Put put

[RSMike]

That's the best way I can describe the changed note from the engine under (hard) acceleration that suddenly happened today. Its the type of sound that I at first expected would be accompanied by some hesitancy or loss of power, but it didn't seem to. I think if I blocked my ears the bike would just feel like normal.

Did a close inspection of the exhaust system and all look sound, Could there be an issue with the breather?

At idle / lower revs everything sounds like normal, Any suggestions what it might be?

[Bob_Roller]

It seems like you are describing, 'pinking' or pinging/detonation in US ternimology.

Have you switched brands of fuel this last tank .

[Barry]

Hard acceleration below 3000 revs is when you might hear pinging but it sounds more like  "tink tink" than put put.  Easiest way to identify pinging for sure is to remember that it sounds to be at a higher frequency than the engine revs.

[RSMike]

Well, yes, I did switch to Texaco from Esso at the last fill, but I have done that before on occasion without any affect,  and also I had pretty much got through the tank of Texaco when this happened, My first reaction was to switch to reserve, thinking low levels might be the cause and then I decided to fill up at the next station about 5 miles on, this time with Topaz ( which I do not normally use in the bike). I rode about another 10 miles on that fill/mix before I got home, and the "put put" sound under acceleration continued.

Weather is unusually warm (for here  ) at the moment, about 28C today ( that is about 83F) so bike is running hotter than normal, and I was pushing along at about 70 just before this happened, but I doubt this should be a problem.

I was actually out to buy oil for the bike, so when I got home I did the oil/filter change as planned, let her cool for about another hour and then did a short ride to check for no leaks and  check oil level, again the changed sound under acceleration but otherwise all OK.

Just reading Barry's reply, yes Barry,  seems like more a lower frequency than high frequency sound.

[nhmaf]

Maybe something like a baffle has come loose in one of the mufflers?

[Matt Chapter]

You could also be looking at a loose exhaust clamp, there are plenty of them on the airhead exhaust.  Think front crossover, or either of the headers to the collector, or the collector to the mufflers.

[Barry]

Mike,

You mentioned the breather as a possibilty.  Does your bike still have the original disc type breather ?   If it has and as the noise started suddenly I wonder if breather disc is damaged.

[RSMike]

Nhmaf & Matt, thanks for the suggestions, I did a fairly thorough 2nd look over the exhaust system yesterday and all seemed ok , but I will check again, rode the bike again yesterday and is running as normal apart from the changed sound.

Barry, yes as far as I know it is still the original breather, I see from the Haynes that you can fit the later reed type, have you done it and is it straightforward?

I have always had the turkey gobble at idle and this sound is somewhat similar. Are there other symptoms of the breather being kaput ?

[Barry]

I haven't replaced mine but others here will have. You need to devise some sort of slide hammer/puller to remove the old breather housing.

This is probably the best article on crankcase breathers.

http://bmwscotter.org/procedures/procedures.htm


Bear in mind that breather problems are not necessarily down to the breather valve itself. It could be that air is get into the crankcase somewhere. Think crank seals or maybe dipstick seal.

[RSMike]

Thanks Barry, vey useful article. Having read through most of it, I am less convinced now that there is a breather issue, especially as it says "A breather failure causes the engine to pump massive amounts of air in and out of the crankcase on every stroke, spewing oil all over the place". Nothing like that happening, thank God, I just had the left carb and inlet duct off and had a look into the airbox, everything looks as I would expect, a few smears of oil form the breather hose and in the ducting, but nothing excessive or abnormal.

Rode the bike again and the most obvious thing due to the lack of any other symptoms is some sort of exhaust issue, just can't find anything obvious, any loose clamps etc or any sealing issues.

I'll probably whip the silencers off next so I can examine them and give them a shake, maybe do a short test run without then to check how that sounds, not too early Saturday morning though,