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Old 04-16-2009, 10:23 PM   #1 (permalink)
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Default Basic Carburetor Tuning Guide

First off, there's 2 basic fuel related problems. You either have a rich mixture, or a lean mixture.

A rich mixture is caused by too much fuel compared to the amount of air being used during combustion. Rich conditions can be detected by the engine spitting and sputtering, blurbling, or acting like a rev limiter, rapidly losing and regaining power. In severely rich conditions, you may be seeing black smoke coming from the exhaust. The black smoke you see is actually raw fuel that is not being burnt and is being wasted. By looking at the spark plug, a rich condition can be detected by a black, sooty plug.

A lean mixture is caused by too little fuel compared to the amount of air being used during combustion. Lean conditions can be detected by the engine losing power, yet retaining it's engine speed. For instance, the engine sounds to be accelerating to higher RPMs, yet feels as if it has no power. By looking at the spark plug, a lean condition can be detected by a white, blistered plug.


Secondly, there are 3 basic carburetor circuits: Pilot Circuit, Mid-range Circuit, and Main Circuit. These 3 carburetor circuits can be troubleshooted by knowing the throttle opening they control.

The Pilot circuit is responsible for throttle openings from Idle (0 throttle) - around 1/4 throttle. This circuit consists of pilot air jet(s), the pilot fuel jet(s), a pilot screw (either fuel or air screw), and pilot ports inside the carburetor throat (a.k.a. Venturi).

There are 2 types of pilot screws: a fuel screw and an air screw.

The fuel screw is located on the engine side of the throttle slide in the carb, and controls the amount of fuel that is drawn into the Venturi by the pilot ports. By turning the fuel screw out, you are allowing more fuel to pass the screw, effectively richening the mixture. By turning the screw in, you are restricting fuel, effectively leaning the mixture. Another way to determine whether it is an air or fuel screw is that a fuel screw has a rubber o-ring to keep air from entering the pilot circuit around the screw.

The air screw is located on the airbox side of the throttle slide in the carb, and controls the amount of air that is drawn into the Venturi by the pilot ports. By turning the air screw out, you are allowing more air to pass the screw, effectively leaning the mixture. By turning the air screw in, you are restricting air, effectively en richening the mixture.

The air jets are hardly ever changed, so we won't go over that. The pilot fuel jet(s) can be changed to bigger (richer) or smaller (leaner), depending upon your problem. A good rule of thumb to use is that if you have to adjust the pilot screw more than two turns either way if it's stock setting, then you need to accommodate by changing the pilot air or pilot fuel jets accordingly.

Remember, the Pilot Circuit is only effective from 0 throttle to around 1/4 throttle. It still functions during the rest of the throttle positions, but it's effect is minimal, and goes un-noticed.

The Mid-range circuit is responsible for throttle openings from 1/4 throttle - 3/4 throttle.

This circuit is controlled by 2 things: the Jet Needle, and Needle Jet (a.k.a. the Main Jet Holder).

The Jet Needle, or needle as many call it, is attached to the throttle slide, and drops into the Needle Jet. All needles are tapered. Either the Jet Needle is adjustable or it is not. If there are more than 1 grooves for the needle clip to sit in, then it is adjustable. By raising the clip on the needle, you are allowing the needle to sit deeper into the needle jet, which restricts fuel, effectively leaning the mixture. By lowering the clip on the needle, you are raising the needle out of the needle jet, which allows more fuel to pass, effectively en richening the mixture.

When the slide raises, it raises the needle out of the needle jet, allowing fuel to pass by the needle and into the Venturi. This is where needle taper comes into play. Unless you are extremely fine tuning the carb, you don't need to worry about taper. You change which part of the taper is in the needle jet by the position of the clip.

Remember, the Mid-range circuit is only effective from 1/4 throttle - 3/4 throttle. None of the other circuits have a drastic effect on this circuit, so if your problem is in the mid-range circuit, then it can't be the main jet or the pilot jet.

The Main circuit is responsible for throttle openings from 3/4 throttle - Wide Open Throttle (you'll see me refer to this at WOT later on).

This circuit is controlled by 2 things: the Main Jet, and the main air jet. The Main Jet is the #1 thing that people change in a carburetor when it comes to tuning them. This is often a big mistake, as it only controls 3/4 - WOT, and NOTHING ELSE. Remember that. A larger main jet will allow more fuel to pass through it, effectively en richening the mixture. A smaller main jet will restrict fuel, effective leaning the mixture. With the main air jet, it allows air to premix with fuel as it goes up into the Venturi.

The Main Jet only functions at 100% when the slide is open and the jet needle is pulled completely out of the needle jet. At this time, the only thing restricting fuel flow into the Venturi is the size of the Main Jet.


Now for tuning.

If you read above, you should know the difference in feel of rich and lean mixtures. By knowing at what throttle opening the problem is occurring at, you can figure out what circuit the problem is occurring at.

If it's the pilot circuit, there are 3 basic way to tune the circuit. You can adjust the pilot screw, change the pilot air jet, or change the pilot jet.

Adjusting the pilot screw is simple. With the engine running at idle, warmed up to normal operating temps, turn the screw in until it starts to idle rough, then turn the screw out until it starts to idle rough, then turn the screw so it's between those two extremes. To check the position of the screw, you can count the number of turns as you turn the screw in until it seats SOFTLY with the carb body. Reason I capitalized SOFTLY is that the screws (especially the fuel screws) are easily damaged if over tightened. So screw them in until they SOFTLY seat the carb body. Compare your counted number of turns to soft seat and compare it to stock settings (stock settings are determined by counting turns until soft seat before you do any adjustments whatsoever). Again, if you had to turn the screw more than 2 turns either way, you need to change pilot jets (air or fuel) accordingly.

In the mid-range circuit, there are 2 basic ways to tune the circuit. You can adjust the jet needle, or change the needle jet. Raising the clip will lower the needle, leaning the mid-range. Lowering the clip will raise the needle, en richening the mid-range. You can also change the needle jet, but only if your jet needle adjustments make no difference in the way the mid-range circuit operated. If you are running lean on the mid-range, and you've raised the needle as far as it will go and it doesn't get any better, then you should go up in the needle jet size. Many carb manufactures don't have different sized needle jets, so the aftermarket may offer them, or they may not.

In the main circuit, there are 2 basic ways to tune the circuit. You can change the main jet, or change the main air jet. Changing to a larger main jet will effectively en richen the circuit. Changing to a smaller main jet will effectively lean the circuit. You can determine which you need to do by first determining whether you are rich or lean. Changing main air jets, again, is for very fine tuning. Once you have the main circuit functioning properly, you shouldn't have to worry about the main air jet, because the air for the circuit is mostly provided by the air passing through the Venturi. On many carbs, the main air jet is not changeable. They may be pressed in.


So there you have it. I basically touched base with carburetor internals and how to adjust them to tune the carb. Every brand carburetor has different ways of accomplishing the same main goal of every carburetor. That goal is to precisely and efficiently mix air and fuel in the right ratios for efficient engine operation. This efficient operation comes from complete combustion, which cannot occur if you are too rich. Whether Mikuni, Keihin, or whatever, they all do the same thing, just in different ways. Hopefully this will help some of you to understand the functions of the carburetors internals.



Lastly, you all need to know...

***This is only a reference guide. This is not to be used as a manual for any specific carburetor, as every carb is different. This is only a guide to be used to base your carb tuning off of. In no way am I responsible for the adjustments, or their results, you make on your own machine.***

If anybody sees any mistakes I may have made, please let me know through a PM or a reply. I would hate to mis-lead someone into tuning their carb wrong.
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Old 04-18-2009, 09:10 AM   #2 (permalink)
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THis is one of the best threads ever everybody should read this because it seems nobody understands a carb and this helps clerify them.
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Old 09-16-2009, 02:07 AM   #3 (permalink)
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Great posting about how carburators work and to also add a very important piece of advice. Go to your local motorcycle shop and get a simple see-through "STONE" FUEL FILTER for your ATV if it doesn't already have one. It's called a "stone" fuel filter in that you'll see what looks like a golden stone inside the filter housing. You won't believe all the crud it'll keep out of your carburator and turning a fun filled day into a nightmare. Some jet holes on these ATV's are just larger than what seems like a human hair, so you know it doesn't take much to plug up. Replace fuel filter every year. It'll be cheaper than that repair bill it'll cost you to rebuild your carb from your service center.
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Old 10-21-2009, 02:47 PM   #4 (permalink)
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Quote:
Originally Posted by Delbert View Post
Great posting about how carburators work and to also add a very important piece of advice. Go to your local motorcycle shop and get a simple see-through "STONE" FUEL FILTER for your ATV if it doesn't already have one. It's called a "stone" fuel filter in that you'll see what looks like a golden stone inside the filter housing. You won't believe all the crud it'll keep out of your carburator and turning a fun filled day into a nightmare. Some jet holes on these ATV's are just larger than what seems like a human hair, so you know it doesn't take much to plug up. Replace fuel filter every year. It'll be cheaper than that repair bill it'll cost you to rebuild your carb from your service center.
Delbert
i was reading that thread started by saber6 (awesome reference)and have a question about the fuel filter on my quad, i just bought a 2004 400 AT rancher and its been blurping and acts like rev limiter, since i am totally new to this, i was wondering if my quad is equipped with a fuel filter or is it something i need to tap in the line.
tx for your help, btw bougth the quad without repair manual, martin
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Old 10-23-2009, 01:18 AM   #5 (permalink)
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Martin,there are so many ATV's out there, it's impossible to know if your model had a inline fuel filter on it when it came from the factory. If it's like most motorcycles(which an ATV engine really is), the factory probably only relied on the "sock filter" that's attached permanently onto the fuel petcock that goes up into the fuel tank, and it's something you don't see or think about until you have carburator and fuel issues. It's ALWAYS good insurance to have an auxillary inline fuel filter that you can positively see, and positively know, it's filtering the fuel before troubles arise. Nothing wrong with the factory petcock "sock filters". They do a good job of filtering down to probably 20 microns or a little more of crud. BUT, when crud can be only 5 microns, isn't it good to also know you have a inline fuel filter that can filter that smaller crud out before it gets to your carburator? These motorcycle filters are heavy plastic, but "glass clear" in appearance to let you know they're filtering properly with crud sticking onto the filter innards or you see things floating around in them, and cost is less than $5.00 and I would also recommend asking the motorcycle shop for some little "spring wire hose clamps". The ATV's carburators work by "gravity feed" and ARE NOT under any pressure at all. NOW, if you have an ATV that's fuel injected, DON'T cut your fuel line as it's under pressure from a fuel pump(probably submerged in fuel tank) and you'll rupture a regular fuel filter and possibly catch your ATV on fire. These require special fuel filters and special designed fuel lines.
As to how your ATV runs, when you say it's blurping and acts like hitting rev limiter(but you know you aren't)that's a sign that one or both of your carburator jets is already clogged with crud and carburator needs to be removed and cleaned out thouroughly. If your careful about disassembly of carb, you can probably save the gaskets and reuse them without getting a new carburator rebuild kit. There's not but a couple gaskets and other items to these carbs, but you need to be watchful, as some items only go on "one way". When I recently bought my '98 Yamaha Big Bear, I was having same problem and I disassembled and cleaned my carb(used old gaskets) and reassembled and ATV worked perfect. My problem was with OLD GAS as it sat for too long(6 months). When this happens the gas will evaporate and can leave the jets clogged, dirty flaky crud in carburator bowl, and possibility that float needle won't seal and you have fuel running out the carb's overflow tube. Now what I do, is when I put in more fuel into my ATV, I'll also put in some "STABIL" fuel treatment. This way the fuel won't go stale from non usage. If you're not comfortable with carburators, just find someone who is(neighbor, friend, family member)and they'll know what to do and how to do it with rebuilding for the least amount of money. If you have Dealership rebuild carb, they'll want you to bring in ATV and they might keep it for 3 weeks, for what should only take a couple hours--and charge you $500 or more. Good Luck and keep us informed.
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Old 10-23-2009, 10:52 AM   #6 (permalink)
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Quote:
Originally Posted by Delbert View Post
Martin,there are so many ATV's out there, it's impossible to know if your model had a inline fuel filter on it when it came from the factory. If it's like most motorcycles(which an ATV engine really is), the factory probably only relied on the "sock filter" that's attached permanently onto the fuel petcock that goes up into the fuel tank, and it's something you don't see or think about until you have carburator and fuel issues. It's ALWAYS good insurance to have an auxillary inline fuel filter that you can positively see, and positively know, it's filtering the fuel before troubles arise. Nothing wrong with the factory petcock "sock filters". They do a good job of filtering down to probably 20 microns or a little more of crud. BUT, when crud can be only 5 microns, isn't it good to also know you have a inline fuel filter that can filter that smaller crud out before it gets to your carburator? These motorcycle filters are heavy plastic, but "glass clear" in appearance to let you know they're filtering properly with crud sticking onto the filter innards or you see things floating around in them, and cost is less than $5.00 and I would also recommend asking the motorcycle shop for some little "spring wire hose clamps". The ATV's carburators work by "gravity feed" and ARE NOT under any pressure at all. NOW, if you have an ATV that's fuel injected, DON'T cut your fuel line as it's under pressure from a fuel pump(probably submerged in fuel tank) and you'll rupture a regular fuel filter and possibly catch your ATV on fire. These require special fuel filters and special designed fuel lines.
As to how your ATV runs, when you say it's blurping and acts like hitting rev limiter(but you know you aren't)that's a sign that one or both of your carburator jets is already clogged with crud and carburator needs to be removed and cleaned out thouroughly. If your careful about disassembly of carb, you can probably save the gaskets and reuse them without getting a new carburator rebuild kit. There's not but a couple gaskets and other items to these carbs, but you need to be watchful, as some items only go on "one way". When I recently bought my '98 Yamaha Big Bear, I was having same problem and I disassembled and cleaned my carb(used old gaskets) and reassembled and ATV worked perfect. My problem was with OLD GAS as it sat for too long(6 months). When this happens the gas will evaporate and can leave the jets clogged, dirty flaky crud in carburator bowl, and possibility that float needle won't seal and you have fuel running out the carb's overflow tube. Now what I do, is when I put in more fuel into my ATV, I'll also put in some "STABIL" fuel treatment. This way the fuel won't go stale from non usage. If you're not comfortable with carburators, just find someone who is(neighbor, friend, family member)and they'll know what to do and how to do it with rebuilding for the least amount of money. If you have Dealership rebuild carb, they'll want you to bring in ATV and they might keep it for 3 weeks, for what should only take a couple hours--and charge you $500 or more. Good Luck and keep us informed.
Del
Hi Delbert,
tx for looping me to all the possible causes of fuel related problems. It appers that i might have 2 symptoms related to that, first, the old men i bought the quad of had left 1/2 tank full, this might be old gas and as you mention micro particles must of got stock or clogg near jets or carb moving system, second and this is my fault, the fuel tank selector was engaged on Reserve sucking all the crap from bottom of tank, i've put new plug and suck remaining gas out and my newborn runs like a charm, thank you for your technical advise, i will later in spring attempt to remove carb and clean it for good, looking forward to adding pics soon, marty
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Old 05-03-2010, 03:21 PM   #7 (permalink)
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Hey Saber6, i was just adjusting my fuel screw and air screw last weekend because my bike does not idle and when starting it, it spits and spudders. I almost always have to choke it to start it even after its warm.
When i adjust my screws nothing happens, the engine does not idle faster or slower. Since you seem to know lots about this sort of thing i am looking for a suggestion. I pulled the carb apart and cleaned it. Could there be dirt or something blocking the adjustments from the inside of the carb or is that impossible?

Any help would be appreciated. It's a polaris Xplorer 400 2001
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Old 12-21-2010, 01:17 PM   #8 (permalink)
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Hello, saber6 or anybody with knowledge of 82 honda atc 200,
Do you have any expierince with the 82 honda atc200 carbs? my wheeler sputters at 0-1/4 throttle, starts with one pull but rpm climbs as it warms up. the plug is wet after running 4 a while it stalls out. I'm a auto mechanic so it's ovious to me that it's runnin rich but when i ajust the carb it only helps for a short time then the problem returns, cleaned carb & fuel tank with no success. please help if possible. thanks pitbull3773. also if you have any specs on this that you could email me that would be great, email is Ampitbullsrule@ yahoo.com
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Old 01-02-2011, 07:26 PM   #9 (permalink)
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Hi, i have a 85 suzuki lt 250 lt, have cleaned the carb, and it starts fine, and idles fine, but when ,i have the gas wide open, it starts to break down,on not running right, i read your post,tryig to figure out if its running too rich or to lean, and what should i do, if you can help me thanks
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Old 01-02-2011, 11:43 PM   #10 (permalink)
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Hi Delbert,
You are definitely a pro, I had no idea about the so called "Stone fuel filter" in fact I never heard about it, must be because I'm not much of a biker.
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