R1150 Overrun Fuel Cutoff

[roger 04 rt]

When I first rode my 2004 R1150RT, one of its most startling characteristics is how rapidly it decelerates from cruise when you closed the throttle quickly. I have read that this function goes by various names but I'll call it Overrun Fuel Cutoff (OFC).

 

It is written that this function aids in fuel economy, recharges the Cerium layer in the CAT with oxygen, improves deceleration (you can say that ) and in some motorcycles, reduces exhaust system backfiring.

 

I was reading through my GS-911 data lists last night, a slow manual process, and made some observations:

 

--When you drop the throttle from cruise or acceleration, within a second, the fuel and spark go full off--nothing.

 

--The fuel stays off until the RPM drops to 1800 RPM and then the fueling and spark come back on.

 

--Fast idle, which is TPS=1.28 degrees on my bike and about 1500 RPM when warm, does not block OFC. There was no data above 2 degrees that resulted in OFC coming on.

 

--When the engine is cold, OFC is still operational.

 

--If the throttle were dropped to 2 degrees open (as opposed to 0.32 degrees which is close throttle on my bike), I believe that OFC would not happen.

 

--If you drop the throttle, and then open it a couple degrees, OFC is canceled. (But feels a bit jerky to me.)

 

--Many times when we shift, the OFC function comes on for 3/4 second, give or take.

 

I think I've adapted to OFC fully with my driving style but I was used to motorcycles that lacked this "benefit". But, I can't think of how this function could be disabled if we wanted to try the bike without it.

[Tobias]

Interesting find,

 

Makes me wonder if this is why my RT shifts differently from every other motorcycle I have owned.

[RPG]

are you offering that this is a problem or just an observation?

 

I don't see it as an issue at all, owning a '04 RT and a couple of older Airheads.

 

RPG

[roger 04 rt]

are you offering that this is a problem or just an observation?

 

I don't see it as an issue at all, owning a '04 RT and a couple of older Airheads.

 

RPG

 

Observation/information. There are so many of us who feel we don't know what the Motronic is doing that I'm trying to decode the major pieces.

 

For the first few weeks after buying the 04RT, I wondered what made this bike so different from others I owned, when I closed the throttle. It seemed very abrupt in its deceleration characteristics. Now, I've learned how to use it to advantage but it took me a while.

 

 

Since I have this wealth of data, I was curious about when the Motronic invoked the OFC function. It took about 20 cases to decipher the rules. The only surprise was the momentary OCF during shifting--I wonder why that was beneficial. But I think I've even adapted to that by unconsciously not quite letting the throttle fully close while I shift.

[Quinn]

The deceleration does seem very abrupt. Guess I was used to vehicles with carberators that lost vacuum more gradually when the throttle was slammed off. Now, at least, I know why it feels better to not close the throttle all the way when decelerating before a turn in. Thank you for the information.

 

 

---

[Michaelr11]

The deceleration does seem very abrupt. Guess I was used to vehicles with carberators that lost vacuum more gradually when the throttle was slammed off. Now, at least, I know why it feels better to not close the throttle all the way when decelerating before a turn in. Thank you for the information.

I go back and forth between my R1100RT and my Airhead bikes. It has taken quite a while and still takes a careful hand to be smooth with throttle transitions on the fuel injected R1100. The carbs on the Airheads are very smooth on and off the gas.

[JamesW]

Michael, I agree I have the same problem going from my '81RT to my '04RT. For 8 years I have never been able to do a consistent smooth shift with the '04. Seems if I ride with the throttle positioner in the fast idle detent I can do pretty good. Now the funny thing is my recently acquired '93 1100RSL shifts very much like the '81RT in fact I would say it is just as smooth. Go figure.

 

I wish there were some way to defeat this fuel over run cutoff. The '04 seems to loose RPM very fast with just a slightly reduced throttle. Very fast as in right now.

 

[roger 04 rt]

I should have titled this thread, "Overrun Fuel Cutoff, Friend or Foe."

 

Now that I know how it works, and I went out and practiced, and now my shifting is as smooth as on my former carburetor fueled bikes. Since I've only owned this motorcyle for 8 months, I'm probably the last person that should explain how to manage the Overrun Fuel Cutoff (OFC) feature but now that I see how it operates I'll give it a go.

 

It's worth mentioning that it was a while before I realized that the Motronic regularly SHUTS OFF the motorcycle while I was driving it--no spark and no fuel equals SHUT OFF to me.

 

Recapping the OFC algorithm:

 

If RPM is falling but above 1800 RPM, and

If the throttle is closed to less that 2 degrees

The Motronic SHUTS OFF the motorcycle until:

 

Either the throttle is opened up more that 2 degrees (just a bit more than the Cold Idle lever), or

Until the RPM has fallen to 1800 RPM

 

Deceleration

When you want to decelerate by closing the throttle, relax the throttle a bit till all the backlash in the drivetrain is taken up and the motorcyle justs starts to slow. Then for fast deceleration you can then drop the throttle to the stop without feeling like you've been thrown forward. Or you can drop the throttle almost to the stop (2 degrees open) and get a more gentle slowing. Or you could start out gently for a while and then drop the throttle. This gives you two deceleration speeds.

 

Accelerate after deceleration started:

If you've reached OFC mode, crack the throttle so the engine can RESTART, and then open the throttle as fully as you want keeping in mind takeup of the drivetrain backlash.

 

Shifting

If you drop the throttle to the stop as you pull in the clutch (above 1800 RPM) the Motronic SHUTS OFF the bike. After you shift and as you let the clutch back out, first crack the throttle 2 degrees so that the motor is restarted, and the finish the shift normally.

 

After an hour of driving with this new idea in mind, my shifts were smooth and on-par with a non-OFC vehicle.

 

 

[JamesW]

Morning to you Roger.

Interesting thought on how to smooth shift the '04RT I will give it a try. This certainly does explain why I have never been able to shift my '04 smoothly. On my '93RSL smooth shifting is not an issue as it shifts just like my '81 airhead. Must be this cut off feature came later. Really wish one could disable this engine cut off but I think it would require internal surgery to the motronic. Not going to go there.

 

Thanks again for your time and expertise, you also DR.

[Boffin]

If you drop the throttle to the stop as you pull in the clutch (above 1800 RPM) the Motronic SHUTS OFF the bike. After you shift and as you let the clutch back out, first crack the throttle 2 degrees so that the motor is restarted, and the finish the shift normally.

 

After an hour of driving with this new idea in mind, my shifts were smooth and on-par with a non-OFC vehicle.

 

 

You are shifting too slowly. On an oilhead a gearshift should take a few hundred milliseconds.

 

Up-shifting. Do not close the throttle, dip it down and up. At the same time part-pull and release the clutch. At the same time pull up the gear lever. This should all take less than 1/4 second.

 

Down-shifting. Do as above except throttle up then release - this is a slightly slower operation, but not much.

 

I find clutchless up-shifts the smoothest - just slightly dip the throttle as I pull up on the lever.

 

Why mess with the fuel-cut-off which saves a LOT of fuel when fixing your riding technique is cheaper and easier.

 

Andy

[roger 04 rt]

Hi Andy,

 

My first clutchless shifts were on the flatbed farm trucks I drove back in the 60s. Man, double-clutching those beasts was tough for a 15 year-old. Even without synchronizers, with some care you could get the shifts slick as ...

 

To your point though, my plans are to leave the OFC just as it is. I was more interested in how/when it operated. Some days I'm flying and the shifts are quick. Other days I'm poking around ...

 

The two-stage deceleration is kind of a neat aspect of the OFC.

 

How're things up in Leicestershire these days, I traveled for about 20 years to Woking in Surrey--nice countryside a little west of there.

[roger 04 rt]

Hi Andy,

 

A little further clarification ...

 

It's not that the engine gets to idle during shifting (unless you take a nap) but that when you shift, as you describe, IF the throttle gets below 2 degrees and it does sometimes even on a quick shift, that the OFC kicks in. It kicks in because the rpm stays above 1800, not that it gets down to 1800. In that sense OFC is ON because the shift is quick.

 

RB

[roger 04 rt]

For another reason today I was looking at the ignition circuit and I noticed that the driver for the secondary ignition coil was also connected to the tachometer. This led me to conclude that the spark must stay on during OFC. So I went back and looked again at my data, sure enough I had been looking at the wrong column--the spark does stay ON during OFC. The spark timing though shows a firing angle that is between 3 and 6 degrees, lower than idle. Sorry for the mistake.

[azccj]

When I first got my 04 RT last rear, one of the things I noticed was the amount of engine braking the bike had. After reading this thread I decided to see if I could feel the engine coming back on at 1800 rpms. Last Tuesday while out on a ride, I was in 2nd gear at 3500 rpms and let the bike coast with the throttle closed. Right at 1800 rpms I could easily feel the engine come back to life. It felt as if I had pulled in the clutch or no longer had the same amount of engine braking.

 

I guess this could be a real fuel saver if you ride down a lot of very steep hills.