Key on "chuff", sometimes a loud bang?

[justabill]

Ok. This was recently rebuilt. Everything is new or reconditioned. I've got a few issues to chase down, and this is one.

Occasionally, when I switch the engine to run but before pressing the start button, I will get a "chuff" sound. Two times, while testing other things, I got a loud bang. More of a single pop than a backfire. I am guessing at this, but I think the ignition is sending a spark to the plugs once it get's power. Actually, about one second after, which seems to align with the timing of the electronic ignition self-check. This spark seems to be igniting residual fumes in the carb or exhaust. Again, all a guess. I will take another look at the carbs to be sure they check out, but wanted to see if anybody else has run into this. The ignition is the EME electronic ignition, if that matters.

You can hear the chuff in this short video.

Bill

 

[ksdcr]

I can’t really confirm, your theory, or explain the “chuff”. I can say that my 1983 R100 will occasionally make that “chuff” sound, after I turn the key on, but before I hit the starter. Maybe it’s a airhead trait?

 

[20774]

I presume this is the '85 R80 shown in this thread:

Two years of spare time and I am 99% done.

It's a 1985 R80 RT that has been given a full rebuild and a new direction. This will be a good week as it only needs some final tuning before it gets off the lift and onto pavement.

forums.bmwmoa.org

Sounds like air to me. There were some things on the post twin shock models that were different. Does it have the pulse-air system? From your previous picture, it seems to have a different gas cap. Maybe this system is pressurizing or something.

 

[omega man]

I think the “sometimes” is a clue. Will/does it do it after sitting for a week?
Residual gas, that doesn’t or hasn’t evaporated can be ignited. My experience in not Airhead specific but with engines in general.
A recent- an prevalent example of this is “big-box” store lawn tractors (mowers). You may have noticed a “bang” to go along with the louder exhaust note from these machines. To combat this residual fuel deal, electronic solenoids have been incorporated into the carburetors to immediately shut off fuel when the engine is turned off.
In your case-
You could see if the noise is louder after you shut the bike off at 1/4, 1/3, 1/2 throttle positions which would allow more residual gas in the cylinders.
You could check and notice if the idle is too high.
Any of that would suggest that something is amiss in the fuel delivery end of things…..like something hanging up one of the carbs, carb out of adjustment or even poor throttle return.
Good luck.
OM

 

[melville]

It's just a magical alignment of crank position, state of charge in the combustion chamber, and the field in the coil collapsing by alternate means.

Happens once in a while on my stuff. Seems more likely to happen when I'm not using my usual order of operations with ignition and kill switch on and off.

 

[Lendog]

Ok. This was recently rebuilt. Everything is new or reconditioned. I've got a few issues to chase down, and this is one.

Occasionally, when I switch the engine to run but before pressing the start button, I will get a "chuff" sound. Two times, while testing other things, I got a loud bang. More of a single pop than a backfire. I am guessing at this, but I think the ignition is sending a spark to the plugs once it get's power. Actually, about one second after, which seems to align with the timing of the electronic ignition self-check. This spark seems to be igniting residual fumes in the carb or exhaust. Again, all a guess. I will take another look at the carbs to be sure they check out, but wanted to see if anybody else has run into this. The ignition is the EME electronic ignition, if that matters.

You can hear the chuff in this short video.

Bill

some electronic ignition systems have a “wasted spark “, the ignition timing could be advanced too far

 

[justabill]

Thanks team. Great thoughts.

• The cap is vented. (it's from flat racer if anybody is interested)
• It is the same bike from the other post. Everything is new and newly rebuilt.
• No pulse air system.
• It's only been rebuilt for a week, so I don't know how time will play into this.
• Residual gas is my current theory. (blended with the wasted spark)

Good to know that others have had something similar. I'll see what EME says as well and follow up.

 

[stevenrankin]

Is this happening every time you turn on the ignition? A rare chuff or pop is noted with the Stock systems, but should not be happening every time you turn on the ignition to run. In my case it is extremely rare, about the only time I get it is when I use the kill switch, which is rare.
Go back over your work and check things out. You might want to contact EME in regard to the electronic ignition system's foibles that may cause this. St.

 

[justabill]

Is this happening every time you turn on the ignition? A rare chuff or pop is noted with the Stock systems, but should not be happening every time you turn on the ignition to run. In my case it is extremely rare, about the only time I get it is when I use the kill switch, which is rare.
Go back over your work and check things out. You might want to contact EME in regard to the electronic ignition system's foibles that may cause this. St.

Not every time. It's more likely to happen a few minutes after a failed start, but I have not been able to nail down how to reproduce it exactly. This is not always the case though. I recorded about 2 dozen start sequences trying to catch this sound. This particular time, the bike had been sitting, tank off for about 2 hours and the chuff happened on the first, and only the first, start sequence.

 

[stevenrankin]

LOL, I think perhaps you should put a few miles on it and see what happens then. The chuff and bang is residual gas and with a wasted spark it will ignite. While a curious thing, I don't believe it is harmful unless it happens all the time when you turn the key on before trying to start the bike. Get my drift?
St.

 

[ebeeby]

I get an occasional "chuff on the R75/5 here...

 

[20774]

I get an occasional "chuff on the R75/5 here...

But not on your R90S? What would be the difference? Ever look into it more?

I've never had a "chuff" on any of my bikes.

 

[kinterridge]

I also get a "chuff" on my R100S ever since I moved to the EME system, not always, just once in a while. I've attributed it to a spark and residual gas firing when I power up the ignition. If I switch back to points, I still have them set up, just one wire switch at the coil it does not do it. It's been several years and no issues I can see from it.

 

[justabill]

I also get a "chuff" on my R100S ever since I moved to the EME system, not always, just once in a while. I've attributed it to a spark and residual gas firing when I power up the ignition. If I switch back to points, I still have them set up, just one wire switch at the coil it does not do it. It's been several years and no issues I can see from it.

That's good to read. I can't easily switch back due to the other updates. I have had a couple dozen starts since this post and the Chuff is now <5% of the time. It is likely more than it should be, because I also have a carb running too rich. Other than the two bangs I had, there hasn't been anything alarming and I am more confident about the wasted spark + residual unburnt fuel hypothesis.

 

[beemerchuck]

The EME system will discharge the coils about 2-3 seconds after power up. If there is a cylinder with sufficient fuel it will pop or chuff. Just put one on a friends 81 RT and it does that fairly often.

 

[mark_bartels]

Our 78 R100S will do the same chuff occasionally with the EME ignition

 

[justabill]

I will stop chasing this down. Seems to be a common EME Ignition artifact. Glad to know there's nothing necessarily wrong with it.

Thanks everyone.