Re: My 304 stumble

[jlandry@domain.elided (John A. Landry)]

When the truth serum kicked in, Tom Mandera <tsm1@domain.elided> blurted out...

>I have no idea which power valve is in the Scout now.. but I also have an 
>off-road spring-loaded needle and seat to install, and some different jet 
>sizes to try for better economy and/or power.. so I'll pull the power 
>valve while I do all of this, and swap up for a larger one.
>
>What's the downside, other than the EcoNazi's war cry, to a larger power 
>valve?  Will fuel economy drop if I keep my foot out of it?  Naturally, 
>when I floor it, economy would go down... ;)

The downside to a too large power valve is wasted fuel.  Your fuel economy
will *really* suck when the vacuum drops below the point at which the valve
opens.  If you have a vacuum gauge and know the opening point, then you can
simply avoid dropping the vacuum that low.

A power valve with a too small orifice caused poor drivability, lack of power
and possible lean pinging.

Choosing a power valve is a two part affair.  You have to know how big it
needs to be for your particular engine setup.  Simply changing the air cleaner
can completely change the power valve requirement (and everything else carb
related).  Next you have to determine when the power valve needs to start open
to give you the performance / fuel economy you want.

For example, for normal commute driving, I have my Edelbrock's "power mode"
starting in at about 10" of vacuum.  I have the mixture set so I get a proper
air / fuel mixture for good acceleration, but no more.  As I'm accelerating at
say 12" of vacuum, the mixture drops quite lean, but as soon as the vacuum
passes 10", the metering rods move up causing the power mode to engage and I
can watch the mixture jump up rich on the air / fuel mixture gauge.

When I tow my travel trailer, I'm usually cruising down the highway at 10 -
12" of vacuum.  So if I left the power mode staged at 10" of vacuum, I'd be
cruising around all of the time in that power mode.  I could watch the gas
gauge moving!  Instead, I change the carb settings so the normal cruise is a
tiny bit richer and I set the power mode to wait until about 6 - 8" of vacuum
before it comes in.  

I can't imagine trying to properly select a power valve without the aid of an
air / fuel mixture and vacuum.  It would be a lot of trial and error, never
really knowing if you have the optimum settings.  I'm sure you could spend a
day at a drag strip with a stop watch to try and figure it out.

Regards,

John
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