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Author [NL] [FR] [ES] [DE] [SE] [IT] [NO] [MY] Topic: Partial throttle 2800rpm Lean stumble,surge cure now with pics!  (Read 6847 times)

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Offline Airlock99

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So my bike an 08 US Versys has been surging and coughing at part throttle worst at about 2800rpm. Sure seemed like a lean stumble to me so i went looking for a cure. Lots of great info on this site but not a cure for this.
Removing the sub butterflies was not the solution though the improved responsiveness is very welcome.

So what it the fix you might ask, well before we get to that if you blow your bike up/void your warranty don't complain to me.  Ok so there are 2 potentiometers on the left side of the throttle bodies, these are connected to the butterfly shafts. They are held in place with torqx screws. The top one tells the computer where the servo is holding the sub butterflies and fast idle cam. Since i don't have sub butterflies any more there is no restriction on the air flow when i open the main butterflies and no ability to close off the intake should the computer decide im going to fast, all it does now is set fast idle cam position. Rotating the lower pot shifts the fuel mixture curve as a whole leaning or richening the mixture everywhere.

I don't have the equipment to sniff the AF mixture but what i can tell you is my bike was WAY lean at 2800-3000rpm. So warm up your bike throughly. Then turn off the bike  disconnect the plug on the lower pot,
the black plug its easy to get to. With a good multimeter check the resistance of the pot on the two outer pins write this number down if you want to go back to the stock stetting later rotate the pot to get this reading again.

Idle speed must be set the same when taking this reading and reseting it should you decide to, rotating the shaft by turning the idle screw will change this reading!!! Reconnect the plug start the bike set the idle about 2800-3000rpm with your torqx bit break loose the screw holding the lower pot in place and gently rotate the pot back and forth noting the effect on engine speed when you find the sweet spot where the speed is the highest lock the pot down and turn the bike off.

 Dont take all day about this we dont want the bike idling at 3K for an hour! but a few minutes wont hurt anything either. You will probably notice the rpm went up quite a bit when you did this let the bike cool a minute restart reset idle for 2800-3000rpm and make sure the pot is in the sweet spot for highest idle lock the pot down  reset you idle to 1300rpm and your good to go! Oh one more thing remember that top pot the on e on the sub butterflies shaft you will more than likely need to rotate this pot to set your cold fast idle unless you want to warm your bike up at about 4000pm  :5:


just loosen screw and rotate the upper pot to get the desired fast idle speed, i leave that speed to you to decide... For the two bike i have done this two the difference was huge millage does not seem to be hurt in fact the one that was coughing went up and best of all very butter smooth and no stumble surge!!! I suspect the mixture is not perfect everywhere, what stock fuel curve is? But i can live with a bike a bit rich on the top, i cant live with one coughing through the intersections...In the end this is much like adjusting a fuel screw... though this fuel screw changes the whole curve not just one area of it. Hope this help someone and sorry to be so long winded just wanted any one who tried this to have all the info they needed. :002:
Best of Luck Chad :031:
« Last Edit: Aug 29, 2008, 06:59 PM by Airlock99 »

Offline stevemersey

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Re: Partial throttle 2800rpm Lean stumble,surge cure.
« Reply #1 on: Aug 28, 2008, 11:21 PM »
Chad,
What is the main advantage of removing the subs? Does it improve airflow and power? Is there a downside?
Steve

Offline Tioli

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  • Tasmania Australia
Re: Partial throttle 2800rpm Lean stumble,surge cure.
« Reply #2 on: Aug 29, 2008, 07:06 AM »
One of those posts I had to print and file.  Will get into the detail if needed.

Thanks
Tioli

Offline bebbe

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Re: Partial throttle 2800rpm Lean stumble,surge cure.
« Reply #3 on: Aug 29, 2008, 02:06 PM »
Superb  :019:

But if you do it to more bikes, take som pictures. I'm having some problem to understand all in English  :187:

But I will probably get it sorted after reading it a couple of times  :015:

Offline Airlock99

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Re: Partial throttle 2800rpm Lean stumble,surge cure.
« Reply #4 on: Aug 29, 2008, 03:32 PM »
As for sub plate removal there is no improvement in horsepower, i doubt you or I could tell the difference on a dyno. Its more of a responsiveness thing very noticeable on the road..... er unless you work the throttle quite smooth and slow. The effect on acceleration is more like flywheel weight than anything, the sub plates/butterflies are controlled by the computer and they always lag behind what you do with the main butterflies. You can see this effect if you watch the right side of the throttle bodies and work the throttles, the upper shaft with the fast idle cam on it will always open a bit slower than you open the main butterflies. As a result if you demand open throttles and twist the main butterflies the throttle is not really open until the computer gets around to opening the secondary butterflies and lets the air into the cylinder.
Sorry to be so damn wordy with this... :180:
Hope that helps Chad :016:

Ps ill try to post some pics soon.

Offline stevemersey

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Re: Partial throttle 2800rpm Lean stumble,surge cure.
« Reply #5 on: Aug 29, 2008, 03:41 PM »
Chad,
Excellent stuff. it wasnt wordy..it was very clear.

Great contribution.
Thanks
Steve

Offline Tioli

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  • Tasmania Australia
I would like to see the pictures when you get them up

Offline invader

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  • Kapuskasing Ontario Canada
Here is a pic of my TB with sub-throttle plate removed... There are many more with instructions at  http://fortycaliber.org/
I also filed down the protruding metal shreds on the sub-throttle shaft's threaded holes.

Airlock99: Did you end up turning the TPS counterclockwise? Ignition timing advance is affected by TPS and RPM inputs. Your idle RPM went up by pre-advancing ignition timing. It's normally 10? BTDC @ 1300 rpm to 33? BTDC @ 5000+ rpm. You could check for changes at idle (and higher rpm's) with a timing light. According to the service manual, standard output voltage at main throttle sensor (TPS) is: DC 1.005 ∼ 1.035 V at idle throttle opening, and DC 4.2 ∼ 4.4 V at full throttle opening. Sub-throttle sensor standard output voltage: DC 0.4 ∼ 0.6 V at idle throttle opening, and DC 3.825 ∼ 3.975 V at full throttle opening.

Offline myorangecamaro

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mY bike has the same exact coughing problem....only that is a bit technical since i cant see the pics... is there any way you could email them to snowboardr12@hotmail.com?

Offline Airlock99

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Yes good thought on the ignition timing Invader  :028: Im sure it has been changed though i suspect a fairly small amount ill check with a light and see what the number is. Im sure the timing contributed to the increase in idle speed but really think most was due to the mixture... Other than changing the flywheel position im not sure how the effect of each would be tested separately though, as a side note i have not experienced any detonation before or after this change. I have email you the pics Myorangecomaro though i have edited the main post to include pics. Are they not showing up??
Chad