Idle Air Bypass
Idle Air Bypass
This modification allows you to adjust the TPS for the recommended 1400 rpm idle speed, but then increase the idle speed by 150 - 200 rpm without affecting the EFI. This modification, along with a flywheel weight, makes the engine nearly impossible to stall.
The idle air bypass is commonly used in the automotive world. It is often implemented as an “air leak” under the control of the ECU via a needle valve moved by a stepping motor. It allows the ECU to compensate for variables like the cycling of an air conditioning compressor or variations in alternator demand while maintaining a constant idle speed.
It works because the idle mixture is often extremely rich (especially on a 2T), and bleeding in a bit of extra air is not harmful.
I found a Clippard MNV-1 needle valve among my brass fittings. It seemed about right size-wise, but the M5 x 0.8 thread was very small. I test-tapped that thread in an old rubber manifold and it had very little holding power. The relatively fresh rubber of the OSSA manifold was even worse. As a temporary measure, I liberally applied Threebond 1207B on the outside, and it seems to be holding well (even years later). On the second iteration, I made an internal “sleeve nut.” Although I never took a photo of the sleeve nut, the image below should give you the right idea.
The yellow hose is a Saint-Gobain Tygon LP-1100 fuel hose, 1/4" OD, 1/8" ID. I made the hose barb fittings from 1/4" brass hex stock.
Interestingly, the M5 x 0.8 thread is almost exactly the same as an Imperial #10-32 thread.
Assembled air bypass on manifold
Location of installed idle air bypass
Location of upstream air-bleed fitting
Example sleeve nut
Clippard MNV-1 needle valve