Lean or Rich: The Source of Popping? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 9 (permalink) Old 05-10-2010, 09:33 AM Thread Starter
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Lean or Rich: The Source of Popping?

This is a question that has come up in various posts countless times, but I am still having a little trouble wrapping my mind around the issue. Hopefully someone can clear this up! It is widely posted about that when experiencing excessive popping, marbling (or coastering) in conjunction with richening the fuel mixture idle screws should solve most of the problem. First off, does the EPA equipment inject air during normal throttle or during idle? If during idle, the marbling makes some sense as a solution, but otherwise, it seems marbling would only affect the airflow under throttle. Secondly, the cause of backfiring is generally excessive unburnt fuel that is suddenly in contact with oxygen and heat. My question is, why do we increase the fuel in the mixture with the idle fuel screw then? It seems to me that this would serve to increase the extra fuel at idle/coasting and exascerbate the popping? Wouldn't you want to lean out the mixture to stop popping at idle/decel conditions? The conventional wisdom seems to say that the mixture is too lean and pops as a result...but why? If a mixture is too lean, additional air doesn't cause combustion...it further hampers it. What am I misunderstanding? I'm sure I'm missing something as those who are more experienced and knowledgable all agree on this. Please help me to understand, I have read all there is on the Vulcan Verses, and I'm still not able to put this to rest!

1992 Vulcan 750
(Brother-in-law's bike, but am caretaker while deployed)
V&H Cruzers
K&N Filters
Re-Jetted (40, 135)
Marbled
Rebuilt ACCTs (TOC kit)

1984 VF700 Magna
(Wife's Bike)
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post #2 of 9 (permalink) Old 05-10-2010, 09:39 AM Thread Starter
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My only thought is that too lean of combustion causes the mixture to not burn properly in the chamber and causes a pooling effect in the exhaust which builds up and then combusts when the fuel is rich enough, but this does not seem to ring true with a constant exhaust flow...just trying to put this together in my mind....(its a mess up there!)

1992 Vulcan 750
(Brother-in-law's bike, but am caretaker while deployed)
V&H Cruzers
K&N Filters
Re-Jetted (40, 135)
Marbled
Rebuilt ACCTs (TOC kit)

1984 VF700 Magna
(Wife's Bike)
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post #3 of 9 (permalink) Old 05-10-2010, 04:55 PM
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Our 'reed air injection' into the headers is the source of most, if not all of our 'popping' issues. When a exhaust pipe sucks fresh air into the pipe as in a exhaust leak, popping will happen. Our reed air system injects fresh air into our exhaust to burn excess hydrocarbons in the goat to lower emissions = happy EPA but we get the pop.

Our bikes run lean from the factory so when we take the crap off (reeds, goat, stock air box, re-breather) we really must re-tune the engine from the air intake to the tip of the exhaust.

Check your vent hose, vacuum hoses, carb needles, jets, air screws, spark plugs for color (light tan), carb sync may cause pop, and no leaks in your exhaust.

Hope this helps,

DT
(BTW I have zero pop)

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post #4 of 9 (permalink) Old 05-10-2010, 05:20 PM Thread Starter
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I grasp the problem with the air injection, but my quandry is that I only experience distracting popping during a hard throttle blip or coming off harder acceleration and it's only a single loud pop followed by perhaps some smaller popcorn pops. So I'm really trying to figure that part out.

As for my question in general as posted here: The EPA equipment is actuated by the suction line to our right side carb (rear cylinder). This means to me that the EPA equipment injects air based upon suction pressure. My first question is thusly put: Is the suction greater at decel/coasting/throttle (ranking?) and aside from that...does suction close the EPA air injection or open the EPA air injection?

The second part of my post is inquiring as to why we enrich a fuel mixture to relieve popping when it seems that additional fuel in would result in additional unburnt product. I think I am figuring this out as such: The lean mixture retards combustion and as such is only partially carried out in the cylinder. Upon reaching the exhaust pipes, the reduction in pressure in conjunction with hot metal allows for a second instantaneous combustion. If this is the case, manipulating the idle screws makes logical sense. Additional fuel encourages a more complete combustion and thus reduces popping. But is that what is going on?

1992 Vulcan 750
(Brother-in-law's bike, but am caretaker while deployed)
V&H Cruzers
K&N Filters
Re-Jetted (40, 135)
Marbled
Rebuilt ACCTs (TOC kit)

1984 VF700 Magna
(Wife's Bike)
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post #5 of 9 (permalink) Old 05-10-2010, 07:12 PM
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the suction gets greater as throttle increases...
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post #6 of 9 (permalink) Old 05-11-2010, 08:43 AM Thread Starter
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Okay, after some helpful postings (Thanks Denny and Slim) and some outside research, I think I have this whole thing figured out. Please understand that I was not questioning your collective wisdom. I fully accept that you all know what you are talking about and I greatly respect you all for being so willing to help us "noobs" out. I am a mechanical engineer by trade though, and having to know "why" seems to be my constant curse and boon. So for those whose interest I spiked here goes my rendering of what all the fuss is about (Feel free to correct me if I misrepresent the subject somewhere!):

First off, there's a fairly good article on Wikipedia about backfiring in general and if you're so inclined, I'd recommend a quick read through.

http://en.wikipedia.org/wiki/Back-fire

Coming from a previous job as a combustion engineer for industrial burners (Natural Gas and various Mixed Gas experience), I was confusing myself a little on the subject. There are two conditions which can cause backfiring: Lean OR Rich.

As most of us know by experience to some degree, fuel can only combust under a certain set of conditions which are driven by things such as temperature, pressure, and perhaps most importantly, air/fuel mixture. A rich mixture has too much fuel and as a result, there is simply not enough air to have complete combustion, which results in unburnt fuel. This unburnt fuel is hot via the partial combustion and the introduction of oxygen causes a secondary burndown. This is what is typically experienced during throttle overrun, which is what I think my original issue with my Vulcan boils down to. Under hard throttling (accelerating or impressing your friends with your new-found noise maker), when you let off the throttle quickly, there is an excess of fuel due to your loaded-up carb which cannot be burnt completely in the cylinder. This causes an overrun backfire and in my case explains why I get a single loud "boom" followed by your typical runback popcorn. I think I can try to tune this a little bit with the idle/fuel screws and marbling, but it's never completely avoidable.

A lean mixture is a bird of a different color. The fuel mixture does combust, but the excessive air serves to retard combustion and as a result, unburnt hydrocarbons are formed. The combustion that does occur tends to be very localized and as a result causes a hotter firing cycle where combustion is taking place. This is part of the problem with running lean. My error in my previous thinking was not understanding why these burn with the addition of air from our air injection systems. Unburnt fuel, and particularly hydrocarbons are problematic in that they have ample temperature for ignition, but are simply not in the right air/fuel ratio for combustion...until they are via random mixing and molecule collisions. This secondary combustion with a lean mixture is unpredictable and as a result is difficult to control. This explains why the typical popping is common on the Vulcans. Kawasaki just wants to meet EPA standards (the government slap on the wrist hurts) and as such uses their version of Aspirated Air Injection to encourage the combustion of the hydrocarbons formed by the initial lean mixture partial combustion. Because the second burndown is unpredictable, Kawasaki uses excess air in conjunction with the "goat's belly" to promote mixing and attempt to control the popping. Those of us who remove the "goat" via de-goatting or after-market pipes notice the popping more so as the more controlled method of secondary burndown has been removed

Now, as for how our EPA "crapola" works. There is another good article on the basics on Wikipedia as well :

http://en.wikipedia.org/wiki/Secondary_air_injection

I highly recommend a cursory reading, but here is the important section:

"Aspirated air injection
Air injection can also be achieved by taking advantage of the negative pressure pulses in the exhaust system at engine idle. A sensitive reed valve assembly called the aspirator valve is placed in the air injection plumbing, which draws its air directly from the clean side of the air filter. During engine idle, brief but periodic negative pressure pulses in the exhaust system draw air through the aspirator valve and into the exhaust stream at the catalytic converter. This system, marketed as Pulse Air, was used by American Motors, Chrysler, and other manufacturers beginning in the 1970s. The aspirator provided advantages in cost, weight, packaging, and simplicity compared to the pump, but the aspirator functions only at idle and so admits significantly less air within a significantly narrower range of engine speeds compared to a pump. This system is still used on modern motorcycle engines, e.g. the Yamaha AIS (Air Injection System)."

So, our EPA equipment is a simple aspiration affair (there is no seperate air pump) and as such is only used at idle and low throttle conditions. Aside from being ineffective at high throttle inputs, I think the additional problem Kawasaki faced was that the Vulcans are tuned VERY lean at high end already, and this causes unwanted engine heat to begin with. The addition of more air at this point could serve to cause further problems. So for this reason, or some other I have yet to figure out, Kawasaki installed the air switch valve which is driven off the suction vent of the Vulcan's right-side carb. As throttle input increases the air valve is driven closed and prevents air from even reaching the reed valves, but opens as the throttle is released and the carb vacuum line returns to a lower vaccuum.

In summary, the popping we get on decellerating can come in two forms. At the first crack off of throttle, there is a chance of an overrun condition (harder throttling to sudden decel or coasting) followed by the much more common lean combustion popping. By eliminating the EPA injection we can remove the "fascilitating" factor presented by the air injection, but we also end up enriching our idle/fuel screws in order to encourage a more complete combustion in the cylinder to begin with, which in turn reduces the formation of those nasty hydrocarbons.

I hope this was helpful to others and that I have not led anyone too far from the beaten path. If I have gotten any of this wrong, please, PLEASE feel free to correct me. I am simply trying to learn how our bikes work and what our modifications are actually doing to the operation of the bike. I apologize for the length as well, but I tend to like to get it all out at once!

Thanks for all your help!

1992 Vulcan 750
(Brother-in-law's bike, but am caretaker while deployed)
V&H Cruzers
K&N Filters
Re-Jetted (40, 135)
Marbled
Rebuilt ACCTs (TOC kit)

1984 VF700 Magna
(Wife's Bike)

Last edited by hoff02; 05-11-2010 at 08:51 AM.
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post #7 of 9 (permalink) Old 03-02-2015, 03:11 AM
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This is exactly the thread I was looking for. Thanks, Hofff for doing the work.

I've been playing around with different settings of the air/fuel mixture after doing the ear shave. I think I had it a bit too rich, and my buddy with the spark plug socket wasn't home today, so I tried to just tune it by trial and error of revving and adjusting the screw. I'll test ride it tomorrow.

Anyway, yeah, good post.
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post #8 of 9 (permalink) Old 03-02-2015, 08:07 AM
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Always enjoy seeing someone dig up a five year old post.....(telling us what almost everyone already knew here)
Good for the newbies that missed it.

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post #9 of 9 (permalink) Old 03-02-2015, 05:49 PM
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Quote:
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Always enjoy seeing someone dig up a five year old post.....(telling us what almost everyone already knew here)
Good for the newbies that missed it.

Quote:
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