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So I'm trying to figure out what's wrong with a Bandit 400 that is currently in my possession.

As far as I know the only performance mods are a slip on. I think the carbs are stock. I'm waiting on a call to let me know if the PO rejetted it.

When the bike is warmed up and at idle, if you blip the throttle to WOT, it will stumble, and often die just off idle.

When running down the road, if you crack the throttle open, the engine bogs. It will slowly accelerate on what feels like two cylinders. After a few seconds, I can feel a third cylinder start to come to life, intermittently. Then, a few seconds after that, the fourth cylinder comes on line, and the engine starts screaming and pulling well.

The fourth cylinder doesn't start firing until about 8k rpm.

I haven't opened anything up and have only put Sea Foam into the tank.

BUT...

If you pull the choke all the way, the bike will idle at 4k rpm. If you take off with the choke pulled, it does move a bit better, but you still get the same symptoms as above.

I have my theories, but they're weak at best. What say the more experienced members here?
 

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THE FUELLLL! there is probably moisture in the fuel. a good cleaning wouldnt hurt, you dont HAVE TO rejet it. it should still run with minor mods, maybe not at peak power but certaily should run.
there is a good possibility of moisture in your fuel which turns into carb icing very easily. there is no need for extremely cold weather, you can get carb icing at about 40-50 degrees.
Go to your auto parts store, buy some HEET and dump it in your tank along some new fresh gas with slightly higher octane. let the bike idle for 5 mins, turn it off. Turn it back on for 5 mins and let it run until it reaches operating tempreture. go on a slow run and see if things improve.
best of luck to you
 

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Pull the plugs and ground them on the motor(one at a time), make sure they are sparking through the rev range. Depending on which cylinders are/aren't firing it could be as simple as a bad coil to the pair of cylinders that are firing intermittedly OR loose ground on a good coil.

Rule out no/weak/inconsistent spark before you pull the carbs to rebuild/clean them. I had an old honda that had a loose ground wire and when the revs would build, so would vibration. It would vibrate just enough that the wires would connect and thrust me forward on all 4 cylinders at about 7k rpm. Kinda scary on a cb400f. Would be funny if that's what yours is doing.

-Vaughan
 

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I rebuilt one of those from the crank up and they're famous for being a real pain in the butt. Does it seem to be firing on all four when it is at idle or only two? If its firing on all four and only bogs when you open it up, it sounds like a fuel problem. Try a fuel additive, but you might need to clean and balance the carbs. You can check to see how the headers heat up, a non firing cylinder will stay cool longer, obviously. As for electrical, could certainly be a bad cable, ground, or coil. Unfortunately, Bandit 400's are also known for loosing the CDI's. When mine went, it lost its timing, so all of the plugs would fire, but as it turns out, at the wrong time. That one had me scratching my head for a while. Another problem I had was a broken cam chain tensioner. As I recall, it tensions the pull side, so the extra slack changed my cam timing and made the bike run like crap. Least ways, when I replaced it, it ran a lot better. That one is a decent tear down to get at, so hope it's one of the others. Have fun with it once you get it running. Those are a hell of a lot of fun to ride. I wish I still had mine.
 
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