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Carburetor Rebuilding

Rebuild the carb? It seems like a very common bit of advice to dirt bike performance problems is rebuilding the carburetor. I think it's in second place, right after "check and/or replace the sparkplug".

The reason a carb rebuild is recommended so often, is because it helps so often. Have fuel pouring out of the carburetor? Rebuild the carb. Engine bogs when you crack the throttle? Rebuild the carb. And so on.

The good news is a carb rebuild is one of those things that seems complicated but is, in fact, simple to do.

First, try and find yourself a clean area to work in or on. There are some small parts, and if you drop them, you'll want to be able to find them easily. Also, you're working with leftover bits of gas and good amounts of degreaser -- so the dining room table is probably not a good idea.

If you can, get hold of a shop manual for your specific dirt bike. That way you'll have a detailed description and diagrams for disassembling and reassembling your carb, plus any settings. If you can't get a manual, go online to someplace you can order parts and print out the exploded view of the carburetor many of these sites have. Also, I find my digital camera valuable for taking pictures of things BEFORE I take them apart. Later, if I can't remember which way something goes, I check one of the before photos.

For the most part, all carbs are pretty much the same and rebuilding the carb on any dirt bike is pretty much the same. If you're into some real vintage dirt bikes I may have a missing or extra step or two here -- but you'll probably figure it out.

So let's get started. First, shut off the fuel from the tank. You wouldn't believe how often I forget that step and pour gasoline all over the floor. When the fuel is shut off, disconnect the fuel line from the carb -- keep a rag handy, because you'll get a little residual fuel coming out of the hose.

Now take the carb off your dirt bike. Usually this involves disconnecting a rubber or plastic sleeve that runs from the air filter to the carb. Some carburetors have a small foam air filter fastened right to them -- common on pit bikes. If you can take it off before removing the carburetor... it'll just make everything easier to handle. Then, if you have the clearance, remove the top of the carb (where the throttle cable goes in) pll the slide out, and leave the slide hanging on the cable. If you're not going to remove and clean the slide and your shop is dusty, slip a plastic bag over it. Then remove the carb from your dirt bike's engine. If you're tight on room above the carburetor, remove it from the block first, THEN remove the top as you move the carb off to the side and down.

The bottom of your dirt bike's carburetor -- the bowl -- is full of fuel. Open the drain screw at the bottom and drain the fuel. By the way, if you're a smoker, you may have already discovered it's a bad habit to indulge in while doing this.

Moving right along... remove the screws that are holding the float bowl on, then remove the float (being careful not to bend anything), and the float needle.

Then remove the idle adjust screw BUT FIRST, turn it all the way in and count the number of turns it takes 'til it stops. Write this down and when you reassemble the carb, turn the screw all the way in and then back it out the same number of turns. That'll give you a good starting point for adjusting the idle later. Do this with any other adjusting mechanisms or screws your dirt bike's carburetor may have.

Remove the choke assembly. It's not necessary to remove the butterfly valve/flap, but on some bikes it pops right out. Choke assemblies are one of those things I always take pictures of before disassembling because I'm lousy at remembering how they go back together.

Finally, remove the jets. Those things that were hidden by the bowl, sticking out from the main body of the carb, with slots in them for your screwdriver.

At this point you have a couple of choices. Some people like to spray the carb with an aerosol carb cleaner (can be very messy), some like to dunk it in solvent and scrub with a small brush (gets a little messy, and takes some work), or some like to soak it overnight in commercial carb cleaner (easy, not too messy, a little more expensive). For me, the extra cost of the carb cleaner is worth the convenience and the shiny, brand new look of the clean carb.

All the loose parts need to be cleaned as well. Do them the same way you choose for the carburetor body. Don't forget the carb slide that you left hanging on the end of the throttle cable.

Don't poke or scape with any metal objects -- especially any holes in the carb body or jets.

When everything is clean, blow it all dry with compressed air if you can -- otherwise, dry it as best you can and give it a couple more hours to let things evaporate.

Reassemble everything in the reverse order you took it apart. If rubber gaskets look worn or torn, replace them.

If you have the specs, or if you were having fuel delivery problems, you can check the float level. Personally, I never worry about the float level unless I'm installing new ones, or I've dropped or banged them around.

Your dirt bike carb is now clean and rebuilt. Once you've done one, you'll wonder why you didn't try it sooner.

It's up to you, but I like to take a little time to clean the air filter and housing before putting the carburetor back on the bike -- I hate the thought of dust and grime getting into that shiny carb.

11 comments:

  1. Thank you so much for sharing this! I just rebuilt and cleaned my first carburetor belonging to a 1980 Honda XR500. The thing was filthy and the jets were clogged.

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  2. i cleaned my carb and put it back together,but now it is leaking fuel out the over flow hose alote.my throttle cable stickes,i have sprayed tried adustments still not working right.

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  3. Unless your carb was doing all those things BEFORE you cleaned it... it looks like you did something wrong.

    You're going to need a manual to take it all apart and carefully put it back together -- making sure you have all the right parts in all the right places, and that nothing is missing.

    I can't think of any individual thing that could cause all your problems. Sorry dude.

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  4. I just cleaned out the carb on my 2000 xr100r,the first 3 or4 times it started up amazing and idled great. lately it has gotten harder and harder to start, and if I try to give it quick gas from idling it bogs down and if I don't let the throttle up quick it dies. if I rev it up slow its fine and if I rev it fast while the rpm's are up its also fine. Does anybody know whats going on??????????????????????????????????????????

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    Replies
    1. I have the exact same bike and had the exact same problem. Turned out the mixture screw on the bottom/front of the carb had vibrated itself out on the trail somewhere. I didn't figure it out until I took the carb apart to clean it, thinking that was the problem.

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  5. To the guy with the 2000 Honda XR 100R, here is what I would do. I would, get a new air filter, get a new spark plug, change the oil, put new gas in the tank, and either:
    1. Purchase a new carburetor.
    OR
    2. Clean and rejet the one you have now.

    This is a basic, yet necessary, maintenance job and can help tremendously. If there are still problems, it would be a good idea to take your 2000 Honda XR 100R to your local Honda Powersports dealership and get them to find what's causing the issue.

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  6. On my yz 125 2005, I took the carb out and it's still leaking. It's been doing this ever sense I got it. What should I do to fix it???

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  7. RE: Your carb. You'll have to be more specific about "leaking". If it's fromthe bottom, could be a loose, missing, or damaged drain screw. If it's coming from what looks like an empty opening, probably a missing set screw. If it's coming from a seam/joint it could be a bad or missing gasket. Your floats may also be set incorrectly, causing the bowl to fill too much and then drain.

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  8. Well I should of read this page first. Because I poked at my carburator in all the jets and holes with a metal object. Plus banged my float and bent the shit outta it, so now it pisses fuel out the back part of carb and out the air filter. when i took my Honda xr5oo carb apart only one jet had a slot for screwdriver and was removable. I seemed like the others had no slots to remove with?. I had a fuel line come off while riding and was not aware ended up running outta gas. When i put gas in from a can that had been outside it had water in it wont fire up and been pissing fuel ever since. How does running the bike out of gas mess up the float or did i just mess it up being a novice carb cleaning girl? Please help Im all alone dealing with this and no book only you tube knowledge of bikes, still a little confused on compression and carbs with broken floats

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  9. How can i set my float correctly if I don't have a manual? I bent it on accident and have no idea what to do!

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  10. Nice post. Thank you for shearing this.
    Carburetor

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