A few honest questions about vapor lock

[djwarner]

In five years in Florida I've yet to experience vapor lock in my Weber down draft equipped 240Z. So my question maybe more academic curiosity raised while I was working on a different problem.

My problem has to do with hard starting after my car sits for several days. I can't say I ever experienced starting problems when it served as my daily driver. Now that I've moved on to others projects, the Z often sits for 3-5 days without starting and hard starting has raised its head. At first I thought I caused it when I converted the sequential 2 barrels into synchronous operation with a kit from Redline. Also I re-jetted the carbs to better handle the ethanol laced gas we all suffer with today.

I suspected the conversion kit because it lacked the device that cracked the carbs open when the choke is applied.

It got to the point that a shot of ether was required to start after 3 days of sitting. Cold starting the next day required no choke, second day required half choke, and third day ether.

I believe I have finally traced it down to evaporation through the float bowl vent. This causes lean mixture until the bowl finally refills. This requires enough pumping from the mechanical pump to raise the fuel manifold pressure and fuel to pass through to the float valve. Getting volume through the mechanical pump is not a problem with a slowly cranking engine, but achieving pressure is another matter.

The first owner had been talked into replacing a defective mechanical fuel pump with an electric pump mounted back near the fuel tank. The second owner decided he didn't like the clicking noise and went back to an engine driven pump. He didn't bother removing the electric pump, just plumbed around it and sealed the electrical pump by connecting tubing from the inlet to outlet. He also disconnected the power lead. I subsequently removed it during other maintenance.

It occurred to me that it might be useful as an on demand booster pump to refill the float bowls. After cleaning and replacing its fuel filter, it checked out, so I installed it yesterday. Verifying it would work inline with the engine driven pump, I removed the rubber return line at the metal line on the fuel manifold. Holding a glass to the metal line, I turned on the pump. At first I got the familiar clickety-clack as the air was purged from the line. Then the pump quieted down as the pressure began to build. Success.

It was then that I realized why the Z never required a fuel pressure regulator. The fuel passing out of the return line was a much thinner stream than the diameter of the rubber line that connected to it. Aha! a closer examination showed there was a restrictor on the end of the metal line with a small hole, perhaps 1mm in diameter. This small hole established a pressure drop and this pressure was presented throughout the fuel manifold and up to the float valves.

So when the engine is cranking before start, the pulse rate of fuel from the pump is much slower that an idling engine and the pressure  is never generated. So any stickiness at the float bowl valve - or in my case - restriction caused by adjustment of the float height means that the ability to re-fill the bowl is greatly diminished. Since the electric pump has the ability to supply pressure and flow rates for max HP without regard to engine rpm, this should be an acceptable solution and I will install a momentary switch near the console. (REDLINE changed the recommended float height to accommodate modern fuels with ethanol. I did this when I re-jetted the carbs.)

This has got me thinking about vapor lock. If the return line is always open through this orifice, how does vapor prevent flow of gas to the carbs? The only way vapors can disrupt fuel flow is for vapor to get into the check valves in the engine driven pump. If the check valves don't seal vapor well, no net flow of fuel occurs.

I have heard it said that is pressure builds in the fuel rails, it can pass to the float bowl causing rich running. But my carbs have float bowls vented to atmosphere and any pressure in the fuel manifold will simply vent through the return line. 

If I am right, there would appear to be little to no advantage in wrapping insulation on the fuel manifold metal lines as is often done. While shielding to prevent heat from reaching the float bowls and vaporizing the fuel there would appear worthwhile.

So what have I got wrong?

[Mark Maras]

http://xenonzcar.com/s30/files/1973 240z 1974 260z fuel system modifications.pdf  Submitted by ZedHead (and maybe others) a while back. Many thanks.

 The factory printed this in 75. It has some history of the problem on 240 and 260 Zs. While I find it very interesting, I also have to keep in mind that the problem seemingly got worse the more solutions they threw at it. They never did solve heat soak and it continued into the 280's. IMO, the answer lies in the past. The early 4 screw carbs, manifold, possible differences in the steel fuel rail (2line-3line), etc. The fact is, the early Z's rarely, if ever, had this problem using a lone mechanical pump. But, I guess heat soak must have been a problem somewhere, or else they wouldn't have tried to fix it. Even now, there seems to be more complaints of this problem with 3 screw carbs even with the water passage blocked.

[djwarner]

Very interesting reading Mark. The major difference with the later 240's and 260's from the earlier models was pollution control requirements. The fact that Nissan was able to meet new regulations and improve performance over the '70s is a testament to their efforts.

So there was a competition hood with side vents.

[Zed Head]

3 hours ago, djwarner said:

While shielding to prevent heat from reaching the float bowls and vaporizing the fuel there would appear worthwhile.

So what have I got wrong?

Most of the reading I've done about vapor lock, some before I ever had a Z, was about the line before the fuel pump.  The pump can't pull vapor so keeping it liquid before the pump was important.

The link that Mark provided describes percolation.  Which is a different problem.  For the EFI systems it seems like hot injector bodies is the problem.  Hence the fans and other remedies from other manufacturers.

Your problem of not starting after several days might be related to fuel volatility.  We all assume that gas is gas and it's been the same over the years but in reality, there are no regulations about what the manufacturers can sell as gasoline, as far as I can tell.  The manufacturers have an agreement but it's all about keeping the whole big giant automotive machine going.  

My EFI engine starts best if I give it a few starting revolutions to get some squirts, then let it sit for 10 or so seconds so the fuel can vaporize.  I don't know if you can work up a similar routine for carbs but maybe....

[Patcon]

I had downdrafts of one of my Z's at one time. They did tend to evaporate the fuel out if they sat for a few days. The electric fuel prump as a prime should be a good solution...

[Mark Maras]

 I once had a stock 621 PU that did the same thing.

[Captain Obvious]

I always wondered if downdraft carbs were more prone to that due to sitting on top of the manifold with all that heat rising.

[Patcon]

Codys 510 does the same thing with the factory down draft car. The float bowl has a glass side so you can actually tell when it dries out and watch it refill while cranking

[SteveJ]

12 hours ago, Captain Obvious said:

I always wondered if downdraft carbs were more prone to that due to sitting on top of the manifold with all that heat rising.

I never noticed any issues when I was running them in the 1990s, and I was driving the car in 100+ degree weather, too. Of course, I wasn't taking many trips where it would have a chance to heat soak.

[Mike W]

I had the exact same problem as djwarner although I run triple weber sidedrafts rather than a down draft.  I ultimately concluded the same thing which was gas evaporation.  My car is a late manufacture 260Z which had both the mechanical pump as well as the electric pump in the back, but the way it was wired from the factory did not allow the electric pump to actually start operating until the engine was running.  So I removed the mechanical pump completely and rewired the electric pump so it turned on when my key was in the "on" position.  This allows the electric pump to prime the carbs after sitting for a few days / weeks and my car now starts right up. 

My concern in the way I configured the system was that in the event of an accident the fuel pump would keep running and the risk of fuel being pumped all over hot engine components.  So I did also install an inertia switch which will kill the power to the pump in the event of an accident. 

 

[djwarner]

Just an update.

Ran wire for the electric pump into the cockpit and dived into the area behind the radio to repair the defroster circuitry. After figuring our where to tap the power, I located the switch on the bottom of the steering column plastic finisher. Plugged the battery back in and the pump ran as expected, but when I went to start the engine, the starter circuit was dead. Finally traced the problem back to the neutral/park switch wiring (AT). While running the wire for the pump, I must have loosened a bullet joint. With the old style soft plastic tubing over the joint, nothing was apparent until I tugged on it.

So back to testing the pump. Earlier in the day I ran the pump until I heard the change in pitch as the fuel manifold pressurized and then another 5-10 seconds. So several hours later when I got the engine to turn over, it started in the first second of cranking with no choke applied!

Looks like I got a winner.

Thinking back to how long it takes to pressurize the manifold, I wonder if the check valves in the engine driven pump may be allowing fuel to slowly drain back into the tank leaving air/vapor in the fuel manifold. An empty manifold could be a partial cause in the delay to re-fill the float bowls after sitting for several days. In any case, the electric pump seems to be doing the trick.