I have a 1991 cbr600f2 i started it up and it fired first time on sunday, then i went to start it again then next day and it would not start atall it would try and turn over and then just backfire with a really loud bang. can anyone tell me what could be the problem??

 

My first thought is that there is an issue with the plugs or battery. If it's back firing then fuel is getting through to the exhaust so there's probably not a fuel flow issue. You might just have a weak spark from a run down battery or poor connection at the lead. Have you checked the voltage on the battery? Does the engine turn over ok or does it sound slow and struggle to crank? Check the battery leads are on tight and either charge the battery or see if you can borrow one that you know is good.

 

+1 If you're getting fuel, and your plugs are now shot, you will get backfiring through the exhaust.

After doing carb work to my new bike, but not replacing the plugs, or at least checking them, before firing it up again , my bike backfired a lot while trying to start.
I wasn't certain, but I thought the previous owner had stated they installed new plugs, but anyway, upon inspection, they were all heavily fouled-out...once I replaced them, it started easily, and ran fine.

I can't believe I didn't even CHECK the plugs ... I'm still feeling like an idiot, for pulling such an idiot move!

 

i have fully charged the battery and the spark plugs are brand new and all connections are tight

 

I can only assume, that the spark plugs were gapped correctly when installed, is this correct?

And, for what it's worth, do you know that the plugs are brand new, as in you installed them yourself, or saw them being installed, or is this just what the previous owner told you?

 

yes and i installed them myself

 

Well, if it were me, the next thing I would do is check for spark at one or more of the plugs, and depending on how that goes, it's time for you to start investigating fueling issues.

 

i am just going to check the plugs now

 

all spark plugs are fine and sparking but still no start

 

I don't suppose you have the tools to perform a compression test, do you?
It would be interesting to know what your leak-down numbers are, but either way, I would be leaning toward fueling issues at this point.

For starters, I would verify that you really are getting enough fuel to start and run - try to get to one of the float-bowl drain screws and verify that you've got fuel coming out when you loosen it... and I know this is going to seem like a very elementary question, but, is your petcock in the "On" or "Reserve" position, and, is the vacuum line to the petcock connected to the tiny vacuum port at cylinder 1?