GSF400 Running Rough and Backfiring

[Demilus]

Alright.  My 1992 GSF400N was running fine one day.  The next day, I start it up and it is running very rough.  It also didn't have nearly as much power as it used to.  The exhaust is also much deeper.  It seems like it is idling unevenly too.  When I get into the lower rpms, it also started backfiring.  I have no idea why it is doing this.  It runs fine one day and then the next it pulls this crap.  Of course I have a guy coming to buy it on Sunday.  The day he checked it out it ran fine.  I'm about ready to freak out right now.  Any and all suggestions would be appreciated.

Thanks,
-Kurt

[andrewsw]

dirt in the carbs. That's the first thing I'd suspect.

And did you let it warm up all the way and did it still show this behavior once fully warmed up?

Any smoke out the tail pipe and what color?

A

[Demilus]

I'm wondering if it's the carbs or the sparkplugs.  Just took it for a spin and got it nice and warm.  Turns out the left center cylinder isn't firing (cylinder second in from left side if you are sitting on the bike.)  The header pipe was barely warm.  The other three, however burned my fingers to the touch.

Any idea what it could be?  I'm thinking fouled plug or dirty carb.  Any chance it could be the timing? (I really hope not.  That is one part of the bike that remains a mystery to me.  I've yet to delve into the timing region.)

Thanks,
-Kurt

[andrewsw]

surely not a timing issue. probably still most likely a carb issue followed by the plug. if those don't pan out, then move up the chain. Oh, check the spark plug wires and maybe it's the coil.

A

[Demilus]

How do I check the coil?  And how do I check to see if the spark plug is working?  Just take out the plug and try to start it? See if I get a spark?

Thanks,
-Kurt

[andrewsw]

Well, what I would do is pull the tank and rig a temporary gas supply (be sure to close the vacuum line on the intake to cylinder 1), then start it up and start poking stuff. You can pull a spark plug wire (watch out, I always get a good shock) and see what happens. If #2 really isn't running, then pulling that wire will have no effect on the running of the engine.

So if you isolate it to one cylinder, then yes, check for spark (watch out... don't get shocked). You can pull the plug and examine it. You can stick the plug back into the wire cap and then hold it to the frame and check for spark at the plug. If you've got spark then it's likely a carb problem.

Did I mention it's most likely a carb problem?

Since you've got the tank off, it'll be straightforward to remove the air box. Then you can look down the barrel of the carb and see if it's pulling fuel (the plug should be in for this). You can stick your hand over the carb barrel and see if it pulls fuel (this will be obvious). Etc etc etc...

You may find that it's getting too much fuel (flooding) from a stuck float or fouled float needle. or maybe the pilot jet is plugged. etc etc etc...

If you find it's carb related, be prepared to tear down that whole carb: float bowl, float, pilot jet, main jet. cap, diaphragm, needle, clean it all really well, blow air through every orifice. run a stiff wire through every orifice etc etc etc.

A

[Demilus]

Alright, that's what I figured.  I was just kind of hoping it was the plug so it would be nice and simple.  I'm gonna tear down everything this evening.

Thanks again,
-Kurt

[Tainted-Meat]

this is worth a repost and a look for anybody wanting to dive into them carbs ; ; ;

http://forums.banditalley.net/index.php?topic=9697.0

Hope it hlps

Aid

[PitterB4]

Definitely check the simple things first.  Due to the suddenness of this issue, I'd put about a 50% chance on a bum connection between the plug wire and the cap (pretty simple to check and fix - tons of posts here about it).  The other 50% is on crap in the carbs... 

[Demilus]

Alright, I took out my sparkplugs and the one in the second cylinder was fouled.  I cleaned it up and got a spark out of it.  then I put it back in the bike and tried it again.  Still no firing in the cylinder.  Took it out and it was soaked with gas.  Wondered if my cylinder was flooded from before so I blew some air into the chamber and let it sit for a while.  Blew on the sparkplug and got it to spark.  Tried again and no luck.
I'm thinking stuck float in the carbs?  Any ideas on how to unstick them if it is the floats?
I've pretty much resigned myself to tearing apart the carbs and cleaning everything.

Thanks,
-Kurt

[andrewsw]

pull the offending float bowl, pull the offending float, look at the needle. sometimes the bit of dirt will be right at the tip and you can just blow it out. Sometimes it works down alongside the needle in which case you'll have to pull it apart to get it out.

A

[gsxr400 racer]

Change your orings if you have never done before

[Banditmax]

Theres nothing scary about pulling your carbs apart. Just work methodically doing one carb at a time making sure it is spotless.

[gsxr400 racer]

but always keep them connected as a bank of 4

[Demilus]

Alright, so I cleaned the carbs and now it won't start.  I figured the carbs just didn't have any gas in them, so I just cranked, rested, cranked, rested, etc.  That was last night.  I tried the same thing this morning.  Then I figured I'd look at the plugs and see if it was flooded.  All the plugs were very wet.  I blew on them and left them out of the bike for about an hour and a half.  Put them in and tried again.  Loud backfire followed by a fwooomph.  Then it started making puttering sounds like it wanted to start.  That brings us to now.  Anybody wanna tell me what's going on?

Thanks,
Kurt