Tuning

By symptom

Indexed by what the machine is doing, not by what the part is called. A symptom appears here only when something is written that addresses it.

Mikuni Super BN carburettor trouble? Find the throttle opening first

1
Is the engine itself sound, with its parts and porting matched to each other?

Mikuni warns that a mismatched engine cannot be jetted around.
- Yes: go to step 2.
- No, or not sure: go to step 8.
2
At what throttle opening does the trouble show?

The circuit is chosen by how far the throttle is open, not by rpm or boat speed. Where two ranges fit, look at the lower number first.
- Idle: go to step 3.
- Just off idle to 1/4: go to step 4.
- Just off idle to 1/3: go to step 5.
- 1/3 to 3/4: go to step 6.
- 3/4 to wide open: go to step 7.
3

Low speed adjuster

Adjust in this order: 1 of 5.
4

Pop-off pressure

Adjust in this order: 2 of 5.
5

Low speed jet

Adjust in this order: 3 of 5.
6

High speed jet

Adjust in this order: 4 of 5.
7

High speed adjuster

Adjust in this order: 5 of 5.
8

Put the engine right before touching the carburettor.

For a Mikuni Super BN carburettor only: it narrows the search to one of its circuits, and it does not diagnose pump, ignition or engine faults.

How we know: the throttle ranges and the order are the rows of the circuit table, taken from the Mikuni Super BN Owner's Manual, pages 14 to 17, Mikuni American Corporation; the engine-first question is that table's own condition.

Bogs just off idle: What pop-off pressure actually is Carburettor
Bogs off the line: What an impeller pitch number actually means Pump
Compression test reads lower than expected: Compression and octane are one decision Engine
Detonation after a head or dome change: Compression and octane are one decision Engine
Engine screams, ski does not move: What an impeller pitch number actually means Pump
Failed inspection over trim tabs: What is actually legal in IJSBA Ski Lites Class rules
Fine on race fuel, breaks on pump gas: Compression and octane are one decision Engine
Flat spot in the midrange: What an aftermarket exhaust pipe actually does Exhaust
Fouls plugs repeatedly: What a spark plug can and cannot tell you Ignition
Hard to restart when hot: What pop-off pressure actually is Carburettor
Holed a piston: Compression and octane are one decision Engine
Lean surge before it comes on the pipe: What pop-off pressure actually is Carburettor
Loads up and blubbers at low speed: What pop-off pressure actually is Carburettor
Lost bottom end after fitting a pipe: What an aftermarket exhaust pipe actually does Exhaust
Need to know what a closed list means for parts nobody mentions: What is legal in UIM Ski GP3, and what the rulebook does not say Class rules
New plug fixes it for one session: What a spark plug can and cannot tell you Ignition
Not sure whether aftermarket reeds are legal in GP3: What is legal in UIM Ski GP3, and what the rulebook does not say Class rules
Not sure whether reeds are allowed in a stock class: What is actually legal in IJSBA Ski Lites Class rules
Not sure which vintage class my build fits: Vintage Ski against Vintage Ski Limited, the same ski and opposite answers Class rules
Over-revs and loses drive in chop: What an impeller pitch number actually means Pump
Pings or rattles under load: Compression and octane are one decision Engine
Plug comes out wet with fuel: What a spark plug can and cannot tell you Ignition
Plug looks white, glazed or blistered: What a spark plug can and cannot tell you Ignition
Pulls hard but only in a narrow band: What an aftermarket exhaust pipe actually does Exhaust
Racing UIM rather than a domestic series: What is legal in UIM Ski GP3, and what the rulebook does not say Class rules
Revs out but will not reach top speed: What an impeller pitch number actually means Pump
Runs hot since the exhaust changed: What an aftermarket exhaust pipe actually does Exhaust
Same ski, two classes, different answers: Vintage Ski against Vintage Ski Limited, the same ski and opposite answers Class rules
Told at tech that my pipe is not legal: What is actually legal in IJSBA Ski Lites Class rules
Told my ignition is illegal in one class but not the other: Vintage Ski against Vintage Ski Limited, the same ski and opposite answers Class rules
Told the exhaust must be entirely original: What is legal in UIM Ski GP3, and what the rulebook does not say Class rules
Want to know what I am actually allowed to change: What is actually legal in IJSBA Ski Lites Class rules
Want to know why one class is called Limited: Vintage Ski against Vintage Ski Limited, the same ski and opposite answers Class rules

33 listed