Pick the right needle valve spring for Tillotson HS-series carbs (HS-175/205/319). Spring stiffness changes pop-off (opening) and reseat (closing) pressures to tune fuel delivery.

Options

• STD — baseline behavior for general use
• Hard — higher pop-off; helps calm richness
• “Suninen” — special spring named after the tester/builder for precision racing setup

Practical targets (that work)

• Pop-off (opening): ~1.0 bar
• Reseat (closing): ~0.8 bar

Every paddock carb guru has a “perfect” number—use the pair above as a solid starting point.

How to get there

1. Install your chosen spring and measure pop-off/reseat.
2. If needed, shorten the spring in tiny steps—start by removing one coil, then retest.
3. Keep the metering side open during adjustment (without diaphragm/cover). When targets are reached, install diaphragm and cover, then re-check; a small shift is normal.

Note: trimming permanently changes the spring. Go slowly and test often.

Usage note
A Tillotson HS rarely becomes “perfect,” but aim for a clean ~1.0 bar pop-off and a quick drop to ~0.8 bar reseat.

Humorous aside
If Mom asks the driver where Dad is: “Probably in the garage pumping Tillotson HS carbs :)” — a fine Saturday-night winter hobby for the mechanic.

Fitment
Tillotson HS-series (HS-175 / HS-205 / HS-319). Verify with your needle/seat spec and always test pressures on your own setup. Fast shipping from stock.