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1998 VTR low miles off idle sluggishness

Old 09-15-2016, 01:28 PM
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1998 VTR low miles off idle sluggishness

Hey folks - This is my first post, but I have read most of the knowledge base. I am pretty sure I know where the vacuum line goes on the gas tank petcock.

Please forgive the post length. Basically, I have cleaned and cleaned the carbs, but the bike is still very sluggish off idle to 4000 rpms. In addition to suggestions on that topic, I have the following two questions if you'd like to chime in without reading the bulkiness of the whole post:
-what is nominal vacuum at idle?
-what is the best routing for the choke cable from the mounting bracket on the left hand side to where the cable splits?

Thanks for any advice you might have. Here's the whole saga:

I bought a 1998 VTR 1000 in 2012, with only 2500 miles. It seems to be bone stock. Upon arrival it ran rather weak, not bad, but not punchy. My regular mechanic tuned it up and found a mouse nest in the airbox. It ran better after that, but still not as strong as I had expected, especially at lower speeds and rpms. To be honest, I rather expected it to power wheelie easily with all that low rpm torque, but it would not.

Unfortunately for me, my regular mechanic retired, and so I found another shop. They never really were able to do anything to make it better – they said the carbs were clean. They replaced the choke cable, but I don't remember why. I kept riding it, but it was vaguely dissatisfying.

Then in the fall of 2013, the bike blew over in a big windstorm. The bike sat for all of 2014. Over the following winter, I finally experimented with ABS glue and was able to rebuild the fairing for about $9… a gratifying experience.

Spring of 2015, the bike predictably didn't run well after essentially sitting for 18 months. I assumed the carbs had gotten dirty. I had become self employed during this time, so money is tight, and I decided to work on it myself. I'm not a great mechanic, but I am a mechanical engineer, and do a lot of work with natural gas combustion controls, and think I ought to be able to handle this job. It's been humbling.

I pulled the carbs. Stock jets and needle. The idle screws have been slotted. Carbs looked really good, no goop or gunk or buildup to speak of. I did find some foam rubber crumbs, from the air box inlet horn, I guess, on both sides of the diaphragm. I blew it out, sprayed it with carb cleaner, did not pull the jets, put it back together, took it for a test ride, and it ran great… for about 15 miles. At which point it would idle, or run wide open throttle, but not much in between. Choke made it worse.

I assumed the tank was rusty and I had sucked a bunch of it into the carbs when I slapped the tank on the bike and drove away. However, when I inspected the tank, it was totally clean. No rust, no dirt, nothing. I then pulled the carbs and cleaned them again, with jets out this time, they looked great. I reassembled it. Same thing, bike ran great cold, fell on it's face when warm. Choke made it worse.

I called my retired old mechanic and he said he bet they didn't get the choke cable routed correctly, as it is easy to pinch it so the cable is actually enrichening the starter circuit even when the cable appears to be in the “off” position.

Sure enough, I found the cable had tension on it. I also determined this was not the correct cable, as it should have had 90degree bends at each end of the cable, and these were straight. I zip tied it all to the frame with a lot of slack, and then just started it without choke, using starter fluid occasionally, and it ran quite well the rest of the summer. It still didn't want to pull the front end up without clutching it, even in first gear, but it ran nice enough. I will admit to only riding it about twice a month from April through October, and letting it sit again over the winter. No, I didn't drain the bowls or start it every month, I just let it sit. Glutton for punishment.

If anyone knows the best routing for the choke cable, please let me know. The shop manual says you can leave the choke on the carbs when removing them, but that only really works if the cable is on top of all the radiator hoses. It seems to kink the cable less if the cable is routed under the front hose, but then I have to take the choke cable off the carbs and feed it through from the other direction.

So this spring I went to go for the first ride and it ran well until it got warm, and then it was very weak and sluggish from off idle to a little more than 1/8th throttle. Basically, it idled fine, and was fine wide open, but the transition was bad. I hoped the choke cable had slipped, recreating the previous condition, but I checked it and it looked pretty good.

This began a series of pulling and cleaning the carbs, including the jets and emulsion tubes I did find a little more foam rubber in the carb, the first time but it really looked pretty clean. I think the third time I cleaned the carbs, I completely disassembled the carbs and dipped them for four hours using KB carb dip, It was somewhat better just off idle, but then would sag, stumble, whatever, until approximately 1/3 throttle. I soaked the carb bodies again, this time for 24 hours each. But it didn't really make a difference. I did put new gas in it – no difference.

I have begun to wonder about the diaphragm and sliders as it would stumble, hesitate, and then “catch” and take off, about throwing me off the back of the bike. Almost like there wasn't enough vacuum to lift the needle, or the diaphragm was stuck, and then released, until it got a big gulp of air and took off. If I feather the throttle, and keep it above 4000 rpm, then it seems to act normally, even if the throttle opening is small. When I test the slides on the bench, it seems to work normally/smoothly, if I blow into the breather vent that goes from the airbox to the bottom of the diaphragm. I tried using a shop vac to generate a venturi through the carb body, and found it would only raise about half way up, but did activate fairly normally. I am pretty certain the diaphragms are installed correctly and not leaking. I inspected for pinholes and don't see any issues.

There is the consistent pattern of stumbling until 4000-5000 rpm then taking off like mad. I wondered about ignition timing and TPS. I had set the TPS to about 500 ohms along the way .I just checked it last night, and it was at 600 ohms after the most recent disassembly/soak/ rebuild operation. As received, it was 880 ohms.

I just had been making do with bench synchronization until last night – the front cylinder was at 5.8 inches HG and the rear was at 6.6 inches, which is just barely in spec, but not off far enough to cause all this trouble I think. I have scoured the internet, and can't find out how much vacuum there had ought to be at idle. 6 inches seems low to me, but I don't have anything to compare it to. So maybe there is a vacuum leak? The carb insulators (boots) look good, so I don't think it's leaking there. I did not remove the butterflies when I soaked the carb bodies, so it's possible the felt seal is leaking, but it was doing the exact same thing before the carb soak, so I don't think so.

Idle screws were set at 2.63 turns out from fully closed. I had been reluctant to change them since it has been running well at idle, but finally broke down and bought the pit pro 90 degree ratcheting tool, and will start the next testing at 2.38 turns off full close front, 2.5 turns rear.

I found the front air cut off valve would not hold a vacuum, and replaced it with one from a newer set of carbs.

It starts easy and is idling pretty nicely, right around 1100 rpm. It does occasionally surge up to 2200 rpm (maybe once per minute) while idling. This causes the vacuum gage to surge further negative (more vacuum). After perhaps a quarter second, it drops immediately back to 1100 rpm without hanging up.

After the most recent test, I noticed there was some thick fluid in the airbox just under the white plastic part which feeds the PAIR valve. I am not sure what that is about, and don't think I've seen it in the past.

I was able to pick up a complete set of carburetors on ebay for a very attractive price – the whole package, for less than a new choke cable! These carbs are from a 2000 VTR that was reported to have been running nicely when parted out. The 2000 year carbs are in decent enough shape, could probably use a cleaning as there is some build up and varnish, but not bad. I have not checked the jets yet, but it does have a jet kit on it, as the needle is not stock. The rear carb throttle was very sticky when I received it, but having worked it back and forth a lot, it seems to have freed up.

This newer set of carbs has three vent holes in the slider bodies, while the 1998 set only has two.

My plan for now is to assume there is still a particle of foam down in the pilot air circuit (even though I see flow through all three jets under the butterfly when blowing carb cleaner through the pilot air vent, and block off the pilot jet opening). I plan to mount the stock jets, needles, etc into the 2000 year carbs to see if changing carb bodies fixes the problem. I am going to try the “three hole” sliders and diaphragms just to see if there is a difference.

When I get it back together, I'm going to check the compression, just to make sure nothing dreadful has happened. I'll also check for a vacuum leak, but the airintake snorkel is so close to the front carb, I'm not sure I'll be able to tell if there is a leak, or if it's just sucking extra fuel into the intake. I'm planning to buy a bunch of spark plugs to play with plug chopping it, if the alternate carb bodies do not do the trick.

What else? What do you all think?
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Old 09-15-2016, 03:08 PM
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While you have the carbs off go ahead and check the petcock diaphram on the underside of the tank. Light check it for pin holes.

The dense fluid you see in the air box is more than likely oil from the pair system. Look into pair removal or block off. It's a simple mod.

Are you using grease to get the carbs fully seated?

I might be corrected on this but it shouldn't matter how your enricher cable is ran just as long as there is not tension on the cable or sharp bends.

Sorry for the short reply but I have to go see my kids teachers and only have a moment.
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Old 09-15-2016, 06:09 PM
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My eyes want to cry from reading this post and everything you've done or tried to do to this bike.

Lets begin and address the simple issues. Get the right choke cables. The ones that are now perhaps have a different profile than the OEM ones, causing the bike to run rich since they do not close off completely. Just off the bat, to me, your issues will be solved once you've address this.

Next, there are a few threads on here that speak of the jets, and idle jets on what is OEM sizing. Compare yours and see if you have stock or not, then, verify to be sure the idle jets are installed in their proper place. The front one is different from the rear. Once this is all done, if you still have issues, then we may begin to look else were.


Originally Posted by Meier Link View Post
The dense fluid you see in the air box is more than likely oil from the pair system. Look into pair removal or block off. It's a simple mod.
Oil does not come from the pair system, but from the valve cover hoses that feed from/to the air box. Oil slowly travels through these hoses, and makes it's way into the air box. The reason for the catch can inside.
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Old 09-15-2016, 06:42 PM
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Originally Posted by Meier Link View Post
While you have the carbs off go ahead and check the petcock diaphram on the underside of the tank. Light check it for pin holes.

The dense fluid you see in the air box is more than likely oil from the pair system. Look into pair removal or block off. It's a simple mod.

Are you using grease to get the carbs fully seated?

I might be corrected on this but it shouldn't matter how your enricher cable is ran just as long as there is not tension on the cable or sharp bends.

Sorry for the short reply but I have to go see my kids teachers and only have a moment.
Hey, don't worry about the short reply. All help is greatly appreciated.

I'm happy to check the petcock diaphragm, but it does seem to work well. Tons of flow under vacuum, no flow without vacuum. What is it the indicates there might be a problem with it? Or is it just a good thing to do while it's apart?

I haven't been using grease to seat the carbs on the boots, or the boots to the heads. Does it matter what kind? I'd love to find that made a difference. I did double check that the boots are lined up with the grooves on the head. My front carb boot clamp does NOT line up with the hole in the heat shield, so I had rotated the clamp on the boot a little. This sheared off the little alignment tab on the boot, which I had noticed before. I then made sure to line it back up with where the tab had been before tightening it up. I drilled a new hole in the heat shield so I can get a screw driver on the boot clamp. Why that couldn't have all been rotated 15 degrees so the boot clamps could be tightened easily is beyond me. I've got some bearing grease and a little bit of silicone grease I could use.

The reason I asked about the choke cable routing was just for ease of assembly/disassembly. It's easier if it lays on top of the water hoses, but is sort of in the way for idle screw adjustment. And it puts some torque on the cable itself being there... doesn't quite kink it, but it doesn't lay straight. If I route it under the water hoses, then it lays nice, and is out of the way for idle mixture adjustment, but then I have to disconnect the cable from the carbs before I can remove them to work on them. Sort of a pain either way.

Thanks for the help!
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Old 09-15-2016, 06:56 PM
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Originally Posted by Jack Flash View Post
My eyes want to cry from reading this post and everything you've done or tried to do to this bike.
Mine too. I'm ready to ride it and be done working on it for a while.

Originally Posted by Jack Flash View Post
Lets begin and address the simple issues. Get the right choke cables. The ones that are now perhaps have a different profile than the OEM ones, causing the bike to run rich since they do not close off completely. Just off the bat, to me, your issues will be solved once you've address this.
I have installed the choke cables from the 2000 year carbs I bought last week, which are the stock configuration. Things do get worse when the choke is applied, so I think the choke cable is activating. I can't swear that it's sealing, but I think it is, just by visual inspection.

Originally Posted by Jack Flash View Post
Next, there are a few threads on here that speak of the jets, and idle jets on what is OEM sizing. Compare yours and see if you have stock or not, then, verify to be sure the idle jets are installed in their proper place. The front one is different from the rear. Once this is all done, if you still have issues, then we may begin to look else were.
I had checked the main jets and jet holder/emulsion tubes against the forum and are pretty sure those are stock and in the right places. I am not positive the pilot/idle jets are stock or in the right carb. I'll double check it again when I have it apart. My plan is to install the 2000 carb bodies with the stock needles and jets, but use the 2000 CV slides and diaphragms since it has three bleed holes into the slider/top side of diaphragm, rather than just the two bleed holes as found on the 1998 original carbs. I'm hoping that will make the slide a little snappier. I have seen several online articles about plugging one of the holes, but I'm going to give it a try.



Originally Posted by Jack Flash View Post
Oil does not come from the pair system, but from the valve cover hoses that feed from/to the air box. Oil slowly travels through these hoses, and makes it's way into the air box. The reason for the catch can inside.
Ok cool. So that's just to let it evaporate in the airbox and get burned in the combustion chamber, right? I didn't run it that long last time when I realized I still didn't have good throttle response, so perhaps that's why I hadn't noticed it on prior teardowns. It doesn't sound like I need to be concerned about it.

Thank you!
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Old 09-15-2016, 09:12 PM
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I did a quick compression test tonight and discovered the o-ring on the tester fitting was split. I hooked it up anyway, and found 140psi on the front and 155 psi on the rear with the carbs and tank off. The shop manual says 164 psi at 350 rpm. Given the cracked o-ring, I think that was pretty good.
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Old 09-16-2016, 05:52 PM
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Jack Flash just read your comment hanging my head in shame because I knew this. Hate having to rush responces. Everything gets jumbled up in my head and it all blurts out at once lmao.
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Old 09-16-2016, 08:08 PM
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Compression should be within 10% of each other, so your on the high end there. I would verify compression again with a good gauge. If its still that far apart check valve lash. Also how old is the gas in the tank? If its more than a few months old I would drain and put some fresh gas in.
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Old 10-22-2016, 01:15 PM
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Ok, I finally have time to jump back on this project. My first question is: "is there a good way to be sure the slide is lifting on each carb?
My next question is "how can I tell if the air bleed passage is clean?" The polyurethane foam filters on the corners of the airbox had deteriorated and there was foam everywhere when I got it. That should no longer be the issue since I have the 2000 year carb bodies in there, but the symptoms are similar still... big sluggishness from 1/8 throttle to 4000 rpm.
I'm going to look at plugs, recheck compression, double check for vacuum leaks and fix what I find, then idle drop, then report back. Currently set up as:
carb bodies: 2000 year
pilot jets: 45S
Front Main jet 175
Rear Main Jet 178
stock needles and shims (one washer)
stock slides, two holes each (not counting needle hole)
stock emulsion tubes in the stock location
idle screw: 2.25 turns front, 2.5 turns rear
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Old 10-22-2016, 04:11 PM
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Man, I would like some good news on this bike for a change. Plug pictures attached. Front on left, super lean. It never looked like that before - they have always looked more like the rear plug, which seems normal to me. The reason it looks like that is most likely the front float needle is stuck mostly closed. I know this because when I went to drain the carbs before the compression test, no fuel came out of the front bowl. I had only run the bike a short while to get it warm, like ten minutes, and then another ten minutes at idle to check syncronization, so hopefully the pistons and the valves are not burnt...

Then compression test, with a new o-ring on the compression tester, I saw 148psi rear and 128 psi front. I'm going to check the valve lash now. Hopefully the low compression in the front is valve lash and nothing burnt from fuel starvation.

Any advice appreciated.
Attached Thumbnails 1998 VTR low miles off idle sluggishness-plug-pix-20161022-front-left.png  
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Old 10-22-2016, 07:31 PM
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Mixture looks more than a bit lean on the left plug.
Is the tip of the plug on the left eroded away? Can't tell from the pic.
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Old 10-23-2016, 02:38 AM
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Originally Posted by xeris View Post
Mixture looks more than a bit lean on the left plug.
Is the tip of the plug on the left eroded away? Can't tell from the pic.
The tip is ok, or at least similar to the rear plug. I didn't get back around to checking valve lash last night... perhaps this evening. I'm not sure what would be causing compression loss at 5500 miles but it's not encouraging.
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Old 10-23-2016, 06:54 AM
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Originally Posted by BKFerrand View Post
The tip is ok, or at least similar to the rear plug. I didn't get back around to checking valve lash last night... perhaps this evening. I'm not sure what would be causing compression loss at 5500 miles but it's not encouraging.
Not that I think that it's the problem, but you should consider using the slides that are still stock with one lift hole.
As you have covered most of the possible simple causes it might be useful to do a leak down test. The loss of compression on the front cylinder is cause for concern as that is the cylinder that the CCT usually fails first. I would also check the timing on the front cylinder while I was at it.
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Old 10-23-2016, 02:49 PM
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Originally Posted by xeris View Post
Not that I think that it's the problem, but you should consider using the slides that are still stock with one lift hole.
As you have covered most of the possible simple causes it might be useful to do a leak down test. The loss of compression on the front cylinder is cause for concern as that is the cylinder that the CCT usually fails first. I would also check the timing on the front cylinder while I was at it.
I read the carb set up thread and intend to plug one lift hole on the front carb next time I'm in there. That's correct, yes? I wasn't planning on the 48 pilot jets just yet as I just wanted it running decently, but I suppose it won't hurt to order them now since it may be out of commission for a while.

I had wanted/hoped I could get the fueling issue straightened out before jumping into the CCT...perhaps not. I was just reading about the valves on the forum, and it seems they are unlikely to be a problem in and of themselves, and require shims to adjust anyway? I am afraid I burnt a valve in the front cylinder running it with the front float valve stuck mostly closed. I don't even want to think about dealing with that, but please point me in the right direction. I replaced the burnt valves on our 1951 Ferguson tractor, but that thing is very simple to work on. Maybe I will just ride the tractor for fun, and park the motorcycle.
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Old 10-23-2016, 03:01 PM
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Originally Posted by BKFerrand View Post
I read the carb set up thread and intend to plug one lift hole on the front carb next time I'm in there. That's correct, yes? I wasn't planning on the 48 pilot jets just yet as I just wanted it running decently, but I suppose it won't hurt to order them now since it may be out of commission for a while.

I had wanted/hoped I could get the fueling issue straightened out before jumping into the CCT...perhaps not. I was just reading about the valves on the forum, and it seems they are unlikely to be a problem in and of themselves, and require shims to adjust anyway? I am afraid I burnt a valve in the front cylinder running it with the front float valve stuck mostly closed. I don't even want to think about dealing with that, but please point me in the right direction. I replaced the burnt valves on our 1951 Ferguson tractor, but that thing is very simple to work on. Maybe I will just ride the tractor for fun, and park the motorcycle.
A leak down test should tell you if the problem lies with the valve. Whether burned or bent same amount of work either way.

Last edited by xeris; 10-24-2016 at 05:55 AM. Reason: duh
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Old 10-23-2016, 06:36 PM
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Originally Posted by xeris View Post
A leak down test should tell you if the problem lies with the valve. Weather berned or bent same amount of work either way.
Thanks. So, I decided to redo the compression test... I remembered to open the throttle this time... and saw 140 on the front and 150 on the rear. So those numbers seem a lot better to me. I will still do the leak down test, hopefully later this week. But I think I can fix that front float needle, check for vacuum leaks, and do the idle drop and synch... hopefully that will get it back to stock, or at least indicate what else is going on.
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