WOT hesitation or subtle bog
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  1. #1
    Senior Member RockyTFS's Avatar
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    I have 2900 miles on my '11 T-dub. I added one shim washer to the needle and opened the pilot screw one turn in the first month because I was going down to sea level. The bike ran perfectly and continued to do so until this spring when I began to notice that it would not always get all the way up to 65 on the same stretch of road it used to.



    It seems to be slowly getting worse. I use 87 octane real gas exclusively. I just adjusted the valves, they were .008 EX, .007 IN, and I brought them back to .005 and .003. No change. I cleaned the tank, a little fuzz came out but the valve filters were perfectly clean. I did a flow test, plenty of flow, ten times what the engine needs.



    I pulled off the top and the needle had some slight wear in two spots, due to the fact that I bent it 2000 miles ago and hadn't got it perfectly straight. It is now and I polished the two tiny spots. A new needle is on order. However for 1500 miles the bike ran fine like that.



    I just did a WOT plug cut and the plug looks fine.



    So, I'm stumped. The bike runs great at any setting except WOT. No hesitation, no difficulty starting, idles fine indifinitely, and so on. It still has the factory .126 main jet, which should run just right at 5,000 to 7,000 feet, and did so for almost a year, as well as performing adequately at near sea level in cooler temperatures.



    So I am about to take the float bowl off to have a look for abrasion of the main jet where the needle was touching it and perhaps the main nozzle too. Considering the tiny amount of wear on the needle I doubt that there will be much. I has occured to me that contact between the needle and the jet would make it sticky, but this would affect acceleration and there is no symptom of that.



    It acts so much like fuel starvation at WOT fuel flow that I'm trying to figure out where else in the system besides the obvious float level this could happen. As soon as I get the bowl off I'll check the float needle. The carb is so new and I've been careful to clean the air filter I can't imagine that there is any goo or varnish in there, but since it will be apart I will clean it ala the Sticky post.



    Any ideas?



    EDIT: I just pulled the float bowl, it's clean as a whistle, same for the main jet and main nozzle. The bushing had a slight burr on the top end, it looked like it had been very slightly crushed around one side of the opening. This is likely where the needle showed some abrasion. I polished it out. What I did notice is that the needle, although appearing straight, did not center, either with the bearing in place or not.



    So if the needle was hanging up as the vacuum tried to pull it all the way up, could that cause the WOT bog? I'll rotate it in the diaphragm to see if it feels better in a different position.
    Rocky
    2018 TW200
    2014 BMW R1200GS LC

  2. #2
    Senior Member lizrdbrth's Avatar
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    Sure could.



    Also if you're not already doing so check the float level using the clear tubing method rather than using the static measurement with the float bowl off. If it checks out ok drain the float bowl and recheck it a couple of additional times just in case it's hanging up intermittently. If you have it apart again measure both floats and make sure one float hasn't become tweeked higher or lower than the other, cuz that one will make you crazy.



    Just my .02. Sometimes you gotta figger out what it ain't



    I can explain it to you, but I can't understand it for you.

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  3. #3
    Senior Member mhomadness's Avatar
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    Liz, what do you think about the diaphragm? I realize it is pretty much brand new, but here is my question… If the diaphragm is removed & reinstalled several times, could this cause enough damage / distortion to it to cause it to react improperly?



    I must point out that I have no experience with the diaphragm actuated carb on TW’s, just going on a gut feeling from my experience with small 2 cycle diaphragm carbs (string trimmers, etc.) and how much of a difference in performance the condition of the diaphragms can make. Just taking a stab at it. m.



    (PS, not obsessing on the word “diaphragm”, no, really!)

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  5. #4
    Senior Member RockyTFS's Avatar
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    Quote Originally Posted by Mikes House of Madness View Post
    Liz, what do you think about the diaphragm? I realize it is pretty much brand new, but here is my question… If the diaphragm is removed & reinstalled several times, could this cause enough damage / distortion to it to cause it to react improperly?



    I must point out that I have no experience with the diaphragm actuated carb on TW’s, just going on a gut feeling from my experience with small 2 cycle diaphragm carbs (string trimmers, etc.) and how much of a difference in performance the condition of the diaphragms can make. Just taking a stab at it. m.



    (PS, not obsessing on the word “diaphragm”, no, really!)




    I looked very very carefully at the diaphragm every time I had it out, which is now about 5 times. It is indeed very easy to damage, and must be handled gently, especially when putting it back in. You can get it out without pulling the throttle cables, but only just, and there is a distinct tendency when the carb is at an angle for the needle not to want to go into the bushing and nozzle....that's how I bent it. So now I insert the assembly part way, rotate the carb and let the needle drop in, then rotate it back to get the screws in. (be very careful with the large diaphragm spring, it can get kinked easily.)



    But, I don't think that the mere removal and re-install, if done carefully, will have any effect on the diaphragm itself.





    I went back in this morning and polished the scratches on both the needle and the bushing some more with 600, fiddled with rotating the needle and got it to fit straight. Went for a ride on the same stretch and noted some slight improvement. It's very subtle and the difference between a rougher patch of pavement and a smooth patch can pretty much mask it.



    It also occurs to me that when the bike seemed to be running WOT better a few months ago, the temps were in the 50's, not the 80's like the last few days. Maybe I'm chasing a chimera. Also, the new Kenda 120/80 front tire may have some more rolling resistance that the original one.
    Rocky
    2018 TW200
    2014 BMW R1200GS LC

  6. #5
    Senior Member lizrdbrth's Avatar
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    Ah, the wunnerful diaphragm (1). The one reason I run slide carbs even though I could probably benefit from a CV. Costs like 70 bucks, er sumpin' and when they let go there's no substitute and you need a thousand watt bulb and eyes like a supreme predator to find the crack? Yay!



    You're correct to handle them like glass.



    I'm not saying it's what's wrong with your carb but once you've exhausted every other possibility I can give you my last-ditch, Mickey Mouse, not-always-surefire method for determining whether it's a micro-crack in the diapragm (2). I can't help you with the possibility of the needle hanging up other than to suggest removing the airbox boot and trying to catch it in the act.



    Take the diaphragm (3) and slather it in Vaseline, Never-Sieze, wheel bearing grease, something thick yet removeable. Both sides, and lay it on thick, up to 1/16" or so. Quickly slap it together and go for a ride, but not so long as to build up a lot of engine heat. If the problem goes away, Bob's yer uncle. This ain't real good for that type of rubber over time so make sure to clean it thoroughly if it turns out there was no change.



    I'm still betting fuel delivery or the bent needle.



    I gotta say diaphragm (4) three more times so Mike's obsession with the werd "diaphragm" (5) won't seem as obvious.



    I can explain it to you, but I can't understand it for you.

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  7. #6
    Senior Member RockyTFS's Avatar
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    Quote Originally Posted by lizrdbrth View Post
    Ah, the wunnerful diaphragm


    A micro crack! I never thought of that! Somewhat unlikely with a year old diaphragm...(there's that word again!).... but one never knows until every other possibility has been exhausted. We'll see what happens on my upcoming trip to Colorado. One thing for sure, if it's runnning a little lean WOT here it sure as hell won't be at 13,000 feet!
    Rocky
    2018 TW200
    2014 BMW R1200GS LC

  8. #7
    Senior Member RockyTFS's Avatar
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    UPDATE:



    After going to Colorado and running up to 12,800 feet with no problem, I think that the second iteration of straightening the needle and polishing the bushing fixed the problem.



    I had it up to 70 indicated today (13 tooth countershaft sprocket) and there was no hesitation at all. I will be replacing both the needle and the bushing, which showed contact wear surfaces and burrs as a direct result of bending the needle 9 months ago.



    While it is difficult to believe that the total flow though the jet and bushing would be much different with an offcenter/bent needle, it doesn't take much imagination to think that the spray pattern and subsequent vaporization would be affected.



    If the problem does not re-occur for several years with a new needle and bushing, I will believe this anaysis.
    Rocky
    2018 TW200
    2014 BMW R1200GS LC

  9. #8
    Senior Member mhomadness's Avatar
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    I'm glad to hear it's not the... almost said it!



    Sounds like a nice ride. I need to make the time to take a good ride 4 summer's gone! m.

  10. #9
    Senior Member RockyTFS's Avatar
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    Update on this thread:



    I just returned from Utah, where I was riding on a paved road at 10,000 feet. I started getting exactly the same behavior as I originally posted, a bog at WOT that got worse up an incline (to the point of having to go down to 4th) and better down same). Riding in the dirt in third or 4th on the side trails off this same road at up to 10,500 produced no problems. I have since realized that when I was in Colorado at 13,000 feet, I was in the dirt and never got out of third until I hit the pavement again at 9500 on a downhill all the way back to Aspen at 7800.



    Now after the replacement needdle and bushing, the bike is fine at 6500 as I said in the previous post, but the symptoms start again at 9000. It seems rather obvious at this point that my main jet is too rich for that altitude at WOT, so I ordered a #122. We shall see if this fixes it! However, we don't HAVE any paved roads in Idaho at 10,000, so it may be a while before I stick that #122 in there.



    (For those with similar problems getting an OEM Yamaha #122 jet, I had to order PN 288-14355-61-00, which is the superceeded PN for 288-14343-61-00, and from its price and description apparently includes both the main jet and the nozzle.) We shall see.
    Rocky
    2018 TW200
    2014 BMW R1200GS LC

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