Yamaha TY175 · 1976–1980 · fuel system

The slide carburettor, cut open and driveable

A Mikuni VM-type round-slide carb has no single “fuelling part.” As the throttle rises, the job of metering fuel is handed from one circuit to the next. Drag the throttle and watch the hand-off happen.

Type: Mikuni VM round-slide
Bore (approx): ≈24 mm · VM24-class
Pilot jet (std): 25
Float: Twin, needle-valve fed
Choke: Lift-up starter plunger

‹ swipe diagram ›

Throttle opening 0%

0–⅛ pilot ⅛–¼ cutaway ¼–¾ needle ¾–full main

Currently metering

Pilot / idle circuit

Air through venturi8%

Fuel delivered14%

The four hand-offs

0 – ⅛ throttle

Pilot circuit

Slide shut. The engine sucks fuel through the small pilot jet; the air screw trims idle mixture. The main system contributes almost nothing.

⅛ – ¼ throttle

Slide cutaway

Just cracked open. The scallop cut into the slide’s intake edge governs how much air slips under it — shaping the off-idle mixture and throttle response.

¼ – ¾ throttle

Jet needle

The tapered needle lifts out of the needle jet. Higher lift = wider fuel gap. Moving the needle clip up or down shifts this entire midrange.

¾ – full

Main jet

Needle is essentially clear of the jet, so raw main-jet size now caps maximum fuel flow. This is the number you change for top-end richness.

What each part does

Slide (throttle valve): A round piston the cable lifts. It both throttles airflow and carries the jet needle.
Slide cutaway: The chamfer on the slide’s air-side bottom edge. Bigger cutaway = leaner just off idle.
Jet needle: Tapered rod hung from the slide. Its taper meters midrange fuel as it rises out of the needle jet.
Needle jet: The tube the needle sits in. The ring-shaped gap between the two passes fuel into the airstream.
Main jet: Brass jet feeding the needle jet, capped by the hex plug that protrudes from the bottom of the float bowl — unscrew the plug to change it.
Pilot jet: Tiny jet feeding the idle and low-throttle circuit. On the TY175 it sits in a long tube that reaches right down to the bowl floor, trimmed by its own air screw.
Float + needle valve: Floats rise with fuel level and shut the inlet needle — holding a constant level in the bowl.
Venturi / throat: The narrowing under the slide. Fast air here drops pressure and draws fuel up out of the jets.

Mechanism shown is the standard Mikuni VM round-slide as fitted to the TY175 — orientation, hand-off ranges and circuit behaviour are accurate to the type, including the long pilot-jet tube reaching the bowl floor and the main-jet access plug under the bowl. Exact jet sizes vary with year, altitude and exhaust/airbox: the pilot 25 is the documented TY175 standard, but confirm main jet, needle (6-series) and clip position against your own Haynes/Yamaha figures before re-jetting. Throttle-band percentages are the conventional rule of thumb, not hard cut-points — the circuits overlap.