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Discussion Starter #1 (Edited)
I've committed and bought a Vance and Hines twin slash staggered exhaust for my 2008 Custom. I also installed a K&N air filter. My question is do I need a Power Commander?
 

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You are probably okay since you are still running the stock airbox, even with a K&N filter. But, would check your plugs after a couple thousand miles to see if they show signs of running lean. I ran debaffled (full) pipes with a K&N and saw no issue of running lean. But, once I went to a new intake (ArlenNess Big Sucker) needed a fuel mgmt system.
 

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Discussion Starter #3
Thanks for the information. I got the exhaust today and will install them tomorrow.
 

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When you get tired of the decel popping you can try plugging the hose going to the secondary air supply control valve (marbling). If it's still a nuisance then buy a cheap Cobra fi2000 three pot fuel manager and you can dial out the popping. No need to spend a lot of cash on a PCV.
 

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Just be aware that every bike is different! Even though others do it without, yours might not.

Also, the cure for the decel pop is often setting the idle where it needs to be (between 950 and 1050rpms). SEVERAL people on this forum have commented the same. Most folks set the idle way way too low, which causes other problems (oil starvation to the top end, etc.). My THEORY is, that the low idle position is creating a 'rich' environment causing the exhaust popping. Remember exhaust popping is the result of running RICH. Higher flow pipes and intake create a LEAN condition on an unmodified fuel system. So the only way you get a RICH condition on a bike that's running LEAN is to reduce the amount of air below what the bike is mapped to be running. You CAN dial this out with a computer, OR, you can fix the issue entirely by setting the idle properly.

Also, running regular gas seems to fix the popping for some. Premium fuel resists burning and can burn longer, meaning it can pop. (Also, because someone always mentions it, Kawasaki recommends 87 octane for the 900. The '91' on the seat sticker is RON, which is the way they measure it overseas. Similar to kmh vs mph. Over here it's measured in R+M/2. 87R+M/2=91RON)

Just throwing that out there! If the bike is running lean you should certainly get some sort of fuel management. But if it's not, and you're getting decel pops- well; either way the idle should be set correctly!
 

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Just be aware that every bike is different! Even though others do it without, yours might not.

Also, the cure for the decel pop is often setting the idle where it needs to be (between 950 and 1050rpms). SEVERAL people on this forum have commented the same. Most folks set the idle way way too low, which causes other problems (oil starvation to the top end, etc.). My THEORY is, that the low idle position is creating a 'rich' environment causing the exhaust popping. Remember exhaust popping is the result of running RICH. Higher flow pipes and intake create a LEAN condition on an unmodified fuel system. So the only way you get a RICH condition on a bike that's running LEAN is to reduce the amount of air below what the bike is mapped to be running. You CAN dial this out with a computer, OR, you can fix the issue entirely by setting the idle properly.

Also, running regular gas seems to fix the popping for some. Premium fuel resists burning and can burn longer, meaning it can pop. (Also, because someone always mentions it, Kawasaki recommends 87 octane for the 900. The '91' on the seat sticker is RON, which is the way they measure it overseas. Similar to kmh vs mph. Over here it's measured in R+M/2. 87R+M/2=91RON)

Just throwing that out there! If the bike is running lean you should certainly get some sort of fuel management. But if it's not, and you're getting decel pops- well; either way the idle should be set correctly!
Boy, I wish this were true. The VN750 has a horrible decel pop and you can't fix it by increasing idle. The popping is caused by the emission system burning exhaust gas. The only true way to get rid of it is to block off the emission system. As with the VN750, richer fuel conditions helps eliminate the popping, which is what the PC will do. Our bikes, as with most factory bikes, are set to run lean which will get the exhaust really hot which leads to random combustion of the exhaust gas via the emission system.
 

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Boy, I wish this were true. The VN750 has a horrible decel pop and you can't fix it by increasing idle. The popping is caused by the emission system burning exhaust gas. The only true way to get rid of it is to block off the emission system. As with the VN750, richer fuel conditions helps eliminate the popping, which is what the PC will do. Our bikes, as with most factory bikes, are set to run lean which will get the exhaust really hot which leads to random combustion of the exhaust gas via the emission system.
That's a VN750. We're talking about Vulcan 900's here. And myself and several others have been able to cure the decel pop by setting the idle correctly and running 87 octane fuel.

Either way, the idle shouldn't be below 950 RPM's, even if it sounds 'cool', this is an overhead cam engine, the oil pump needs to be turning fast enough to get the oil up there so those cams and valves don't starve of oil. Among other issues, like potentially reduced charging output which can stress the battery and charging system, etc. Kawasaki designed the bike to run at 950+ RPM's, so idling it at 700 or 800 RPM's is not so good!

I've got fully debaffled pipes, no power commander, and don't have a lick of popping. But if I lower my idle and/or start running the high octane stuff it'll pop like crazy.
 

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Discussion Starter #9
OMG this exhaust sounds GREAT!

Installed this morning and it only took about 2 hours to remove the old and install the new. Have not had any popping as of yet. When I started the bike and gave it a rev the neighbor kid across the street came to their window.
 

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Interesting conversation here. I'm confused however in what the connection is between keeping the idle at factory specs. and decel popping which occurs when the bike is coming down from an RPM well above the idle setting. My decel popping occurs when I'm trailing the throttle, say at 2%. Above that and at zero throttle no popping. That's with a PCV & Autotune.
 

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Interesting conversation here. I'm confused however in what the connection is between keeping the idle at factory specs. and decel popping which occurs when the bike is coming down from an RPM well above the idle setting. My decel popping occurs when I'm trailing the throttle, say at 2%. Above that and at zero throttle no popping. That's with a PCV & Autotune.
Well I don't know why folks would WANT the idle below specs, considering it's ability to cause damage to the engine.

BUT, my thought is that, PCV or not, the bike is mapped to run from factory idle on up. The whole engine is tuned to do just that. So below 950rpms, it's quite feasible that you're introducing the amount of fuel required for 950-1050 RPM's despite being much lower than that, creating a rich condition (i.e. backfiring). It's not SO rich that it backfires as it idles, but rich enough that when it's coming down from a higher RPM it certainly can. Exhaust gas temps are probably higher as it comes down from a higher RPM than when idling as well, improving the chances of igniting that spent fuel. So you could lean out the setting for around 1k RPM's, but then when you hit those RPM's it'll be lean and you'll have a flat spot, etc.
 

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Well I don't know why folks would WANT the idle below specs, considering it's ability to cause damage to the engine.

BUT, my thought is that, PCV or not, the bike is mapped to run from factory idle on up. The whole engine is tuned to do just that. So below 950rpms, it's quite feasible that you're introducing the amount of fuel required for 950-1050 RPM's despite being much lower than that, creating a rich condition (i.e. backfiring). It's not SO rich that it backfires as it idles, but rich enough that when it's coming down from a higher RPM it certainly can. Exhaust gas temps are probably higher as it comes down from a higher RPM than when idling as well, improving the chances of igniting that spent fuel. So you could lean out the setting for around 1k RPM's, but then when you hit those RPM's it'll be lean and you'll have a flat spot, etc.
Your explanation still doesn't provide an answer for the popping while maintain speed. Its not unspent fuel that is popping, it is un burned exhaust gas igniting in the headers/exhaust. You can thank the emission system for that. Get rid of it and the popping should stop. Just like blocking the reed valvkes on a 750 which also has DOHC.
 

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Your explanation still doesn't provide an answer for the popping while maintain speed. Its not unspent fuel that is popping, it is un burned exhaust gas igniting in the headers/exhaust. You can thank the emission system for that. Get rid of it and the popping should stop. Just like blocking the reed valvkes on a 750 which also has DOHC.
The 900 has SOHC. Two very different bikes. I dunno what to tell you. When you set the idle correctly on a 900 and use the proper fuel it doesn't pop. Myself and several others can attest to that. At least in our experience, it goes away without needing to marble or modify the emissions control system.
 

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I just don't see the correlation between idle speed and decel pop. Once you give the bike gas, it comes off the idle circuit and returns when back at idle. Letting the motor slow the bike down will produce decel pops due to the exhaust getting reburned. Maybe it was a coincidence that upping the idle masked the popping. But I can say that total removal of emission system (marbling) will eliminate the popping.

My bike doesn't pop at idle nor at highway speeds. Only when I let the motor slow it down. About the only thing I experience at speed is seems to be constant misfires w/o any hendersnce to performance. Bike has cobra longshots installed and nothing else. Air filter and plugs were replaced 2 weeks with no change. Old plugs were brown in color.
 

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Interesting conversation here. I'm confused however in what the connection is between keeping the idle at factory specs. and decel popping which occurs when the bike is coming down from an RPM well above the idle setting. My decel popping occurs when I'm trailing the throttle, say at 2%. Above that and at zero throttle no popping. That's with a PCV & Autotune.
There is no correlation between the two factors. The emissions system is the key factor.

Let me provide a related example. At one point, I had 2002 Ford Focus ZX3. I got the modding bug while I had and replaced the entire exhaust system with better header, hi-flo cat, and a 2.5 inch flowmaster exhaust kit. Before the install, the car did not have any decel pop. After the install, it had the popping. Something about the exhaust swap caused the car to have or was it already there? the stock exhaust had a restrictive system comprised of non muffler and 3 mini cats. This, most likely, kept the decel pop to an absolute minimum. Once the less restrictive system was installed, there it was. And was it there at all? The two O2 sensors. One before and one after the 1st cat. If I had bypassed them, the popping would have been eliminated. I know our bikes don't have asystem like that, but it does refeed the exhaust back to the motor for reburning and some of it produces popping, especially on decel.

If simply bumping the idle speed fixed the decel pop, then no one would ever suggest costering / marbling the emissions system? But they do. Why, because it causes the decel popping.
 

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Ok, for shits and giggles, I spent 1 minute and found THISexplanation on decel popping. It should satisfy everyone's palette. When in doubt, search.

Seems Roman and I were right about how decel popping works.
 

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Well, with nothing to do on a hot summer's day I took the Vulcan out for a ride and purposely dropped the idle down to about 800 RPM. Sure enough the decel popping returned as I was dropping down the gears and coming to a stop. No high revs involved. I jacked up the idle at what I would guess to be 1000 RPM and finished my ride with 80% of the popping gone. Go figure.
Romans5.8, you nailed it for me. Cheers.
 
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