5th gen rough under load below 3.5k

Hello all! First off, wanted to thank y'all for such a great site and excellent tech discussion. As a disclaimer, hoping it had been discussed already, I tried searching the forums (as well as "the other VFR site") for my issue but found nothing. Here goes.

I bought a '99 VFR800 (so, 5th gen) about three months ago with 62k on the clock (now with 67K). It's a California bike. Mechanically it's stock except for:

• Remus exhaust
• Iridium spark plugs
• Part of the emissions system looks like it's been removed

The bike runs fine, except for the following case: at 3500 rpm and below, if the engine is under any load, it runs rough and stumbles a bit. Not like a "running on two cylinders" thing, but almost like it's not producing enough power. As you can guess, I experience this often in start/stop traffic, stop lights, and parking lots. As soon as the engine hits 3500-4000 rpm, the problem vanishes and the bike runs fine. So I'm forced to rev it up when taking off from lights, and fan the clutch when going slowly, otherwise it will stumble. It can stall this way, typically if I take off from a light without giving it enough gas to get it up to 4000 RPM.

Doesn't matter if the engine is warm or cold. Idle seems pretty smooth; I haven't heard another VFR but the needle bounces around maybe +/- 20 RPM. In neutral, revving the engine does not cause this "hiccup," so it's strictly when under load. It hasn't gotten any better or worse with the 5K miles I've put on it. Keeping the choke on full seems to make it less obvious, but I really will have to double-check that, as it might be all in my head.

During the sale, the PO mentioned two things: the roughness was probably due to a "pinched vacuum tube." He also said that the iridium plugs only needed to be cleaned, never replaced. In an attempt to quash this bug, I've done the following:

• New battery
• New voltage regulator (yes, used the "fixed" one)
• Cleaned contacts to ignition coils and ECU
• Using vacuum tool, verified "flapper" valve in airbox works fine. Snorkel is intact as well.
• Examined air filter media; looks fine
• Removed air box and completely cleaned it
• Cleaned out crankcase breather hoses
• Checked choke operation
• Checked hoses for cracks and pinching (the hoses I could find, at least)
• Checked fuel lines, etc. for pinching

None of these things made any difference. At this point, I'm stumped. :unsure: The only things I can think of would be possibly the CARB (California emissions) mods and/or the Remus... maybe the plugs are dirty as well?

To describe what's been done to the emissions system: the charcoal canister has been removed. The PAIR valves seem to be intact; their hoses run to the four-way-splitter thing under the seat. Instead of a hose coming out of the splitter, however, somebody jammed a screw tightly into the spot where the fifth central hose should be, effectively sealing it off. I see no evidence of the purge valve solenoid, or a tube running to the atmosphere. One question in my mind is: would modifying the emissions system in this way cause this behavior? (I have photos of the plugged four-way-thing and the missing canister, but will have to figure out how to post them, if my description wasn't enough to go on.)

Coming from a CX500 - essentially a sofa on wheels - I was so happy to have a "fancy" bike that the stumbling didn't matter much, but it's gotten annoying now that I'm trying to ride it smoothly in town. I appreciate any advice that you VFR gurus can offer.

Other than that, what a great bike!

 

I would bet you have a bad spark plug wire. You can check them with an ohm meter to see if they have uniform resistence.

 

Hi Dr.

Have you synced the starter valves (lots of info on this site), they are connected to your fast idle lever.

 

Now I can look at all this stuff

Thanks for the insightful replies, everyone.

Y2Kviffer, when I had the VFR apart a few weeks ago, I noticed the 12V leads to the ignition coils were pretty dirty. Contact cleaned all connections and reseated the cables to the plugs. No change. However, I did not check resistance on the plug wires, which I will do. Good preventive maintenance if nothing else, as I understand running failing plug wires tends to be pretty bad for a bike.

Kingsley: I started reading about starter valve / fast idle wax unit issues; it sounds exactly like my issue. Good thing I bought some vacuum gauges a few months ago. I'll make up some adapters for them and let y'all know what happens.

mello dude: The stumbling happens in any gear, though when I'm doing under 3000 rpm in 3rd or 4th I'm typically just ambling along a straight, level road. In this case, I can still "feel" that it's running rough, but there's enough momentum that the bike runs along without hitching. Funny that you bring this up as it is a perfect example of this issue. OK, say I wanted to accelerate in 4th, from 3000rpm. I'd give it some gas, the engine would noticeably work harder and run rough, my revs and speed would slowly increase, then at about 4000 RPM, BAM, it would shoot forward and accelerate quickly. Sort of like a turbo. Same thing in 1st or 2nd, but of course at that gearing, the engine flies past 4000 pretty quickly, so it's not as much of an issue. San Francisco hills are interesting, though.

Also, thanks for the info on the hose routing. It sounds like you did a cleaner PAIR block-off than the PO did on mine. I will check for the presence of any loose hoses, but I think I have it all worked out. I don't think at this point it's the CARB stuff, but I will check to make sure everything's capped. For environmental reasons :treehugger: I'd actually prefer to bring it back to stock, with the CARB equipment back on. If you or anyone else is interesting in offering me the bits from a de-neutered bike, let me know!

To recap, I'm thinking it's the starter valve, as performance below 4000 is just abysmal, in all gears (thx mello dude for asking the right question). Given it has 67k on it, I also want to check my plug wires now. I will open the beast up this weekend and see what's going on.

 

Hi Witch Doctor,
It sounds like lugging to me. I never run my '95 below 4k, except coming off idle, of course. These engines are happier at higher revs than a twin would be.
If it runs bad below 4k, don't run it there!
But that's just my opinion.

Cheers!
Craig :smile:

 

Yeah, that's what the previous owner said! "Just run it at high revs, it likes it!" Which it does.

Still, with a "flat spot" from 0-4000, on weekends it means I can't take off smoothly from lights. On weekends, when I'm romping out in "the hills" and it drops to 4000 just before a corner, it means I really have to watch the throttle... or just ride in 1st/2nd all the time and use a surgeon's touch on the right grip. :smile:

 

You're right about former twin riders! :smile: Took me forever to get into the habit of running at high revs on the VFR. I'm used to the jumpy, instant torque of a twin, albeit a 500. I've started riding it at >5000 now all the time... below 4000 the VFR feels like an econobox car, absolutely no acceleration. Let's say I'm at a dead stop, and take it up to 3500, then let out the clutch slowly. That will usually stall the engine. I have to take it up to 4 or 5 grand, then let the clutch out, in order for it to take off properly. Even my little 150cc Bajaj Chetak wasn't this feeble at low revs. (OK, maybe a slight exaggeration.)

When I lug a bike, I typically feel the engine struggling to turn over, and poor acceleration, in the bottom of the rev range. This feels much broader, and for lack of a better description I'd say it "feels" like it's restricted somehow, kind of like when a bike is lean surging / about to run out of fuel.

Another reason why I think it's the bike and not me, is the previous owner explicitly mentioned the issue, and he put about 50,000 miles on it, so I figure it was something that developed at some point, rather than being "normal" for the bike. I'm going to check out everything this weekend. I'll let everyone know what I find.

 

Well, the weather's been so bad I haven't been working on, much less riding the VFR.

However, I do have an update: the one nice day I took it out (Friday the 13th), I got the stuttering even worse than usual. Bike stalled in traffic almost immediately after. Jumped the bike, got about 10 miles before it completely died.

The stator appears to be knackered! All three wires have continuity to ground, just like when my CX's stator was toasted. I put a voltmeter on it; the battery voltage actually went as low as 8V when it was being revved. I'm ordering a new stator now.

Once it's in, I'm going to sync the starter valves and check the resistance of the spark plug wires. The bike has almost 70K miles on it and I know they run hot, so I figure it's the stator's time to go...

 

The beast is apart

Given that this was the first non-rainy weekend in a while out here, I took advantage of the weather and spent most of today working on the VFR. I rebuilt the brake system, replaced the air filter, and began working on the stator. When I pulled the cover, I saw that 1/3 of the stator was its normal coppery color, and the other 2/3 was a nasty, burnt color. This might just be due to the oil level in the engine, but it still looks pretty beat. (I'll see if I can attach a photo later.) I will finish the stator installation tomorrow. (Man, I hate scraping gaskets... I'd have the install done by now!)

Tomorrow I'm going to test the ignition wires. Once it's back together I'm going to start it up, see if the stumbling is gone, and if not, will proceed with the starter valve synchronization.

 

Long time, no post! I had a brake system issue in the spring that took weeks to diagnose, and then I sort of stopped caring about the stumbling for a while. Anyways, back into it.

When the engine is warmed up, the idle is noticeably rougher, and it's possible to stall the bike from a stoplight if I don't really watch the throttle. I tested the ignition cables; though they all checked out fine (all had zero resistance!), I ended up replacing them anyway. My local NAPA Auto Parts had bulk ignition wire in stock (Belden 7mm spark plug wire, BEL734803, about $0.50 / foot); for the record, you want 7mm cable, and I ordered 10 feet of the "metallic" stuff, which should be enough for 2 VFRs! A resistor is built into the spark plug cap so I bet you could use "racing" cable, but eh, I used the stuff that they said was for "street" use, due to my concerns about the ignition system's RF interfering with the ECU. To change the cable, you just loosen the black grip on the ignition coil side and pull it out, and it comes right out on the plug cap side with a little force. This smoothed out the engine a little bit. Hey, it was something like 1/20th of what Honda was charging ($70 for 4' of uncapped ignition wires? Really?) so hey, might as well!

I tested each spark plug cap (the rubber boot). Resistance was approx. 6Kohms on each cap, except for the #2 cylinder cap, which was all over the place... my meter couldn't get a lock on it. It started at 0.6Megohms and then went down to almost nothing, crept back up, then down, etc. I swapped this cap to the #1 cylinder and it changed the idle sound, so I think it's bad. I ordered a new one from Service Honda. We shall see!

 

OK everyone, it was a bad spark plug boot in the #3 cylinder. Y2Kviffer, you get the point!

All four spark plug wires showed 0 ohms resistance; the replacement wire was the same (of course). I then removed the wires from each boot and tested the boots. Each boot showed approx. 6K ohms resistance, deviation -/+ 100 ohms from boot to boot. However, the #3 boot, when cold, read all over the place, several megohms initially, then going to several tens of kilohms, and finally going up and up as I held the meter leads across the boot terminals. Warm, it showed up as infinite resistance (but not open). This jives with the behavior I was getting: a decent (if rough) idle when cold, then approx 5 minutes later, poor idle, slow revving, and sluggish acceleration (relatively). Basically the #3 plug was cutting out once the engine warmed up. I never noticed the acceleration/rev issues as much, as my previous bikes had all been 50hp or less and "non-sporty."

Ordered a new plug boot ($25) and it's literally a new bike! Instead of a weird gurgling when accelerating from 5K, it sounds like a little V8. I can take off at lights from engine speeds as low as 1500rpm without stalling (though there's still a little hesitation around 3k, probably due to it needing a throttle body sync). The engine revs much quicker and it runs like a bat out of hell. It doesn't need to start at 7000 or 8000 to accelerate on the highway... 5000 is fine.

I suspect the high engine temps probably did in the boot. There was no obvious physical damage to it. I'm going to cut it open eventually. It may have been damaged by a previous owner trying to change plugs; I'm not sure.

Thanks for the advice, everyone! I now have gone full circle and understand why everyone is OBSESSED with these bikes.

 

thats awesome! im happy that you finally got this sorted out. its been a long time coming!

 

Excellent diagnosis work. Thanks for taking the time to post it up - that will be helpful for later use, I'm sure.

 

Well, turns out it's not been fixed. I've been putting up with it through the winter. Changing the boot helped, for about a week, and then it started running like crap again. Since then I've replaced all spark plug wires, and all spark plugs, and done a complete PAIR removal (with blocking plate installation) to no effect. After each tweak to the electrical system, it seemed to run better for the first 30 seconds while still cold, and then the idle becomes rough and the power drops as before.

The symptoms are that idle is rough - it's +/- 100 RPM at random times, and it "feels" like it's running on two cylinders under 5000 RPM. To take off from a light, I have to rev it and slowly dump the clutch, or else it'll stall. At that point, it feels like it's only running on two cylinders, so I have to rip on the throttle. Once it hits 5000 it's like a turbo kicks in and it seems to run "normally" (I say normally, because acceleration, etc. is still not great, but at least it's smooth.) The warmer the engine gets, the worse the symptoms are... my temp was at 220F and it almost stalled on me on a medium-size hill.

I'm starting to swap out the ignition coils now. I noticed when I bought it that the vacuum hoses had been plugged and the evap canister and purge control valve had been removed. (See pic below... the fifth hole in the 5-way connector has been plugged airtight.) I'm tempted to ask around for these replacement parts... with so much plumbing I'd like to "return it to stock" to try to figure out the issue. I'm going to do the starter valve sync as best as I'm able... as you know from the service manual, it basically assumes the evap system is present afterwards, so I wonder how well that will match up with my current setup.

 

I always tend to stay around 5k. Any thing lower than 4500 rpms is luggy.


You probably need your starter valves adjusted.

 

Sweet action Bro,nice work ride safe.

 

if you havent already done so, you might want to go through each vacuumm line, replace it, and ensure that it is in fact "capped off" correctly. i too have just about all of my emmissions equipment removed and vacuum lines capped off, but my bike runs like a dream.
also try going through every electrical connection that you can find, pull it apart and clean it as best you can, and then pack full of dielectric grease before reassembling...
good luck brother...

 

Techron actually works

Two years later - and I finally fixed my engine problem.

About 2 months ago, the symptoms became noticeably worse. (To recap: lumpy idle and almost no throttle response below 3500RPM, now as high as 5000RPM.) Low-speed maneuvers required me to rev the engine to 6 grand and slip the clutch, as if I was riding a two-stroke dirtbike. I noticed that the symptoms were less apparent when the engine was cold, and after the thermostat hit 175 degrees F, it was almost as if a "switch" had been triggered, and the throttle response immediately became terrible.

This thermostat "switchover" made me suspect the injectors, either bad connectors or dirty/clogged nozzles. I don't know anything about the '99 VFR's ECU, but I figured it could be running rich when the engine was warming up, and once it hit 175, the ECU told it to lean out the mixture. On clean injectors, this wouldn't be an issue, but on dirty ones, it would severely restrict fuel flow and cause the lack of power I was experiencing.

I figured I'd pull the injectors and have them cleaned. This is my commuter bike, so I was planning to get my "beater" CX500 up and running, then use that for the week or so the injectors were away. Two months later and I hadn't made time to do any of it.

I'm not a fan of snake-oil solutions. Believe me, I like to quantify, dissect, and examine everything, and frown on "magic" solutions. However, given that I hadn't made any progress with my spare bike or the injectors in two months, I figured, "why not?" and bought a "12-gallon treatment" size bottle of Techron yesterday. Dumped the entire thing into my VFR's half-full tank, nice and concentrated. (3 gallons... I'd say that's about 4x the concentration they recommend on the bottle.)

Rode to work this morning (20 miles):
- Idle purrs like a kitten and got smoother through the commute. Needle sits steady on the tach now instead of jumping around.
- Idle went up 100-150 RPM! Need to turn it down again.
- No insane engine braking like before... it would almost buck you off the seat if you chopped throttle closed.
- Can accelerate on the freeway in 6th gear and actually pull past cars now.
- aaaand... the poor response below 5k has been eliminated. I can now engage the clutch at 1500 RPM and smoothly go through traffic.

Lubed the throttle cable as well and it basically feels like a new bike. I'm still kind of shocked that Techron actually works. Who woulda thunk it?

Thanks to all for your thoughtful advice. I WILL be tearing the bike down at some point to clean it/adjust valves etc. and will have the injectors properly cleaned, as well as starter valve sync / etc. done. But for now, it's basically eliminated the problem that's been dogging me for years.