How We Made a 38mm Carb Flow Like a 40mm — and Stayed Lites Class Legal

Small changes can make a big difference.
Inside every carburetor are small bumps, casting flaws, and edges that disturb airflow.
Even though they seem tiny, these imperfections hurt performance — especially in high-RPM racing engines.

We found a way to fix that.

By designing a smarter flame arrestor adapter that smooths the airflow inside the carburetor, we recovered almost 10% more usable airflow.
The result?
Our 38mm carburetor now flows like a 40mm carb — without changing the carb itself and without breaking Lites class rules.

In racing classes like Lites, where you’re restricted to a smaller carburetor by regulation, this gives you real gains without stepping outside the rulebook.
And because the improvement is built into the flame arrestor adapter, it’s fully IJSBA Lites legal.

For those who want the technical details:

When we inspected the 38mm carburetor, we found several internal casting features — small bosses and supports — sticking into the airstream.
Measured together:

• Three small supports blocked around 19.8mm² of flow area.

• One larger sloped feature accounted for 86.61mm² of area.

That’s a total of 106.41mm² blocked, or about 9.4% of the total cross-sectional area.

However, it’s not just the blocked area that matters — it’s the turbulence and flow separation they cause inside the venturi.
Sharp edges inside a carburetor cause air to separate violently, creating swirling wakes and dead zones.
In a high-speed venturi, this turbulence magnifies the real-world flow loss far beyond what the numbers suggest.

We calculated that these obstructions were causing up to a 20% effective loss in airflow — not just 9.4%.

How We Fixed It

We designed a flame arrestor adapter that:

• Smoothed the transition around the sharp-edged bosses.

• Gently re-shaped the large sloped area to reduce flow separation.

• Created an aerodynamic, blended path for air to follow.

We also spent time testing different setups.
We trialed short adapters, long adapters, with and without blending, and found very little difference between them in terms of real-world performance.
The real gain came from simply smoothing the critical airflow area inside the carburetor itself.

That means you can bolt these inserts in between your existing flame arrestor adapters, no need to change anything else — and instantly enjoy the speed and throttle response benefits.

Important Ordering Information:
These adapters are sold individually.

• You will need two units for a Yamaha SuperJet (SJ).

• You will need three units for a Yamaha 1100 Raider.

Please adjust your quantity accordingly when ordering.

After installing the adapter:

• Effective flow loss dropped from around 20% to close to 10–9%.

• We recovered almost half of the lost airflow inside the carburetor.

• We picked up 50 RPM at full throttle — a real, measurable performance increase.

Why It Matters

This shows that even small sharp-edged obstructions, when placed inside critical airflow areas like a carburetor venturi, can create far greater losses than most people realize.

While the physical blockage was only 9.4%, the effective airflow loss was closer to 20% before the fix — purely because of turbulence and flow separation magnifying the disturbance.
After installing our flow-straightening insert, we estimate the disturbance has been cut dramatically, bringing effective flow loss down closer to 10%–9%.

In high-performance engines, small details matter — and this is a perfect example of real, measurable gains by improving the airflow where it matters most.

In particular, in restricted racing classes like Lites, where competitors are forced to use smaller carburetors by rule, this level of improvement can make the difference between being mid-pack and leading the race.
By fixing internal turbulence, we recovered enough efficiency that our 38mm carb now flows similarly to a 40mm carb — effectively gaining around 10% more usable airflow without changing jetting, venturi size, or breaking class rules.

And here’s the best part:
Because our flow-straightening insert is integrated into the flame arrestor adapter, it remains fully legal under IJSBA Lites class regulations.
No modification to the carburetor itself — just a smarter, better-designed adapter that unlocks hidden performance.

In Lites racing, where every small advantage counts, this is like bolting on a 40mm carb legally — and it shows on the stopwatch.