Mopar Muscle – October 2019

TECH | FUEL FOR THOUGHT

air filter that connected to the crankcase

breather via a rubber hose.

Back at the Pennsylvania College of

Technology Mustang chassis dyno, we

connected the O 2 sensor to our new

sensor bung. With the 273 idling, we

noted the air/fuel (A/F) ratio was 13:1,

which was way too rich and confirmed

our previous belief about the over-fuel-

ing condition. The idle mixture screws

did not help to lean out the condition

at all. Seeing we were at the dyno, we

strapped the Dart down and did a few

pulls to see how close the tune was.

The first pull was extremely lean (13.7:1

on E10 87-octane pump gasoline at

WOT), and there was a very notice-

able bog when the secondaries opened.

To make matters worse, the torque was

down almost 30 lb-ft when compared

to the 2-bbl setup. We made a few jet

changes, an accelerator-pump

discharge nozzle change, float-height

corrections, and adjusted the vacuum-

secondary screw to slow down the open-

ing of the secondaries, but nothing

worked like we’d hoped. It was time

to regroup, so we returned to our garage

to analyze the data we had acquired.

Changing from the 2-bbl to the

Brawler had greatly changed the air

velocity through the intake of the 273.

The vacuum signal at the Brawler’s

boosters was not as strong (when com-

pared to the 2-bbl); consequently the

amount of fuel being pulled into the

engine had been reduced. Based on the

dyno pulls we had made and the jet

changes we had performed, we devel-

oped a ratio of jet size to A/F ratio and

determined that a bigger swing was nec-

essary. The primary jets were bumped

from stock 66 jets to 74 jets, and the

secondaries were also increased eight jet

sizes from 70s to 78s. We figured the

larger jets would lower the A/F ratio to

about 12.3:1 at WOT. To handle the idle

A/F ratio, the primary air bleeds were

increased from 73s to 79s, and the sec-

ondary air bleeds were opened up from

73s to 80s. The vacuum secondary was

adjusted to one turn out. We retained

the discharge nozzle at 0.030 inch,

which had provided decent results

on the previous dyno runs.

Back at the Penn College dyno,

we connected the O 2 sensor, and we

were able to adjust the four corner idle

screws to about 1.5 turns out, and the

A/F ratio was 14.4:1. This was much

more acceptable, and the rich odor that

had accompanied the Dart was greatly

diminished. We secured the Dart to the

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