Carburettor

Group K's carburettor setup sequence

Group K's order for setting up a watercraft carburettor, step by step, with their own figures and what their guide refuses, from a company that sells carburettor work.

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One company's guidance, and they sell the kits

Everything on this page is Group K's, transcribed from their own pages on 8 September 2026. They sell carburettor work and the parts that go with it, so read it as a supplier's method rather than as a neutral standard. It is published because knowing what a good carburettor job looks like is what lets you judge the one you paid for. The order matters more than any single figure: Group K's sequence is the method, not a checklist you can start in the middle of.

1. Fix the mechanical faults first

Air leaks, fuel delivery and pop-off before any jet moves. Group K's position is that a mechanical fault defeats every jetting change made after it, and that air leaks only ever get worse.

2. Match the symptom to the throttle position

Find the throttle percentage where the problem appears, then adjust the circuit that owns that range: low speed 0 to 35 per cent, transition 20 to 50, high speed 30 to 100. Second source: Mikuni's own Super BN manual arrives at the same split independently.

3. Set pop-off to the carburettor generation, not to a number you read

Group K give 8 to 12 psi for round-pump carburettors and a 25 to 35 psi minimum for square-pump designs, and say paired carburettors must be within 1 psi of each other. Round-pump is 38mm and 44mm pre-1989; square-pump designs run from 1990. Second source: Mikuni say the same thing about method: pop-off is measured with a pump, not looked up.

4. Re-test when the weather moves, not when the setting feels old

Their thresholds are 1,000 ft of altitude, 20 degrees Fahrenheit, or 20 per cent humidity. Water temperature between 55 and 85 Fahrenheit they say has little effect on mixture.

What the guide refuses

Kept apart from what it recommends, because a guide is usually more informative where it says no. Group K refuse five things. Reading plug colour above the porcelain base, and treating plug colour as a strong diagnostic at all, because throttle position varies constantly in real riding. Masking a lean mid-range by over-richening the low speed circuit. The idea that maximum power sits right at the edge of seizure, which they reject outright. Lowering pop-off pressure to cure starting hesitation, except as a last resort. A return line restrictor larger than 0.030 inch on 44mm round-pump carburettors.

Where the figures behind steps two and three are published

The figures behind step two and step three are published here in full, out of Mikuni's own Super BN Owner's Manual rather than out of Group K: which circuit owns which part of the throttle, and needle, spring and pop-off. Both tables are listed under Spec tables below.

Before you act on this. Every step and figure above is Group K's, and Group K sell carburettor work and its parts. Treat it as one supplier's method, set against the Mikuni tables below, and not as a neutral standard.

Spec tables

• Mikuni Super BN, circuit by throttle opening
• Mikuni Super BN needle and arm spring

Cite this page

“Group K's carburettor setup sequence”, BaddJet archive, https://baddjetarchive.com/tech/carb/group-k-carburettor-setup-sequence/

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