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Discussion Starter #1
I hoping someone here has the skinny on this little issue I am having with my 01 St4. Anyway here is the issue:

First off, I have D&D exhaust with upgraded DP chip, K&N filter.

Over the last couple weeks on random occassions I have had an acceleration hesitation when throttling on. This seems to occur just after warming her up and only happens one time and then its smooth sailing. The strange thing is that sometimes no issue initially and then I shut it down and do whatever and come back, fire it up after 10-30 minutes, runs fine and then 1-2 minutes later I get the hesitation. It stumbles and grumbles for less then 30 seconds and clears up with no further issues.

I'm thinking its some type of fueling situtation but not sure if its in the brain box or something more old skool mechanical such as hoses air flow or what have you.

Anyone experience soething similiar that they figured out. I don't think its anything too too serious but would like to figure it out since its annoying and potentially dangerous when you get on the throttle and sputter, sputter, sputter happens.....

Thanks in Advance Boys...........
 

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Thermostat or temp-sensor maybe? Only reason I think this is because it only happens once (when the thermostat opens, possibly).
 

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Have you tried new plugs or checked the breather hose for mis-routing ?
 

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Thermostat or temp-sensor maybe? Only reason I think this is because it only happens once (when the thermostat opens, possibly).
I'd check this first too. If it continued with a leaning out issue my bad experience was with the air pressure sensor. One thing you could do is unplug the temp sensor and try it and see if that makes a difference.
 

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I had a problem like this on my ex 916 at the race track. It would hesitate on acceleration, but it was a random problem. It was not consistent. I checked all my temp and pressure sensors. Found the pressure sensor to be bad. They are easy to check with a voltmeter/ohmeter. Usually they either work or they dont. I do have the sensor test procedures from the workshop manual on PDF. Let me know if you want it.

Mike
 

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Have you tried new plugs or checked the breather hose for mis-routing ?
I would check all of the hoses that vent the fuel tank. Also check the charcoal cannister to see if it is saturated with gasoline.
 

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If it continued with a leaning out issue my bad experience was with the air pressure sensor. One thing you could do is unplug the temp sensor and try it and see if that makes a difference.
It would hesitate on acceleration, but it was a random problem. It was not consistent. I checked all my temp and pressure sensors. Found the pressure sensor to be bad.
You guys just gave me something else to check on my wife's ST2. It will hestitate under acceleration on occasion, but it is not consistent. It is almost like dropping a cylinder for a few seconds, and then it clears up and never causes another issue....for a while. Sometimes when riding in the mountains and riding hard, I can get it to act up several times in a few miles. Other times, with regular and casual riding, I can get it to act up once every 70-150 moles.

I tried differences with the airbox (stock vs chopped), filters (OEM vs. K&N), chips (stock, Euro, and aftermarket), swapped out timing sensors, and pipes (OEM vs. D&D). I finally got fed up with it and figured I'd just ride it until whatever it was finally broke. I installed the chopped box lid, Triton eprom, and the D&D slip-ons and the bike ran great for about 500 miles before it started acting up, again. I think I have a spare pressure sensor in my toolbox or can pull the one off my other ST2. I'll have to swap that out and see if that makes a difference. Thanks for another thing to check.
 

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I'd go with the temp sender to the ECU. This one is on the vert. cylinder plumbing, right behind the "X" is the frame. It's tough to get to with the body work on, but can be done if you remove the large side fairing bolt, and 2 of the belly bolts. Then flex the fairing out a bit, and shove something soft in there to keep it out (like the roll of DUC tape).

Unplugging the sender will make the ECU think that the coolant (engine temp) is COLD.
See if that get's rid of the intermittent. Speaking of which, these senders are a known sore-spot with me. BOTH of them were flakey on my 1998. Replaced them with the Bosch senders (do a search in here), and she was like new!

If you want to experiment, a 220 Ohm resistor makes yields a coolant temp of 207 F.
You can verify this by plugging the 220 res. into the ECU readout sender's plug. It's on the horizontal cyl., and easily accessed thru the side air conveyer opening in the fairing.

Note that the ECU controls when the fan comes on, so when unplugging or substituting the resistor, the fan will never turn on. Should not be an issue unless you are in a big city. BTW, a 150 Ohm resistor into the ECU connector will force the fan to turn on...

Also note that while these things can be flakey, I'm convinced that mine were reading back an erroneous temp., about 25 or 30 F too cool. So my fan was actually coming on at over 250F. !!!
and note that on my readout, I would get no reading (or a flashing "LO"), but if/when allowed to get hot, then I'd see a temp reading. Sounds to me like that's what your ECU sender is doing--unstable resistance value, and then stable after some warm-up period.

Let us know how you make out.
 

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I'd go with the temp sender to the ECU. This one is on the vert. cylinder plumbing, right behind the "X" is the frame.
<snip>
Note that the ECU controls when the fan comes on, so when unplugging or substituting the resistor, the fan will never turn on.
This reminds me. IIRC, I checked this over a year ago when I first got the bike. This bike has the fan kick on for about two seconds when I first turn the ignition on, so I was trying to troubleshoot that, also. IIRC, there are two sending units that are the same, and I checked both. My memory won't let me remember what each one does. I know I did not check the temp or pressure sensors back then (I did check them on my 748 when I had an issue with that two years ago, though).

I may have to find some time to check or swap out the pressure and temp sensors. Having two '98 bikes can make troubleshooting easy and without any cost. ;)
 

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Another place to look is the large connector on the left side below the fuses...this connects the rear harness to the front. look for green gremlins. One gent in here looked, and found a broken wire (snapped off the pin that's on one of the connectors).

But since the original symptom seems to go away after a bit, I'd lean towards temp. sender.
DUCATIMIKE:
Barometric pressure is another interesting input. I'd be interested to see what's in this procedure--how can I obtain a copy of what you have?
 

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Discussion Starter #11
Thanks fella's I'll look to these areas this weekend and report back.
 
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