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Discussion Starter #1
I have a 300 fourtrax. I'm mechanically capable of doing about anything, but as the title says me and carbs don't know each other very well.

I have taken the carb apart and cleaned it. Not very well I guess, as there is some varnish but all the passage ways are clear and I got a carb rebuild kit. Everything seems to install one way with the exception of two pieces. The idle mixture screw (I understand this) and the needle. What does this needle do, and how does positioning it differently change the power?

I'm also thinking of changing the exhaust and modifying the intake for better flow. I've read that this should be rejetted, but not one thread I've found has given an exact answer. I'm not even sure how to tell if I'm running lean or rich. I'm used to plugging in a "programmer" and knowing so please excuse my ignorance.. I'll take a shot in the dark and say I'd need a 128 and keep the stock 40?

While I'm here another question.. The atv seems to stumble or sputter during partial throttle. What could cause this?
 

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NvrDwn , I have cleaned a lot of carbs , and have cleaned some like 5 times before getting it right , sometimes all it takes is a can of spray carb cleaner , sometimes you need to soak it in a gallon can of cleaner and blow it out with compressed air ---------- but the biggest thing about cleaning a carb is cleaning out the tank , petcock and fuel hose as you will clean the carb then reintroduce trash right back into the carb , I run a small fuel filter before the carb , never use ethanol gas , always buy the premium gas and always add fuel stabilizer and Sea Foam to the 5 gallon cans I fill my bikes with
 

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Discussion Starter #3
I use that one gallon stuff. I let it soak for 30 minutes and blow it out. It needs a scrubbing to clean everything. I lowered the needle in the carb and it runs good now except it doesn't have very good low end, but doesn't sputter like it did. The idle is a little and I mean just a little rough. I moved the screw around and it made little to no difference. I've never had this bike running right since I had it so Idk if the no low end is normal. It really picks up the last 50%.
 

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If your pilot jet screw (for idle mixture) won't cause the motor to respond to adjustments then the carb is either dirty/corroded and partially plugged somewhere and/or the float level is way off. I would completely disassemble the carb (100% stripped) and let it soak overnight (or longer if its corroded inside) in your gallon of carb solvent. Then use compressed air to blow every passage and hole out good before carefully reassembling the carb to factory specs found in the service manual. Install ONLY the original OEM Honda jets and other hard parts back in the original OEM carb, else you may be chasing problems (china knockoff parts/kits/carbs won't work) no one can fix.

Let us know how it goes...
 

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You want that e-clip set to stock, so that's the centre groove, also, the pilot jet is somewhere around 2 1/4 to 2 1/2 turns out according to the service manual.
Just take into account that every carb will need setting differently so the pilot will possibly need tweaking from that position to suit your machine.
 

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what does your spark plug say ? ------ the spark plug condition is a good indicator of whats going on with the carb , take a pic of the plug tip and post it up ------- but before you take that pic , if you have been adjusting it up and down , then either put in a fresh plug or clean the one in there , then run it and then pull the plug , there are picture charts with descriptions of different senerios -------------- an over looked carb problem is the rubber boot between the carb and head gets a crack or hole in it and throws off the mix , spray some starting fluid around the rubber boot and listen for an increase in rpms ----------------- try running it with the air box cover off ----- are you sure you got the right spark plug in the engine , personally I find that my bikes don't like the Champions and Autolites , NGK last longer
 

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what does your spark plug say ? ------ the spark plug condition is a good indicator of whats going on with the carb , take a pic of the plug tip and post it up

Plug tip will tell you if you have the correct heat range. The bottom round edge of the plug will let you know if your jetting is off.
 

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what does your spark plug say ? ------ the spark plug condition is a good indicator of whats going on with the carb , take a pic of the plug tip and post it up

Plug tip will tell you if you have the correct heat range. The bottom round edge of the plug will let you know if your jetting is off.
Black soot ring and black soot tip , all dry black soot , what would you do ?
 

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what does your spark plug say ? ------ the spark plug condition is a good indicator of whats going on with the carb , take a pic of the plug tip and post it up

Plug tip will tell you if you have the correct heat range. The bottom round edge of the plug will let you know if your jetting is off.
Black soot ring and black soot tip , all dry black soot , what would you do ?

If it's black, where does the issue present itself? Idle, mid, or WOT? Then once thats determined, lean it out..
 

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Plug tip will tell you if you have the correct heat range. The bottom round edge of the plug will let you know if your jetting is off.
Plug tip will tell you if the motor is experiencing a lean condition, rich condition, a just right condition, correct or incorrect heat range, oil consumption, pre-ignition, detonation... fuel type & quality... etc... etc... There's usually more to learn than meets the eye from a spark plug.
 

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I am going to have to take a pic of my black sooted plug as the chart Wheeler put up doesn't show anything close , if I had to call it something , I call it black out , the carb on this one is set at 1 1/2 , way below specs of 2 5/8 turns , might have to move the needle on this one , yesterday I bought one number hotter plug and going to try that
 

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Discussion Starter #13
I'll try to work on it today. I'll move the needle back down. I forgot to mention that the vacuum piston has a small rip right at the seal. I was think it was sealing but maybe it isn't, causing full power at low rpm to suffer. I have a new one on the way. That should be Monday. I'm changing the oil in this thing today. I believe it's been 2 maybe even 3 years since it was last changed. When I get the new part if that doesn't fix my issue I'll tear it completely down again and let it soak for a few hours, clean it out with carb cleaner and then take it over to my dads shop to blow all the crevasses, I'll probably go a head and use a metal brush to get the remaining stuff stuck to the walls out as well.
 

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Discussion Starter #14
Also, I broke a gasket yesterday replacing the starter. The gasket for the gears of the started to the crank. I think it was 5 bolts. What is that gasket called? I see it in full gasket kits, but I don't need all of that.
 

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That sooty plug looks like it come out a motor running with the choke on.... hope you find that problem right away.
 

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That's a great point retro!!! IF the choke cable isn't returning to the closed position it will definitely carbon up!!
 

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Discussion Starter #19
Doesn't look like i can add pictures through my phone. But I am running super rich. Everything is black even the tip of the electrode. I'm disassembling the whole carb now. I'll add all the original pieces back after a nice soaking. I got my new diaphragm in today instead of Monday. I tried that first, and put the C clip back to stock Location. I gained some top end for sure.
 

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Discussion Starter #20
That picture, that's how my plug looks except the black goes up a few of the threads. I checked my choke, I seems to work fine to me. It runs on the "on" position, however it never starts or it kills the engine if it's already running when ever I move it to anything other than on. Which I know isn't right.
 
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