Thursday, 26 September 2013

Dumping the Gizmo

My Honda CBF250 has recently developed a small but annoying fault. At its usual cruising speed of about 60 mph it occasionally stutters. It feels like it's about to run out of fuel. Otherwise the bike is running fine. I checked the spark plug, air & fuel filters, cleaned the carb and even replaced the coil with no success.

I then found reference to running problems associated with a faulty air injection valve. This is located behind the cylinder head and has a pipe running to the cam cover and the air filter. A smaller pipe runs to the inlet. I assumed that this was for emissions reduction. I don't know how it works. A breather from the engine to the air box is ok and I guessed that the pipe to the inlet was a vacuum switch but why? The unit is called the "air injection control valve" in the parts book which implies that it supplies air to the inlet, but that would seem to affect the mixture. Anyway I disconnected the inlet tube and plugged it. After a run today it seems that problem has gone.

The valve does not appear in the Brazilian repair manual I have so it seems it's not vital.

Air injection valve
Pipe to inlet blocked



  1. My Triumph Bonneville has air injection and no one seems to like it as it causes popping and banging on the overrun. The idea of it is to burn unburnt fuel in the exhaust. I have disabled it too by pulling out the tube from the airbox, plugging it and putting it back in. I was told on the forums that the bike should be idled for 10 mins to allow the fuel injection to "learn" and adjust to the modification. I was sceptical about this but did it anyway. Popping and banging now completely gone and the previous snatchy throttle response is now smooth!

  2. Thanks, I have the same problem on my CBF 250 will try this tomorrow.
    I have also changed the drive sprocket to a 14 tooth which has raised the cruising speed
    Barry from Perth WA

    1. As it turned out removing the air injection valve did not fix the problem. After a long time I found that the stuttering was caused by a leak in the carburettor at the top cover. This allowed loss of vacuum above the diaphragm causing the slide to fall. I sealed it with silicone gasket. My bike is now running perfectly.

      I also tried a 14t gearbox sprocket but I found the gearing was too steep. With it I had to remove the steel plate between the outer cover and the crankcase to avoid fouling.

    2. Hi thanks for the quick reply it does seem better since removing the air injection valve. I will take the seat and tank off again and look at sealing the diaphragm cover.
      Western Australia is very flat and I have not found the higher gearing a problem.
      I ground the steel plate to make room for the larger sprocket, the problem here is the strong head and cross winds, I would not mind a bit more horsepower sometimes when hit by a gust of wind on the freeway.

      Thanks for your help
      Barry from WA