Hi all, I recently acquired an 851 (1990) and have been enjoying my first rides on it. Lovely, raw machines. I have noticed that the bike backfires on closed throttle when using engine breaking for deceleration. It isn't severe but it does bang every so often. The bike is on stock pipes with baffles, all standard airbox etc from what I can tell. Do these bikes run lean? Can I can adjust the fuel mixture (ECU?) and make it run richer? Or is the backfiring as part of the 851 experience !? Thanks.
Indeed, could be the case where the downpipes meet the exhaust box, there are no clamps, they just seem to slot one into the other.
Owned a 851 back in the nineties mine did it, when I took it into Baines for other work they checked it over as I commented on it to them. They found nothing wrong with the bike and I hand no problems with thereafter.
For peace of mind, firstly I would get the fuelling checked as under, or over fuelling can contribute to backfiring. The other thing to consider is the valve gaps as if the exhaust ports are partly open, as the open exhaust moves the burning mixture to the exhaust system, where it explodes when the combustion process is over.
Thanks for the input, very helpful. The valves were checked and adjusted 200 miles ago from my Ducati mechanic. I may ask for the fueling to be checked again although the bike had a thorough service earlier in the year. Could be a feature of the 851 as DucatiScud also experienced with his in the past.
If the fuelling is good and it still concerns you, it might also be worth considering if the cans could do with repacking. With regards to your thoughts about the slip-on connections, unless they are really loose, they shouldn’t be the cause of extra air in the system.
One of the exhausts (left) does wobble a bit. The other one doesn't have any slack. Not sure why the wobble on the left exhaust box, I think I will dismantle it and replace the rubber grommet. All screws are tight so until i investigate I am not sure why the slack. Could an air leak cause the backfire?
When I repacked my 888 Termignoni slipons the brackets inside the cans had rotted away. The inside of the standard cans are not the same as yours, but I am sure you get the idea of what could be happening inside. See if the movement looks to be from within the can itself.
Ok, I took the wobbly left hand side exhaust box off and I can confirm there is an air leak between the box and the downpipe. The downpipe needs to be slightly longer (5mm?) to make a good seal with the box. I managed to move the two parts closer a bit but it's still short by a tiny amount, so the air leak is still there. Potentially the backfiring may not be as frequent as it was but I expect it to still pop occasionally. Does backfiring cause any engine (valves?) damage?
Re-set your TPS, balance the throttle bodies and seal the exhausts joints with RTV. After that if it still back fires get it on a dyno for a fuel/air ratio check. Steve R
I had same on my old 996 , was indeed a gap in exhaust joins letting air in, could also be that it may have had pipes in the past and was mapped to suit, they may have left the old chip in the ecu and just whacked the standard pipes back on.
