Thursday, 8 July 2010

The sound of silence.

If you've ever heard a big bore Husky at full chat you know it's loud. Proper loud. Little children screaming, car alarms going off at 50 yards - loud. Then imagine that you have an 84-decibel limit to adhere to. That's exactly what I was up against today when I did the mandatory noise test for my street tracker.
I always thought that passing the noise test would be the hardest part of the whole M.O.T-process and I knew from the start that there was no way in h**l that I could get my bike under 84 dB at full throttle – not unless the test was done in deep space. So first on my list was finding a way of making it impossible to give the bike the full beans. After some head scratching I found a nice little hole on the underside of the plastic carburettor lid. I put a 50 mm long screw in there, which made it impossible to lift the throttle more than about 40 % of the way. That went some way towards getting the noise down. Next up was filling the SuperTrapp silencer with sound deadening fibre glass "wool" and taking out 11 of the 12 discs at the end of the silencer.

At this point I was kind of happy with the results but the induction sound from the carb was still too loud to my ears (I was guessing at this point since I didn't have a dB-meter). So yesterday night I started panicking a bit and began botching together a sort of external air box over the exposed KN filter. And when I say botch, I really mean botch. Or how does cutting out plastic pieces from IKEA boxes, filling it with cell foam and gaffer taping it all up sound? Botch Engineering Ltd.

After a few hours sleep I brought the bike up from the garage to do a test start before I was to load it and go to Malmö (some 65 clicks away). It absolutely refused to start! No matter how many times I kicked the thing – nothing. Obviously it didn't get enough air and I had to modify my "air box" one more time, with one hour to go to the test. I finally got it started and it sure was quiet:-) I also ran like Chitti Chitti Bang Bang with a bad cold. Ah, to hell with it I thought and loaded up.

After waiting in line for an hour or so, watching all sorts of machinery go through the sound trap, I handed my bike over to the guy who was going to do the test ride past a dB-meter and watched him "speed off" with the engine coughing and farting on "full gas" (40% throttle). On his return pass I could see the frown on his face and he gave me the bike back stating that it "just wasn't working". I felt the panic rise since I knew that there wasn't going to be a new test for some weeks. I told him I was going to do some mods and could he please give it one more go? At this point the other bikes and cars had passed their tests and it had started raining. So I started tearing frantically at my "air box" to try to give the bike some more air. Guess what? The bike refused to start. I kicked and kicked. I checked the plug, the fuel and everything else I could come up with. Nothing. After some 15 minutes of this I was drenched in sweat. Seconds from giving up, the last remaining biker told me to throw away that stupid air box since "I was screwed anyway". I did and the bike fired up and I handed it over to the tester, expecting disaster.

But do you know what? IT PASSED! I was actually one dB under the limit even without the stupid airbox. I haven't felt so relieved in years. On Monday I'm going back to get the final clearance on the frame and then the final M.O.T will be a mere formality. Wish me luck!


  1. ah, c'mon, man, you simply HAVE to post pictures of your beauty with the uggly airbox!!!!

    Well congrats on passing the 84db limit test.

  2. SOUNDS GOOD!!! :-)) Next show us a good wheelie past the sound meter without restricors!

  3. ^^^wot he said.

    (actually that airbox baffle doesn't look half bad -- nothing wrong with a practical bodge now and then...)