Vapor Lock

[rmel]

I had an interesting case of Vapor Lock on my Carb'd 712 that initially surprised me.
Turned out to be a classic case of heat soak. Some context, I have a Hardi 24V
electric fuel pump, located external to the engine about center line of the tank,
well within the siphoning limit of this diaphragm pump.

So....motoring up in the Sierra's at about 7,000' and a warm day. Stopped on a steep
grade to spot a couple of trucks through an obstacle. Thereafter went back to my
712 and she just would not start, absolutely no hit of any ignition. Removed the
dog house cover to do a visual, nothing obvious. Checked the fuel pump and it was
no warmer than the gas in the tank. After scratching my head for 15 minuets or so,
went back to the truck and turned the IGN switch on, heard the Hardi pump make
it's chug chug noise -- Oh! OK, the pump works...So I hit the starter and she started
like there was never anything wrong -- WTF?? Well, that was just enough time to
drop the temp in the intake manifolds and stop gas from over vaporizing. I must
have had an extremely lean condition.

Made me think about the two major types of intake manifolds (Pants). One set
are bare aluminum, as was in my case, the other is insulated. Talked to Jim L. the
insulated version was done for the Hot Climate version of the Pinz, That's interesting.
It all makes sense to me now, I believe this was a mode to prevent heat soak. Since
the intakes are directly in the cooling air stream, as long as the engine is running I
don't think insulated or not matters -but- when you turn the motor off, then insulated
would make a huge difference as the intakes will get very hot sitting directly over the
heads and cylinders.

Ordered the pre-cut insulation from SAV, they can be installed by removing the top
tins, Carbs, and the side tin. Pix below of before and after, I used Stainless Steel ties
but ran short. I'll gussy up that one Pant leg next time I'm in there. I won't know how
much this staves off the issue until next time I'm up in elevation and it's hot.

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[VinceAtReal4x4s]

Would idling in place for a minute before shut down in that situation be a good practice? Kinda like when you cool a turbo engine down after a hard run.

[ChickenPinz]

Were you running straight gasoline or ethanol blend?
I ask because I have two vehicles (one carb and one FI) that do notably worse with heat soak on ethanol blends vs. non-blends.

Is it possible the engine was actually flooded,and not lean? I could imagine flooded from the fuel in the bowls expanding and overflowing into the engine.

Also I have a similar issue on a Honda V4 motorcycle engine. The carbs are tucked right in the hot-zone between the 90-deg V so when you shut down the engine they get backed. Restarts are fine within a few 10's of minutes, but there's a window of time from then until maybe an hour that the bike will be very difficult to start. In talking with a Honda Sr. Mechanic, I was told it was a known issue with engines where the carbs get heat-soaked by the cylinders after shutdown.

[rmel]

To answer Vince's question -- idling before turning the motor off might reduce
Vapor lock due to heat soak. I say might because it all depends on the circumstances;
idle RPM effects air flow, your current elevation, ambient temp, and fuel your using.
But, when your on a very steep grade, need to jump out of the truck to help spot, I
would not 100% trust the parking brake so off the engine goes and in 1'st gear.

[rmel]

I have little choice but to run E10 gas. It's a matter of practical convince.
Almost W/O exception that's what your friendly gas stations sell. I've
looked for pure-gas https://www.pure-gas.org/ and the options
are spartan. At least I am running summer blend.

E10 does bump up the Reid Vapor Pressure by 1 additional psi, ergo, adding
another component that boils off early.

I suspect there was some bowl evaporation, but not a lot. Since I have a
Hardi electric fuel pump, it has a characteristic "chug chug" sound when it's
filling the bowls -- when I turned the IGN ON I heard one or two chugs.
That was enough to tell me the pump was working AND little fuel was needed
to fill the bowls. So I'm still betting on the intake "pants" heat soaked and
evaporating the fuel very lean. Puch added the intake insulation especially
for hot climate -- it then became standard.