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Discussion Starter · #1 ·
Okay so a little back story first, So I bought an 05 KFX400 for $800 with a cracked case. Found a used case on eBay and went ahead and began installing it. Well I realized while installing the piston that I had an over-sized piston. Took some measurements and discovered it had a 434 bbk installed on the bike prior to me purchasing it. I thought to myself “well that’s a plus”. Finished the install went to crank it and nothing. Decided to buy a ignition coil and a new carb. Installed them once they arrived and it fired right up, literally instantly. Well drove it around a bit and realized it would bog down on 1/2 throttle and beyond. If I ease into it it’ll accelerate just fine. So i started playing with the jetting. Now the carb I bought on Amazon has a 130 or 135 main jet and a 22.5 pilot jet When I received it. Not sure the needle clip position. Well I removed the fuel screw plug and started adding different sized main jets. Went to a 140 with the screw 2 and 1/2 turns out. Still rich based on checking the spark plug. Tried the 130 jet and moved the clip to the leanest position possible. Still rich. Ordered some main jets online. Put in a 120 jet. Still rich. I’m at a loss. I have no idea why I’m still running rich. I’ve got the air box lid off, from what I can tell stock exhaust and the rest is stock minus the bbk. The weird thing about it is if I put the fuel screw all the way in, and I mean ALL the way in it shows some improvement but not much. Right now it has a 100 jet in it. Yes that’s right a 100 main jet in it and it’s still running rich. I’ve tried every combination possible with no real improvement. The bike has very little power and I’m about to give up. The valves have been checked and adjusted to spec and all other work that was done was done per service manual instructions. Any and all help would be greatly appreciated.
 

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Start by getting a OEM Carb or better yet an FCR, These cheap Knock off carbs cause people all types of issues.
 

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Discussion Starter · #3 ·
Start by getting a OEM Carb or better yet an FCR, These cheap Knock off carbs cause people all types of issues.
Ahhh that was my next purchase. When I’ve looked online for em there seems to be sooo many different options. Is there a right and a wrong one to get or is it just “buy and FCR and send it” kinda thing?
 

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You are not rich if you have a 100 main jet in it. Don't go by the plug, go by how it runs.. Unless the low speed circuit is completely jacked, you should be about a 155 or 160 DJ main with the lid off on a big block, unless you live on mount Everest..
 
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If you are not stock, then the jetting in that new carb will be wrong. Assuming that the old carb was jetted right, you should reference whatever that one had in it for jets. Honestly, you should fix whatever is wrong with the original carb and throw the chinese replacement away. Those are all kinds of questionable anyway.

When checking jetting, it's important to know a few things. Don't try to read the spark plug. It won't show you anything useful on pump gas anyway. Replace it if you haven't, but other than that, ignore it.

It's important to know that you can isolate carb issues according to throttle position. Idle is only affected by the pilot jet and fuel screw. When you first start to open the throttle, the transition from idle to pulling fuel up the main well is handled by the pilot. The needle is responsible for supplying fuel from 1/4 to 3/4 throttle. 3/4 to wide open is handled by the main jet.
 

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There are different model FCR, I am very happy with the FCR 39 I got from Jet Labs, pretty much plug and play.
 

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Discussion Starter · #7 ·
If you are not stock, then the jetting in that new carb will be wrong. Assuming that the old carb was jetted right, you should reference whatever that one had in it for jets. Honestly, you should fix whatever is wrong with the original carb and throw the chinese replacement away. Those are all kinds of questionable anyway.

When checking jetting, it's important to know a few things. Don't try to read the spark plug. It won't show you anything useful on pump gas anyway. Replace it if you haven't, but other than that, ignore it.

It's important to know that you can isolate carb issues according to throttle position. Idle is only affected by the pilot jet and fuel screw. When you first start to open the throttle, the transition from idle to pulling fuel up the main well is handled by the pilot. The needle is responsible for supplying fuel from 1/4 to 3/4 throttle. 3/4 to wide open is handled by the main jet.
unfortunately it was also a Chinese carb. Just much, much older. I was looking at the FCR online specifically the FCR 40 and they kinda pricey for my pocket atm due to the holidays being in full swing. Is it the same for the FCR clones on like EBay and Amazon? Are they also garbage or could I get away with pulling the trigger on one of those and trying that route? Sorry if I’m asking some dumb questions, I just really want this bike to run like it’s supposed to. Thank you for the input so far, I really appreciate it!
 

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I am not sure if I would trust the clones, I have read people having decent luck with the ZOOM kit which is a clone but maybe a better one and the price is better. I think if I was you I would get a OEM used off of Ebay and start there, it will be cheaper.
 
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