Why do pods "cough"?

[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.

 

[Spyder]

Hmmm, That's starting to sound like a mixture issue... Are you still getting sooty black plugs?

 

[salty_monk]

My skunk will do it on Choke after it's first started up. (VM's), more so if I haven't ridden it in a couple of weeks or more.

Can't remember my CV's ever doing it on the 1000G or the 750ES.

They both had K&N's on at some point.

 

[pitbullmccoy]

I have the same issue but I can say I really don't think it is all pods. On my bike now '78 gs1000 it has a few coughs at warm up and maybe a couple at say 1\3 throttle like going up the back road from my house but when i hit the hwy and run 70mph or really give it gas while shifting no prob at all. "BUT" I had the exact same problem with a sporster. And i mean exact. Pope's at warm up and mid throttle. I never got it completely worked out on that bike but after several tries, and its way easier with a single Carb,I got the mixture very close to perfect. I personally wouldn't get overly worried but that's just me. I haven't had my gs very long but rode the absolute **** out of the sportster with it doing the same thing. Other than a being annoying I had no real issues. But that's just my 2 cents. I think it would take a seriously seasoned mechanic way beyond my years to get all 4 working flawlessly.

Just be sure to let us know if you track down the cause and get it worked out. Good luck with it.

 

[tbirdhd]

hi new to this link but i had that and figured out i had a air leak in the exhaust , ,, changed exhaust backfire gone , just some thing to check ,

 

[Steve]

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

Not mine.

.

 

[MisterCinders]

Not mine.

.

Steve has special bikes. He's not allowed to have a chain.

 

[Seana23]

With my bikes particular cough is actually a spitback... I have seen it happen. A spray ov vapour blows back out the carb/filter manifold...
usually, its number 1 carb... Less often its number 4... Not noticed it with either 2 or 3.

Xxx

 

[keman]

This can also be caused by weak valve springs. The weak spring allows the intake valve to hang for a split second. This hang allows gasses to escape back into the intake and 'cough' back through the Carb. IF 'IF' this is the cause, no amount of tuning well make it go away. 30 year old springs get weaker with age, especially on bikes that tend to run hot.

 

[TheCafeKid]

Not to be contrarian, but in my experience in tuning carbs (and while it's not the decades of experience of some guys on here, I can say I've tuned more than a "couple" sets) popping or backfire through the carb is generally cause by a rich mix. Unburnt fuel vapor hanging around the intake combines with a sudden rush of cooler dense air on the next intake stroke and will pre-ignite. Pay attention to when it happens next. We're you blipping the throttle softly at the light? Sitting and idling for a good while??
Could be your fuel screw needs backed off a hair, air screw needs opened a hair, or float levels are off.
Much the same reason the bikes pop a bit through the carbs on choke when first started. The cylinder is up to temp, fuel condenses on the cylinder wall, or intake rail, and when enough builds up, "cough". On "choke" (actually an enrichener, not a choke) you're dumping nearly straight fuel into the cylinders.

Which, brings up another point of contention for me. Often times you will see people say that opening up your mix screws on the CV carbs will allow the bike to warm up quicker. This seems a bit counter-intuitive to me.
If you're adding more fuel/air mix (which is what the mix screws control on the CVs, the fuel and air are pre-mixed, the screw simply controls how much of that mix goes in) more fuel will keep the cylinder cool. The longer it stays cool, the longer it takes to warm up. Open them up too much and you're just wasting fuel.
I dunno, just things I think about sometimes.

Anyway, with your "cough" I'd look at being rich more than lean. A lean mix would mostly be sounded by poppin through the exhaust, although a rich mix can do that as well...

Plug chops, or a sniffer...
Pods pop. Just the nature of the beast...

 

[Guest]

Mine will do it once in a very blue moon, but I believe it's as TheCafeKid described "blipping the throttle softly at a light".