"Heat Soak"

[Steve G.]

Re: "Heat Soak"

I will need to go back and read this complete post but I want to add on a Holley carburetor fuel can boil in the main well and push out the nozzle that's located just inside the booster venturi. The main well is located vertically inside the metering block, fuel does not have to go up and back down to exit the booster nozzle like the idle circuit. This fuel usually drips down onto the throttle blades and out the side of the carburetor onto the intake manifold.

The idle circuit fuel is drawn by vacuum so the up and over/down breaks suction where the main circuit fuel is delivered by air flow through the booster venturi.

All the fuel is drawn by vacuum (technically it's all pushed by atmospheric toward the point with lower pressure, which we call vacuum,) except accel pump, which is pressurized. If that were not the case and fuel did not have to go up to a height above the level of the fuel in the bowl then back down or straight out it would run continuously. Gravity will do it's job and liquid will seek it's level. It's physics.

What you may be confusing this with is when you have a cross leak at the metering block gaskets. Fuel will leak across instead of traveling up and over.



Steve

[Timothy B.]

Re: "Heat Soak"

I agree with you but want to add the source of low pressure in the main well is created by air flow through the booster venturi. The source for low pressure in the idle circuit is created by manifold vacuum. The fuel level in the main well is the same as the float level at low speeds and at rest and if fuel boils in the float bowl it will boil in the main well and push up and out the nozzle causing the issues many experience. This can be corrected to a certain extent by lowering the float level providing you don't create a stumble when the main circuit starts feeding.

[Steve G.]

Re: "Heat Soak"

All true, but given the size (diameter) of the vertical transfer tube you will not get the effect of a rolling boil splashing fuel. What you would more likely get is the vapor boiling off the the surface. The level of the fuel in the tubes is at the same height as the fuel level in the bowl. How does it rise up well over the height of the fuel in the bowl and make it's way to the nozzle. All this in an environment with a lower temperature than the bowl itself? Additionally, if this were the case the first place you would see the fuel is at the air bleeds at the top of the transfer tube.

What strikes me most about this entire subject is that the diagnosis appears to have started from data sheets on the volatility of fuel. Then looking at temps under the hood it was decided that it must be causing a problem. What problem could it be causing? Here's one in a component we don't know a lot about, so this must be it. So this lead to, if you have a drip in the carb the first thing to do is lower the heat because of what we know about the fuel from the people that make it.

To me, this is backwards. Fuel can boil in the bowl all it wants. It's been happening forever, even with the old fuels. The only problem it ever caused was rich hot idle. So the car engineers designed a fix. Let the fuel boil all it want, as long as the vapour doesn't make it down the throat, who cares. Eventually we cared because fuel vapour venting into the atmosphere was an emissions problem. So we captured the vapour and burned it when purging the vapour canister.

If you have a drip in the carb the assumption that should be made is that the most likely cause is the same one that has been causing drips in carbs for the last 50 years, floats, needle and seat and gaskets. It is backwards to assume that any of these long known problems can no longer be the source of malfunction and believe instead instead we should move first to reducing the heat by wiring heat risers open and changing engine running parameters because the volatility of the fuel has changed.

And it's pretty easy to check on a Holley. Think you've got a rolling boil splashing fuel all over the place and down the throat? Pull the sight plug and take a look inside.

Steve

[Timothy B.]

Re: "Heat Soak"

With regard to your statements, you need to consider what's changed over the last few years and it's not the heat, it's the fuel. Given this change the most helpful thing to try is lower the heat by opening the heat riser valve. I think that's all others are stating and with a stock engine many times that's all that's needed. The pressurized fuel injections on modern cars raise boiling temps but these old cars are at atmospheric.

I agree with your comments that other issues can cause symptoms like internal leakage etc.

[Steve G.]

Re: "Heat Soak"

With regard to your statements, you need to consider what's changed over the last few years and it's not the heat, it's the fuel. Given this change the most helpful thing to try is lower the heat by opening the heat riser valve. I think that's all others are stating and with a stock engine many times that's all that's needed. The pressurized fuel injections on modern cars raise boiling temps but these old cars are at atmospheric.

I agree with your comments that other issues can cause symptoms like internal leakage etc.

As I've said thruout this thread, I have considered immensely what's happened to the fuel. What's missing is all the connects between the fuel change and the diagnosis of "heat soak". All the supposition about what the high volatility of the fuel is doing appears to have never been confirmed and the theory of the affects of this fuel change defies logic and science.

If you read through the rest of the thread you will see the part that explains why I oppose altering the heat riser. Especially when it is attempting to treat an issue that is not the real cause of the problem.

I contend that the drip is due to either gasket leakage or fuel level. You stated that you've been able to offset the effects of the heat soak by altering fuel level. I say you are offsetting the effects of a high fuel level by altering the fuel level. After shut down heat soak will thermally expand the fuel in the bowl which raises the level. With a correctly adjusted and properly operating fuel level system this expansion will not bring the level high enough to spill out. If the fuel level is too high or erratically controlled this expansion on a hot day, hot engine, can bring it up high enough to spill out the various orifices in the throat. The repair is to correct the fuel level issues so it does not rise to that point.

Steve

[Domenic T.]

Re: "Heat Soak"

I would like to share 2 experiences I had with fuel pressure forcing the fuel past the needle on carbs done here. Both were on Q-jets and on the first I was using a electric fuel pump and had to add a fuel pressure regulator as the pump pushed past the needle which was new.
The next was a mechanical fuel pump failure. I wired a electric fuel pump that I borrowed to the car so I could drive it to the shop. It to flooded past the needle.
So this brings up my question. Fuel trapped between the fowl pump valve and the needle, if heated and expanded, creating a high pressure causing it to enter the fuel bowl past the needle just as the HP electric pumps did.

DOM

[Steve G.]

Re: "Heat Soak"

I would like to share 2 experiences I had with fuel pressure forcing the fuel past the needle on carbs done here. Both were on Q-jets and on the first I was using a electric fuel pump and had to add a fuel pressure regulator as the pump pushed past the needle which was new.
The next was a mechanical fuel pump failure. I wired a electric fuel pump that I borrowed to the car so I could drive it to the shop. It to flooded past the needle.
So this brings up my question. Fuel trapped between the fowl pump valve and the needle, if heated and expanded, creating a high pressure causing it to enter the fuel bowl past the needle just as the HP electric pumps did.

DOM

Dom,
There are several things that make the solid fuel expansion an unlikely cause of this fuel drip.

One is that in all the cars we deal with here, there is a fuel filter in the line. If you've ever looked at a correctly installed (arrow pointing the right way) opaque or clear fuel filter you will notice that there is actually very little fuel in it. It's mostly air with some fuel. You actually can't get the air out and completely fill it with fuel because of the way the outlet is positioned. So the expansion of the non compressable liquid simply compresses this air. When you consider the tiny amount of expansion and the vast amount of air, it will never raise the pressure enough to unseat the needle.

Now let's suppose we have a line with no filter solidly filled with fuel. The coefficient for thermal expansion of gasoline is .00095 per 1 degree C rise. That means for every degree C rise the volume increases by .00095 times it's starting volume. Before doing this calculation, let's think about how much the temp rises. Keep in mind that this is temp rise in the line only, between the needle and the fuel pump. At shut down the coolant temp rises because the heat from combustion and exhaust soak into the coolant surrounding those sources. There is none of that around the fuel line. The temp of coolant in the radiator actually starts to drop almost right after shut down. It's just the block that warms up before it starts to cool down. So how much does the temp rise in this line ? I'd be surprised if it rose much at all, but let's even say it rose 10 degrees C.

Now calculate the volume of the fuel in the line. Let's say 18" of 5/16" steel tube. v=pi x r squared x 18". Now multiply it by .0095 to calculate the volume of the increase over the 10 degree C temp rise.

Now with no air at all the fuel expansion will definitely unseat the needle, but the only thing that will flow into the bowl is the amount of the expansion. The pumps not pushing fuel through an open valve. It's not running. Again, if the fuel level in the bowl is correctly set this minute amount, should it actually get in (won't in a stock configuration) is not enough to raise the level enough to spill over into the throat, even when combined with the thermal expansion of the fuel in the bowl.

And once again, the solid fuel scenario is not perc. It's thermal expansion. How much has that changed in the last 40 years? Perhaps Duke has access to that info from the fuel people.

Steve

[Domenic T.]

Re: "Heat Soak"

Steve,
I understand what you are saying. For some reason it didn't SOAK in when I read it in a earlier post.

Dom

[Stuart F.]

Re: "Heat Soak"

Dom;

That's because first it has to "heat" up before it can "soak" in.

Stu Fox

[Steve G.]

Re: "Heat Soak"

Dom;

That's because first it has to "heat" up before it can "soak" in.

Stu Fox

All this in the context of a "cool" conversation.
Steve