Backfiring through airbox

Purplegerbil

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I have a 2003 r1150gs and have changed the exhaust to y piece and black widow can. It was running just fine but would occasionally cut out at junctions, lights etc. Always re-started no problem. It didn't do this before I changed the exhaust. I couldn't get the old lambda sensor out of the original exhaust so used a spare one, I figured this could be part of the problem so swapped in a new one. It now backfires through both exhaust and airbox whilst sitting on the bench. I haven't ridden it.
Whilst plugging the new sensor in one of the quick release fuel pipe connectors got damaged so I have now changed both of these to the metal type.
I haven't been able to balance the throttles yet because of the poor running. All pies etc under the tank do not look to be kinked and both throttle cables are correctly seated.
Any ideas on where I should start, I don't get much time off so don't want to be retracing every step if there's something simple I'm missing.
Thank you in advance.
 
Try disconnecting the lambda probe and see if that helps. That should if I remember right make it run a bit richer.

Also check resistance on your coils to make sure they’re in spec and not degraded.

Could also be an air leak - Did you take throttle bodies off, or lose a bung on the little vacuum connector on the throttle body?

Do one thing at a time tho … as I’m sure you know.
 
Thanks Nin
The only thing different to when it was running well is the new lambda sensor and the fuel quick disconnects. I have checked all round the throttle bodies and all looks to be good so I don't think I have introduced an air leak.
Just to be clear once I changed both of these the backfiring started immediately when I fired it up.
I will pop the lambda plug to see if that makes a difference if I get a chance later.
I also cannot be sure the cutting out previously wasn't caused by the same thing just now its worse.
I will also give the coils a check.
 
Quick update, I have disconnected the lambda sensor and it's still the same, i.e backfiring.
 
I’m sure someone more knowledgeable will be along to correct me if I’m wrong, but I think backfiring into the air box is caused either by timing being out of whack, or mixture is too lean.

On my SE a failing stick coil gave me similar symptoms though.
 
Any significant exhaust backfires need a little unburnt hydrocarbon, usually available when the motor is cold, along with an oxygen supply upstream in the exhaust, usually a significant air leak in the pipework.

This leaves your y-piece connection the primary suspect. Once corrected you can get down & dirty with balancing the TBs.
 
Any chance you have some diesel in the system ?

back fire in Airbox = possible burnt valve (Compression check should clear up that mystery)

or as I have asked Diesel in fuel system

The diesel burns much slower and is still alight when the next fuel arrives hence Bang in airbox

Oh and TPS and tickover (big air screws on Throttle bodies) likely need reset when you have the freer flowing Y piece fitted

I have heard of exhaust air leaks causing backfires but never so far in 20 years of doing beemers

One last one! :rob Does it stop backfiring if you squeeze the Fuel return line with a pair of pliers? (N.B. NOT completely closed)
 
Thank you for the responses.
I am certain the exhaust is sealed correctly, I'm pretty sure it wasn't backfiring before I changed the sensor.
I know the throttles need balancing etc and figured this was probably the reason for it cutting out before I switched the sensor.
It is a bit like a tractor compared to my other bikes but don't think I've put diesel in it, I generally only use premium branded fuel in my bikes.
Recently adjusted valves etc prior to exhaust swap. Fully charged battery.
Not done a compression check so will do that when checking the coils.
I did have some issues with sealing the fuel disconnects, several o-rings before I got one to seal on delivery side. Incidentally this has split when I unclipped them to get to the sensor but I do have spares. I suspect this as it is the inly thing that's been changed other than the sensor. Will pinch the return pipe when I next get a chance.
 
I will check when I next get a chance, I have not really left it running long as it cuts out each time it backfires. I assume it will have to run for a while to allow them to get up to a decent temperature.
 
I can't remember but it has done roughly 1000 miles since with no issues.
The fault manifested as soon as I changed the lambda sensor/fuel disconnects. Rode it into tha garage working fine and it hasn't left the bench since.
 
I say adjusted valve, should have been checked really as they were all within tolerance anyway.
 
Is this right?
Bike running fine.
Swapped exhaust, bike running fine but would occasionally cut out.
Changed lambda now backfiring through airbox etc.
Faulty lambda then? Put the one you took out back in.

When you do swap out the exhaust system it takes a few tank fulls for the ECU to adapt.
 
Yes that's right.
I have disconnected the lambda and it was still backfiring.
I did also change the fuel disconnects and am thinking this is where the fault lies but am open to any suggestions of anything I can check.
It's probably had around three tanks since I changed the exhaust. I do need to balance the throttles and suspect this was the reason for it cutting out. It wasn't cutting out every time I pulled up.
I will get a chance to pop out and pinch the return pipe soon, will also check exhaust temp.
 
I also could not get the lambda sensor out of the original exhaust so fitted a second hand one when I first fitted the exhaust, I have now changed this for a new one.
 
Assuming you have connected the fuel quick disconnects correctly.

Lower pipe to front - Supply.
Upper pipe to rear - Return.

Then pop your injectors out and check the flow pattern. That should tell you if you have a fuelling problem.
 

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Ian J Hartley said:
Assuming you have connected the fuel quick disconnects correctly.

Lower pipe to front - Supply.
Upper pipe to rear - Return.

Then pop your injectors out and check the flow pattern. That should tell you if you have a fuelling problem.
Click to expand...

That would do it - if connections were reversed.


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