Vapor lock questions for the hotter climate guys

[Captain Obvious]

It's a 73 with all the 72 intake stuff. I have the 73 recall manual for the VL mods, but aside from the pusher pump I've done all the recommended mod's. I won't have to worry about VL as it will be nice and cool by the time I can drive it again, thanks for the well wishes.

So from the factory, you originally had the earlier version of the flat tops, right? I've got a mostly stock 260 with the later version of the flat-tops. Haven't had it long enough to know if I've got a vapor lock issue or not.

So why is it that the 73's and 74's are so notorious for VL? It might just be a mistaken impression on my part, but it seems to me you might hear mention about 70-72 occasionally. However, it's not the resounding cry of "known issue" that you get when you mention 73 or 74.

The first, most obvious answer that everyone jumps all over is the flat-tops. Well, you've taken that out of the equation already. Next is that the 74 got an electric pusher pump from the factory. But the 70-72 got along OK without one, right?

So is there something else about the 73 and 74 that makes them more prone to VL than the previous years? Fuel rail design maybe?

Back to the VL. he fuel filter is before the pump, and mounted at about the same height, thats how I came to the VL conclusion so quickly. I could see that the filter was empty, and when it did begin to fill the fuel was boiling within the filter itself.

You're saying that you parked the car hot and the fuel filter was full when you shut if off, but after sitting for 15 minutes in the sun, the filter was empty? Is that before or after it runs for half a block?

I won't have to worry about VL as it will be nice and cool by the time I can drive it again, thanks for the well wishes.

That's just sad...

[5thhorsemann]

I think it may have something to do with the coolent flow through the flat top carbs, thats just a hunch. The hood vents on the 74 will ease the kind of issue I'm experiencing.

The fuel boils off when the car is parked and it won't start without cooling the lines with water. Once I get it in motion the engine bay cools off and the car runs okay.

[Captain Obvious]

I think it may have something to do with the coolent flow through the flat top carbs, thats just a hunch. The hood vents on the 74 will ease the kind of issue I'm experiencing.

The fuel boils off when the car is parked and it won't start without cooling the lines with water. Once I get it in motion the engine bay cools off and the car runs okay.

But the 72 round tops have coolant flowing through them as well though. And you don't even have flat tops anymore.

I don't know... I'm just trying to figure out why you (with a 73) are seemingly having more problems than the 70, 71, and 72 owners out there. Even the ones in hot climates. Not that MD is "cold", but there are lots hotter. I'm wondering if there's some other 73 remnant that's complicating matters.

About the fuel boiling in the filter... I doubt that's where the fuel is boiling. The filter is cool compared to anything that actually touches the engine. Maybe it's boiling in the intake side of the fuel pump and the resultant vapor is backflushing the fuel out of the filter and back into the tank? They don't run any check valves in that line to prevent that, do they? My 74 has an electric pusher pump that probably acts as a check valve, but you don't (currently) have a pump.

You could park it hot and then use locking pliers to clamp off the hose on the intake side of the fuel filter and see what happens then.:bulb: If you don't let it go back into the tank, the pressure will go up, and raise the boiling point?

BTW - My 74 doesn't have hood vents. I don't think that started until the 280.

[ConchZ]

Here's an idea that I just got from an old Datsun mechanic who used to work at a Datsun dealer in Phoenix during the early 80s. I haven't tried it, as I've been warned by another guy, Bonzi Lon, not to do it. On a carbed Z, he says the fuel return outlet of the fuel rail has a restriction in it that helps maintain pressure. He says they drilled it open just a bit, which allowed more fuel flow out, thereby increasing circulation, and cooling the fuel rail, resulting in zero vapor lock in Phoenix Zs. Bonzi Lon warned me that the restriction is necessary in order to maintain fuel pressure in the rail. Has anyone tried this? I have an extra fuel rail that I'm tempted to try it on, as this summer heat is causing vapor lock for me. I did get some improvement from richening up the mixture. Another source of my vapor lock is probably a weak cooling fan clutch and the fact that I have no fan shroud. I just ordered an electric fan and shroud kit from MSA that I'm going to try out, in hopes that it helps not only my vapor lock, but mostly that it helps improve my AC's cooling capacity. That is a nice looking piece of kit. If it helps my AC, eliminates the drag from the stock fan, makes the engine run cooler, PLUS helps the vapor lock, it'll be worth the money. If it doesn't fix the vapor lock, maybe I'll opening up the return outlet on my fuel rail just a little bit.

[5thhorsemann]

That seems reasonable, but I would think you should go slow with the drillling, no more than one or two wire sizes at a time. Just keep going till the problem eases, too fast and you might end up with too low a pressure which will agrivate the VL.

[Captain Obvious]

On a carbed Z, he says the fuel return outlet of the fuel rail has a restriction in it that helps maintain pressure. He says they drilled it open just a bit, which allowed more fuel flow out, thereby increasing circulation, and cooling the fuel rail, resulting in zero vapor lock

That orifice is a rudimentary fuel pressure regulator.

If you enlarge that restriction, it will lower the fuel rail pressure and if you drill it out too much, it will cause a problem. There needs to be enough pressure under all load conditions to force fuel through the fuel lines (which is uphill in spots), filter screens, and push open the float valves.

How far can you go (if at all) before it becomes a problem? I have no idea.

I slept through that part of class, but I don't think the extra flow will do much for a temperature change in the rail. I can see my return flow because I've currentlly got my return line flowing into a catch can. At idle (light load) there's a surprisingly large amount of fuel being returned to the tank already.

Can you take a couple thousandths off without causing a problem? Probably.

Would it do anything good for you? Probably not.

I'm no expert, but I consider it risky.

Haha! Now you've heard two divergent opinions... Good luck with whatever you do, and I hope you kick your VL problems.

[jalexquijano]

Also have this same problem! Nobody seems to know the answer!!

[Mark Maras]

  I agree with the Captain's observation about the early SUs having little to no problem. My early 71, for example. 20+ years driven daily, year round, never had VL, even once. Stock set up. No elec. f. pump, just the mechanical one on the engine.  Pressure was slightly more than 3 psi as I recall. (BTW, we used to run a 327 in a dragster that had plenty of fuel at 3 psi.) Clear filter on the firewall was never full. It ran at 1/4 full or less. No vents in the hood. Seemingly everything that should have caused VL didn't.

 So, we know the later round tops and flat tops were plumbed for hot water and that can contribute to VL. It seems that the addition of an elec. pump did little to nothing to eliminate VL, if that was it's purpose. Nor did the hood vents seem to help much. They did change the fuel rail from two lines to three later on. Could that play into this? I don't know the reason for that change. What was the third line for?

 Drilling the restriction in the line is an interesting idea. It would be interesting to put a  f.p. gauge on it before and after. You're not going to hurt anything until the pressure drops well below 3 psi. It doesn't take much pressure to fill the float bowls. There's no restriction until the needle and seat close. Usually running low on gas in the bowls is from lack of fuel volume not poor pressure.

[zKars]

One clarification on the hot water flow through the manifold and carbs. On 72-74 carb systems there is a valve at the end of the intake where coolant exits, that CLOSES once the coolant reaches operating temps. Hence coolant flow is only used to aid the warm up portion of usage. Now if that valve is plugged open, perhaps it contributes. I'd love to know what normal everyday carb bowl temperatures actually run at then what temps rise to after shut down, including the fuel rail.

BTW I'm having hot re-start issues right now as well on my ZT Su's. Electric pump only, Non stock fuel line routing, SS -6 lines, no return. wrapped headers, inspection door vents, 0 problem running once started at any temperature. Only happens after hot shut down. 

[jalexquijano]

Well last night i intalled a couple of heat shield extensions which isolate the float bowls of each of my Round top SU Carbs. It seems to have lowered the temperature! Any other recommendations?? swapping the fan clutch with 2 electric fans? removing the fuel rail and replacing it with a modern type with other type of gasoline hoses (AN1) as the picture attached??? My fuel pressure is at 4.3 to 4.5 psi.

[Jeff G 78]

After struggling with heat related issues while racing for several years, I finally ditched the steel OE fuel rail and the problem was solved.  I had tried every type of heat shielding with no luck.  In the end, I believe that the heat was coming through the steel rail mounts and into the rail.  The insulation on the rail did a great job of holding the heat in.  Once I went to an all-rubber supply, I have no issues.

[Stanley]

My only experience with vapor lock in my 240 was a few years back, in Blythe, going west on the 10. Temp gauge showed the engine to be hot, don't remember exactly but at least 220F. Stopped at Albertson's to get lunch (sign at the bank said 117F), and couldn't start it until it sat about 20 minutes. Stopped at a motel; several other cars that came in were overheating. Left at 4:30 AM to beat the heat, but it was still 99F. Had to turn around on the Arizona side, overheating again. Found a little garage open on Sunday. They put a new fan clutch and I limped on to Tucson. A mechanic there told me to pop the hood which I found would drop coolant temp about 10 deg. Ordered an oil cooler and installed it before heading back, and got a Nissan thermostat. Back in the South Bay, head gasket gave out, found head was corroded, put JDM head and metal gasket, big aluminum radiator, flushed engine three times, added fan shroud, and removed plastic tube to cabin air vent to get more air in the engine bay (didn't like it blowing hot air on my feet anyway).

Using mech. pump only, put filter by tank and removed the front one. Using a metal fuel rail that's more isolated from the engine, with no return line. Engine temp remains normal 180F even on a long upgrade on a hot day, like Palm Springs to the summit. Haven't had vapor lock, but I have a FP gauge (connected after the fuel pump) in the cockpit and I've noticed the fuel pressure bouncing (engine temp was still 180F) around on hot days when traffic on the freeway comes to a stop. So maybe that signals a pre-vapor lock condition.

I wonder where that vaporization is happening on a carbed car with mech. pump. The steel fuel line comes under the car away from the engine and exhaust, which seems like a relatively cool area. If any vaporization happened in the float bowl, seems like it wouldn't do anything at all unless it all vaporized which seems unlikely. But I've seen the pressure dropping (and quickly rising again, measured right at the fuel pump, on the carb side. Normally it's steady.