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Discussion Starter #1
so I have an 03 ltz400 with the 440 kit ported polished head kibble white valves and a full white brothers carbon pro exhaust. I decided to buy an fcr 39mm carb and I put a size 50 pilot jet in it like I was told by a shop and it will only run with the choke on. If you turn the choke off it’ll idle great but as soon as you hit the gas it shuts off. Would that be it needing a bigger pilot jet?
 

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I’m not too great on carbs...

Is it smoking any? That sounds way too rich to me... but the way you describe with the choke it sounds too lean.... what’s your main? Is your pilot jet clogged? Running with the choke sounds like a clogged jet..

Mines a 462, kw valves, full HMF, fcr39... and I’m running 180m 50p and it’s almost spot on, super close, not perfect...

Too lean it’ll backfire heavy once u left off throttle (loud pop, and you’ll see a flame most times) at night you’ll see the pipe glow cherry red when it’s idling. That’s too lean. (On pilot jet, most likely)

Too rich and it’ll seem like you just barely touch the throttle it acts like you’re punching it half throttle or better (revs quick and hard instead of a steady slight change) that’s the main (sometimes the pilot acts that way, but too rich on the pilot and it’ll cut off once the fuel starts to hit (throttle, floods the spark out)

I would clean the jet really good, maybe even whole carburetor. If it still does it, I would change to a 42 maybe a 45 pilot....

What’s your air fuel mixture? Between 1-2 turns out, give or take, you’re in the ball part. Less than one, smaller pilot. More than 2-3, bigger pilot.

Btw, I’m roughly around 1,000-1,200 in elevation
 

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Discussion Starter #3
My main is a 160 my dad built the quad and I bought it off him so o had him jet it and I’ve been riding it and it rides great with the choke on so he told me to try a 52 pilot but I’m not 100% sure if that’s gonna fix it. So I got on here to see if anyone else has had this problem before or is they have dealt with the fcr before.
 

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I’d take the carb off (or at least the bolt in the bowl) and take out the pilot, clean it out with some brake cleaner. That should fix your problem, then you’ll know whether or not if your jets are in the ballpark from the statement above..

Running only with the choke on means (not always but 9/10x) that the pilot is stopped up. Me personally, I’d take the whole carb off and clean every port, jet, and any little hole in it out completely. (Be careful with brake cleaner, it eats plastic and rubber) then use compressed air to be for certain it blew everything out.

I have a Honda 250 and drove it for about a month with the choke on, only way it would run (on weekends) because I didn’t wanna take the carb off all bc it’s such a pain.... finally got around to it one day, cleaned my pilot jet and been running like a champ ever since
 

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Compare your setup to others in here:

I’d say 50 is close, between 45 and 55 anyway. Lots of other info that makes a difference, like airbox lid on or off, elevation, clip position on the needle, how many turns out on the mixture screw, etc.
I agree, need to make sure it’s clean first. Clean carb and clean air filter or you might be barking up the wrong tree. Make sure the pilot is clear and the slide moves freely, and the AP shot is timed correctly.
 

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Discussion Starter #6
The carbs clean it’s about 4 clicks out running an ltz box with 400ex intake boot pro design filter for a 400ex
 

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What needle do you have? Is this a slow throttle opening problem, or when you move the throttle quickly?

Your main sounds a little lean, but I don't think it comes into effect yet at the point you describe.

The key to understanding the FCR carb is to first know the components, and what they're responsible for, so you know what to change.

The fuel screw:
It's only significant at idle, and it gets it's fuel from the pilot jet. The fuel screw should be set while the engine is idling, which isn't the easiest thing to do. Fuel screw settings are referenced as turns out from seated. In is leaner, out is richer. Because the fuel screw gets it's fuel from the pilot, if the pilot is plugged, or too small, or too large, it will have an effect on the fuel screw. Typically, if the pilot is the correct size, the fuel screw will be happy somewhere near 2-2.5 turns out. This is not law, just something to consider.

The pilot jet:
The pilot supplies the fuel screw, and is responsible for the transition from idle to 1/4 throttle. The best way to determine if the pilot is sized correctly or not, is to see how it acts at very light throttle. Just above an idle. That's the only point where you are pretty much using only the pilot. I've seen anywhere from a 40-52 in the FCR. I don't exactly know why, but they're not extremely consistent. Just give it what it wants. Keep in mind, that a pilot jet change, will make it necessary to set the fuel screw again.

The needle:
The needle is responsible for the broadest range of throttle. Pretty much it's from 1/4 to 3/4 throttle. There are many needles available with different tapers and diameters. Typically the NCVQ on the 3rd or 4th is a pretty good fit for something like this. If the needle needs a minor adjustment, there are slots at the top where you can move the clip to different positions. Raising or lowering the needle has more or less taper blocking the main jet throughout the range that the needle is responsible for. To reference the position, use the clip position, from the top. NCVQ in the 3rd or 4th slot from the top is usually correct. Something that's a little different with the FCR, is that the needle position has an effect on the main jet size. This is something I learned on the dyno. This is also why what works for someone else, may sound completely wrong for yours.

The main, is as it is with any carb. It's responsible for the fuel supply from 3/4 to wide open. The main is the biggest wild card on sizing, because many things effect it. The engine build, and the dynamics of the intake system can bring about really big swings in main jet size. I've used anything from a 160 to a 230 in a LTZ.
 
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