Update: 11-19-2010 added some stuff..
This list should go for almost all ATV/motorcycle carbs. CV or mechanical. I tried to list them by frequency
1)
Old fuel - this is the number one cause of carburetor problems. as it sits it dries out and varnishes the small ports/orifices in the bowl. All these problems can be cleaned with carb cleaner, air compressor , fine bristle brush, and a thin brass wire.
2)
Clogged idle jet - the orafice is small in this jet and it is the first to get clogged with tiny varnish particles and/or dirt.
Symptoms- poor idle or no idle, you may need to feather the choke to keep the engine running at idle.
Fix - See carb cleaning section below
Prevent - install inline fuel filter and always store machine with a full tank of gas. Stabil can help but will not totally prevent the issue.
3)
Clogged main jet - This will only happen with extreme dirt and varnish. more then likely your idle jet will be clogged also.
Symptoms - machine will not rev has no power, will stall when the throttle is opened.
fix - See carb cleaning section
prevent - install inline fuel filter and always store machine with a full tank of gas. Stabil can help but will not totally prevent the issue.
4)
Fuel over flowing from the bowl - this is caused by a bad float needle or varnish/dirt preventing the float needle from seating, or the float is out of adjustment.
symptoms - worst case is fuel continually flowing from the carb into your engine or airbox or out the overflow. This can also be show up as a very rich running engine, or slow drip from overflow when engine is running. The engine will stumble and stall, running rich and choking itself out. Spark plug will be coated with black soot.
Fix - see carb cleaning, additionally, check the adjustment of the float arm to make sure it is perfectly parallel to the bowl gasket seat. If not you need to bend the little brass tang on the float arm until it is resting parallel when the carb is held upside down.
prevent - install inline fuel filter and always store machine with a full tank of gas. Stabil can help but will not totally prevent the issue.
5)
fuel flow problems - Fuel not properly flowing into the carb, caused by clogged fuel filter, clogged petcock filter, kinked hose, clogged float needle/seat, clogged breather cap (although this one will run longer before dieing)
symptom - machine runs for a few seconds up to a minute or two then sputters and dies. Worst case the engine will not run at all (no gas in carb)
Fix- it's best to remove and clean the tank with water and detergent, remove and clean the petcock, check for kinks in the fuel line replace if there are any problems, replace the fuel filter. Check the flow by filling the tank and opening the petcock holding the carb end of the fuel line over a pan. Fuel should pour ot at a good rate. Then reconnect the carb and remove the drain plug from the bottom of the bowl. Hold a pan under your carb and turn on the fuel. It should flow at about the same rate out of the bowl.
6)
Choke clogged out out of adjustment - varnish is clogging the choke orafice in the bowl, or the choke cable is not opening the choke valve properly.
symptom - engine will start ok when it is warm but when it is cold out (50 degrees or less) it will take a long time to start, or not start at all.
fix - see carb cleaning section, additionally adjust your choke cable so there is no more then about 1/8" of play
7)
Vent Lines clogged/kinked - The vent lines on the carb are pretty important when it comes to keeping excess fuel out of the intake.
Symptom - every time you hit a hard bump your engine boggs/stumbles/or cuts out.
Fix - clean/replace the vent lines and drain lines on the carb/bowl. Make sure they vent tubes are routed to a place high up around the snorkel for the carb. Also make sure they are not kinked or close to the exhaust. The drain lines (usually 2 going to a T to one) from the bowl are routed so gas will drip on the ground and not your engine/transmission. They need to be clear and free flowing. Also it is good to check the float level to make sure it is not set too high.
Carburetor Basics
As you can see most of these problem come from old fuel and varnish caused by old fuel. One needs to simply understand the basics of a carburetor to be able to figure out what is going on. It's sole purpose is to properly mix the right ratio of fuel to air at any RPM range. There are basically 3 circuits that allow for this.
-The first one is the idle circuit. It consists of an idle jet and an idle mixture screw (or air mixture on some models). The stock setup of most idle circuits (98%) is to turn the screw in all the way. Not too hard or you will damage the screw. Then back the screw out 1 1/2 turns. This setting should get you in the ballpark enough for the engine to idle. You then need to adjust the idle down and re-adjust the idle screw until you attain the best idle. You need to listen to the engine, you will be able to hear when the idle mixture gets better or worse. This cicuit will effect the idle mixture with a little overlap into mid range. So from closed throttle to cracked throttle. If your engine stalls a lot when you close the throttle you need to suspect the idle circuit.
-The next circuit is the main - This circuit actually has two inherent circuits the first one controls mid range throttle/RPM. The second controls Wide open throttle. It does this with the use of an adjustable needle in the slide of the carb that varies the size of the opening in the main jet circuit. In the racing world there are infinite setups/possibilities here but for home use just understand that the needle position controls everything from cracking the throttle to about 5/8 throttle. After this the main jet takes over and all fuel flow is metered by it. If you have a mid range stumble check your needle position. If your engine doesn't want to pull at WOT then check your main jet. if both are problem them your main jet is way off.
-Cold start/Choke circuit - just as the name implies there is usually some form of fuel bypass valve that allows additional fuel to be drawn directly from the bowl into the intake stream. This allows a cold engine to start on cold days, when a much richer mixture is required. It's usually controlled by a manual lever either on the carb body or through a cable connection.
These three circuits work together with an intricate network of small tubes that siphon fuel from the bowl in various ways. Understanding this will help to pinpoint issues while you are working with your carb.
Carb Cleaning and other misc tips----
Carburetor should be removed form the machine.
Start by first setting up a clean towel or large rag to lay your parts out on. You need to keep these tiny parts clean. Light color works best to provide high contrast for the tiny screws and parts. I've used paper towels and in a pinch on the trail I took off my white under shirt to fix a friends bike. Proper preparation will make this job so much easier. Any carb cleaner spray will work fine, have the finest nozzle you can for your air compressor. When blowing off parts hold on Tight!! you do not want your tiny jet flying across the garage at Mach 2!!
take the bowl off and remove the idle(low speed) jet. Then blow carb cleaner directly into the hole where the idle jet came from you should see it coome out of a few tiny ports in the engine side of the carb. Immediately after blowing carb cleaner through take an air compressor and blow directly into the same low speed jet hole. You should repeat this a few times each time looking at the flow coming through the ports. Then take your low speed jet and look real close at the openings they should be round and perfectly clear. Take some carb clean and blow through each of the holes in the jet then use the compressor and blow it off (hold onto that thing tight so you don't blow it across your garage... been there done that!!). Most of the time idle problems lie in this jet. Next remove the idle mixture screw and blow through it with carb cleaner and air. When re-assembling it turn it in all the way and then out 1 1/2 turns (don't tighten it too hard!! or it will damage the screw and the seat).The next thing you need to do is remove the float by pulling the pin that slides into the carb body. Be careful when removing this there is a tiny spring clip that holds the needle to the float arm don't lose it. Clean the mating surfaces of the needle and the needle seat. next check your needle to make sure the spring inside it moves freely. If it does not spray it with carb cleaner and work it in and out with your fingers this will usually clean it out and allow it to flex again. Then reinstall the parts and check your float level. Stock setup on most mikuni carbs is the float tab set parallel with the bowl flange. So when you hold the carb upside down your float tab (the brass lever the floats use to close the needle) should be perfectly parallel with the flat surface that the float bowl mates to. if it is not parallel then take a small screw driver and adjust it by bending the brass tab until the float arm is parallel. Now remove the main jet and spray it with carb cleaner, again viewing the hole to make sure it is perfectly clean and round. In the worst cases I use a piece of fine brass or copper wire to run through the holes to clear them out, or just the bristle of your brush. Now finally before you put the bowl back on take your brush and clean out all the gunk that is dried in there. If it is heavily varnished spray it down it carb cleaner and allow it to dry and it will rub right off like a powder. The most important thing is an eye for detail blow through all the small holes that you see and look for the carb cleaner to flow through freely. Not only will it clean your carb it will also give you a good understanding of how this seemingly complex peice of equipment is very basic and easy to understand.
Choke Adjustment----
Hmmm its easier to show someone then try to explain in text but let's see if I can get it close. This is the best way I found.
-Start with the adjuster screwed in almost the whole way before you assemble the brass slide valve back in the carb. The carb should be installed already before you start unless you cannot access the choke when the carb is installed
- now insert the valve back in the carb and tighten in down, not too much just snug.
- this is where it gets a little tricky. With one hand on the choke lever and the other on the choke adjuster start to spin the adjuster out slowly. While doing this the other hand should be working the choke lever to feel the amount of slack left in the cable. You should be able to feel when the choke cable starts to raise the choke valve. This can be tricky if your choke cable has a lot of drag. If so you should think about oiling the cable. The choke valve spring should offer enough resistance to feel when the cable picks it up..
- keep working the adjuster until you feel about 1/8" of slack then tighten the adjuster lock nut.
As a test when the engine is warm and you pull the choke it should kill, or almost kill your engine. On my TB if I just touch my choke it will die. If there is no effect either the adjustment is wrong, the idle mixture is too lean, or the choke is still clogged.
Here is a Typical Mikuni Carb blow up. There are many variations but they all use the same basic design.
Parts referenced in this writeup:
#8 Needle Clip - used to adjust the needle height
#9 Needle - this is what is used to meter midrange rpm fuel
#36 Main Jet - this is what meters the high RPM fuel
#21 Idle/Pilot Jet - Meters idle RPM fuel
#32 Idle set screw - adjusts the idle RPM by setting the stop position of the slide (part #10)
#29 Idle mixture screw - used to fine tune the idle mixture
#23 Float arm - notice the small tab in the middle this is what you bend to adjust the float levels
I hope that all makes sense I was writing this between work phone calls so it may seem a little chaotic....
Cheers..
Rusty
