Uneven four-bowl carburetor levels — why octane repeatability suffers

The same fuel through four bowls should reach a similar maximum knock point. When levels diverge, needle seats leak, or bowls cross-feed, repeatability collapses even if the knockmeter zero is perfect.

Level first

Set each bowl to the laboratory’s reference mark. A bowl that runs low leans the mixture; one that runs high richens it. The absolute knock number may still look “in band” while bowl-to-bowl scatter grows.

Needles, floats, cover gaskets

Cleaning removes gum. Grooved needles, fuel-soaked floats or leaking cover gaskets need parts and a fresh level set. After work, run one fuel through all four bowls and compare maximum knock.

Cross-feed

If reference and sample meet inside the bowls, conclusions can invert. Isolate lines and confirm bowl independence before blaming the fuel.

Related: octane repair, knockmeter zero.