For those of you doing an HS4 SU card conversion on your Spitfires,
the following is the result of many hours of experimenting .
My Spitfire has a low compression engine.
The lower the compression, the richer the jetting
must be -- all else being the same. The header is a 4 into 2 into 1 with
the balance of the system stock. The K&N's are the 1.75" thick open
side individual filters with internal ram stacks installed.
The engine is in perfect condition (no compression loss) and the carbs
are totally rebuilt as new. Fuel is 88 octane unleaded. After many
calculations and needle profile plots, I have settled on the following
combination:
main jet: 0.090"
piston springs: red (4.5 oz.)
Needles: AAS
Initial ignition timing: 15 deg. BTDC.
Mechanical advance: 16 deg. by 4000rpm
Vacuum advance - ported after idle: 18 deg. @ 20" hg.
Float height: 3/16" (set low to help prevent heat soak flooding)
Main Jet set 0.065" below the carb throat
Damper fluid: ATF filled to 0.25" below the top of the piston tube
Spark plugs - NGK BPR6ES @ 0.028"
Idle speed = 850 rpm.
I live at 2,500 ft. above sea level with the air temp usually 20 to 25
deg. C and draw hot air into the carbs from under the hood. There is no
water heating of the intake manifold.
This state of tune may be on the lean side for some combinations and
I've found that going to yellow piston springs (8 oz.) will bring it in
slightly richer but causes the engine to rev up at a more leisurely rate
(slower response to throttle). If you want to stay with the red spring
throttle response, substitute ABD needles.
If you are running the high compression (9:1) back off about 2 deg. on
the initial timing and substitute AAT or if a more richer mixture is
required -- AAQ needles.
Hope this helps one and all with any carb tuning problem you may be
experiencing.
Larry Quirk
80 Spit (30K km. -- still has some factory air in the tires)
Edmonton, Canada
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