Why do pods "cough"?

[MisterCinders]

I have seen threads about pod spitting or coughing as a lean symptom.

My pods will sporadically cough and suck in a big gulp of air, causing a slight stumble that disappears after the cough. It happens only rarely, and I haven't figured out what scenario triggers it or even what circuit might be the culprit.

While I root out this hiccup, what about the carb function causes this to happen in the first place? It feels like the carb is starved for air (like a person with asthma even) and that the pop clears it out. It's not spitting fuel and sounds feels like the pop is air going into the system, not something backfiring out through the pod.

What's the science to this?

 

[Big Rich]

I actually am just interested in the replies.........sorry.

But could it be from the length of the intake being so much shorter than stock? Maybe causing a hiccup in the intake pulses? Obviously just a guess.

 

[ssbrazo]

The first part of the science is to either make a mental or physical note pertaining to when the "Hiccup" occurs.

How far open was the throttle, how fast were you going, what gear were you in, etc....

Any information you can gather will eventually solve your problem

 

[Seana23]

Hi babes.
I know.from experience, this cough happens at idle more than under.load....
It jyst spits back every now and again...
Ive never worked.out exactly why this happens...
Sorry.
But im interested too...

Xxx

 

[Steve]

And it's not just pods. :-k

Last year, I put adapters on the carb sync fittings to make the carb sync process easier. A couple of "coughs" later, the rubber caps were history and I had a SERIOUS air leak. This actually happened while on a ride. I happened to notice a missing cap when I looked down at a stop to see why the engine was not running "right". Stopped at Auto Zone, got a rubber cap assortment, got back to the motel, took the adapters off.

This was on "Junior", a mechanically-stock '80 850G(K).

.

 

[razor02097]

And it's not just pods. :-k

Last year, I put adapters on the carb sync fittings to make the carb sync process easier. A couple of "coughs" later, the rubber caps were history and I had a SERIOUS air leak. This actually happened while on a ride. I happened to notice a missing cap when I looked down at a stop to see why the engine was not running "right". Stopped at Auto Zone, got a rubber cap assortment, got back to the motel, took the adapters off.

This was on "Junior", a mechanically-stock '80 850G(K).

.

Same happened to me. That is the reason I don't run those brass adapters anymore. I guess the airbox muffles the "cough".

 

[tkent02]

The cough at idle is caused by being lean, to prove this to yourself turn the choke on a little bit, it goes away. I think somehow all of the fuel air mix between the head and the carburetor is burning, the next stroke there is none to burn in the cylinder, so the engine misses once, and after that it runs fine for a while, until it happens again. Not sure why this burning happens in the first place. Maybe a little piece of left over flame comes out the intake valve when it opens? There should not be any flames after the exhaust stroke, like I said I don't really understand it.

To prevent it, leave the choke on a little bit longer warming it up.
If it does it with no choke on a warm engine richen the idle mixture a little bit.

 

[Steve]

The cough at idle is caused by being lean, to prove this to yourself turn the choke on a little bit, it goes away. ...

To prevent it, leave the choke on a little bit longer warming it up.
If it does it with no choke on a warm engine richen the idle mixture a little bit.

Actually, it coughed a time or three while warming up on "choke", too. :-k

Not hard enough to spit the cap off the adapter, though.

.

 

[MisterCinders]

The first part of the science is to either make a mental or physical note pertaining to when the "Hiccup" occurs.

How far open was the throttle, how fast were you going, what gear were you in, etc....

Any information you can gather will eventually solve your problem

That's what I am still working out. It doesn't happen consistently or predictably, so I am keeping an eye out to try and isolate what's going on when I have the cough -- e.g., throttle position, engine warm or cold, etc.

Also, when it does cough, it happens once only and then runs fine for quite a while.

The cough does not really happen at starting or idle. Maybe once during an initial warm-up, but not often. Most warm-ups, starts are fine. It seems more likely to emerge once the engine is warm and then during acceleration from a stop or very low speeds. With most (80%) of my riding in the city, I am always at a stop or low speed, so "low speed/stop-and-go" is not the end of the equation, or she'd be coughing like a beast. She doesn't.

The cough at idle is caused by being lean, to prove this to yourself turn the choke on a little bit, it goes away. I think somehow all of the fuel air mix between the head and the carburetor is burning, the next stroke there is none to burn in the cylinder, so the engine misses once, and after that it runs fine for a while, until it happens again. Not sure why this burning happens in the first place. Maybe a little piece of left over flame comes out the intake valve when it opens? There should not be any flames after the exhaust stroke, like I said I don't really understand it.

To prevent it, leave the choke on a little bit longer warming it up.
If it does it with no choke on a warm engine richen the idle mixture a little bit.

It doesn't tend to happen at times where I could play with the choke, since I am riding. Also, when there is a cough, even if I could add choke, it would be impossible to tell the difference, because when it coughs, it coughs once and then behaves for quite some time.

Will continue to check plugs and play with the Colortune on the idle mixture, since my city riding puts me in that circuit most of the time. So far the plugs are reading pretty good. If anything, they still show a tiny bit rich. Not as rich as they were before my series of carb tear-downs, overflow problems, etc.

I wonder if it relates to the needle height, since I need to dig into that circuit anyway. Between my urban geography and still newbish riding skills, I am wary of the mid-range plug chop. There just aren't many good places to do that chop and safely pull over to inspect the hot plug without traffic, speed bumps, stop signs etc. Right now I have the clip in the 4th position with a shim, but I am not quite happy with the performance in the mid-range (even without the odd cough thing). Cannot tell if it's slightly flat or mildly bogging. I suspect the latter, if only because I was pretty aggressive on richening the upper circuit mixes. Of course, the plugs will tell the real story.

 

[MisterCinders]

Same happened to me. That is the reason I don't run those brass adapters anymore. I guess the airbox muffles the "cough".

I tried the vacuum linked "permenant" adapter mod. It seemed neat, but I prefer the longer adapters for synching. Then I snapped one of the "permanent" adapters the other day when reinstalling them after synching the slides. So I am back to the original vacuum port screws. Still have the random cough though.

 

[mvalenti]

Well I'm glad to hear I'm not the only one with an occasional cough, and not running the stock airbox, or pods. Maybe it has something to do with area 51, northern lights, bermuda triangle, or that dam sasquatch!

 

[MisterCinders]

Well I'm glad to hear I'm not the only one with an occasional cough, and not running the stock airbox, or pods. Maybe it has something to do with area 51, northern lights, bermuda triangle, or that dam sasquatch!

Who would steal 30 bag lunches?

 

[ssbrazo]

Who would steal 30 bag lunches?

I'll tell you who! It's that damn sasquatch!

 

[MisterCinders]

I'll tell you who! It's that damn sasquatch!

Consider me Miles Davis.

 

[Seana23]

Not sure what this adds to the mix but ny needles are one clip higher than standard and im running 142.5 main jets... With 3 turns on the mixture screw...
Also ive sheathed the pods with short pieces ov old inner tube so only 15-20% air is flowing into the carbs..
So im reallt not sure the cough is happening because ov a lean muxture....
Xxx

 

[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? :-k

.

 

[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.