Fueling under 5k is appalling

I've got a gen 6 with just over 10k miles and I find that the fueling under 5k is appalling.

Taking off in 1st is ok most of the time, but in pretty much all gears it bogs down quite badly if it's runnung under about 5K. Over 5K it seems fine - I can roll on the throttle and it just responds.

So...

Is this normal? Should this be something simple to fix? I haven't done any mods myself but I also haven't lifted the tank yet to see if the old owner did any himself.

Any thoughts?

//db

 

The VFR's Honda PGM FI runs in 2 different modes depending on RPMs.

At lower RPMs (where you're having problems) the system uses a "speed density" approach which determines fueling based on the signal provided by the Manifold Absolute Pressure (MAP) sensor. So it could be that something is affecting the accuracy of the bike's MAP sensor.

Also, there are inputs from other sensors that are used as "trims" in the fueling algorithm (they are little nudges in one direction or the other, making the mixture richer or leaner depending on the situation).

For instance, the PGM FI pays attention to feedback delivered from the oxygen sensors mounted in the exhaust system. If one or both of these O2 sensors was out of calibration it would negatively impact the math of the fueling algorithm, possibly trimming it in the wrong direction.

The fact your VFR isn't putting out FI malfunction codes suggests that everything is working, but something isn't working quite right.

Could be innacurate O2 sensor (one or both)
Could be an out-of-calibration fuel pressure regulator
Could be the MAP sensor isn't accurate
Could be dirty injectors (one or more of them)
Or maybe you've got corrosion in your bike's wiring harness (usually it would be see in a connection plug) that is affecting the voltages transmitted by one or more of the sensors.

But the fact that everything goes to great when you rev over 5,000 RPM makes me very suspicious of your bike's MAP sensor.

At higher RPMs the fueling algorithm is switched to "Alpha-N" which uses the throttle position sensor in place of the MAP sensor as the main parameter to determine how much fuel to inject.

Good luck with your troubleshooting.

 

Also check for vacuum leaks.

 

Thanks for the ideas.

SO... would an airbox mod possibly be a reason for this sort of thing?

 

Maybe if the airbox has been seriously f-ed with. There's a lot of stuff connected to it.

If you carefully read Chapter 21 of the VFR800 Service Manual (titled "Technical Features") you'll learn a lot about the engine (I'm assuming that Chapter 21 in the 6th Generation VFR800 Service Manual is the same chapter subject matter as it is in the 5th Generation Service Manual).

 

Ok, cool. I have all the original documentation for the bike so I'll take a look. I really need to get into the airbox and see what's happening in there. I strongly suspect it's either been messed with or there's a rat's nest in there. Lol. Regardless, I want to replace air and oil filters with K&N and replace all the fluids so I know what's in there. I just haven't had time to do since I picked her up.

I'll check a few things and let you know what I find. If nothing obvious, I'll take it to a tuner and get them to shack it on a dyno and see if they can work out what's happening.

 

Careful with K&N Oil filters. They tend to leak.

Also, unless you get the rare K&N Air Filter with the larger filter area it's not worth the extra $$.

regarding the fueling under 5k I noticed the same thing when I went from a 4th gen to a 6th gen. The fueling on the carb'd bike was beautiful compared to the PGM-FI of the 6th.
Oddly enough the 5th gen felt a bit better in the lower RPM's compared to the 6th.

Did you go from a different bike to the 6th gen?

 

Last bike I had was a K7 GSXR-1000, which was fuelled perfectly. Before that I had an '07 Fazer S2 which was also fine and I never noticed anything 'off'.

It's just weird, I test rode a couple of other 6th gen bikes and they were fine, so I assume something's amiss somewhere. Like I said, I still need to give it a good once over myself and get everything the way I want it and then I can really start troubleshooting.

I've always run K&N on all my bikes and never had any trouble with leaking, but thanks for the advice. I'll keep an eye on it.

 

Sounds like you know what you're doing. Could very well be the previous owner did something to the airbox like you suspect.

The K&N oil filter issue is usually a leak or catastrophic failure around where the nut is. I've done oil analysis tests with my VFR's and the Honda OEM Oil filter works very well.
But it's your bike

Keep us posted.

 

I'm voting for the 'rat nest' theory. Lol. We'll see.

 

You mean you haven't looked into it yet? That it's so recently new-to-you there's no idea of what's "under the bonnet"?

It's just me but I can't bring myself to ride a newly-purchased used motorcycle until I've gone through it thoroughly.

 

With a multitude of quality oil filters around K&N is NOT one of them. Stock Honda is good, check this forum for the "Oil Thread".
This is why we don't like the K&N, almost lost one of dear members!

 

Ouch! Fair enough. Lucky to keep that one right side up and not on fire.

 

Ooooook. So out of curiosity, I lifted the tank to see what's happening under there and guess what? It looks to me like the flapper mod has been done since the hose has been disconnected. I can see the other hoses are all still connected and the snorkel is there too so the PAIR hasn't been done either. Would that alone be enough to mess with the fueling?

Not sure what the stock Honda air filter looks like but the one that's in there is bright red and looks pretty new to me.

My helper (son) took a couple of pics while I held the tank up. Will post them once they've uploaded.

 

Where can I find the MAP sensor?

 

This shows where all the PGM-FI components are located.

http://www.lifford-cooke.com/motorbiking/documents/VFR VTEC PGM-FI Diagram 2002.pdf


SkiMad