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520 RS jetting question

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New 520 RS owner. As a die hard two stroke guy, I have a basic understanding of jetting, but as this is my first real 4 stroke, I'm a little confused as to the role of the fuel screw. The bike is running lean, and on a two stroke I would start with the main and go from there one circuit at a time, and that is my plan here, but I want to know exactly how the fuel screw comes into play. I have searched threads here and gotten an idea of the adjustment range in terms of how many turns out form lightly seated. So, my intention was to turn it to that point and experiment form there. There are two problems with this: first, I'd like to know exactly what throttle range(s) the fuel screw affects; and secondly, and more importantly, I am having difficulty in finding "lightly seated" because the remote screw is extremely hard to turn in either direction. I know this is probably a dumb question, and I have searched threads prior to posting this one, but I would rather ask a dumb question before I dive in and hopefully save myself some aggravation.

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Here is a picture of the fuel screw I have.

IMG_0793.JPG

Edited by Bryan139

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The fuel screw meters the fuel flow for the pilot and idle circuit.

So, basically it's a fine tuner for the pilot jet....0 to 1/4 throttle

The preferred method for adjustment is to

Let the bike get hot

Turn the screw inwards until it lightly bottoms.

Turn it back out, the idle will begin to get stronger, at a point the exhaust note will become flat....stop

And turn it back in a 1/4 turn.

If you end up roughly between 1 and 2 turns out your pilot is in the correct range.

As you ride in different conditions, elevation, you can adjust the fuel screw throughout the day to crispen the off throttle response.

If I remember correctly, that's an fcr carb right?

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In the pic are you sure that's not the idle adjustment screw.

On the older fcr carbs the fuel screw is underneath the bowl.

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I thought I might be asking a dumb question, and. man, was I right. Yeah, that's the idle screw. The original owner said he installed a remote screw so I assumed that was it since it was the only thing that resembled a remote screw. If I would have at least started the bike and tried to adjust it before posting, it would have been obvious right away, I wanted to get a baseline for where it was at relative to all the way in before changing jets and/or adjusting the needle so I was just messing around with it last night when it was way too late to start it and wake up the neighbors. But late night is the perfect time to waste way too much time on the internet reading about dirt bikes...

 

In my searching though, I did locate a helpful thread on disassembly of the carb which is evidently a bit more complex than the PWK 28 in my KTM, so there is a bright side to my stupidity. Bookmarked the thread and got my digital camera charging.

 

But your explanation of the fuel screw is exactly what I was looking for. Thanks.

 

BTW, the fuel screw that the original owner installed isn't exactly remote, but it works for me. I can reach it just fine and it is nice and simple.

 

IMG_0794.JPG

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Most of us use the aftermarket fuel screws that can be reached from the left side on the bottom of the carb. But be careful, I've received bikes from friends to tune that never added the o-ring and even the spring. 

The FCR on the older Honda's were worse than the Beta FCR's, the Beta version is newer/better than my old Honda...

 

If your bike adjusts correctly as previously suggested then cool, if not look towards the JD jetting mindset, as he has a different idea on tuning the Beta 4-strokes....

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There are a lot of cool mods you can do to the fcr carb to help it perform better, like

wire the ap lik closed, shave the ap diaphragm button down, like sugg a remote adj fuel screw

merg pump kit which has an adj for the leak jet....very cool item

JD jet kit, intake wing

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Thanks for the tips.I'm going to start with the main and work my way down from there just to get a feel for the bike. Actually it runs nice and responsive off the bottom, is lean from 1/2 to 3/4 throttle, and cleans up nicely once opened up, so a change in needle position or taper is going to do it, I'm thinking. I will definitely look into some of the mods mentioned once I get it dialed. Thanks.

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Thanks for the tips.I'm going to start with the main and work my way down from there just to get a feel for the bike. Actually it runs nice and responsive off the bottom, is lean from 1/2 to 3/4 throttle, and cleans up nicely once opened up, so a change in needle position or taper is going to do it, I'm thinking. I will definitely look into some of the mods mentioned once I get it dialed. Thanks.

 

Enjoy!

 

Beta on my 2014 450RS did rich jetting and a lean taper on the needle. Which for me was kinda wonky. As the choke never worked right well and had instant wheel (rich) spin off idle and then lean world throughout the rest of the RPM  :facepalm:

 

I installed JD jetting kit, and his thoughts are leaner jets and a more progressive (richer) needle. Which works better for me, off-idle, much better on warm up on the choke, slightly softer off-idle then consistent progressive power throughout the rpm range.

 

While I tried the different jets that also came in his kit, his suggestion on initial jetting was spot on and went back to his base-line suggestion, other that adjusting the needle a little..

I have ridden so far from 0-3500 feet normally and sometimes over 7500 feet no issues no popping very linear power. and no back firing  :ride:

 

For $75.00 for the JD kit it was nicer that the OEM concept, but ya gotta have adjustable fuel screw!

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The fuel screw on the 4 stroke FCR carb controls the fuel flow through the low speed pilot jet system.  Turning the fuel screw in clockwise leans the low speed fuel mixture while turning it out counterclockwise richens it.  (Just the opposite of the air screw in 2 stroke carbs.)  The range is from 0 (lightly seated) to 3 turns out.  The standard setting is 1 1/2 to 2 turns out.  If the engine runs better when the fuel screw more than 3 turns out, this indicates that the pilot jet is too small (lean).  If the engine runs better at 1 turn or less out indicates that the pilot jet is too large (rich).  If your carb does not have a finger adjustable fuel screw, you should install one as it makes adjustments much easier. 

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