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Bogs down when throttle opens "too far".

Old 05-13-2006, 08:47 AM
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Bogs down when throttle opens "too far".

So I recently bought a 2000 SuperHawk from someone who'd neglected it pretty badly. Regulator/rectifier died promptly (I suspect it was already dead and he'd just replaced the battery), so I replaced that with a non-Honda one, and with that and a new battery it's much happier. New tires, some fairing repair, a new tank, new plugs, etc, etc...

I pulled the carbs (I think I'm up to about six times now, sigh) and cleaned them extensively, cleaned the jets, got no small amount of crud out of them. Diaphragms seem good, the slides move well when I rev the engine (now), I appear to have good fuel flow. The bike seems to have had its PAIR system disabled, or at least the tubing from airbox to heads doesn't match what either manual I have says it should be. I have two lines that run straight from the valve on the heads to the airbox with nothing in between. I haven't pulled the valves to see if they've been flipped around to seal them off, though. I found at least one vacuum line that didn't seem properly sealed off, so I sealed it off -- I'm suspecting it was the actuator line for the PAIR valve. (It comes off the accelerator pump for the front carb. Fixing that one made a nontrivial difference in how well the bike handled having the throttle blipped...)

The jets appear to be stock, but I'm not 100% sure. I don't see any extra holes in the slides, though I could be missing them. Cans are stock. As far as I can tell, someone messed around with the carbs but didn't really know what they were doing (the coolant lines to the carbs had been bypassed and so one line ran straight from the water pump to the temp sensor, for example, and the TPS was unplugged). I've been fiddling with the TPS this morning, trying to figure out if it's working properly, and I'm pretty sure it is.

I have not yet done an actual carb sync.

And I live at 5k feet, regularly ride up to 10k feet. Cold, thin air.

Anyway, with all that, I've managed to take care of most of the problems it was having. I have pretty good, smooth power off idle, the bike runs great if I moderate my throttle hand. However, if I get the throttle too far open (where "too far" is something like "more than 1/4 open" at low RPMs, and something like "more than 2/3 open" at high RPMs), the bike bogs badly, acting like it's fuel-starved or like it's got no spark. (Only, as far as I've been able to tell so far, I have good fuel flow even at high demand, and the spark seems reasonable.)

When I do this with the airbox off and watching, I get what looks like backpressure through the carb throats -- it stops short of an actual backfire, but air's definitely not supposed to push back out through the carbs!

When I'm riding and it bogs like this, if I just back off the throttle a bit, it'll pick back up again. The critical point seems to be dependent entirely on RPM, not on how fast or slow I open the throttle, so it seems like something is overwhelming the engine, but I have yet to sort out exactly what.

Any suggestions?
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Old 05-15-2006, 05:03 AM
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I would be looking at the needles first. If they carbs have been messed with who knows how they are set. Sounds like it could be running too rich. If you have adjustable needles try lowering them one notch. If the standard non-adjustable needles have been shimmed, take the shim out.

Not guaranteed, but a possibility by what you describe.
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Old 05-19-2006, 07:04 PM
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Pretty sure this is a fuel problem. Sounds like you're not getting enough fuel at higher rpm and it's leaning out. This can be a couple things.

1) Improper airbox modifications. Make sure the rubber duct is there by the steering stem.
2) Poor carburetion either from clogged jets, clogged fuel strainer, or improper petcock hose locations. Most likely I think the carbs are starving from the latter. Check that the smallest vacuum line is attached to the nipple on the REAR of the petcock, NOT the bottom - that's the vent for the diaphragm. Also, check the petcock is full open (using an 8mm box end wrench).

Hope this helps.

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Old 05-19-2006, 09:25 PM
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I'm betting that the fuel petcock is the problem. I did indeed have the vacuum tube on the wrong nipple, and fixing that made things a little better but still not quite right. Looks like I have it turned on (full CCW) but on looking at the diagrams the correct nipple should be pointing left, and instead it's also pointing down. When I swapped tanks I pulled it apart to clean it and put it back the way I found it, rather than digging out a diagram of how it should go together. (Silly me. This was before I realized how badly connected many of the other hoses and such were.)

I'm thinkin' I should pull the petcock off again and pull it apart and put it back together right and test it. I'm sort of surprised I was getting fuel in the lines at all...
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Old 05-19-2006, 11:17 PM
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Oh, and good to see you back. As someone else noted, yours was the first site I found when researching the VTR...
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Old 05-20-2006, 03:40 AM
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Sounds like the bike was molested! One option I recommend is rejetting down to 172/175 instead of 175/178. Bought the 172 jet at Sun Honda in Denver for $9. Bike is snappy at 5k-10k altitude. A pressure gauge will help your carb sync immensley.
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Old 05-20-2006, 07:18 AM
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There are three large hoses and two small ones under the tank.

The large ones shouldn't be mixed up as two of them are for the carbs and really aren't long enough to connect to anything else by mistake. The other much longer one is the fuel over flow hose.

The two smaller ones are very similiar and can easily be hooked up wrong. One is the tank vent hose that connects to the smaller of the two tank nipples and the other is the petcock vacuum hose which goes to the petcock's rear nipple. TIP: I have a t-adapter for carb synching spliced into mine so I can't get it wrong. The vacuum line will connect to the rear cylinder for its vacuum signal and the tank vent line runs to the underside of the motor (usually thru the belly pan). If these are backwards, the bike should only run but only for a short time (poorly or not) until the carbs run dry.

The two large fuel supply hoses can kink easily; especially the left one, which would inhibit high fuel rates. It's a poor design IMO that defers to sytle to keep the hoses out of sight. My VFR tank has much more room and the hoses are MUCH easier to get at. I consider myself somewhat of an expert at tank removal! LOL
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Old 05-20-2006, 09:08 AM
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True. Also the small line that goes to the petcock *should* have a small wire-type hose clamp on it.
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Old 05-20-2006, 08:26 PM
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Ysabel,

Try this old tuner's trick. When you're in a safe area, away from traffic, get the bike to bog, and at the same time, pull the choke out a little bit. If the bog gets better or goes away, it's leaning out. If it gets worse then it's too rich.
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Old 05-20-2006, 10:08 PM
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Pulling the choke out helps a lot but doesn't fix it, so it seems likely it's way too lean, I would think. With any luck I'll get to pulling the tank tomorrow; trying to decide if I want to go ahead and do another pass through the carbs while I'm at it and see if I missed anything/check the sizes of the jets, since I didn't actually write that down last time, I just cleaned 'em out.
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Old 05-20-2006, 10:09 PM
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I took your definition of "bogging" to be the same as mine, which generally indicates a richness of fuel. It seems that this is not what you meant.

If it is the petcock that is a "too easy" fix!
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Old 05-20-2006, 10:45 PM
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I think your problem still lies with the petcock. The VTR is not THAT sensitive to jetting changes (unless you're fouling plugs that is). Pull your tank and set it upside down on a soft towel. Remove the petcock assembly. Careful not to damage the gasket or o-ring. *Carefully* disassemble the petcock and inspect it. Do all this on a clean white rag so any crap that comes out you'll be able to see. Clean the internals with silicone spray and a soft cloth. Carefully reassemble. Take your time with this. Exercise/test it using a spare vacuum hose and using your mouth or MityVac to draw the vacuum. Let the gas spill into a oil drain pan or similar. Ensure it flows fuel freely. Also ensure that when the vacuum is released, there is no more flow. If this doesn't solve your problem then we'll move on to the next place in line - the floats and float valves.
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Old 05-26-2006, 08:42 PM
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Okay, petcock reworked, tested. As far as I can tell, it was sticking partway before (neither fully on nor fully off), seems to be flowing fuel well now.

Midrange performance is significantly better. (Damn! Got the front end up accidentally as the low-end stutter went away and the midrange kicked in. Whups!)

The stuttering at very low RPM (just off-idle, basically, pilot circuit territory?) is now very obvious by comparison, like only one cylinder is actually firing and the other is firing/not-firing/firing/not-firing semi-randomly. I thought I'd gotten rid of that, but obviously not. Still acts fuel-starved at full throttle but it takes a good bit more to get there now. Pulling out the choke while I've got the throttle full open and the bike is bogging doesn't do anything that I can tell, now.

Time to pull the carbs back off? What should I be looking for, specifically? Last time I did a bunch of carb work was on my 4-bbl quadrajet I had on my Jeep; I'm familiar enough with the guts of Mikuni-style carbs to do the disassembly/clean/reassembly, but I clearly missed something when I went through them the first time. I haven't tried to adjust the floats, I'll note -- the books I have basically say, "Replace the floats if they're out of whack, you can't adjust them."

Thanks for all the help, guys. This has been quite baffling so far.
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Old 05-26-2006, 08:46 PM
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Originally Posted by shayne
I took your definition of "bogging" to be the same as mine, which generally indicates a richness of fuel. It seems that this is not what you meant.
Yeah, mostly it's that the really quite impressive acceleration just suddenly goes away and the bike acts pretty much like it would if you'd just run out of gas, sounds like it's not firing, that sort of thing, until I let off the throttle far enough, at which point it suddenly kicks back in.

The (now much more obvious) low-RPM stuttering is similar, only far more chaotic. And a little disturbing when it suddenly goes away and the front wheel is suddenly in the air.
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Old 05-26-2006, 09:42 PM
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Okay, lets try some stuff. Seems fixing the petcock helped. Let's attack one problem at a time. We'll start with the stutter off idle.

Pull the carbs. Adjust idle mixture screws to 2 turns out with stock cans, 2.25-2.5 turns out with aftermarket cans.

Has this bike been rejetted? If so, raise needles one notch (lower the clip). If it has not been rejetted and you DO have aftermarket cans I recommend a jet kit. You can get away without one if you shim the needles but this will not give you good WOT performance, which we will address later.

Reassemble and reinstall carbs. Synchronize the carbs (very important for idle and just off idle). Test ride.

Let me know how it turns out.
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Old 05-26-2006, 10:43 PM
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Stock cans. I'll work on this tomorrow and see what I see when I get back into the jets again. I probably need to go buy a vacuum gauge for doing a proper carb sync...
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Old 05-27-2006, 01:31 AM
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Do it right, borrow or buy mercury sticks. That's the best way. Too hard to compare them if they are not dancing side by side.

...or you could buy a Twinmax! Google it!

Damn, stock cans. I was actually hoping you had aftermarket. That would have made things simpler to troubleshoot. Okay, well lets get the mixture screws set anyway and do the carb sync. I don't see any reason to open the carbs up unless you think they're gummed up from sitting for months at a time. If they have been sitting, then you really must open them and clean the small passages. I'll explain more if necessary.

Oh yeah, you'll need a special D-shaped tool to adjust the mixture screws. Alternatively you can use needle-nose pliars to get them out far enough to slot them with a Dremel, then use a flat head screwdriver. Or I can mail you my special tool if you promise to mail it back. PM me if you want to go that route.
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Old 05-27-2006, 08:36 AM
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I've actually had the carbs open once before, cleaned out all the small passages and so forth (and I did find a lot of gunk, including what looked like mud (!) in one of the float bowls, so there is certainly the possibility I missed something when I did that). I didn't check the jet sizing at the time, though.

I'll see what I can find for doing the sync and work on that. And maybe see if anyone has that silly D-shaped tool in stock.
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Old 05-27-2006, 08:40 AM
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Oh, and I want a set of high-mount aftermarket cans at some point, but that'll have to wait for a while for right now. I figured I'd do that and the jetting and a K&N all as one package.
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Old 05-28-2006, 07:17 PM
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Okay, so I haven't managed to adjust the idle air yet -- haven't found a tool so far -- but I did manage to sync the carbs (and took a little video, front cylinder is left, rear is right, sound quality is not great but ah well). Huge, huge difference. I still have the issues with WOT but up to that point it's much, much happier.

I may still fiddle with the carb sync more (I put a cap on the front, and a T and cap on the back, so I can do it again easily) since I'm not sure I have quite the right compromise so far, but seriously, it's hugely better up 'till the WOT hesitation (which seems to me to be a little less pronounced, but I think that's probably just because I'm not getting weird issues while getting there).

I also noticed four holes in the top of my airbox that look drilled. Are these normal? See pics for reference:

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Old 05-28-2006, 10:46 PM
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Hmmm, put some tape over those holes. Don't think that's helping your cause any. Good to hear syncing the carbs helped. Were they out pretty far?

Okay, now for the WOT....

Well, you know what, go for a nice long ride and get some fresh gas in it. Let me reason this out loud for a minute...

You bought the bike used, right? Any history on it, like was it a high altitude bike or low altitude bike? You're in Denver, so you're a mile high. You require less fuel and therefor should (optimally) have probably one size smaller jets. If the previous owner (PO) was in Denver, that accounts for the holes in the airbox to compensate. If the PO did not live in Denver, the bike is still better off with more air. Is holes in the airbox the way to go? IMO, no. I'd rather go with a BMC or K&N filter. Speaking of filters, have you checked yours to make sure it's clean? You say choking the bike doesn't help the stumble on WOT. Did that change any?

Get back to me with some of these answers and we'll reason further.
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Old 05-29-2006, 06:47 AM
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I'll tape 'em, and I'm going to go put 60 miles or so on it this morning. The previous owner lived in Denver, but it may or may not be a high altitude bike. The PO was completely clueless, I'm fairly sure many of the incorrect tweaks were made by the guy he bought it from.

And yeah, the carbs were way out to start with. I now have the rear a tiny bit high at idle and it drops a tiny bit lower than the front on opening the throttle -- before it was way high at idle and still somewhat high on opening.

Thanks again for all your help.
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Old 05-29-2006, 08:09 AM
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Okay, I don't know what the #$%@ is up.

Tried to take it out this morning, it was almost unrideable. I swear one cylinder wasn't firing at all for most of it. I came back, checked carb sync, it still looked pretty much the same.

The bike idles on one cylinder, I think. The other cylinder catches periodically, though, so it does this thrum-thrum-THRUM-thrum-thrum-THRUM thing. Opening the throttle very slowly brings the second cylinder online relatively smoothly. Opening it at all quickly makes it stumble and then catch and suddenly the RPMs come up. Watching this on the vacuum gauges is interesting, though I don't know that I understand how to interpret what I'm seeing. When the other cylinder catches, the relative vacuum readings change dramatically, though.

It's possible the air screw in one of them is just totally out of whack, or something, I suppose.

I suspect I should just pull the carbs and open them up again, though. Would make it easy to check the air screws without needing the Special Tool...

*sigh*
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Old 05-29-2006, 08:11 AM
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Oh, and the air filter is practically new, btw. I forgot to mention that.
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Old 05-29-2006, 12:27 PM
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Just confirmed it's jetted 175/178, so it's not jetted for altitude.
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Old 05-29-2006, 07:20 PM
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Guess you opened the carbs up then. There is one emulsion tube in each carb. It's a screw in type jet. It has tiny tiny holes that if clogged will completely screw your idle up. Pull those and check to make sure there is no dirt/grime/gumming. You've obviously already pulled your main jets. Those are the biggest holes and will clog last.

Did you find anything odd while the carbs were open?

Oh, and while you have them off, adjust the mixture screws.
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Old 06-07-2006, 09:59 PM
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Unhappy Ignition, mebbe?

Okay, the bike now has a Factory Pro jet kit and BMC street filter.

165fr/172rr, stock pilot (#45), needles on #3 notch. Air screws at 2.5 turns out.

Resynched the carbs, replaced the fuel lines, tested the fuel petcock extensively, went though everything in the carbs extensively, soaked all the small bits in carb cleaner, etc, etc.

When it's on, it's much smoother, much happier.

Still have a good stutter throughout most of the idle and off-idle. When sitting idling, the bike percolates, like it's running on one cylinder but the other catches periodically. Midrange isn't bad if I keep a light touch on the throttle. WOT the bike still acts like it's fuel-starved or something.

I'm now strongly wondering whether the R/R failure cooked something in the ignition system.
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Old 06-08-2006, 04:03 AM
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Try replacing the spark plugs. Get back to us.
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Old 06-08-2006, 06:13 AM
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Did that already, though I was thinking about pulling 'em again and peering a bit.
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Old 06-08-2006, 10:41 AM
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this may help, may not...
this is how i tune fuel screws..

carb synch must be within 10mm of each other... i use mercury sticks
engine must be at operating temp... this can be done by idling and keeping fans on the radiators to keep it from overheating... may take 5-10 minutes, but as you adjust the carbs, it will warm up and by the time your done with this procedure, you'll be close enough to op-temp

step 1... with engine running, set idle at 1000-1200 rpm
step 2, slowly open (turn out) front cylinder fuel screw... if idle increases, continue turning out slowly untill idle stops increasing... RESET IDLE to above
step 3, slowly open rear cylinder fuel screw, if idle increases, keep turning slowly untill it stops increasing... reset idle to 1000-1200

NOTE: if idle does not increase by turning out the fuel screws, you're already too rich... proceed to next step

step 4... very slowly turn one cylinder fuel screw in untill you begin to hear the engine miss slightly... this is the lean point... back out screw from this point, 1/8th to 1/4 turn for optimum, repeat for rear cylinder... if turning the fuel screws has NO effect, you probably have an air jet clogged, or the emulsion tube holes on the pilot jet clogged ,or the pilot jet itself... reclean your carbs..

to recheck your settings, slowly turn in the fuel screws and see if it starts to hiccup, if you only go in 1/8th to 1/4 to 1/2 turn, your just about perfect (per honda spec, it's the idle drop procedure, outlined in the service manual)
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