Motorcycle Repair: Honda CB125S 1976 fuel, premium gasoline, vent hole

Question
Hi Bill,

Just wanna ask if Honda CB125S 1976 can take UNLEADED gas.

The engine works fine for years using LEADED GAS until Taiwan banned the sale and use of leaded gas about 10 years ago. Tried additives with UNLEADED gas but the engine overheats easily then just shuts down.

I am trying to restore it now slowly as it's been sitting in the garage for almost 10 years now. I've never done any bike repairs before but I just love older bike models more than the "plastic scooters".

Any tips on how to go about this restoration project will be much appreciated.

Thanks in advance and all the best,
Mhike/Taiwan

Answer
Mhike, Honda engines work fine with unleaded fuels. That's all we have had here in the US for the past 20 years and I run my 1961 CB77 on it with no additives and no problems. Adding "additives" can change the fuel's viscosity in some cases, which will lean the engine out dangerously.

What is important is that the octane rating be maintained. I always have run premium gasoline in my motorcycles, unless regular is specified (late model machines).

Do a full tune-up on the bike, checking valve clearances, camchain tension, spark advancer function, ignition timing and clean the whole fuel system. If the gas cap vent hole plugs up, it will create a vacuum condition in the fuel tank and the carburetor will run out of fuel. Check the petcock passages and gaskets for restrictions/corrosion. The idle jet and main jet holders have cross-drilled holes which must be clear and the air passages in the carburetor throat must be open and clean.

With our alcohol-based fuels here, I have had to re-jet the carburetors up about one size to maintain the power output. The engine ignition system is battery powered, so the battery must be fully-charged and load-tested to provide maximum voltage to the ignition coil.

Bill Silver