Hi Scott and list -
Scott, you inadvertently said the answer to the mystery problem - and only
because a "racer" knows this - me...
"lifting" the throttle a bit allowing the engine to rev more is a sign of
weak mixture and/or insufficient power. Remember these cars are not that
aerodynamic and it is entirely possible that the spitfire "could" be running
out of steam at 70 mph. Hell, they only got 23 BHP on a good day anyway
<BG>!. Seriously, under race conditions, if you hit the wall o'revs on a
straight and backing off a bit either does nothing or buys you a few revs,
the motor has reached a lean condition. Remember, "condition" is the word.
There is a graphable curve (see front of Haynes Stromberg or SU or Weber
book) of A/F ratio to power and economy. 14:1 is optimal for combustion but
does not produce optimal power, that is about 12.5:1 from memory.
Likewise, about 15.5:1 is about the limit for lean surge/misfire. Given
that our cars were "smogged", if you look then at the corresponding graphs
for emissions, it should be clear that a richer mixture produces more
emissions.
Now, think a bit and take all the emissions/ignition timing/etc. crap into
play, and you can guess that if anything, the car will run lean at high
RPM's. Maybe only enough that it is "down on power", but enough that you
could hit the wall into a headwind. Float level could be the problem (too
low, despite your measurements, for the condition). Also, a lean mixture is
more difficult to ignite, thus you could be exposing a coil/ignition
weakness. Further, the highest combustion pressures are achieved under
load, also creating a need for more spark. Lifting the gas relieves some of
this pressure and allows a bit more revs. (when we race, "feathering" the
throttle buys a few revs each time up to a limit).
All in, look at the mixture first. Get thee to a quiet road, top the car
out in 4th, and while still on the throttle, kill the ignition and coast to
a stop in neutral. Pull all the spark plugs out and examine them. If they
look whiter than normal (almost guaranteed), you have a lean condition. Fix
it. (BTW, this exercise is called a "plug cut" and is only valuable if you
shut down ALL power to the motor at the EXACT moment you want to measure
(ie: return to idle or downshifting will taint the results enough to be
useless)). All racers have used this for longer than I've been alive.
Before ignition computers, etc., this was the only way to set the car up -
no plug in fuel maps based on a test bed. Bla Bla Bla. Gotta love old tech
that needs a car, a driver, a road, a plug wrench, and a screwdriver (oh ya,
and this list helps too)
Dave T
Sunny Winnipeg in the 60's
(the temp, not the decade)
-----Original Message-----
From: ScottBarr8@aol.com <ScottBarr8@aol.com>
To: faustus@inconnect.com <faustus@inconnect.com>; spitfires@autox.team.net
<spitfires@autox.team.net>
Date: May 5, 1999 7:55 PM
Subject: Re: Power struggles
>
>In a message dated 5/3/99 11:35:45 AM Central Daylight Time,
>faustus@inconnect.com writes:
>
><< When traveling on the freeway my power dies out at about 70
> MPH it fourth gear. I don't notice any power loss through the RPM range
> in any other gear. Last fall I found my carb (ZS CD40) full of sludge, so
> I thought it was fuel starvation. I've cleaned the carb and added a fuel
> filter. The carb was professionally rebuilt about 2000 miles ago, though
> it may be in need of adjustment. Anybody have any suggestions on what
> might be causing this?
> >>
>
>Well, Andy, I see there have been no magical solutions proposed to your
power
>loss question. I was hoping there would be as I experienced much the same
>thing last year -- the engine in the GT6 would pull cleanly (if weakly) up
to
>about 4500 rpm and then begin to miss and sputter -- lift off on the loud
>pedal slightly and it would clean up a little and pull a few more revs and
go
>back to sputtering.
>
>I was thinking, like Mike, that it was fuel starvation. Or carburation.
Or
>coil overheating. Or weak valve springs. Certainly, adjusting the fuel
>mixture didn't help either way (leaner or richer). Valve clearances set at
>10 thou. Fuel pressure was correct. Float level set properly.
>
>I'm hoping changing to the triple Weber setup will improve matters. And an
>electric fuel pump. And some larger diameter fuel piping for a little more
>volume.
>
>Scott
>ScottBarr8@aol.com
>
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