in. As Duke mentioned the 1-1/2 turns out is not gospel. It's just for initial start

Here's how to learn about FI units guys. When I went to the GM training center in 1961-62 I took all the United Delco Courses.
When I took the fuel injection class the instructor from Butler, PA told us to concentrate on the theory. His name was Paul Glagola. Paul said if you don't know theory then you don't know squat about fuel injections. To learn takes hands on experience he says and lots of studying and reading the manuals. The final exam for the course was 100% theory. Most of the Chevy mechanics in class flunked the course.
I took the class twice in the old days. The 2nd time I learned some about theory but it took many years of learning the hard way to grasp the finest detail. Still learning.
The '64-65 units are the best ones that RP made. Easiest to start also.
By the way Larry etal.
Here's how to start the car when it's cold.
Open the door. Get in. Touch the accelerator pedal an inch or so.
Get out of the car. Close the door with the window open. Reach in the window and turn the key. This info came from Michael Hanson and I absolutely love it.
To start the car when it's hot: Get in the car. Put the accelerator pedal to the floor (manual says 2/3 the way). Hold the pedal there. Turn the key in the start position until the engine fires. Do not pump the accelerator pedal. Info came from me and the manuals.
Apply this procedure to all '57 to '65 FI cars.
If you own a '58 to '65 fuel car buy "Corvette Servicing Guide". In it you will find the best fuel injection section. If you own a midyear still buy this manual along with the '63 shop manual. The '64 or '65 supplement (I forget which) has a 1/4 page just on starting. Nothing else.
John D.

**Steve G.** · July 20, 2015, 02:04 PM

Re: 64 FI Vacuum Measurement

Originally posted by John DeGregory (2855)

HI Larry, Nice to see that your '64 with a 30-30 cam has so much vacuum. Not typical though but consider yourself lucky.
I just refinished restored a very early '64 FI unit. It has the slotted plug on top of the air meter. It's usage was accurately described by Jim Lockwood.

Some of the early '64's have this slotted plug and some don't. RP was using left over '63 air meter body castings. But inconsistently on the early '64's. Don't try and remove that slotted plug as they get frozen in there. Nothing to clean out.
Although the hex shaped idle fuel screw on an FI unit looks identical(similiar) to one from a carburetor it is all one it's own. Taper on the end if different. You didn't ask this but I threw it in. As Duke mentioned the 1-1/2 turns out is not gospel. It's just for initial start

Here's how to learn about FI units guys. When I went to the GM training center in 1961-62 I took all the United Delco Courses.
When I took the fuel injection class the instructor from Butler, PA told us to concentrate on the theory. His name was Paul Glagola. Paul said if you don't know theory then you don't know squat about fuel injections. To learn takes hands on experience he says and lots of studying and reading the manuals. The final exam for the course was 100% theory. Most of the Chevy mechanics in class flunked the course.
I took the class twice in the old days. The 2nd time I learned some about theory but it took many years of learning the hard way to grasp the finest detail. Still learning.
The '64-65 units are the best ones that RP made. Easiest to start also.
By the way Larry etal.
Here's how to start the car when it's cold.
Open the door. Get in. Touch the accelerator pedal an inch or so.
Get out of the car. Close the door with the window open. Reach in the window and turn the key. This info came from Michael Hanson and I absolutely love it.
To start the car when it's hot: Get in the car. Put the accelerator pedal to the floor (manual says 2/3 the way). Hold the pedal there. Turn the key in the start position until the engine fires. Do not pump the accelerator pedal. Info came from me and the manuals.
Apply this procedure to all '57 to '65 FI cars.
If you own a '58 to '65 fuel car buy "Corvette Servicing Guide". In it you will find the best fuel injection section. If you own a midyear still buy this manual along with the '63 shop manual. The '64 or '65 supplement (I forget which) has a 1/4 page just on starting. Nothing else.
John D.

John,

Yes, I'm sure there's a lot more reading that can be done for a more complete working knowledge of these things, but I guess my experience in all the various other types of injection systems and carbs gave me a bit of a leg up. I had no trouble understanding the explanation of operation given in the service manual.

That said, am I understanding it wrong? Are the idle systems and main system not independent of one another? Is there a purpose for the vents in the nozzle blocks different from what I took away from my reading of the manual?

Want to make sure I'm not missing something.
Steve

**John S.** · July 20, 2015, 06:16 PM

Re: 64 FI Vacuum Measurement

John,
I quoted that star procedure today at the nationals. Remembered it from a previous post of yours. Excellent advice. I don't think I have to tell anyone on this board that John D. is among the very few fuelie gurus left on this earth. His advice should be considered gospel. He helped me thru rebuilding and adjusting my first fuel unit this year. I owe home a debt of gratitude for this help. Thank you John!

**Larry M.** · July 20, 2015, 07:34 PM

Re: 64 FI Vacuum Measurement

I got a lot of good information here and I appreciate it very much. I did not expect this much help. I'm planning to experiment on the car. So just a couple of post scripts.

1. I also changed the idle air feed hoses under the unit. One of these was supplemented with tape but I don't think I had a serious vacuum leak there.
2. I did an electrical tune up recently using the NAPA points and condenser that Duke had listed in a Restorer article. Didn't see much help starting or at idle but a big difference under load....as Duke said...got me most of the way to where I needed to be.
3. I have a 236 16 VAC. It works but I will check to ensure it doesn't leak down.
4. I'll study this more but on a FI unit I think the idle speed screw is like pressing the accelerator or rotating the bell crank. It simply keeps the throttle plate from closing completely. I'm fairly confident I could go to a lower idle speed and keep it running.
5. What I meant by surging is that the rpm varies...drops and recovers over and over again...probably what Duke called a lumpy lopey idle. I'd prefer a strong smooth idle. I'll use an elctronic tach and play with all this.
6. I also have never adjusted the valves. I'm not a mechanic or gear head but suspect this could have some marginal impact idle performance. I have an article that Duke and John Hinkley wrote on how to do this with the engine not running.

Thanks again for your help.

**Duke W.** · July 20, 2015, 09:38 PM

Re: 64 FI Vacuum Measurement

Originally posted by Larry Meyer (37196)

I got a lot of good information here and I appreciate it very much. I did not expect this much help. I'm planning to experiment on the car. So just a couple of post scripts.

5. What I meant by surging is that the rpm varies...drops and recovers over and over again...probably what Duke called a lumpy lopey idle. I'd prefer a strong smooth idle. I'll use an elctronic tach and play with all this.

A lumpy or lopey idle from a high overlap cam means about a +/- 50-100 RPM variation from the nominal median idle speed, with a frequency of about 1-2 Hz. "Surging" by my definition is the idle cycling over several hundred RPM at much less than one Hz.

I recall observing this on a '63 FI engine back in the mid sixties. It was cycling between about 800 and 1200 with a period of about 2-3 seconds. I didn't know what caused it then, and I still don't know what could cause this behavior, but there are a number of possibilities such as a vacuum leak, the spark advance map, and probably several other potential causes.

Duke

**Larry M.** · July 21, 2015, 06:34 AM

Re: 64 FI Vacuum Measurement

Before the electrical, vacuum and nozzle work I probably had surging but suspect now it is lopey. I'll report back once I put a tach on it and try to reduce the idle or turn in the idle fuel screw. The car is certainly drivable now...took it to a cruise last night. After sitting two hours I used the cold start routine....it started without touching the throttle.

John D. - I have the original mimeographed (purple ink) starting instructions which came in the glove box paperwork. When hot, if I depress the throttle just enough for the bell crank to engage the micro switch it starts fine without over revving the engine. I'm sure you have this but if not I can post a picture.

**G B.** · July 21, 2015, 06:53 AM

My anecdotal experience.

A 327/375hp injected engine with a 7380 FI unit will idle at 800 - 850 rpm and make 11 - 12 inches of steady vacuum if:

it is properly calibrated,

the fuel is undiluted racing gas,

the Forward Balance Tube is in place,

the cam is an accurate copy of the factory 30 - 30 grind,

the valve lash is set at .030" / .030",

the nozzles are flow-matched within 5%,

the spider central hub is working right,

there are no vacuum leaks at the plenum base,

the ignition timing is set for 12 - 16 degrees initial (and 36 total),

the points are set correctly or the distributor has an electronic trigger that's working right,

the advance weight springs are a little shorter or stiffer than stock,

the vacuum advance canister is a 236 or B28 and working properly,

the spark plugs are equivalent to an AC46 heat range and do NOT have platinum electrodes,

the spark plug wires do NOT have the "correct dates",

the axle link is properly balanced and NOT resurfaced,

the gear pump has proper clearances,

the combined resistance of the coil and ballast resister is between 2 and 3 ohms,

the engine easily passes compression and leak-down tests,

the nail/thumbtack spill valve has stock dimensions and clearances,

the stock PCV system is in place (including the front .090" restriction fitting),

the main diaphragm is soft, wrinkle-free, and has a big "hump" around the magnesium wafer,

and the idle boost passage in the air meter has been cleaned out thoroughly with a drill bit.

If the spider is a good one and the unit is calibrated correctly, the idle mixture screw will be 1.25 to 2 turns out from seated and will kill the engine when screwed in 1/2 to 3/4 of a turn. You may or may not have enough screw sensitivity to kill the engine rich at idle.

NOTE: Cheap vacuum gauges (like those found on a vacuum gun) are often inaccurate, especially if they are more than two years old.

**James G.** · July 21, 2015, 07:47 PM

Re: 64 FI Vacuum Measurement

I am reading and watching.

**Larry M.** · July 22, 2015, 09:20 AM

Re: 64 FI Vacuum Measurement

Ok, so now there are four guys with numbers under 3000, two of them under 1000 looking so I'd better be on my toes. Seriously, I had time last night and this morning to test.

1. I don't have a vacuum pump to test the VAC trigger points as Duke suggest but I did test the leak down by pinching the vacuum test hose with the engine running and then cutting the engine. I had trapped vacuum at 16 inches (running on the fast idle cam) and it held forever. I think my 236 16 is basically good.
2. On my first try at the above I lost the vacuum which turned out to be the test hose having a large I.D. and not sealing. Once replaced I got the result above but this got me thinking. I did not use the new hose from John's kit and was running the original plenum to VAC hose (I like original). But it goes on and off too easily....like my test hose that was too big. So it has to go bye-bye.
3. I reduced the idle to 850 with no issue and had 13 inches of vac and fluctuation 800-900 RPM 20-30 times a minute...certainly acceptable.
4. If I turn the idle fuel screw in to 1 3/4 the Rpm fluctuation becomes unacceptable. As I back it out the idle becomes more stable and at 2 1/4 or 2 1/2 it seems fine.
5. It still starts the way it should (establishes idle without feathering the throttle).

I'm going to stay with this for a while and see how it goes. Worst case I'll go back to a higher RPM but I think I have a decent running unit and from this post I have a baseline to continue to monitor the vacuum to ensure an issue does not creep in. At some point I'm going to replace the main diaphram and set the valves. Thanks for everyone's help.

**Duke W.** · July 22, 2015, 10:10 AM

Re: 64 FI Vacuum Measurement

If the 236 16 holds vacuum then it's probably reasonably within spec; 13" at 850 doesn't sound like a 30-30 cam... more like a Duntov cam.

...don't know what history you have but most 50 year old engines have been rebuilt somewhere along the line. One way to get a good indication is to measure head gasket thickness with feeler gages at the front corners of the head-block interface. The OE .018" shim gasket was not available in service (except for maybe in the early sixties), so if it's thicker, the heads have certainly been off.

When you check valve lash, suggest you measure all before making any adjustments, and that data may give you a clue as to the installed cam. For sure if a Duntov cam is lashed to the 30-30 specs the valvetrain will be VERY noisy.

Duke

64 FI Vacuum Measurement

64 FI Vacuum Measurement

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