I've spent a couple of evenings tuning a friends 1979 1500 Spitfire. I
can't say I ever got the car to run right and am open to ideas from
those who are Stromberg literate.
The Problem: The car is running rich at idle but will then go lean at
high RPMs
Background: Car is non-stock. He had the motor rebuilt a few years ago.
He went from dished to flat-top pistons which supposedly upped the
compression to 9.0 (from 7.5). Ignition is the stock electronic. He is
running a 4 into 1 header. No emissions equipment. Running a Kent cam
but he isn't sure which one.
Good wires and plugs, timing seemed okay although I failed to find an
obvious timing mark on the pulley. Vacuum advance operational.
Compression on my gauge was a tad under 150 and even all the way across.
Valves all adjusted to 0.012. Plugs cleaned and gaped to 0.030. Did not
check float level.
The carb is the stock Stromberg but has been massaged by one of our
local tuners a few years ago. Converted to manual choke. Running a K&N
pancake filter that seemed only slightly dirty -- could still see light
through a strong lamp. Diaphram is fine.
When the car came into the garage, the jet needle was backed all the way
up into the stop and the car wouldn't idle. I checked over the motor but
the only real non-carb change was setting the rocker clearence. Added
light oil to the dashpot.
I ran the jet needle all the way down with only 1/2 turn left on the
threads. I could get an erratic idle at around 900 rpm. Very lumpy idle
and running rich. When the engine was hot, idle was problematic.
On a test drive, I hammered the car up to 70 MPH then cut and coasted to
check the plugs. They were clean and going towards white with a bright
white hallow around the base of the insualtor.
Steady state cruising at 4,000 rpm and the plugs were about the same.
Steady cruising at 3,000 rpm and the plugs looked normal to a little
lean.
Steady cruising at 2,000 rpm and the plugs got to a dark brownish yellow
with black carbon intruding on the electrode.
Idling around the block they go to black and sooty and you can feel the
motor loading up.
I don't know what to do now. I would guess that he would be into buying
some new needles but does anyone have a reference or any advice on which
way to go? My limited experience with the stromberg tells me that rich
conditions can only be corrected through the jet needle. The bypass and
compensator can only cause it to go lean -- not overly rich at idle.
That's the story to this point. Any advice appreciated.
Bob Allen, Kansas City
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