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Discussion Starter #1
Last night, a friend and I were looking at a few things on my TW, the intake vacuum tube that comes off the top of the manifold and curls over the top of the carb... those are supposed to be capped, correct? Mine is not, and I'm wondering if I may have had a vacuum leak (it's since been vacuum capped at the vent tube port on the intake).

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A leak, or missing part, in the circled assembly will cause a lean running condition with associated spitting and back firing on deceleration.. The TW is not the only bike that has some form of surge tower creating an air plenum to supposedly even out some of the pressure pulses in the intake tract. I would imagine the fancy aluminum fitting #23 could be replaced with a simple cap if one wished to maintain the plenum's air volume that Yamaha's engineers had designed in for the constant velocity carb..
 

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Thanks Fred, so is that component part of an emissions system? I just capped it at the port on top of the manifold, I don't imagine changing where the plug was should matter much.

You are correct, after I initial plug reads around the 200mi mark, I had already rejetted and adjusted pilots screw to 2 and 1/4 turns out, and added an additional 1/4 turn to adjust for the plug condition and lean symptoms running the bike.

After reading some older posts discussing the same issue, capped the port, adjusted back to 2 1/4 turns out and elected to move off of the iridium E3 plug I've been using back to a standard plug. My friend mentioned while it's a neat spark plug, the temperature range might not be within tolerances and would be better to move back to a standard plug and monitor for the time being.

Bike sounds and responds better from an initial run. I'll do another plug check when I take the bike out this weekend and see how she's looking.
 

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I figure Fred has it about right – there’s likely to be some back pressure from the intake valve closing that you wouldn’t want going back to the carb – seems a bit ineffective until you consider the unburnt fuel aspect

Yamaha used to use a “boost bottle” on their two strokes to recover unburnt mixture and provide the same “echo chamber”, but the length of that component on the TW is likely to be carefully calculated by the Mothership, and yes, it needs to be capped (though it’s a neat access point for Nitrous)

As for spark plugs, go back to standard until things settle down, and take it from there. Iridium is nice, but probably quite unnecessary …… ;)
 

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I ran the E3 plug for a bit but did not like the fact that it took a different size spark plug socket which was bulky and heavy to carry afield.

Would not know if the #23 fitting is considered emissions related, I really only had one credible person explain the fitting once so take whatever I say with a grain of salt. Would almost have to cut one open to understand its construction and function. However would not be surprised to find some sort of flexible membrane/ drum head to further dampen or average out intake pressure pulses thus mimicking a larger plenum. Was looking at another bike the other day & it had a ~100cc plastic bottle shape attached to intake manifold seemingly accomplishing same task.

These constant velocity carbs seem very sensitive to pressure variations between both sides of carb and atmosphere. Odd that pinching off, or submerging the carb atmospheric vents seems to stall these bikes within 5 seconds or so while gas tank vent stoppages will often take a few minutes to cause a flame out. Or so has been my experience.
 

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Discussion Starter #6
I ran the E3 plug for a bit but did not like the fact that it took a different size spark plug socket which was bulky and heavy to carry afield.
Yea, I was just thinking the same thing last night, I can yank that bulky socket out of my kit now.
 

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The volume of extra space in that hose keeps the diaphragm in your CV carb from oscillating harder at idle and just off idle scooting around slow. One side of the diaphragm in your carb is basically getting atmospheric pressure or higher on the air box side, while the other is responding to vacuum draw from the cylinder. It's not emissions related, and keeping that extra volume to smooth out the harder vacuum pulses on the diaphragm/suction piston will probably extended it's life... especially the flat sides of the plastic suction piston that slide inside the aluminum carb body. I wouldn't worry about that hose being absent and capped at the little manifold only if I was running a PWK or flat slide carb (even though it would still serve the purpose of smoothing out the idle circuit operation pulling fuel from the float bowl.)
 

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Discussion Starter #9
The volume of extra space in that hose keeps the diaphragm in your CV carb from oscillating harder at idle and just off idle scooting around slow. One side of the diaphragm in your carb is basically getting atmospheric pressure or higher on the air box side, while the other is responding to vacuum draw from the cylinder. It's not emissions related, and keeping that extra volume to smooth out the harder vacuum pulses on the diaphragm/suction piston will probably extended it's life... especially the flat sides of the plastic suction piston that slide inside the aluminum carb body. I wouldn't worry about that hose being absent and capped at the little manifold only if I was running a PWK or flat slide carb (even though it would still serve the purpose of smoothing out the idle circuit operation pulling fuel from the float bowl.)
Thanks MtnMan, I was a little concerned about capping at the manifold, but this info will be good to have in my notebook should I run into any diaphragm related carb issues down the road, in which case I will reinstall said tube assembly.

Fred, you can never have enough space for adult beverages.
 
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