Bad fuel question - (symptoms)

[Duc]

This weekend... sorted my Honda Cafe Racer 550 /4 Supersport ( 76/77) and got her started up again after laying her up for 18 months.

Starts and runs sweet but I have a bit of a "throttle down" backfire thing going on as I de-accelerate under the throttle. Kind of unburnt gas maybe

I had a full fuel tank when I layed her up.. Could the fuel have gone "off" and be causing this.

Otherwise....what could it be?

All 4 carbs are balanced and jetted correctly. Backfire wasnt there before.

[NJ83]

I would put some fresh fuel in her, see how she runs after that.

[Duc]

I would put some fresh fuel in her, see how she runs after that.

I am guessing and hoping that fresh gas is going to make a difference but how/where do you dump 15 litres of old gas ...and because the tank is virtually full....I cant just dilute it with fresh gas top up.

Question was also kind of academic ...I wanted to know what is the effect of old stale gas.

[MSTRS]

How old is the fuel?
If the bike has been sitting for 'ages' then chances are the fuel is going to cause trouble. Usually rough running, rather than what you describe.
Are the carbs on boots? Old rubber does perish and crack. If this happens, then you get an air leak between the carbs and the engine, which will cause your symptoms.

[schrodingers cat]

The more volitile aromatics evaporate out and leave low quality base stock.

Put it in a container and run your lawnmower on it. Or put 5l in your car every time you fill the tank or give it to street kids to snort or...