I've been trying to fix this "cold-start, warm-start" issue for quite a while now...like since I bought this one...and I want to share my path to success:
First of all, a summary:
- With either of my previous models, I could cold-start them then walk away to get my gear, etc while they fast-idled until "warm". Then I could push in the choke and they would idle at factory-spec, at which point I could ride away. Not this one...cold starts were awful; with full choke she would sputter and cough and run like crap...I would literally have to babysit for the first few minutes, until she warmed up enough to idle on her own without choke.
- On warm-starts with the previous models, all I had to do was touch the starter button for a split-second and they would restart instantly...this one took a bit more effort, although they weren't as bad as the cold-starts.
- I lost track of how many times I changed out the pilot jet...from a #31 to a #34 trying to find a happy medium...I even changed out the main jet to a #128, hoping this would solve the problem...it didn't.
So...I concluded that this carb needed a good ultra-sonic cleaning, so I dismantled it completely and placed it in the U/S with a Dawn dishsoap solution.
It was incredible how much dirt and crap was removed by this process, but when I went to dump the solution afterwards, I noticed a small black disc in the waste water. Unable to grab it, it disappeared down the drain. At first, I thought it was some plug for an unused port of the OUTSIDE of the float bowl. I reassembled the carb and reinstalled it and the first "cold-start" I tried was the worst ever; lots of backfiring and rough idle...once warmed, it eventually calmed down and even rode decently, so I didn't give it much thought.
On the next cold-start a few days later, it was backfiring and running rough again and I concluded that this wasn't normal, so I removed the carb one more time and dismantled it.
This is when I discovered where that "small black disc" came from...

Apparently, this "plug" (OEM# 50M-14355-01-00) is vital for proper operation of the CHOKE CIRCUIT...
A quick phone call to the closest Yamaha dealer located a new plug. But before reassembling, I gave the entire carb another U/S cleaning in a solution of water and PineSol (90/10)...rotating the carb body every 10 minutes to ensure that every passage and port in there would get flooded with solution and zapped with the ultrasonic waves.
Once complete, I reassembled all the bits and pieces and reinstalled the carb onto the bike.
I should also note that I reverted back to STOCK jetting; #31 Pilot jet and #126 Main jet. I wanted to get a base-line that I could refer back to if I ever decided to mess with the jetting again at a later date.
I had to make some minor adjustments for idle speed and mixture on the first start, and once warmed fully, I set the mixture between 1¾ and 2 turns open. This gave the best throttle response as well a the best hot-idle speed. Then I tried a warm-start...she re-started with barely a "blip" of the starter button...
THIS IS WHERE I WANT TO BE!!!!
The final test would be a cold-start the following day...which worked PERFECTLY. I pulled the choke all the way out and cranked the starter, she jumped to life and fast-idled. After about 30 seconds, I pushed the choke in to the ½ way notch and the idle speed dropped slightly.
After a minute or two of fast-idling, I closed the choke off completely and the idle dropped down to factory spec and she sat there purring like a kitten.
I left her to idle for about 5 minutes while checking out other things, then shut her off. The following warm-start was perfect as well...just a brief touch of the starter button brought her back to life and purring anew. EXCELLENT!
So, for all of those frustrated TW owners out there with (stock) carb issues, my suggestions are:
a) Go back to stock jetting and settings and see if the problem(s) disappear.
b) Get yourself an ultrasonic cleaner.
c) When using the Ultrasonic cleaner...REMOVE the plug so that you can access the choke/idle circuit associated with this plug, then...
d)...even if it appears in good condition, replace the "plug". It's odd that it isn't included in any rebuild kits, when it is so vital to proper choke-circuit operation.
I was so pleased with the success I achieved with the TW carb, that I took the newly rebuilt carb from my wife's CT90, and gave it the U/S treatment as well. It released an incredible amount of "gunk" and once reassembled and reinstalled, the bike idles better than ever.