A tale of fuel mixture and blue smoke... - MVAgusta.net
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post #1 of 3 (permalink) Old 06-11-2017, 05:33 PM Thread Starter
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Location: Au'n Colorado
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A tale of fuel mixture and blue smoke...

I have had my 04 Brutale 750 S since 2010. Bought it in Lost Wages with an Arrow non cat full titanium exhaust. Which means it also had the Arrow chip installed to compensate for the improved breathing. Finally escaped LV and moved back home to Colorado and on a couple of occasions, had riders following me comment that the exhaust smelled rather rich. Carbon on the right turn signal confirmed what I was being told. A year ago I took the brute to the local dealing for a few items including the 15K service and asked them to check the fuel mixture for being overly rich. When I picked up the bike, was told the mixture was okay.

Now this is the same Denver dealer who seemingly failed to lube the rear hub before it failed about 600 miles after the service. There was no indication of fresh grease in the hub when I tore it apart so you can kind of see where I am headed with this story. As it would happen, on the very ride on which my hub failed, a riding buddy told me I was gassing him out while he was following me. So after installing a Mitch's Hub I started to dig around of found the procedure for adjusting fuel mixture. Off came the tank and pulled the ECU and checked the potentiometer and found it was in a nearly full anti-clockwise position. In other words, full-on rich mode. The pot only has about one full turn of adjustment so I made an educated guess and leaned out the mixture by about one third turn clockwise.

Put her back together and off for a test run. Better start and smoother idle, check, Improved throttle response coming off of closed throttle, check. Smoother engine performance across the rpm range, check. Runs like a scalded ape, check. Parked the bike on side stand and started contemplating how to get my hands on an Exhaust Gas Analyzer to set CO more precisely and retired for the day.

The next morning I went out to the garage and noted a small puddle of oil on the floor just below the water pump, perhaps half a teaspoon. The oil had come from an unknown location but had followed the coolant pipe from just above the water pump on its journey to terra firma. Great. I checked the oil level and found it was fine so I started her up. Yikes! Blue smoke-o-ramma! WTF? It cleared after a few seconds but the bike had never smoked one iota before. Obviously it was an oil issue.

Reasoning and deduction led me to believe that the brute had been running so rich that there was a good chance that gas had diluted the oil and was probably -5-0W by now. Time to drain and replenish. Grabbed a few quarts of Mobil 1 20-50W motorcycle oil and did the deed. On start up she still smoked but was a mere puff compared to the cloud that she belched out with the prior batch.

I took a morning romp up my favorite canyon today and she still is running much better than before, especially when coming off throttle. No signs of further oil leaks so that is a big mystery though I suspect that some blow by oil had collected in the air cleaner box and drained while on side stand. We shall see.

So I guess the moral of the story is that one should not let fuel mixture issues slide for very long as they can be bad enough to adversely affect the oil and thus could adversely affect your pocket book as a result.


I sure do feel a lot more like I do now than when I first got here....

2004 Brutale S
2016 BMW S1000XR
2001 BMW R1100S
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post #2 of 3 (permalink) Old 06-11-2017, 11:36 PM
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Interesting to see if range is affected by leaning it out a little... I could set my watch to the 750s fuel warning light 186-189km almost every time.

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post #3 of 3 (permalink) Old 06-12-2017, 07:18 AM Thread Starter
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Location: Au'n Colorado
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Thanks, I failed to add fuel milage to the list of pluses.....before leaning the mixture out I could barely get 100 miles or 160 km out of a tank. The first tank since, the range jumped up to 143 miles...but to be fair this was before the oil change and I did not push the bike that hard.

I am still looking for a convenient place to measure CO as I definitely don't want to run too lean either. In the meantime I am keeping an eye on the inside of the exhaust tips to make sure they don't turn from carbon black to ash white on me....


I sure do feel a lot more like I do now than when I first got here....

2004 Brutale S
2016 BMW S1000XR
2001 BMW R1100S
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