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03' RT Light Backfire at 62k miles

B

bigbear

New member
Bike developed an occasional light backfire on deceleration/drop throttle after an extensive “tune-up” last spring at 62k miles. Other than backfire, bike runs well: starts quickly, smooth idle (which is a little low), linear throttle response, no surging, with good gas mileage (45mpg). I have two questions: Is the backfire a concern? And if it is, any suggestions to eliminate it? Maybe caused by a leak in TB intake?
More info on the bike: tune-up included professional injector cleaning, cleaning of TB and BBS ports, removal/cleaning of the Bowden cable box (a real PIA.) and throttle body sync per roger 04 rt instructions. TPS reads 250mv at idle position. Installation of Autolite 3923 plugs @0.25” gap in June did not affect backfire. Valves were last adjusted at 55k miles on 9/16.
 
My WAG would be to back out each BBS 1/8 turn and see what happens. More educated minds will likely prevail...

PS I think your bike is not backfiring, I think you have a lean idle setting which doesn't allow complete combustion thereby allowing unburned fuel to collect in the exhaust system and then "pop" when hot gases ignite that fuel.
 
Before chasing anything in the intake system make sure there are no air leaks in the exhaust system. Check where the header meets the muffler assy. I solved a popping issue on an 1150RT 2 years ago with high temperature silicone and reassembling that joint. Also check the head to pipe joints.
 
My '03 R1150RT has done this since I bought it a little over a year ago. Being the first BMW I've ever owned, and the first motorcycle I've owned in 30 years, I thought it was normal.

How does one go about testing for exhaust leaks in those spots?
 
BC1100S said:
My WAG would be to back out each BBS 1/8 turn and see what happens. More educated minds will likely prevail...

PS I think your bike is not backfiring, I think you have a lean idle setting which doesn't allow complete combustion thereby allowing unburned fuel to collect in the exhaust system and then "pop" when hot gases ignite that fuel.
Click to expand...

BC1100S, why back out the BBS? Doesn't backing out the BBS make the idle leaner?

"Light" popping when you shut down the throttle and allow the engine to slow the bike is normal.
 
rogerc60 said:
My '03 R1150RT has done this since I bought it a little over a year ago. Being the first BMW I've ever owned, and the first motorcycle I've owned in 30 years, I thought it was normal.

How does one go about testing for exhaust leaks in those spots?
Click to expand...

A dead giveaway is black soot at the fitting/junction.
 
nosvamos said:
Bike developed an occasional light backfire on deceleration/drop throttle after an extensive “tune-up” last spring at 62k miles. Other than backfire, bike runs well: starts quickly, smooth idle (which is a little low), linear throttle response, no surging, with good gas mileage (45mpg). I have two questions: Is the backfire a concern? And if it is, any suggestions to eliminate it? Maybe caused by a leak in TB intake?
More info on the bike: tune-up included professional injector cleaning, cleaning of TB and BBS ports, removal/cleaning of the Bowden cable box (a real PIA.) and throttle body sync per roger 04 rt instructions. TPS reads 250mv at idle position. Installation of Autolite 3923 plugs @0.25” gap in June did not affect backfire. Valves were last adjusted at 55k miles on 9/16.
Click to expand...

Note: a better plug for the 1150 is the 3922 (1 step cooler)
The 3923 is typically used on the 1100's
Stock gap @.032"
 
m_stock10506 said:
BC1100S, why back out the BBS? Doesn't backing out the BBS make the idle leaner?
Click to expand...

Y'know, I wasn't totally sure which way to turn the screws on that point which is why I noted it with a WAG (wild assed guess). Thanks for catching that. I was also fairly sure that GSAddict would chime in to clean up any mess I made. :)

I never experienced any audible popping on my S but I had an AX-FIED installed. I also typically travel faster than the speed of sound.
 
GSAddict said:
Note: a better plug for the 1150 is the 3922 (1 step cooler)
The 3923 is typically used on the 1100's
Stock gap @.032"
Click to expand...

Pull the plugs and check the gap. I had a bike that did this after I change the plugs. To stubborn to check and when I did one was almost 5 thousands to tight regap and the backfiring went away.
 
Okay - I know this is thread drift - but I just have to ask while the thought is fresh in my head - what is the performance effect of slightly widening the plug gap, pushing it out to .035 or or so ?
 
Thanks for all of the suggestions. Replacing the Autolite 3923 plug with the cooler 3922 all but eliminated pop/backfire. Below is photo of the 3923 plugs @.032” gap after about 5k miles. Any insight into difference in color? I did find a slight leak in exhaust system at the header to catalyst joint (soap bubbles when I pressurized the exhaust system at the muffler outlet) but no soot.

3F988C71-A077-4284-B507-70687B40F58F.jpeg
 

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