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I have an 1981 Honda CB750 bike that's been sitting in a garage in Arizona for 5 years. So far I've changed the battery and found all electrical components to be working. The engine isn't seized and the bike cranks, but won't start up.

I've worked on cars for a long time but motorcycles are somehow completely foreign to me. Any suggestions?

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    I would check sparkplugs, and the fuel line and carbs; make sure fuel is getting in there. But overall, sounds like a question for @ducatikiller – Stewart Jan 5 '18 at 10:50
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Stupid thing but easy to forget, make sure you have the fuel stopcock on, and that the fuel hasn't evaporated completely.

I had a very similar situation with my first bike, a 77 CB500T that had sat for years and I bought for $100. Motorcycles are no different from cars in basic principles, so you need to do the same checks that you'd do with any traditionally carbureted engine. It cranks, which is excellent, so you need to check fuel and spark:

  • The fuel itself may have oxidized to the point where it's unusable, or at least won't perform the way you want, so I'd drain the system and put in fresh gas
  • You may have gasoline gelled in your carburetor jets and other parts, in which case the carbs need to be cleaned. I say carbs because there's one per cylinder, which makes it a time consuming job. Ideally you'd take the whole assembly off, strip it, replace all the o-rings, jets, etc. If you don't have the stomach or time for that then you could try spraying lots of carb cleaner in them to get them going, then use seafoam to try and get them clean the rest of the way once it runs
  • You may not have fuel flow if the system is completely dry, putting a bit of fuel in the carbs just before cranking can get suction going again. A spray of carb cleaner may also work
  • Your plugs may be fouled, or there's no spark, test spark using the traditional methods while cranking. If you have no spark at all on any plug then it's probably a problem in the coil. It would be worth replacing the plugs at any rate

Other things I found with my bike from long term storage:

  • The front brake caliper had seized, I had to take it off and clean it out, then bleed the system. I had to replace the front master cylinder too as the seals had gone
  • The clutch stopped moving smoothly soon after I got it working, needed a couple of squirts from a grease gun
  • The tires were totally shot, they were old when they were put into storage and were cracked all over, check yours thoroughly

I hope you get it running again, they are great classics.

  • Definitely going to need some carb work if it was stored with unstabilized fuel with ethanol in it. I'm surprised it isn't leaking. – Eric Hauenstein Jan 5 '18 at 13:31
  • Good point @EricHauenstein, ethanol is hygroscopic, so water would get absorbed into the fuel over time. – GdD Jan 5 '18 at 13:44
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By "Won't run" is apparently meant "Can't run until starts, which it refuses to do." Perhaps priming with a little spray of fuel or starter fluid in cylinders would implicate, respectively, the fuel system (if fire up) or ignition system (if no fire up). That narrows the anomaly.

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