[Tigers] Carb Fire Research

[Stu]

The video length?  That's why they invented fast forward buttons, progress
sliders, etc.

I haven't measured it, but I'll guess our underhood temps while moving, or
even while idling with our poorly positioned fans running, is not high
enough to boil the fuel.  When we stop the engine and the airflow pretty
much stops (except for what escapes through the hood gaps, etc), then the
air temp goes up, to the vicinity of the temp of that big hunk of iron
below the carb.  A spacer may reduce the conducted heat, but if the air all
around the carb is hot, the carb temp will go up.

For an experiment I added a drain to the carb vent, and did see fuel coming
out after stopping.  This weekend I hope to open the carb and check the
fuel level, etc.  I used my IR thermometer to read temps on stuff that was
nearly black, and there were many spots that were initially well over 170,
some even after 15 minutes with the hood open.

My first generation Jetta had large flexible flaps over large holes in the
panel the radiator mounted on.  The would apparently open when the car was
in motion, but close when the car stopped, to prevent hot air recirculation.

I already thought of using the electric fan many of us have as a "bilge
blower" (like on boats) on hot restarts.

Stu

On Tue, Sep 15, 2015 at 2:48 PM, Tom Witt via Tigers <tigers at autox.team.net>
wrote:

> I finally got around to watching this video. It was like watching paint
> dry. It could have been 30 seconds long, not 20 minutes. But anyway (it’s
> not Stu’s fault), it did eventually shows the gas boil at a low
> temperature.  I’ll just throw these things out there:
>
> 1. No one ever mentions it, but there has to be some cooling effect of
> both from the air being drawn into the carburetor and the evaporation
> effect of the gas (like when you apply rubbing alcohol to your body).  Then
> again, without cold air ducting..., maybe not.
>
> 2. A cooling effect likely offsets some of the heat soak while the car is
> in use. However, when the engine is stopped that cooling also ceases.
>
> 3. It would seem best to isolate and insulate the carburetor from the rest
> of the engine compartment.  This, however would require some type of hood
> scoop (or duct) and and an isolation enclosure under an already crowded
> hood.
>
> 4. I’m wondering if the horn opening block off plates some use (that work
> at idle/stopped) to prevent recirculation are a detriment at higher speeds
> not allowing cooler air to pass through the engine compartment?  I’ve
> always through a spring loaded door seemed a better concept. Closed at an
> idle/stopped condition, but opened from the flow of air as the car moved
> forward.
>
> 5. If anyone is interested, some of the 280Z’s had a fan and ducting that
> blew air onto the injector rail. The concept would seem somewhat adaptable
> to blowing cool air on the carburetor after the car shut off. A 555 Timer
> could control the run time after shut-off.
>
> It seems any attempt for correction deviates from a stock appearance. And,
> a crowded area under the hood doesn’t leave many options for isolation or
> ducting. Has anyone run without their hood to see it it makes a difference?
>
> Just some rambling thoughts.
>
> *From:* Stu via Tigers <tigers at autox.team.net>
> *Sent:* Friday, September 11, 2015 5:42 PM
> *To:* Tiger Net <Tigers at autox.team.net>
> *Subject:* [Tigers] Carb Fire Research
>
> I've been doing some research, and found an interesting video:
>
> https://www.youtube.com/watch?v=d7AkcjnCTVU
>
> To summarize, this guy does a good experiment that proves that some
> components of normal gasoline boil at around 170F.
>
> Our engines are usually well above this, so when we stop it's safe to
> assume that the underhood temperatures spike, heating up our carbs, and
> everything else.  So there could easily be some boiling in the carb  bowl.
> And once the 170F components boil off,  the temperature rises even more.
> When trying a hot restart, new fuel is introduced to the much hotter carb
> when the pump comes back on, perhaps there could be a few moments of
> additional, perhaps more violent boiling, blowing vapor and fuel out the
> vent.
>
> Last weekend I tried recreating the problem.  Similar hot day, drove
> around the same loop, etc.  Parked and waited 10 minutes, about the same
> time as before.  I had cleaned the top of the carb, so it was obvious that
> nothing had come out when I opened the hood and looked  But listening
> closely, I thought I could hear a faint sizzling sound from somewhere
> within the carb...   I did not try a restart, since I hadn't seen this
> video at that point.
>
> Any opinions?
>
> Stu
>
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