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Discussion Starter #1
I have a 06 Brute Force 4x4i with about 40 hrs 120 mi. It cranks with choke and idles really fast. push choke off and it spits back throught the carbs and backfires. It does it cold or warmed up. It will accelerate and not miss in a pull but it misses on or a little above idle. This machine has been run very easy and not too often. It just started this all at once. I'm starting to hear after it warms a very light tapping around the engine.
I replaced plugs, drained carbs, checked wires and hoses best I could.
I need some of you geniuses to tell me
What could this be from simplest to hardest?
Thanks........
 

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Discussion Starter #3
?

I have not checked them, didn't want to turn anything yet. Nothing has been adjusted since it was bought.
 

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This may help many of you to gain confidence to adjust your carburetor yourself. Many people believe that carbs are very complex, but with a basic understanding of how carbs work, you will have no problem with performing your own carb adjustments.


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 restricing 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 restricing air, effectively enrichening 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 accomodate 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 attatched 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 enrichening 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 enrichening 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 occuring at, you can figure out what circuit the problem is occuring 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, enrichening 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 enrichen 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.

If you have any questions, feel free to ask. Hope this helps you guys!

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.***
 

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If it runs fine with choke on then it is starving for fuel for some reason. Could be simply a fuel filter but probably something else.

The above information is very good but had one point that is incorrect.

The main jet does control fueling all the way down to 1/4 throttle. This is the reason when tuning you start at WOT then work backwards. The needle in essence sits down in the main jet by way of needle jet so if you choke your main jet down then you will also choke mid throttle down.

Example if you set mid throttle first and got it right then found you needed a smaller main jet cause it was rich at WOT then it would be leaner at mid throttle now so you would drop the clip on the needle to richen mid throttle back up.

Always work backwards.

Todd
 

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has this bike sat a while?does it run better when you put the choke on? how did the plugs look when you replaced them. were any either of them dark colored?i know i had problems with the choke plungers sticking.topdrive gave good lesson on carbs
 

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Discussion Starter #7
Wow .......Thanks to you guys for the responses.


Wow .......Thanks to you guys for the responses.
All this may be very helpful once I digest it......

Ok I have isolated the problem cylinder. Its the front. With the rear cylinder plug wire pulled and the front cylinder running it has the missing, spitting, and won't accelerate fully. It seems to bogg down. When I engage the choke it runs faster and stays running.
With the cover off of the air filter box it gets worse, but when I spayed a touch of starting fluid at the filter it accelerated more.
Both plugs seem to be firing fine. Both cylinders seem to have the same compression. .....When I first drained the carbs the front carb did have a small amount of very tiny trash and a small amount of water (about half the size of a BB).
Does this machine have a fuel filter?
I don't have pics or a service manual so its hard to locate the jets on the carbs. Are the jets visible on the outside or are they on the inside?
What jet would be most likely to have trash in it? Could an air jet have trash in it?
The carbs are K........ brand.
When I first pulled the spark plugs they seemed to be blackened but might have been due to over choking.
The machine usually gets run every couple of weeks or so and has ran very well. But this time when starting to load it would never smooth out.
Thanks to you guys.......Feel free to give all the info you want.

I also have a 750 Brute F 2005 4x4i and the only trouble I've had with it is staying on at 80+mph and keeping the front end down.
 

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dirt in carb jets most likely you have to pull carb off pull bottom bowl off pull carb apart clean everything .pull out mainjet etc ck choke plungers one in each carb maybe someone can chime in where to find diagram of carb.my 05 brute did same as yours i also put seafoam in gastank to stablize fuel and clean carb its good stuff
 

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Discussion Starter #9
has this bike sat a while?does it run better when you put the choke on? how did the plugs look when you replaced them. were any either of them dark colored?i know i had problems with the choke plungers sticking.topdrive gave good lesson on carbs


What do you check or look for if its the choke plungers sticking?
thanks.........joe
 

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pull the plungers out of the carbs there is a small bracket and phillips screw you take off the plunger will come out when you pull on thecable they are a painto get to.i clean the plungers real good if they are scraped up get some fine sand paper to clean them up i alsso strech the plunger spring out some you will see what i mean when you get them out.i wish some one could chime in with a diagram what im talking about
 

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also check the carbs make sure they are tight on rubber manifold they loosen up sometimes.when i worked on my carbs i always took the plastics off the front airbox etc so its easier to get to carbs i leave it off till im done but you have to put the airbox back on when you test drive it
 

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Discussion Starter #14
Update on the 650 Brute Force!!!!!!!!!!!

OK........Got back to the machine tore it down took off the carbs.....found a flat piece of micro trash half the size of a grain of sand lodged in the pilot jet of the problem carb. Cleaned both carbs and installed an inline clear fuel filter just inside the side panel and added sea foam to the fuel tank. Fired it up, warmed it up, adjusted idle speed, .....It now runs better than new. The engine cranks up and is running with only a half round of start. It idles as smooth as good whiskey!
Thanks to you guys for your info!
Now gonna service and do the 750 BruteF with the filter and sea foam, .....also the belt light just started flashing on it ..so more work in the shop. It has 165 hrs on it and it was taken in to the dealer at the 100 hr belt light flash in which they may have only reset the switch. So the belt may actually need attention now.........I do have a service manual .........But I will appreciate all the info anyone can give on inspecting, adjusting, and getting the drive belt in tip top shape......................Thanks again! Camo Joe
 
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