Hesitation on WOT

[Timmer]

On the way to Torrey my 1200ST developed a "hesitation" when WOT (wide open throttle)was quickly applied. The engine responds close to normal if throttle is applied in a slow smooth manner. Gas mileage was the same for the rest of the trip, and the bike will still go to 130+ MPH as long as the throttle is applied smoothly. It was only a problem when I wanted to make a quick pass around a car.

 

At first I thought it might be the altitude, but then I've been over high altitude Sierra passes with the ST and it never did this before. When I got back down close to sea level the problem remains.

 

Someone else who rode it described it as if the rev limiter was kicking in, but the rev limiter isn't as rough as this feels. The engine just seems to bog down. The hesitation would last as long as I kept WOT. As soon as I back off the throttle a bit, it accelerates smoothly again.

 

Any thoughts?

[ClearwaterBMW]

Left side of bike, on top of the engine pod: air return tube

did it loosen up and fall off like mine did?

i used a twisty tie to put it back on and keep it on

 

at work, so have no pic now

i have the new GS.... same motor

 

check it out

again: left side of bike

greg

[Timmer]

Greg,

I checked and it's all connected. Looked for other loose connections both electical and non-electrical, but they all seem to be fine.

I guess we'll let the pros figure this one out.

Tim

[ShovelStrokeEd]

Tim,

The symptom you descrive is a classic one of too small a main jet. Bike stutters and misses at WOT but runs better when the throttle is backed off a bit.

 

Since there are no main jets to deal with there may be other problems. Fuel filter comes to mind or fuel pressure regulator. It is also possible that the screen on the fuel pump has become fouled. Time for some diagnostics.

 

First, a fuel pressure gauge, T'd into the line between the regulator and the injectors. Now observe fuel pressure, should be about 45 PSI or so, as throttle is wound open. If it maintains, that ain't the problem. If it drops, chances are it is the screen on the inlet to the pump as a clogged filter will still, for the most part, allow pressure to continue, up to the limits of the pump to overcome the restriction.

 

Secondary could be a fuel line that has not quite come adrift but is leaking enough to limit fuel flow to the injectors. That should show up in the test outlined above.

 

It could be ignition as well as combustion chamber pressures go way up at WOT and a failing coil might just manifest only under that condition. Ditto plug wires as to overcome the increased chamber pressure it requires more iginition voltage/current.

[Timmer]

Ed,

I first thought it was bad fuel resulting in a clogged filter at first, but two other guys I was riding with filled up and the same place and experienced no problems. This problem began after about 450 miles into the Torrey trip last Thursday. The problem was first noticed after about an hour of running at high speeds.

 

I don't have any way to check fuel line pressure, so I'll need to leave it to the dealer.

 

I'll try to check more as time and resources allow.

 

Tim

[Corkus]

Tim, please do post about how this goes.

 

Good luck. Hope it turns out to be trivial.

[Timmer]

Well, we finally figured it out.

 

It was a bad ignition coil to the right cylinder main spark plug.

[Corkus]

Bad ignition coil. That was a nuisance I'm sure, but nothing to worry about.

 

Cool. That's good news.