On my road race Morgan SS with 45 dcoe-9 carbs, with 40 mm chokes, 5.0
auxiliary venturies, 35 pump jets (these are a major factor in high speed
enrichment), we use 145 main jets, f-11 emulsion tubes, 140 air correction
jets. The carbs come onto the main feed system around 3000 rpm at WOT. the
engine comes on the cam at 3800-4000 rpm and will pull past 6800 rpm. The
larger the choke the richer the idle jet needs to be because the main system
comes in at higher rpm as the choke size goes up. On an autocross car, you
need to look at the lowest rpm you think you are going to see at full
throttle and set up the main choke size from that. On a full race engine at
2187 cc, where the cam comes on at around 4,000 rpm, the bigger the choke
the more power the engine will make. going smaller than 36 mm on the choke
will have a tendency to push the beginning of the power curve down a couple
hundred rpm but will noticeably reduce power above 6,000 rpm.
For street use with the same tune engine, we use 34 mm chokes and
smaller jets all around, the idle jets can be 50 f9 instead of 55 f6 and the
cruise fuel economy will pick up from 17 mpg to about 22 mpg.
With smaller chokes you can also use more total advance because they
reduce the WOT cylinder filling. In full race tune on race gas the engine
might use 32 degrees total advance, in street tune with the smaller chokes
and pump gas you might be able to use 34 degrees total advance (but you also
might have to be careful about any real long time full throttle use!) The
more advance will help with better fuel economy on the street (or more
important to me, range).
Greg Solow
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