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Strange RPM drop-off with a corresponding surge effect when adding a bit of throttle (at only 5500 RPM)


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G'day, gentlemen. I have a recently acquired 2002 '6th-gen', California model VFR that I am just getting used to. Previously had a yellow '5th-gen' 2000 Viffer that I loved (but stupidly sold!). The new 2002 Viffer has the controversial VTEC system, of course, which I embrace rather reluctantly...but went ahead with the 2002 purchase anyway, hoping for the best. The bike has 32687 miles on the clock and was previously used for highway commutes by its prior  owner (so the story goes).

 

At any rate, I find that at an RPM of about 5500 (more or less) and in 3rd/4th gear, the throttle is a bit difficult to keep steady; this is somewhat annoying to me. It tends to want to slack off by itself, so when I apply just the slightest bit of throttle, I consequently notice a bit of a perplexing surge in the bike's response. Whether this is VTEC related or not (or fuel mixture related), I have no best guess to offer. As far as I am are also, the exhaust system is stock 2002 California VFR spec (not a custom exhaust, with its 'leaner running' tendencies). My understanding of the 'stock' OEM specs for the 2002 Viffer is that VTEC is supposed to cut in at about 7000 RPM, not 5500, hence my uncertainty as to what's causing this minor glitch in the power curve.

 

My 2000 Viffer only had 8000 miles on it (yeah, as I said: STUPID-STUPID-STUPID!) and its throttle was rock-solid, smooth as silk and absolutely non-problematic! Now, 32XXX miles on a Viffer is nothing to write home about, but I can't help but wonder if the VTEC issue is raising its (arguably) ugly head here, or whether I might have another element causing this throttle instability.  Any ideas or opinions would be welcome. Thanks.

 

 

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Hi, greetings & salutations :biggrin:

 

As quick test, disconnect O2-sensors and test again for stumbles @5500rpms.

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Thanks. I'll do that and get back w/the results.

Cheers, K2

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VTEC = Vacillating Torque Engine Compartment... Honda car owners love it but it's a polarizing subject for their bike customers...
I think Honda learned that their linear V4 power ban is more popular than their engineered bump in the power band for the sake of mileage...
 
My friend Makota San previous job was Chief Engineer Honda R&D who
invented Honda's VTEC... he calls VTEC "his baby" and recalls his boss
being super skeptical of the idea working at all...
 
gallery_3131_5511_17541.jpg

Makota San down on cannery row...
gallery_3131_5511_48694.jpg
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At 5000rpm VTEC is not operating. The early VTEC 6gens had a noticeable transition from 2 valve to 4 valve operation, refined and smoothed out somewhat in later models. Don't think you have a VTEC issue.

You may have the signs of a dirty fuel filter, bad fuel, or excessive water condensation in the fuel.

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Assuming it's the VTEC system is IMHO going down the wrong rabbit hole.  Before jumping to conclusions, start with the basics.  Anything and everything could have happened to this thing in its time with the P.O.  Check for clean, fresh fuel, clean filter, secure vacuum hoses (no cracked hoses and that all are present and in the correct location), no "performance mods" the P.O. may have thought a good idea, all items around the airbox present and tight (sensors, lid tight, air horns secure, etc),  battery checks good on a load test, main fuse and wiring at starter relay is good,  etc.  Without all those, it will not perform up to expectation.  You don't want to chase symptoms but narrow down possible causes.  Now that these bikes are closing in on 20 y.o., they can have poor fuel spray patterns as fuel has been sitting in them for long periods of time.  The ECU can't know that and the system is not very sophisticated by modern standards.   Cleaning my 6G injectors did a world of good for smoothing out the power delivery. 

 

Also, does it have a Power Commander or similar module attached to the harness on the right side of the airbox? 

 

My editorial on 6G VTEC is that while it seems to add nothing incremental vis a vis a 5th gen experience (power, delivery, rideability, etc) , it works fine as long as everything is in good working order.  My main beef with it is the complexity of service and the ridiculous cost of the  VTEC buckets if they need changing (around $32 to $40 + each!). 

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Kalikiano, what you're describing is the ECU going into closed-loop control with the O2 sensors, and it is 100% normal behavior for a 6th gen unfortunately.  Between 4K and 7K RPM (forget the exact numbers, but around there) at steady cruise, the ECU will lean out/dial back the mixture.  This is noticeable at the throttle where the bike leans out, you advance the throttle, it catches up and...wash, rinse, repeat.  There is a super easy fix the message boards finally discovered ten years after we first bought these bikes though.  You can defeat the closed loop mode very easily:

 

- Turn on bike, key on, kill switch on.  Let fuel pump whine and dash light process complete.  Now turn off at key switch.

- Turn back on, hit start button before whine completes.  Now when you ride, the bike will stay in open loop (fixed fuel injection map).

- The throttle lag/surge should be gone.  

 

I did all of the other things we do around here including O2 sensor eliminators.  None of it works.  I used to do starter valve syncs regularly and a lot of other over the top maintenance trying to make it all better but it's just how it is.  It is not related to VTEC in any way, this is just early motorcycle fuel injection technology, and an early attempt at closed loop control on a bike.  I only discovered the start up/interrupt ECU method last year when I re-purchased my VTEC back, and sure wish I'd known about this 18 years ago.  I can tell you that once you do this a few times, you will notice right away when you forget to do it and the ECU comes back trying to manage you on the highway.

 

There is a more permanent solution, and that's the Rapid Bike Evo or Racing module.  Either of those will trick the ECU, take over the O2 sensor inputs for themselves, and manage your inejction and ignition in real time.  I put mine in last week and no longer have to do the key dance.

 

The PC3 and PC5 will do the same thing but they are older stuff, and people have had to futz with them a lot to get all of the bad behavior out.  YMMV.  I was very satisfied with the key-dance but happier with the bike overall with the Rapid Bike Racing in it.  

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Excellent response Patrick... you're the VFRfixer as well as ships... so thanks for pointing to the problem... I saved it all in my VTEC file...

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On 5/1/2021 at 2:05 PM, BusyLittleShop said:

Excellent response Patrick... you're the VFRfixer as well as ships... so thanks for pointing to the problem... I saved it all in my VTEC file...

Wish I could say I was the one who figured this out, but someone on the other VFR forum just randomly chanced across it when they short cycled the start sequence.  But glad it's there!

 

Some really interesting things occurred to me with it though, like even with the O2 sensors at steady signal, Honda obviously still has logic in there to dial back mixture when the bike is at steady cruise.  Probably for overall emissions control, and maybe to protect the catalytic converter.  I had completely forgotten I did the O2 eliminator thing until I hooked up the Rapid Bike unit...the closed loop behavior was still there.  

 

This also means, at least to me, that unless a Power Commander, Rapid Bike, etc. type solution is completely in between the ECU and the injection and ignition systems, taking its own sensor input in from throttle, O2, MAF, etc, it can't ever really be perfect at cruise.  Rapid Bike does it for sure, and in 2021 that's not too big of a technology ask.  I know of some very fast FPGA-based devices that sit between much, much faster radio frequency systems for comms and radars that are also sort of correcting OEM logic in a way, and they have been around for 5+ years.  But I don't believe PC3 was doing it.  I could be wrong but I think it just applies a fixed correction map on top of the inputs and outputs of the ECU, so the ECU could still be dialing back the mixture and the PC3's output with it.

 

That would explain all of the complaints about PC3 and O2 eliminators over the years, and the debate on VFRD as to whether they work or not. 

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