Why do pods "cough"?

[Steve]

Why do pods "cough"?

Have you considered adding some Robitussin DM to the gas?

... and im running 142.5 main jets... With 3 turns on the mixture screw...

WOW!!! 142.5 mains???

Can anybody following you even breathe? That is ELEVEN sizes richer than stock.

Are you running ram air on the intake and open pipes for your exhaust?

.

[salty_monk]

Might not be that much richer on a British bike... sometimes the air jets, emulsion tubes etc are different. Sometimes it's even a different carb over there.
Just hazarding a guess

[BeerHunter]

Any coughing action that I have experienced with my pods has usually been corrected by leaning out the pilot fuel screw on the offending carb.

[hampshirehog]

WOW!!! 142.5 mains???

Can anybody following you even breathe? That is ELEVEN sizes richer than stock.

.

That's probably about right. I would pull off the inner tubing on the pods and see what happens.

The cough is caused by fuel / air mixture bouncing back in to the throat of the carb from the closed valve and if it's weak it is more easily ignited.

[MisterCinders]

The cough is caused by fuel / air mixture bouncing back in to the throat of the carb from the closed valve and if it's weak it is more easily ignited.

Can you elaborate on this more?

When it "coughs" at the pod, it seems like a rapid intake of air, not a backfire out. It's not a loud POP at all, more like a wheezing inhale sound, during which the engine misses -> coughs -> runs. Of course, it happens randomly while riding, so like that damn Sasquatch, I never actually see the offending pod behavior with my own eyes.

I have looked everywhere for visual evidence to match or refute the "gasping intake" noise I hear in the pod. I have seen no scorching or fuel sprays to signal that the pod-cough is a backfire of ignited fuel.

Seriously, I saw a pic of a guy's bike that burned up in seconds when a bad carb backfired, so that crap scares me to death.

Rooting around for information, some folks suggest that the lean condition causes the cough at low RPM and rapid throttle opening. That matches what I am finding, though my cough is not repeatable. Sometimes it happens, but I cannot recreate the cause.

[hampshirehog]

Not easy to explain so if the following sounds like gibberish (which it probably is) apologies.

When your engine is running and on the intake stroke the fuel / air mix is rushing in through the inlet port at very high speed. Most of this mix passes in to the engine and gets ignited. However, some of this fast moving vapour doesn't make it in to the combustion chamber before the valve closes, hits the valve at high speed and bounces back - potentially back through carb and out in to the atmosphere. You can see this really easily on a piston port 2 stroke that is in a high state of tune - they can literally spit petrol straight back out; it's the same thing happening on a 4 stroke.

Now this vapour could be lost to the atmosphere (as I said above) or, ideally, 'trapped' and drawn back in to the engine for another go on the next intake stroke. This 'trapping' is done by the bellmouth design on the carb. The original airbox usually has a good design and cheap pods don't. You can get all sorts of swirl effect etc with poor designs and an inconsistent supply of this vapour - hence a cough every now and then for example. So sometimes your bike will fire with a weak mixture, sometimes it will be fine. Crosswinds can play havoc with this as well.

So the weak mixture that you get when you pull the airbox off is not only because you have an unhindered supply of air (no filter getting in the way) but you are also losing fuel vapour out of the back of the carbs.

[salty_monk]

I suspect you feel an IN rush of air because as the mixture ignites it tries to draw oxygen into the flame... It can't get it from the engine so you feel it being drawn into the filter.

[MisterCinders]

OK - Intake backfires are actually backfires. Ugh.

I'll richen up the offending carb then in the pilot. When that cough hits in a low speed turn, it's pretty unnerving.

Thanks for all the science.

[Spyder]

To everyone that has this issue. Are you using K&N pods or generic??? This has made me curious because I have K&N's and do not experience this... WOOHOO ! ! ! A problem that mine DOESN'T have ! ! !

[MisterCinders]

I have K&N on my bike.

[Spyder]

Hmmm, I'm thinking timing, or a intermittent bad seating valve...

One other question. Are all of these "coughing" bike running VM carbs?

Just to be clear, a VM carb is like the carb on a 2 stroke dirt bike right? The throttle lifts the slide instead of a butterfly valve?

My old dirt bikes would "cough" lightly when the reed valve started to get weak...

[MisterCinders]

Yes, I have VM carbs where the throttle lifts the slide.

[hampshirehog]

Same thing will happen with CVs.

[Spyder]

interesting. can you FEEL it when it happens? What I mean is, is it like a hesitation, or more of just a sound that it makes? I am Merely curious about such anomalies. I was an automotive technician for 9 years. A symptom like this would usually be timing or mixture. but, also usually would do this almost every time you hit the throttle. Not actually a hesitation, but a "cough"...

[MisterCinders]

interesting. can you FEEL it when it happens? What I mean is, is it like a hesitation, or more of just a sound that it makes? I am Merely curious about such anomalies. I was an automotive technician for 9 years. A symptom like this would usually be timing or mixture. but, also usually would do this almost every time you hit the throttle. Not actually a hesitation, but a "cough"...

I can feel it, but usually only after. In other words, the hesitation is there, but more noticeable after the pop clears it. It happens when I throttle up from a stop or low speed, so I think it's in the pilot circuit. But it doesn't happen every time I wind it up, maybe 1:20 chances.

If it were just an oddity that came with pulling from the line now and then, I'd chalk it up to an annoyance. Because it also can happen when I accelerate at low speeds (like when coming out of a turn), that raises its priority. Stumbling, coughing and popping out of a corner is no way to go through life, son.