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Discussion Starter · #1 ·
Hey Guys,

I know everyone sees a million of these a day with all having ambiguous solutions so I'll try to give a brief synopsis of what I think may be happening and then go in to detail for those who have more time and how I think it may effect the outcome.

The brief: The bike runs great full out and will even idle fine (around 1500 RPM, I know spec is 1350 or something). But when I'm running at full out or even getting there the throttle is hanging big time. Likes to hang anywhere between 3-5K RPM. It takes a long while and eventually if I mess with my throttle adjustment screw it will come back down. Makes it even difficult to shift at times because it is not responsive when decreasing throttle. I got tons of power, cranks up fine.

The long: I don't want to jump to conclusions about a big problem, but I picked up this track bike for really cheap. It probably hadn't been run in about 10 years. One day after putting new spark plugs in (chasing a big electrical debacle as well, which HISOWN6 may remember) and doing a complete electrical check of the bike I was able to get it to fire. After cleaning the carbs I had the bike running like a Swiss watch. Now, honestly its been a little while but in Nov, I thought that the throttle was super responsive, rising and falling like it should and everything ran great. It idled great, at a idle lower than it will now without quitting (I still think though that I remember the throttle being responsive). But I was pretty excited about getting it going in the first place so maybe I over looked it.

Flash forward about a month or so later i had to let it sit over christmas. I came back and cranked it up and had a hard time getting it to fire. Got it going on ether, but really didn't want to go. So I thought, fair enough, cleaned the carbs again, blew out all the needles with compressed air. I did not disassemble the throttle bodies, but I did take out main jet, pilot jet, needle jet etc. I did NOT mess with any screw settings.

So after that the bike actually cranked up and would idle (for the most part) but the the bike still had the throttle hanging. I know for a fact that steering and mechanical connections have nothing to do with it hanging. The lines are completely unobstructed.

I have manually choked the bike with the airbox off, and it dies which apparently means there is no problem with a vacuum issue.

Based on my research it appears as though a throttle hanging problem can be adjusted by screwing the idle/mixture screws out a little (within the carb itself... you know the screw they tell no one to touch). I just dont' know how this could have changed from when it was not having any issue and for me to walk away and for it to me happening.

That being said, I have no idea what the previous owner did to this thing. Since it was a track bike at one time maybe they rejetted it and maybe I just missed the signs earlier... Tottally could have happened.

Right now I'm thinking about turning out the idle/mixture screw out a little bit on all 4 carbs (maybe a half a turn)... to see if this might help... Any suggestions any thoughts? Sorry for the novel!
 

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Discussion Starter · #3 ·
Going to dig into this today. Just took the carbs off and decided to pull the plugs while I was at it. It appears that all of the spark plugs are heavily oil/carbon fouled. Makes me think for sure there is a incorrect air/fuel mixture. None of the other likely candidates seem like a good option at this point in time. The plugs were new just a month ago so maybe something going on here. Anyway Will check back in after I mess with the pilot screws and hit the forum back up in a bit. Just trying to help other people down the way that mess with the same thing.

Even if no one cares... i guess... lol
 

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Discussion Starter · #4 ·
So I'm pretty sure I'm runnin a rich condition. Lots of after fire in exhaust pipe, heavily fouled plugs, excessive fuel consumption, and really high idling, never feels like engine stutters or is starving for gas. So for mixture adjustment this is what I've learned and will apply to my screws which were 2.5 turns out originally:

So two things we know for a fact 1) a rich condition can be caused by either too much gas or not enough air 2) a lean condition can be caused by not enough gas or too much air.

On carbs there is generally one fuel/air mixture screw. The screw either controls for air or fuel but not both. If the screw is located more towards the air intake side of the carb then it controls air. If the screw is located more towards the valves or engine side of the carb then it controls fuel.

By screwing "in" either of these components you "lessen" the amount of that element associated with the screw. So when you screw an air screw in you lessen the amount of air thereby creating a rich condition (see rule #1), when you screw it out you increase the amount of air thereby creating a lean condition (see rule #2).

Now, exact opposite can be said about the mixture screw If it is controlling for fuel. Remember, when you screw "in" you get less of the element it controls for. So screw "in" would mean less fuel which would mean lean condition (see rule #2), whereas out would be more fuel or a rich condition (see rule #1).

You do the exact opposite to rich/lean condition depending on if you have a fuel or air mixture screw so you have to keep this in mind when figuring out what your condition is and where to adjust from there depending on what type of screw you have.

So I actually believe in my case since I'm running rich. And I have a fuel screw, I need to turn my screw in for a more lean condition. I'll let you know how it goes.
 

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Discussion Starter · #5 ·
Well this is frustrating, turned the fuel screws in 1.5 turns. Still having a hanging throttle issue. I was still getting some after fire (literally flamings coming out of the back pipe here and there I think indicating still a rich condition. I guess I'm going to try making my fuel screws only one turn out total and seeing if that helps.

Based on my research it appears as though if I need it to go in too far (past 1.5 turns out) then I may likely have to rich of pilot jet. Which I guess could makes sense, since this thing was tricked out for racing before.
 

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IMO its not a 'jetting' problem, its a mechanical one in the carbs themselves. Do the carb slides glide freely in their bores (raise and lower) ? The engine won't run 3k-5k RPM without the slides being raised, as there isn't enough air flow to. Do all plugs look the same ? Have the float needles & seats been replaced ?
 

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Discussion Starter · #7 · (Edited)
IMO its not a 'jetting' problem, its a mechanical one in the carbs themselves. Do the carb slides glide freely in their bores (raise and lower) ? The engine won't run 3k-5k RPM without the slides being raised, as there isn't enough air flow to. Do all plugs look the same ? Have the float needles & seats been replaced ?
The carb slides to seem to be gliding freely in their bores indeed. Have the airbox off so can watch them do their thing. They all seem to be in the same position and are pretty smooth. All the plugs did the look the same. I have not replaced the needle valve attached to the float. In fact nothing has been replaced since I got it.

It was very strange because, in November I though it was running great. But let it sit for a month and all of a sudden it seems like throttle hanging is a new issue. I'm assuming that guy before me rejetted it (because it was a track bike), but I cannot confirm. Since I've had it I have just cleaned the crap out of all the jets including the jet needle, main jet, needle jet, and pilot jet. On inspection everything looks ok.

I tried going in to almost half a turn with the fuel mixture screw and I would say it definitely did not get any better, maybe even got worse. So since my baseline was 2.5 turns out I just tried 3... It may have been a little better but I'm going to try 3.5 turns out. I read in a few places that just because my plugs are fouled and looks like they are running rich, it could be more of an indication of whats going through my main jet. So I'm going to try to richen the pilot jet a little bit more and see what happens.
 

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Discussion Starter · #8 ·
Well - this kind of blows. Was really hoping that I would be able to find a sweet spot while messing with these things. But I've tried as extreme as 1 turn out to 4 turns out... Throttle is still hanging like crazy. Takes forever to call back to idle. Then doesn't really want to idle that much... Not sure what I learned here today.
 

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Discussion Starter · #9 · (Edited)
Kind of out of immediate options right now. So since adjusting the the fuel mixture screw it appears to me as though if it was the answer to my issues I would have seen some differences in throttle responsiveness, in other words some sort of setting where the throttle didn't hang. It makes me think I have a component issue. So I'm taking all the carbs apart again... Just for fun to see what I see. So far looks like my bike may have a jet kit. Apparently if there are little ribs on the jet needle with washers and a setting that that means there is a kit. I noticed on carb 1 on the main jet it has the number: 120. The pilot jet has a funny symbol that looks like half a star and there is written: 38S.

Looks like the star symbol might be keihin...starting to make sense... does not look like a stock main jet but the pilot jet according to my book is supposed to be a 38 for carb 1.
 

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Discussion Starter · #10 ·
Just an update-decided to give it another clean out. This time taking the pilot screw all the way out and using compressed air to clean that pathway. Going to let it sit for a night and see if I can come up with any other brilliant things. Otherwise may just reassemble and see if for some crazy reason I'm able to bring my idle back into check.
 

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Ok, this is a tricky one, but lots of things to think about. First off, you're on the right track with your research. When the idle hangs, there's too much air. Similar thing as if you kept the throttle open. But often severe problems with rich/lean symptoms can appear to be one when it's actually the other. Your fouled plugs indicate excess fuel. But you don't know if it's overly rich everywhere, or rich on one end and lean on the other.

Air could be leaking through a variety of places, and on a 16 year old machine there's potential for cracked hoses or diaphragms. Sounds like the diaphragms are ok (they provide vacuum seal to pull the slides back) but go ahead and inspect them when you've got the carbs out. There's also vacuum port nipples under the throttle plates used for balancing that should be capped off. Take a good look at all the vacuum tubing and replace any that have cracks.

Another possibility is that the idle screw is actually open way more than it should be. Sometimes people arrive at this setting because there is actually excess fuel. The overly rich mixture causes poor idle and opening up the idle setting appears to fix it because the additional air gives a better mixture. But then you get a lot of hanging until it can settle out. This situation can potentially allow fuel to be pulled from the entire pilot circuit, not just the holes metered by the fuel screw. At idle the fuel flow rate should be dominated by your fuel screw setting. Off idle the throttle plates expose more holes and the size of your pilot jet becomes the limiting factor. You should visually be able to see the throttle plates block off some hole (or holes) when it's closed.

It does sound like you've got Keihin jets. The funny half star is actually a "K". Even with a pipe and high flow filter, the stock pilot size should work, so you're probably good there, although I don't know if 38S means anything different than 38. What does concern me is your main jet labeled as a 120, where the stock size is 148 (for cylinders 2 & 3) and 152 (for 1 & 4). Now since you've got an aftermarket kit, the needle shape can be very different, so just swapping a few OEM jets is not likely to fix things. I know Chiefsmokedawg said that these bikes used smaller jets when modified. It's backwards from everything else, but I've never had a 1st gen and he does so he knows more about the specifics of this bike than me. 120 is a huge difference though. I would expect to only see it 1 or 2 steps away from stock. But there are instances where the jet number size from different manufacturers are different. So MAYBE some of your sizing is off, but at this point I'm very reluctant to tell you to shell out $100+ for a full kit just to verify that you've got a properly sized set. And I have a hard time believing that in an apples-to-apples size comparison you've got a 120 but should have a 140+. Maybe someone more familiar with 1st gen jet sizes and kit options can chime in here. I don't want to focus too much on your mixture screw setting either because that is the last part to fine tune and it's pretty much irrelevant when there are other issues. But try finding info on what sizes or kits tend to be run on bikes with similar mods to yours.

Another potential issue is that you're leaking fuel from somewhere. Perhaps a bowl needle isn't sealing and it's dumping in extra fuel. You may have already done so, but inspect those needle tips, make sure the bowl can compress the spring and seat the needle when it floats, and verify the float height. Float height is more critical than many people realize. With the airbox still opened, immediately after shutting off the engine, open the throttle and shine a flashlight down each carb throat. Look for signs of fuel dribbling down the wall. Check each one and do it quickly because fuel evaporates quickly. Try this both after idling, and after revving it up a bit. When there is a continuous fuel leak like that, a lot of it goes right through the combustion chamber and burns once it gets in the exhaust system.

Going back to the fact that is was running ok, or at least you think it was... perhaps you just didn't get the pilot jets as clean as you thought you did. The passages are so small they are the first parts to clog up. You could be in the situation I mentioned earlier where you opened the idle setting so much that additional fuel ports are exposed and they are making up for some clogged passages.

Sorry to just dump a ton of ideas like that, but hopefully it gives you a few things to rule out and maybe lead you to some suspect parts.
 

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I read the whole thread, and very well could have missed this, but have you tried synchronizing the carbs yet? Sometimes this can make a huge difference. Other than that, I totally agree that hang up on decel, is a lean condition.
 

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Discussion Starter · #14 ·
Ok, this is a tricky one, but lots of things to think about. First off, you're on the right track with your research. When the idle hangs, there's too much air. Similar thing as if you kept the throttle open. But often severe problems with rich/lean symptoms can appear to be one when it's actually the other. Your fouled plugs indicate excess fuel. But you don't know if it's overly rich everywhere, or rich on one end and lean on the other.

Air could be leaking through a variety of places, and on a 16 year old machine there's potential for cracked hoses or diaphragms. Sounds like the diaphragms are ok (they provide vacuum seal to pull the slides back) but go ahead and inspect them when you've got the carbs out. There's also vacuum port nipples under the throttle plates used for balancing that should be capped off. Take a good look at all the vacuum tubing and replace any that have cracks.

Another possibility is that the idle screw is actually open way more than it should be. Sometimes people arrive at this setting because there is actually excess fuel. The overly rich mixture causes poor idle and opening up the idle setting appears to fix it because the additional air gives a better mixture. But then you get a lot of hanging until it can settle out. This situation can potentially allow fuel to be pulled from the entire pilot circuit, not just the holes metered by the fuel screw. At idle the fuel flow rate should be dominated by your fuel screw setting. Off idle the throttle plates expose more holes and the size of your pilot jet becomes the limiting factor. You should visually be able to see the throttle plates block off some hole (or holes) when it's closed.

It does sound like you've got Keihin jets. The funny half star is actually a "K". Even with a pipe and high flow filter, the stock pilot size should work, so you're probably good there, although I don't know if 38S means anything different than 38. What does concern me is your main jet labeled as a 120, where the stock size is 148 (for cylinders 2 & 3) and 152 (for 1 & 4). Now since you've got an aftermarket kit, the needle shape can be very different, so just swapping a few OEM jets is not likely to fix things. I know Chiefsmokedawg said that these bikes used smaller jets when modified. It's backwards from everything else, but I've never had a 1st gen and he does so he knows more about the specifics of this bike than me. 120 is a huge difference though. I would expect to only see it 1 or 2 steps away from stock. But there are instances where the jet number size from different manufacturers are different. So MAYBE some of your sizing is off, but at this point I'm very reluctant to tell you to shell out $100+ for a full kit just to verify that you've got a properly sized set. And I have a hard time believing that in an apples-to-apples size comparison you've got a 120 but should have a 140+. Maybe someone more familiar with 1st gen jet sizes and kit options can chime in here. I don't want to focus too much on your mixture screw setting either because that is the last part to fine tune and it's pretty much irrelevant when there are other issues. But try finding info on what sizes or kits tend to be run on bikes with similar mods to yours.

Another potential issue is that you're leaking fuel from somewhere. Perhaps a bowl needle isn't sealing and it's dumping in extra fuel. You may have already done so, but inspect those needle tips, make sure the bowl can compress the spring and seat the needle when it floats, and verify the float height. Float height is more critical than many people realize. With the airbox still opened, immediately after shutting off the engine, open the throttle and shine a flashlight down each carb throat. Look for signs of fuel dribbling down the wall. Check each one and do it quickly because fuel evaporates quickly. Try this both after idling, and after revving it up a bit. When there is a continuous fuel leak like that, a lot of it goes right through the combustion chamber and burns once it gets in the exhaust system.

Going back to the fact that is was running ok, or at least you think it was... perhaps you just didn't get the pilot jets as clean as you thought you did. The passages are so small they are the first parts to clog up. You could be in the situation I mentioned earlier where you opened the idle setting so much that additional fuel ports are exposed and they are making up for some clogged passages.

Sorry to just dump a ton of ideas like that, but hopefully it gives you a few things to rule out and maybe lead you to some suspect parts.
Plethora of awesome knowledge here. Didn't overwhelm me at all. Pretty much all of what you said made lots of sense. So I'm slowly putting things back together after a thorough cleaning - I wanted to check float height, but it appears the only factory spec is for fuel level. It appears as though my float apex hangs just a little less than half an inch past where the float bowl cover sits.

I really appreciate all the advice. I'll report back once I get this thing put back together. Taking my time for now.
 

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Discussion Starter · #15 ·
:toocool: GOT IT!

Ok - so no one ever seems to post follow-ups. So here I am posting a follow up.

Thanks to everyone who chimed in. Have to give another shout out to HISOWN6, since he took the time to really run through some differentials with situation.

I'm kind of starting to feel like a moron. But I'm glad I have stuck with it and not started just buying parts or anything before exhausting all options. Frankly I was going to be a little stumped after this last to at it.

I want to state one thing that keeps me going here. I'm in the medical profession, and if there is one thing we always come upon is that "common things are common,", this can be said many ways, keep it simple stupid, whatever, but at the end of the day - Look to what makes sense. :bash

I stated in the beginning that I had a hanging throttle after letting my bike sit for a month. I don't have a ton of faith in this thing yet because I bought it after it sitting for 10 years or more and who knows what the nature of the beast is. So when I finally did get it going (just a new set of spark plugs, a rap of the hammer on the starter, and a THOROUGH carb clean) I though it was all good. Still, it needed lights and all sorts of other things before it could be street worthy - just an electrical nightmare but I figured it out. Anyway - after letting it sit for a month the idle was hanging bad at around 4-5K sometimes more, never wanted to idle really well (and no it wasn't obvious things like idle adjustment screw, I know that crap). It wasn't pinched cables. I don't have time for that kind of BS (good things to check first though I guess). Anyway I was SUPER bummed that the bike wasn't running well, so of course I panicked and cleaned the carbs immediately. Took me about 45 min to an hour. Gettin quick at this!:D A little too quick come to find out...

Even after reassembly I still had the same problem. So I went through all the steps in the aforementioned posts and still no good. Well this last time I was out of options, so I very slowly and very carefully cleaned all the carbs (I did not separate the bank because I think for the most part everything is synched ok, so didn't want to mess with that). This time I actually investigated the pilot screws very carefully. While you could see through all of them just barely, one definitely seemed like you could see through it more. So I took some copper wire (I know I know, not supposed to use wire but HEY, I wasn't buying new jets - even if they are pretty cheap), and rang the other three out, and hey what do you know, they looked just a little more open. Not dramatically though, I wasn't at all confident that that was the issue. So I put everything back together SLOWLY!

Cranked it up and :flame She runs like a swiss watch. No more hanging idle. All is good. I really think it was clogged pilot jets. Even though there was passage there just wasn't enough. One of these days I may need to just get some new ones to start anew with any of that oxidation buildup that I feel like gets worse as time goes on regardless of cleaning.

Anyway, thanks for the help. It was a simple carb clean that did the job. So my final note is that common things are common so look to the easy things first before getting panicked.

That's all!:cheers
 
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