O/T Backfire on FI V-twin Engined bike

[charlie_pank]

I've posted this query on some bike forums, but haven't met any satisfactory responses. So I thought I'd try the intelligencia of BC.

 

It's a very simple problem, I just want to know where to start looking for the cause... basically I have a desmodromic V-twin engined FI bike which is backfiring into the airbox sometimes when the engine is up to temp. when I blip the throttle for downshifts and moving away from stationary. It's not causing any other problems but is rather annoying and I would like to get the engine running properly again.

 

I'm pretty sure it's only 1 cylinder that's acting up, my thoughts on what it could be are:

1. Dodgy mixture in 1 cylinder - possible causes:

- TPS needs adjusting

- Throttle bodies need balancing

- One of the ECU sensors misbehaving

- Sticky injector

 

2. Dodgy inlet valve clearance in 1 cylinder

 

3. I think that although 1 cylinder is right out in the airstream, and the other is tucked away between my legs, they both use exactly the same map, which, therefore must be an approximation. Could the backfire here just be the result of one cylinder being much cooler than the other?

 

 

Any suggestions about what could be causing this problem, and/or how to test out these hypotheses is much appreciated...

 

 

 

Edited by - charlie_pank on 27 Aug 2004 13:01:47

[Morls]

I've no experiance of FI, but certainly on Carb'd stuff, a leak in the exhaust header, at the head or silencer junction, can cause this, as can a leak on the intake side between carb and head.

 

Typical causes I've seen in the past are perished intake rubbers (on bikes only 5 years old, so not just on Classics), poorly refitted intake rubbers, loose exhaust bolts onto head, slack springs at junction on header pipe and rusty pin-holes.

 

The intake faults can sometimes be spotted by spraying easi-start or wd40 around the manifold and listening for a rise in revs, while a carbon deposit near the leak often gives away something on the exhaust.

 

What Ducati is it anyway 🤔

 

 

 

 

Mark

 

Look it's Mavis here and Jubilee 7 here

[charlie_pank]

2002 620 Monster ie.

 

Thanks for the info.

 

I think it might be the air temp sensor playing up. As it seems to have a tickover that varies between 700 and 2000 rpm depending on the ambient temperature and the temp in the area immediately surrounding the engine (ie I think that the air temp sensor is too close to the fins of the air-cooled engine). If there was a workshop manual available it would be an easy job to take it off, clean it up, test it and refit or relocate it.

[Tony Martyr]

Surely the air temp sensor has a global effect on the ecu map and should not have an effect on the one cylinder. My expereince supports Charlie's comments about a leak except it may be a 'suck' leak that doesn't leave much evidence rather than 'blow' leak that does