The stock intake was not designed with highest horsepower in mind. The intake tract is too large in volume and the carburetor is too small. The 1986 CR250R had a reed block and boot cross sectional area roughly 10% larger than it's 38mm carb. At 34mm, the stock ATC/TRX's large volume cage indicates a different design objective than highest performance.
According to A Graham Bell, the cross sectional area of the intake tract should not exceed 10%-15% of the bore size of the carburetor. Air is a compressible fluid and as such an intake tract that is large will respond slowly if the displacement of the motor is disproportionately small. This condition creates predictable but mellow pulses on the carburetor. Coupled with the fact that the TRX/ATC carb sits roughly 1/2" further from the piston than the CR, Honda has dramatically mellowed the intake tract. The smaller carb is the only saving grace for low end power, however that means wide open throttle will be weak in the fuel delivery department, limiting peak hp. This is all a problem that is easy to fix with simple bolt on parts. Here are some stock observations below.
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This is a photo of the stock TRX250R reed cage. The ATC version is similar. Notice how open the cage is. When designers talk about flow, it is measured as resistance compared to an opening that has none. Unimpeded flow is a good thing, but intake design is not as simple as making the holes as large as possible. It's easier for your mouth to suck fluid through a straw than a pvc pipe, correct?
Here is the stock cage next to an early model CR cage. Notice how Honda "stuffed" the cage. This hole will be easier for the engine to sip on the carb from. Another way to state it is this design will make the carb give more fuel to the engine. Keep in mind that these volumes are relative to motor size - in this case 250cc.
*The CR cage in this case is not completely stock. You can see where the stuffer was radiused to make the outer diameter larger at the opening. This was done so the stock TRX carb boot could fit to it and minimize the amount of drag and turbulence as the fluid passes into the cage.
Compared to a later model CR cage with boot acting as the stuffer:
In this photo on the left is an 86 CR250R boot and an 88 TRX250R boot on the right. The cages below are Moto Tassinari's V-Force cage for the CR and TRX. The TRX version because it has to match the stock boot is large - about 40% larger than a 38mm carburetor.
I've heard cases of high horspower engine builders running the TRX style cage for their application. For sure this design allows a higher volume of fluid to pass through when demands are high. There is nothing wrong with running the large cage, provided your application runs well on it. The stock TRX 250cc motor with a 34mm carb attached is not one of them. It simply will be too lazy.
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