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Discussion Starter #1
Hi everyone!



When I ride at sustained speed, I sometimes feel like the engine "miss a beat". Is it what is called a lean surge? I've red qwerty's post about carb tuning and he describes it like a quick roll off/on of the throttle, that's what makes me think my problem might be.



Am I looking in the right direction?



The only thing I've done to the carb was to add a 130 main jet, shim the needle to 0.03 and open the pilot screw to 2 1/4. It did not cure the lean condition. Spark plug went from snowy white to light grey. I have a stock and clean air box.



Thank you for your help!
 

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With situations like this, you need to be very 'specific'. Folks can make guesses but likely can provide a great deal more insight if YOU provided more (detailed) information.. I suspect, those that can help you would like to know rpm. I gather per your other posts, you do not have a tach. No problem, let folks know the speed and gear you are at when you experience this "surge".



My bike has lots of mods. I as well have played with the carb, but it seems the process is not as yet, correct. I get a bit of hesitation/surge around 3800rpm's. I have not raised the needle as much as others, perhaps this is the issue. As I am not asking the question, I will not belabor my 'situation'.



Guitarman, we likely would profit from more information. Gerry
 

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Discussion Starter #3
I'll try to give you as much info as I can, but since it doesn't happen all the time, it's hard for me to be very precise.



It usually happen when I'm in 5th gear, sustaining a speed between 50-60 mph.



It seems to be more frequent when I'm climbing a hill, and even more if my girlfriend is on the back.



Throttle is somewhere between 3/4 (maybe a bit less) and full.



The "surge" is like a quick miss, I feel pushed forward for a split second (the bike decelerate).



Is there any other info that could be useful?



Thanks again!
 

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A lean surge is a slight speeding up and then slowing down when the throttle is held steady in one position.



It should happen in any gear, but usually more noticeable at smaller throttle openings.



With the larger main jet and shims, it shouldn't be lean... unless there is a vacuum leak.



Your "miss" sounds like something else... but that's just a guess.



jb
 

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My 2005 does the same thing and it is driving me crazy. Always at high rpm when sustained. Doesn't mater if I'm in 4th or 5th. I also have a 130 main, 2 1/2 turns and 3 shims. I'm really starting to think the problem is electrical. Because it is so sudden and of such short duration. Only cuts off for a second then picks right up. I'm open to suggestions but I'm pretty much done with clean the carb or petcock. I have! Again and again! It is a new tank, new petcock, new float needle assembly, new needle, carn is spotless inside and out, ran guitar strings thru every oraface. Bike really runs great other wise. Just cuts out now and then while holding sustained high rpm, I almost thought it was a rev limiter. It was really bad when riding up long highway hills. I'm hoping qwerty might have a suggestion.
 

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Discussion Starter #7

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My 2005 does the same thing and it is driving me crazy. Always at high rpm when sustained. Doesn't mater if I'm in 4th or 5th. I also have a 130 main, 2 1/2 turns and 3 shims. I'm really starting to think the problem is electrical. Because it is so sudden and of such short duration. Only cuts off for a second then picks right up. I'm open to suggestions but I'm pretty much done with clean the carb or petcock. I have! Again and again! It is a new tank, new petcock, new float needle assembly, new needle, carn is spotless inside and out, ran guitar strings thru every oraface. Bike really runs great other wise. Just cuts out now and then while holding sustained high rpm, I almost thought it was a rev limiter. It was really bad when riding up long highway hills. I'm hoping qwerty might have a suggestion.
Hi N2o2,



I have the exact same carb setup as you on my '05, and regularly ride long uphill stretches in the mountains. There is no hesitation or miss...just a slowing down of speed if the hill is too steep.



It won't hurt to check the float level.



I suspect it may be something else, but am not sure what.



Sorry I can't offer a definitive cure.



jb
 

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Discussion Starter #11
I get three guesses, right?



Spark plug, vacuum leak, float level.


Thank you for your help!



I just changed the spark plug and checked if there was a vacuum leak, both joints seem to be ok.



I've checked the fuel level with the technique described in the shop manual (the supplement) and it was a bit low (just under the line on the float chamber).



Do you know if the carb has to be vertical like in the schema of the manual? Even if the bike is on a level surface and standing straight up, it seems to lean forward a bit. Should I lift the front wheel in order to have the carb totally vertical before checking the fuel level??
 

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Those questions I cannot answer. I always remove the carb and measure the actual float level. I would guess that as long as the bike is level side-to-side, and you take the reading at the fore-and-aft center of the float bowl, you'll be close enough.



There are other potential vacuum leaks, such as the various seals, hoses, and diagphrams exposed to vacuum. Any hard part can be cracked.
 

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Those questions I cannot answer. I always remove the carb and measure the actual float level. I would guess that as long as the bike is level side-to-side, and you take the reading at the fore-and-aft center of the float bowl, you'll be close enough.



There are other potential vacuum leaks, such as the various seals, hoses, and diagphrams exposed to vacuum. Any hard part can be cracked.


Just to get my name on the list along with n2o2driver, jbfla, and Guitarman, I too have the 'magical miss" at sustained rpm with my 09. Carb is clean, has a 130 main, 2 3/4 turns out, and 0.60 in needle shims. So far I have blamed it on bad gas, float level, and a dirty petcock (wasn't). During today's ride I decided it was the wrong size pilot, which is the stock #31. But NOBODY on the Forum has reported needing to change their pilot jet. BTW, I get a sort of similar miss when crossing creeks, so I'm now suspicious of the kickstand switch. More symptoms, no solutions from me! So I'll just be sitting over here on the sidelines and wait until somebody figures it out!





UPDATING THIS POST AS OF JUNE 21, 2012: Happy to report that I've accidentally solved mine by simply reinstalling the original stock plug. I was running some type of a fancy NGK. Only change I made, and poof! no more miss! Can't tell you why, but it is G O N E!
 

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I'll add to the pile.



The manifold vacuum storage tube connection at the manifold.



Powdered and/or charred manifold o-ring.
 

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I'll join the club. My 1995 has done the same thing. It's like I hit the kill switch momentarily. Off, then immediately back on, but only at sustained high rpm as has been described. Otherwise it runs fine with a totally stock carburetor. It acts to me more electrical than fuel related. On mine it's a miss, not a bog like you shut off the fuel. Anyway, I don't spend much time at high rpm so it hasn't been much of an issue to me, but I hope we can figure out what's going on.
 

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I was experiencing the same symptoms not long ago. I purchased the bike with 800 miles on it, a ’97, and the carb was toast. So I threw a brand new carb on it. Ran poorly. After much denial and error, I ended up shimming the needle .040”, 2&1/2 turns on pilot, & leaving the stock main jet in. Also had to set the float level. The bike performs almost right at this point, but not perfect.

By shimming the needle & installing the stock main jet, I stumbled into a near perfect setting. But I think there is room for improvement. The bike runs much better on warm to hot days than cool to cold. I’m pretty much at sea level, I wonder what would be a good direction for me to go on the main jet to see if I can smooth it out just a little more? If my memory serves me right, I think my main is 118? What’s your take guys? Thanks,
 

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Discussion Starter #17
Oh my my!




I changed my NGK Iridium spark plug for a NGK Standard one. I've put about 100 miles on the bike since then and what I can say is "so far so good". I've not noticed a miss yet. I will report back again as miles are piling up.
 

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Looks like this will soon become the fabled 'TW miss' much like the TW front end clunk. Perhaps like the all to common faulty clutch spring hook-up there is some mis-written assembly document circa 1989 that has frequently gets some new Yamaha tech. to hookup the coil ground wire to the neutral lite.



For me, the hic-up came when I started messing with my carb (jets/shims). I assumed it is a less than perfect transition from low to high speed. It seems like, for a milli second somewhere between 3500 and 4200 rpm's the carb sends the combustion chamber nothing but air. The sensation seems to be an exact 'spot' and if I were to ride hard through the gears I would never notice it, it's only when I am leisurely moving up the rpm range with a light load on the engine. It is very seldom that I go over 40mph and when it happens I generally don't hold that speed very long.



What strikes me as odd is the lack of commonality. Apart from being Yamaha TW's we have a range of years of manufacture, jetting, stock or modified carbs, even different brands of carburetors. It is hard for me to imagine my issue would have come as a result of a 'change in float height'. Can float height change on its own?



For me, when I get motivated I will play a bit more with my needle height. Just recently brought it up from around .022 to .032 (I think) to deal with my hesitation. Maybe,, just maybe the increase shorten the duration of my 'pause'.



As well, was having some reluctance getting the engine started quickly when warm using the electric starter. Seemed once warm, the kick start was the only way to go. A new spark plug cured that pronto. Now, just give the electric start button a quick tap, and the engine comes to life...... Good luck to all. Sure would be nice to cure the mystery hesitation. Gerry

I
 

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Discussion Starter #19
I went with a bigger jet today, a 132 (I had a 130).



The quick misses came back.




We were riding two up (me and my gf, 320lbs total). Each time we climbed a hill at high speed (between 50 and 60, WOT) the miss happened (as the bike was decelerating).



Moreover, I was sometime getting a miss in 3rd if reved high at WOT.



I went back home and put the stock 126 jet back in the carb.



Will give you updates as soon as I get more results.
 

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I'll add to the pile.



The manifold vacuum storage tube connection at the manifold.



Powdered and/or charred manifold o-ring.


Help me out; The "manifold vacuum storage tube connection at the manifold" is refering to the tube that exits the top of the manifold and eventually exits up near the gas tanks pucks? The tube is capped off on my 09 model. I filed its presence away in my brain as the hose required by the CA models for their canisters and haven't given it a thought since first seeing it! Since then that tube has simply been invisible to me! High rpm definitely produce the highest engine vacuum, and so well may be the culprit.



I've even accidently created such misses in the past by simply failing to tighten the connection manifolds after loosening them to rotate the carb for work! Leaving the manifolds loose magnifies the problem so much that it makes the same issue look like two completely different issues!
 
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