My neighbor asked me to look at his CBR 150. He claims he rides 6-8 kms and then he has no power. (insert CBR150 lack of power joke here)

I followed him on a ride and sure enough on the Hwy he just runs out of speed. His homegrown fix is to stop and remove the gas cap. This is accompanied by a large hiss, and then he can run at speed for a while.

Anyone else experience this?

We took the tank off and there is no vent pipe to block, so one can only assume the tank vents through the cap, but inspection doesn't reveal anything obvious. I thought maybe the vacuum fuel cock was leaking and pulling a partial vacuum in the tank but because the cock is below the fuel level that would also result in the bike running very rich, which doesn't appear to be the case?

Geez! The labour-rate for Honda mechanics is so cheap here why is he trying home remedies? If you can't diagnose and fix a problem instantly, you are wasting your time.

And then wasting more of your time posting. Get thee (and your friend) to a Honda shop!

Try a Honda shop, even a good dealer like I tried when my CBR150 carburetor had fuel problems. Then don't be surprised if it takes them two days and 1000 baht to NOT fix the problem. It seems pretty obvious to be a carb problem, but they may blame the choke, or gasohol, or diesel fuel, or the ambient temperature, or their mother in law's somtam. My carb had to be rebuilt twice; it really is the only problem I ever had with the bike.

The problem is with the tank. It has to vent somewhere

The problem is with the tank. It has to vent somewhere

Spot on

A common problem on german-made "Zundapp" mopeds - my favourite brand ever by the way. The problem is a non-functioning tank-cap-ventilation.

When he drives, petrol goes from the tank to the carb, and air goes thru said ventilation into the tank. If that ventilation is clogged up, a vacuum is generated - until this vacuum is strong enough to prevent further petrol from reaching the carb.

Opening the tank cap, the hissing sound is from the air flowing into the tank. And instantly petrol goes to the carb again and he can drive until the vacuum once again has built up.

Easy solution to this problem: Replace tank cap, don't need any mechanic for that

Best regards.....

Thanh