How-To: Clean a 4 stroke MX/Quad carburetor

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In this 45 minute long How-To video, I show the cleaning process of a modern 4 stroke Keihin FCR carburetor. All of today’s high performance 4 strokes (dirt bikes and quads) use the same style of carburetor, so this video will apply to most 250 & 450F’s. If your carburetor looks like the version in the video and pictures below, then this video will help you.

 

 

Year, Make, Model

The bike featured in this video is a 2006 KX250F, however video will apply to many makes and models, both 250 & 450f’s.

 

Symptoms of a dirty carburetor

  • Only runs with choke on (usually a clogged pilot jet is the culprit)
  • Fuel is leaking from overflow tube (stuck or dirty float needle)
  • Poor performance (exhaust popping, bogging, loss of power, etc)

 

Service Manual

Click here to see if I have a service manual for your bike.

 

Tools

Here is a list of tools used in the video, which can be purchased from Amazon.

 

Vessel Megadora 980 Impacta P2x150 #2 JIS Cross Point Impact Screwdriver

 

Motion Pro 08-0023 6mm Main Jet Wrench

 

Berryman 0996 Chem-Dip Carburetor and Part Cleaner – 0.75 Gallon

 

Parts

I highly recommend installing an adjustable fuel screw if you don’t already have one.

 

MSR Fuel Mix Screw Black 4-Stroke With Keihin Carb

 

Checklist

In the video I mentioned I’ll be giving out a worksheet. Instead I’m going to post it below for everyone to see.

 

Before you begin

If your bike runs, check for vacuum leaks before taking apart. Removing these carbs are a PITA, and you don’t want to do it twice.

Get a hold of the service manual for your bike (I have a bunch of FREE ones here). The fuel system section will state important things such as jetting and float level height. Now your bike may be jetted differently than stock, however I have come across carbs that were tinkered by people who think they knew what they were doing. People love messing around with carbs! Be prepared to write down what stock and current carb jetting specs. Here are the important ones to note:

Main Jet: ?

Pilot Jet: ?

Fuel screw setting: Hopefully 1-2.5 turn out

Needle: ?

Needle clip position (always counted from top): ?

Leak jet: ? (note some KTM’s and other bikes may not have a leak circuit)

 

Pilot jet & fuel screw

In my experience and climate (Chicago area), I have found that most of these bikes are lean on the pilot circuit from the factory. I find that they like anywhere from stock to 2 sizes larger on the pilot jet. For more info, see my video/post on fuel screw tuning here.

 

Hot start

Make sure the hot start plunger o-ring is in good shape. The carb body that accepts the plunger should be clean as well. After everything is assembled, make sure hot start cable is adjusted properly. You don’t want the cable to be tight, opening the plunger too soon, causing a lean condition. Check this post out for more info on hot start.

 

Wear items & gaskets

The carb body and brass jets can be cleaned with Berryman Chem Dip and DIY soda blasting. Carb parts that wear out include the float needle and accelerator pump diaphragm. Inspect these parts closely for wear and replace if necessary. Make sure you pressure test the float needle prior to reassembly. Check the condition of all gaskets, and replace if necessary.

 

Float height

Make sure to verify the float height as this can affect how the bike runs. I higher float height will cause the bike to run richer, and a lower height leaner. The video shows the correct method on a FCR carb.

 

Other helpful videos

DIY soda blasting:

 

Pressure testing float needle:

 

Diagnosing vacuum leaks:

 

And finally, fuel screw tuning:

 

Comments

Has this video/article helped you? Please comment below.

 

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  • Yura Nakonechnyy

    Hi!

    I’ve just disassebled the carb and noticed that leak jet was damaged and it’s impossible to unscrew now – please refer to the picture. Is it ok to leave it as it is (its orifice looks still normal) and how it may affect functioning of carb? Or I definitely should extract and replace it? If yes, then how would you recommend extracting it – using extractor set?

    Thanks in advance!

    • Yura, I think you can manage to get that out. Go buy a cheap slotted screwdriver and grind it for a perfect fit. Often times the leak jet needs to be changed out for a smaller one to get rid of a bog also.

      • Yura Nakonechnyy

        Ok, thanks for the response – I’ll also try to somehow grind the head of the jet, because although it looks fine, but it’s quite flat so screwdriver can’t grab it good.

        • Yura Nakonechnyy

          Ok, so I didn’t manage to get leak jet out, because it’s head is really damaged and when I’m trying to turn screwdriver – leak jet is getting more damaged.

          I’ll order new leak jet and replace old damaged one when new one arrives, because I don’t want to miss summer riding season 🙂

          Nevertheless I disassembled and cleaned the carb as per this article and bought video and it now works much better. One question though – it turned out that there are plenty of air jets which were not mentioned in the video – does it make sense to take them out and clean separately or not?

          • The air jets usually do not require cleaning. I sometimes spray carb clean and compressed air in those passages to make sure they are clean.

            Did you try the screwdriver trick on the leak jet?

          • Yura Nakonechnyy

            Regarding leak jet – well I tried to remove it using thinner screwdriver, however it damages leak jet head. Or do you mean insert even smaller screwdriver into orifice and try to screw it out?

            And another question regarding leak jet – it seems that my leak jet doesn’t have holes in it i.e. it looks like it doesn’t return any fuel into bowl – is it at all possible?

          • Buy a cheap screwdriver that is wider and thicker than what you need. Carefully grind or file it to fit perfectly inside the leak jet. It will come out this way.

            The leak jet should bleed back fuel into the bowl, if not, something is clogged.

          • Yura Nakonechnyy

            Ok, now I got it – will try to do during next week and write back here about the result.

            So far I assembled the bike with cleaned carburetor and was riding it – it works quite well, though jetting is different from stock one which is why I think it’s not working perfectly at higher rpm but still quite good.

          • OK, keep me posted.

 

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