I have a motorcycle that has been garaged and unused for the better part of the last five or six months. I’ve got it running again, but it’s still kind of rough at point. Now I’m not looking for a mechanic diagnosis because I know there are a number of things that need to be done to the bike, but I’m wondering if having “old” gas might be a contributing factor. Should I empty and refill the tank? How long can gas sit in a vehicle and still be “good,” or does quality decrease at all?
Yeah, your gas is probably old.
Sta-Bil fuel stabalizer.
Cecil comments
Yes, fresh gas can help.
-Different blends are sold in the summer and winter months. Using “summer” gas in the winter can lead to poor starting performance.
-The more volatile compounds in the gasoline evaporate first, leading to poor starting performance. “Winter” fuel degrades the most, as it contains more volatiles to help with cold starting.
-A partially empty tank, vented to atmosphere, sitting outdoors in an unheated garage will accumulate water due to condensation.
At a minimum, dump your carb bowl(s) and drain a bit from the bottom of the tank (use the reserve setting of the petcock) to get the worst of the water out of the system.
I’m not sold on it, but there is a product “Stabil” available that claims to be a gas “preservative.” It is intended for use in motor sports vehicles and power equipment that is used only seasonally.
Is this true in all climates? For some reason, I didn’t think this was done in Southern California.
I regularly have to clean out carbs on bikes that have sat too long. The pilot jets and internal passages clog easily, and can be difficult to get clean without removing and stripping the carbs. I’m in Houston, TX and the gas we get goes bad fairly quickly, couple of months at best. Spoiled gas has a distinctive, foul odor. If the carbs were drained before storage, just drain the tank & carbs, and you should be set. The carbs might be only slightly clogged, which can clear up over time, but your best bet is to remove them and clean them out thoroughly. Replacing the viton rubber o-rings and bowl gaskets while you’re in there is a good idea, too. Be sure to get the proper parts, since plain rubber o-rings will deteriorate quickly when immersed in gasoline.
On preview, if you have water in your tank, remove it and take off the petcock to get all of it out, most tanks don’t drain fully, even on reserve. This is a good time to check for rust, which can get past filters and clog your carbs back up. Mild rust can be treated with Kreem, a cleaner/coating kit.
Former motorcycle mechanic here. I assume that your bike is a multi-cylinder four stroke street bike.
I would replace the gas and change the oil. Motorcycles are very sensitive to stale gas due to their having weaker ignitions than cars. Over months, the gas can also seep through the fuel petcock, overfill the carburetor bowls and drain through the cylinders and into the oil. The old gas can go into your car’s gas tank. Mixed with fresh gas, it won’t cause any problems.
You may want to swap out the spark plugs as well. Use the recommended plugs for the bike. In any case, don’t use Champion plugs (Champion plugs for small engines are terrible, at least they used to be.)
After changing the oil and gas, drive the bike around for an afternoon. Accelerate to high rpm a few times, slowing down using engine braking (i.e., don’t use the brakes.) If the bike still stumbles right off of idle, then the carburetors may need to be cleaned.
Winter/summer fuel variation is probably only done in places that actually have a winter. 
(*) The refineries do it at higher latitudes, but it is not super critical…if it were there would be tons of stranded motorists every time there was a very late or very early cold snap.
It is fairly important for Diesels as well, the #1 fuel oil (pretty much the same as kerosene) is much less likely to gel (wax precipitation) in cold temperatures than the #2 sold in the hotter months.
*I recall my parents being stunned when we visited my uncle in San Diego, and he showed them his tomato plants that were several years old…you don’t see that in Colorado.
As a counterexample…14 years ago my father took his 72 Ford LTD, parked it in a barn and walked away. 2 years ago (after being in storage for 12 years). I started it back up. Granted it needed a couple of hours of work, but it started on (untreated) gas that was at least 12 years old and it’s been running fine since.
Also, we took NO special precautions when parking it. He just parked it as if he would be using it again the next day.
Correct. 1983 Honda Nighthawk 450.
I actually bought spark plugs several months ago for the bike, and I still haven’t installed them. Mainly because I don’t have the tools for it at home, and my brother-in-law, who does, lives about 25 miles away. Since the bike isn’t really freeway-worthy, it’s a pain to get there. But I need to get it done. The existing spark plugs are being held in place with duct tape (it was that way when I bought the bike).
Uh oh. Are the spark plugs loose in their holes, or do you mean that the caps are loose on the plugs? If the plugs themselves are loose, you may need to replace the spark plug threads with a Helicoil or something of the sort. It’s not a hard job, but it takes a little effort to not get any metal into the cylinders (I coated the tap with grease and cleaned it off a lot.) You may want to give the responsibility to a mechanic.
Plug caps are easy to replace, if they are loose.
Ah, I remember asking for advice when my Dad tried to get rid of petrol that he considered “old”. He was caught by Mum trying to get rid of it in her car, but for some strange reason he poured it into his own diesel car instead 
I have a somewhat related question…I left the cap off of a big jug of motor oil in the garage. Would it be a bad idea to use that? I’m wondering if it will have any similar problems like evaporation, or water or anything.