99 VFR dies on hard acceleration

Hello all,
Just joined here as I have been troubleshooting my 99 VFR and figured I would offer it up for input from the wise ones before I start to tear into more stuff on the bike.

Currently I have a 1999 VFR 800 with around 34k miles that I have owned for about 3 years. I live in the north so the bike wintered in my heated shop, intermittently on a battery tender. When I got it out this year I have been having issues as the title says with it stalling only on hard acceleration. It was running and riding fine before it was put away with a small amount of ethanol free fuel in the tank (no stabilizer).

Symptoms are as follows:
-Bike starts and idles fine, and revs as usual through all rpms, responsive to both gentle and quick inputs.
-When riding with gentle throttle (1/4-1/2) will accelerate through all gears and rpms
-In any gear however if you roll on the throttle quickly or open wider under load the bike will immediately and completely cut out.
-After stall gauges and lights are still on but it needs to be completely turned of, turned back on, and allow fuel pump to prime again before restart.
-I have tried to "bump start" by dropping the clutch in a lower gear after stall however that is unsuccessful.

Things I have tried so far:
-drained fuel and refilled with fresh ethanol free
-added injector cleaner to tank, ran through about 1 gallon of fuel, and let sit for about 2 weeks
-replaced fuel pump and filter
-without removing more than the tank have briefly checked hoses and wiring
-recharged battery on tender

I also have fresh spark plugs and oil for the upcoming season but haven't gotten to it yet.

My next thoughts were possibly the injectors or fuel pressure regulator? It seems odd that it stalls so dramatically rather than just stumbling and losing power.

Thanks for any suggestions on next steps.

 

Check coils/wires for spec?

 

Maybe the tipover switch on the front subframe is loose; that would also explain why you need to reset the ignition to get things started again.

You could also have an intermittent fault at something like the sidestand switch that is causing the ignition to cut out in gear.

 

It could be something dumb, like an airbox full of mouse nesting material.

If I translate your description of the situation into "fuel-air-ratio" speak, you said, "the engine can produce a usable fuel-air-ratio when the demand is kept at a low level, but when I ask the engine to take a big, deep breath it just can't do it, something is preventing enough air from entering the system while the Honda PGM-FI system is commanding a high rate of fuel input (due to the wide open throttle position) and the engine instantly goes over-rich and can't sustain combustion and ends up shutting down."

I'm betting in this direction because you didn't mention the PGM-FI system annunciating any trouble codes. This means that your bike's problem is something that's being caused by something that the PGM-FI system can't sense (like a TPS, throttle position sensor, problem or a MAP, manifold absolute pressure sensor, problem).

Also, a sudden "dramatic stall" WITHOUT popping or banging indicates an over-rich condition. A sudden too-lean condition is usually accompanied by a pop or bang.

 

So the airbox is clean. That was one of the first things I checked. I am not sure about the FI codes because a previous owner put on a gp style gauge after a shipping mishap. I will check the tip sensor and kickstand connections. It seems like after it occurs there is no fuel and or spark until I reset the ignition so I’m starting to wonder if this is an electrical issue.

 

Okay, good to hear you've checked and there's no mouse nest in the intake.

(Now class, if you would all please open your 1998-2001 Honda VFR800 Service Manual to chapter 21, Technical Features, on page 11 you will find the section labeled "Basic Injection Volume" which describes how the ECM determines how much fuel to inject)

Here's something most people don't know about their VFR800: As operating conditions vary during a ride, the VFR800 uses two completely different methods of determining how much fuel to inject.

At low RPMs/low throttle demand the Honda PGM-FI ECM determines "Basic Injection Volume" by using the current value provided from the Intake Manifold Absolute Pressure Sensor (MAP).

At high RPMs/wider open throttle demand the Honda PGM-FI ECM switches to using the current angle value of the Throttle Position Sensor (TPS) to determine "Basic Injection Volume".

I'm wondering if your VFR800 has a problem with its Throttle Position Sensor. As you have described, your VFR800 runs just fine at low throttle openings and low demand, this is when the ECM is using the MAP sensor. Then, when you put a wider throttle opening and high demand on the system the ECM tries to switch to using the TPS for fuel computations and something prevents this from happening successfully.

There is a fuel system trouble-shooting section in the Service Manual Chapter 5, the specific entry for trouble-shooting the Throttle Position Sensor begins on page 5-18.

 

Agree with Terry. At least the Bank Angle Sensor or Tip Over Sensor needs to be checked or isolated.
Another clue that could point to the BAS is when the bike stalls, is the Fi light On?
And cycling your Ignition primes the bike, Fi light goes out and all appears normal - Yes? A stall shouldn't require you to cycle the Ignition, just pull the clutch press the Starter and Go
For test purposes only, it is very easy to bypass the BAS as this just provides a ground for the ESR engine stop relay coil to energize it. Refer your wiring diagram.
You can simply unplug the BAS and place a jumper between the Green and the Red/Orange wire of the connector (wire harness side connector). See how it goes, just don't drop your bike!! BUT make sure the Green BAS wire is properly grounded, make sure it has zero ohms back to the battery Negative terminal.

Are you sure you are not seeing any active Fi fault code when the fault occurs? You'll need to have Sidestand down when the fault occurs before the Fi light will go from fully On to flashing a code.

If the BAS has been isolated TPS should be checked.

 

UPDATE and successful fix:

Everyone's hunches were correct about the BAS. As I mentioned before this bike was purchased as a fighter after all of the fairings had been removed due to shipping accident. Without the stock cluster and FI light I just started going through all of the trouble shooting checks as it seems the aftermarket gauge was not giving me the codes correctly. I also could not find the BAS in the front of the bike and assumed the previous owner had bypassed it when he redid the wiring harness/headlights/speedo. As I was digging into the back of the bike and tracing wires however I found he relocated the BAS and it had broken loose so that under hard acceleration it would trip. Bypassed it for sure and no more issues.

Thank you everyone for your thoughts.