I've got a few questions about the Keihin FCR carburetor if anybody has a minute to think about it...
My question stems from a problem my brother's '01 KTM 640 supermoto is having:
The other day, while riding, the bike suddenly began to rev pretty quickly with the throttle closed. After some diagnostics, I found that the vacuum release plate on the slide is cracked badly, and a good size chunk of it is GONE .
I hate to even think about where that piece is right now -- I'm hoping the motor was able to pass it somehow. I have a hard time wrapping my mind around that, but he rode it about 1.5 miles back home with no issues aside from some pretty energetic launches (haha).
Anyway, the plate is shot (I'll post pics if this piques anyone's interest at all -- I can't figure out what caused it). They're $90 + shipping +$17 for a new seal. All of this is back ordered until Mid June at the earliest.
This brings me to my question:
Can anyone explain exactly what the vacuum release plate does and HOW? I think it's there to allow the slide to reach the bottom of its travel under high vacuum (idle), but I haven't got a clue how it does it. I haven't studied this much, because I haven't had it in front of me since I learned of the price. The price has inspired me to care how it works, and consider having a replacement machined.
I was discussing a replacement made from stainless (I saw this idea posted by tattootodd on this forum) with another guy, and he suggested that the weight of the plate could effect it's performance. I'm far too ignorant about the function of this part to have a real opinion on that, but it seems to me that MORE weight wouldn't really hurt, but less could. Anyone have thoughts on this?
ANY information or explanation would be appreciated. FWIW, the guy at Sudco who's talking to me about replacement parts had this to say:
"It was most likely a bad seal. The seal is really sensitive, it may still look good but it is not. I recommend changing them at least every 2 years. Change them more often if you use race gas or fuels with alcohol/ ethanol. If you spray carb cleaner or any solvents on the seal, replace it immediately."
Really helpful information, and something I had NEVER thought about. Might be something to consider for other FCR owners.
(I changed the color of this last bit hoping that even if people don't know the answer, they'll still git the info. Sorry if this is confusing or if this is common knowledge)