[Old rider]

Following the twists and turns of getting my 999 running properly over the Winter, all is well, except that under fierce acceleration, I am getting a loud bang from the exhaust accompanied by a hesitation.

Any ideas anyone??

This wasn't happening before the problems I had over the Winter but is very much there now
999with replacement ecu flashed with standard but immo-off map
replacement pump assembly
PC3
new cps
tps reset

 

[Old rider]

No suggestions as to the cause at all??

 

[Slowdive]

Did you do any of the following?:

Throttle body sync
Vacuum leak test
Compression check
Leak down test

Do the plugs indicate whether your running too lean or to rich? Seems like a bunch of basic engine stuff you could do to help track this down.

 

[Yorik]

Sounds like a fuel build up. We used to hit the kill switch in tunnels for a moment and then turn it back on. Now that was a big bang! Scared the shit out of people not expecting it. >

 

[Old rider]

Sounds like a fuel build up. We used to hit the kill switch in tunnels for a moment and then turn it back on. Now that was a big bang! Scared the shit out of people not expecting it. >

Yes, it does seem a lot like that, except for the power hesitation.
If there was an excess of unburnt fuel in the exhaust, I can well imagine that the extra heat and speed of the gasses when really giving it the beans could ignite it but how would that fit with a pause in power delivery at the same time??

This does tend to happen when overtaking cars - it must really make them jump as you say.:surprise:

 

[Old rider]

Did you do any of the following?:

Throttle body sync
Vacuum leak test
Compression check
Leak down test

Do the plugs indicate whether your running too lean or to rich? Seems like a bunch of basic engine stuff you could do to help track this down.

Thanks for the suggestions
This is happening immediately after doing a bunch of stuff to get the bike running, it was fine beforehand. It could be coincidence of course but I think it's very likely to be related to what I did to it to get it running. I don't think anything I did could have affected compression or throttle-body synchronisation.
I did renew the front plug though, just to see if it would fire and to check for sparks, so that is a good possibility, as is mixture.
The strange thing is that it only does this at wot and high revs. Otherwise, it is faultless.

 

[duc96cr]

Does it happen under no load , just bringing up the revs while sitting still ? You understand how to make it backfire, cutting the spark momentarily causing an unburnt fuel build up in the exhaust. But why would an intermittent spark only be happening at WOT ? The only thing that comes to mind is the ECU, which you did play with . The other thing would be an exhaust leak contributing to this.

 

[Old rider]

Does it happen under no load , just bringing up the revs while sitting still ? You understand how to make it backfire, cutting the spark momentarily causing an unburnt fuel build up in the exhaust. But why would an intermittent spark only be happening at WOT ? The only thing that comes to mind is the ECU, which you did play with . The other thing would be an exhaust leak contributing to this.

Thanks,

As it didn't do this with the original spark plug, I stuck the old one back in the horizontal cylinder where it came from and after warming the motor up, ran it gently up to 9,000 rpm and it didn't do it the first time. However, on subsequent runs up to 10,000 it did.
I could also hear and feel some slight roughness and misfiring that I couldn't detect while riding.

So, it doesn't have to be on wot to do it but when riding, that's probably what I'd be using when in that kind of rev range.

 

[1198SP]

Check the side stand/switch. Happened to my old GSXR750. I’d hit the gas and during the acceleration, bike would kinda hesitate, then BANG, and take off again. Turned out the spring on the side stand was a little worn and it would allow the thing to drop just enough to kill the ignition for a split second...

 

[Old rider]

Check the side stand/switch. Happened to my old GSXR750. I’d hit the gas and during the acceleration, bike would kinda hesitate, then BANG, and take off again. Turned out the spring on the side stand was a little worn and it would allow the thing to drop just enough to kill the ignition for a split second...

Interesting, thanks.

If the bike did it while riding but not in the garage, I'd definitely give this priority but it does it in neutral with the stand down too, so I doubt it's the cause here.

Also, imho, there's a 95% chance that the problem is related to something I've done in my attempts to get it working again, as it wasn't there before but appeared immediately it was running again.

 

[1198SP]

Ah, my bad. I must have misread that. Thought it only did it while riding.

In that case, I'd go straight for the PC3 - disconnect it and eliminate it from consideration.