Losing vacuum

[Kenneth F.]

Idle vacuum is 19" on a 1962, 300H.P. It now runs well in town, but if I am going at about 65MPH up a hill, for about 1/2 mile the vacuum drops slowly to about 4," and missing occurs. If I open the throttle more, it usually backfires through the carb. When this is happening, I notice that the water temp. has gone up from 180 to about 200 degs.

I put a pressure tap just forward on one of the oval mufflers, and read 1 pound pressure going up the same hill with the engine missing and vacuum had dropped to under 5 ". I then put the hose adapter tap into a tapped hole in the heat riser that has the insides removed. The pressure going up the same hill as the previous test, was under 2 pounds. I checked the accuracy of the gauge to a digital tire pressure gauge, and the gauge I am testing with actually is reading a pound higher at 3 pounds pressure from an air compressor


Some time ago, I had to block all the exhaust going to the heat slot under the carb. to get it to run well in town. I had the distributor checked by a vendor mentioned on this site, and now have the timing at 14 degs initial, with the vacuum hose disconnected.

If someone has an idea, I am interested.

Ken

[Mark E.]

Re: Losing vacuum

This sounds like a classic high speed miss, which is more noticeable under load (as you go up the hill). This has many possible causes (or combination of causes, jut to make the trouble shooting more fun). Here are the more common causes, as listed in my 1969 Motor Auto Repair Manual:

Fouled or defective spark plugs
Weak spark plug cables or connections (check resistance)
Spark plug range too hot
Point dwell incorrect
Weak points or condenser
Poor ground in ignition primary wiring (including inside the distributor)
Poor or loose connections in ignition primary wiring
Bad coil; incorrect primary voltage to coil
Improper ignition timing (initial or the advance curve)

Insufficient fuel delivery to carb
Clogged air filter
Choke not opening all the way or fluttering
Mild vapor lock
Bad gas

Exhaust restricted (stuck heat riser is a common culprit)
Incorrect valve timing
Leaky or stuck valve(s)
... and there's more

It's a long list, so it's best to be methodical to rule out each possibility one at a time. Start with the ignition system, then fuel system, then exhaust system, then mechanical (valves).

[Duke W.]

Re: Losing vacuum

I had the distributor checked by a vendor mentioned on this site, and now have the timing at 14 degs initial, with the vacuum hose disconnected.

Ken

Why?

The problem sounds like lack of fuel delivery under load... fuel filter clogged, fuel pipe or hose crimped are possible causes.
Also there is a fuel filter sock on the tank pickup. Has the sending unit ever been removed for inspection and replacement of the sock?

Duke

[Frank D.]

Re: Losing vacuum

You aren't "losing vacuum"; highest vacuum is achieved at cruise not under load or at WOT - that symptom is a distraction.
You are missing due to fuel starvation or ignition problem.
Check the above items per the Duke; also make sure the gas tank is properly vented...

[Steve G.]

Re: Losing vacuum

Further to what Frank said, the dropping vacuum is in relation to your foot moving further to the floor attempting to maintain speed going up the hill.

What may not be normal is the amount of throttle required to climb that hill and the 20 degree temp rise. First things first, deal with the missfire.

Ignition demands higher voltage to fire the plug under load, and as said, you may have an ign problem.

And, also as said, it can be fuel starvation. A quick and easy method to advance that diagnosis is to pump the accelerator pedal when the miss starts. If you get a momentary recovery when doing this, you've pretty much identified the problem as fuel.

If you've ever been at the wheel of a carburated car when the tank runs empty you will have found you can coax an extra half block out of it by doing that. The accelerator pump is picking up the fuel from the bottom of the bowl that can't make it out the main metering system and squirting it into the throat.

If the accel pump test suggests fuel, instead of dropping the fuel tank to visually inspect the suction screen I would do a proper fuel pump test. This measures fuel pressure, pump suction and volume. Specs are in the manual. This,along with a check of the cap and filter, confirms proper operation of the fuel system right up to the carb in probably 99.9% of the cases.Everyone has a story about an oddball situation that's happened, like no suction screen and a leaf or something floating in the tank that would periodically block the intake.

Steve

[Kenneth F.]

Re: Losing vacuum

This morning I installed a T in the fuel line between the glass sediment bowl and the carb. I checked cranking fuel pressure and measured 7#. I also did a fuel volume test and collected a pint when I cranked the engine for 15 secs. The engine was cold. I then took the car for a road test. When I started up the same hill, the vacuum was 16 in., and the fuel pressure was 7#. The car was going 65MPH, and toward the top of the hill, the vacuum had dropped to about 5". I made sure that the accelerator pedal was held at the same position going up the hill as the vacuum was dropping. I noticed the vacuum gauge bouncing about 1 " when it lost power. As soon as I crested this hill, the vacuum always recovered, and the engine returned to normal. Today I was watching the fuel pressure more closely and about 5 mins later, I noticed the fuel gauge reading had dropped to 3#. This gauge gives low false reads when warm, but I had the gauge outside of the engine compartment. When I got home, I noticed a large bubble would form in the the glass fuel bowl about every 15 secs. I did a fuel volume test, and collected a pint in 15 secs cranking. The fuel in the container was 81 degs. I then measured fuel pressure at 6#, up 3# compared to before the fuel volume test.

I appreciate the input.

Ken

[Steve G.]

Re: Losing vacuum

Ken,
Forget about your vacuum reading. Leave the gauge in the tool box. If you were driving up the hill not knowing what your vacuum reading is, do you have a driveability problem?

Vacuum drops as load increases. That's the way it's supposed to be. Load increases as you climb a hill, or accelerate away. That's why the power valves or metering rods in the carb work in conjunction with manifold vacuum. At high vacuum the metering rod pistons are held down against their spring and the rods are plugging the holes. As engine load increases and vacuum drops the springs under the pistons overcome the vacuum and raise the rods out of the jets allowing more fuel. That's where this fuel starvation issue comes in. When there is a higher demand for fuel, the metering rods have opened up, and there isn't enough fuel reaching the carb to accommodate them, you can get lean misfire.

And that's the problem you have, not the low vacuum.

I said the low vacuum and eng temp rise may be a cause for concern. Where I was going with that is that if your complaint were that previously your car climbed that hill with no effort and now it seems to take more throttle to maintain the speed and now you notice the eng temp climb when it never did that before, there is another problem. Again, unless you took a vacuum reading when it climbed the hill with ease and compared it to today's reading, the vacuum gauge reading is meaningless. There is no chart that says for a climb of x degrees vacuum should drop by y amount.

A power loss (from previous performance) without missing or hesitation, engine otherwise pulling smoothly, accompanied by a noticeable increase in engine temp over previous, is often a sign of late ign timing.

But you're a long ways from looking at that. You have a misfire going on. That is the complaint to deal with first, and maybe the only one to deal with.

Steve

[Joe C.]

Re: Losing vacuum

Downshift.

[Roger W.]

Re: Losing vacuum

When was the last time that you changed the plugs? That would be the simplest thing to try first.

[Kenneth F.]

Re: Losing vacuum

Steve, I installed a set of metering rod springs that are 5/8" longer than the ones I removed. The same road test had the same results.

I have the thick carb mounting gasket, phenolic spacer, and stainless plate under the carb. I added paper gaskets top and below the s/s plate, which might not usually be done. I read about 135-150 temp at the carb base, on different days.

Roger, I changed plugs. I have Champion J12YC. The wires are Packard TVRS. I think the first set of plugs were Autolite 295.

Thank you for the help,
Ken

[Jim L.]

Re: Losing vacuum

1962, 300H.P. It now runs well in town, but if I am going at about 65MPH up a hill, for about 1/2 mile the vacuum drops slowly to about 4," and missing occurs. If I open the throttle more, it usually backfires through the carb. When this is happening, I notice that the water temp. has gone up from 180 to about 200 degs.
If someone has an idea, I am interested.

If you can rule out fuel delivery, take a look at the distributor. Your 300 HP engine should have a dual point distributor, as far as I know.

If it does, there is the possibility that in normal driving the plugs are firing when the FIRST set of points opens and that when you load the engine the plugs don't fire until the SECOND set of points opens. The symptoms you've described are entirely consistent with this scenario.

Test for the possibility by disabling the set of points which close first and open first, then go for a test drive. If you don't notice the miss, loss of power, and rise in temperature, then you've found the problem. A suitable solution, IMHO, is to continue driving with one set of points disabled and to adjust dwell of the remaining set and ignition timing as if you have a single-point distributor..... which you effectively do.

Jim

[Steve G.]

Re: Losing vacuum

Steve, I installed a set of metering rod springs that are 5/8" longer than the ones I removed. The same road test had the same results.

I have the thick carb mounting gasket, phenolic spacer, and stainless plate under the carb. I added paper gaskets top and below the s/s plate, which might not usually be done. I read about 135-150 temp at the carb base, on different days.

Roger, I changed plugs. I have Champion J12YC. The wires are Packard TVRS. I think the first set of plugs were Autolite 295.

Thank you for the help,
Ken

Ken,

Am I correct that the problem we are still looking for is the misfire that happens under the load that develops on this hill pull?

If yes, did you do the thing I suggested about pumping the throttle when the misfire is occurring. Your fuel delivery check only tested the system up to the carb. You're not at the end of the line. There's a whole bunch more that happens inside the carb. If what you have is a lean misfire, pumping the throttle introduces additional fuel which it is taking from the bottom of the bowl. Even if the bowl is half empty and the other circuits can't access the fuel (low head pressure) the accelerator pump will squirt this into the airstream. If, while the miss is occurring, pumping the throttle causes the engine to recover for a short time, you have pretty much diagnosed this as a fuel issue. If it doesn't change it at all, move on to ign.

The metering rod test is not a good test. You don't know if the longer rods are just raising the parked height of the piston and what it is actually doing to fuel flow.

Yes, 130-150F at the carb base is too low. However we don't have enough information to make a diagnosis of that.

If you look at the fuel distillation charts that were posted in an attempt to prove fuel perc, and read the Chevron article in which it was pulled from, you will see that it states that liquid fuel does not burn. Only the vapour. It describes how we need some of the fuel to vaporize at lower temps otherwise we would not be able to start the cars and they would run terribly until warm. Warm means not just combustion chamber temps, but air air charge temps as well.

According to the chart, between 60 and 80% of the fuel will not vapourize under 150F. The top of the curve is about 400F. Certainly the highest range of the chart is reached in the combustion chamber, but a great deal of it is occuring in the air stream between entry into the carb and it's open intake valve.

I have not seen published here, or anywhere else I've looked, any information about how much heat rise is expected to occur in what portion.

There is also a question of air density. Between 30F and 100F air density changes by about 12%. Don't think about the outside temps, think about the range of temps of the charge air. The oe jetting, metering rod settings were all calculated on a presumption of air density that would have resulted from being heated by the exhaust crossover/heat riser system. The choke is designed to compensate for that density change through warmup and once the choke is off it is presumed air charge is up to appropriate temp and metering is correct for tthat fuel density.

But I do know this. With an operating heat riser and the crossover opened as design, we can safely eliminate inadequate vaporization and air density change as a any part of the problem.

Hopefully the advocates of heat riser removal and cross over elimination will let us know more on how to deal with these changes in AFR diagnosis.

Steve

[Duke W.]

Re: Losing vacuum

"According to the chart, between 60 and 80% of the fuel will not vapourize under 150F."

Absolutely, utterly false! You lack a basic understanding of the physical properties of liquids. The distillation curves show the percent of the fuel's components that are at or above their boililng points. Liquids vaporize at below their boiling points. It just takes a little longer. Otherwise streets would never dry after it rains.

In a carbureted engine the turbulent air and fuel flow through the inlet system cause rapid mixing and vaporization of essentially the all the fuel even at low to moderate temperatures.

Duke

[Steve G.]

Re: Losing vacuum

"According to the chart, between 60 and 80% of the fuel will not vapourize under 150F."

Absolutely, utterly false! You lack a basic understanding of the physical properties of liquids. The distillation curves show the percent of the fuel's components that are at or above their boililng points. Liquids vaporize at below their boiling points. It just takes a little longer. Otherwise streets would never dry after it rains.

In a carbureted engine the turbulent air and fuel flow through the inlet system cause rapid mixing and vaporization of the all the fuel even at low to moderate temperatures.

Duke

I stand corrected. Let me restate. According to the chart 60-80 % of the fuel is below it's boiling point at 150F.

So let's think about the assumptions that were being made about bowl temps of 130 (not fuel temp) and 40% of the fuel being above it's bp and it's relationship to fuel perc and hard hot start.

You can't have it both ways. Is the distillation curve of relevance or not?

Yes Duke, and it can take all day, or longer, to evaporate a cup full of fuel with a lid on with a 1/4" vent hole. We are taking about what goes on here in a very small fraction of a second. And we are not spilling the fuel on the ground and giving it a huge surface area to advance evaporation.

We're getting off topic. Even you stated previously that heat riser operation had to do only ( in your view) with vapourization of the fuel. Now the question is being asked, can we safely take air temp and fuel vapourization (and density in my view) out of the diagnostic question with an inop heat riser?

Steve

I'll start a new thread on the distillation curves and we can discuss further what I know or don't know about distillation of fuel and temp rise of the liquid thruout that process.

Steve

[Kenneth F.]

Re: Losing vacuum

Jim, I have a hydraulic lifter engine with the single point setup.
Steve, I did not pump the throttle. I will try this.
This engine has about 1K miles since rebuild. The 2 things done to improve performance was to eliminate heat to the heat slot. This really helped, but it still had a miss in town going into 4th gear going up a hill. This miss was eliminated by raising the fuel level in the bowls with a .245 float level. The previous level was the spec in the ST-12 of 5/16." The .245 was recommended by a fuel systems vendor who advertises in the Driveline.

Thank you for the input..
Ken