05 AC 400 4x4 - Running Rich - Pitch black plug, soot - ATVConnection.com ATV Enthusiast Community



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Old 01-21-2018, 09:18 PM
Weekend Warrior
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Default 05 AC 400 4x4 - Running Rich - Pitch black plug, soot

I have a 2005 Arctic Cat 400 4x4 FIS Manual. I had a speedwerx exhaust on it but decided to put the stock exhaust back on it since I moved and want to ride it through our community to get my mail without waking the dead. In doing so, I had to change my carb jets back to stock. So I bought a stock jet rebuild kit for my atv: https://www.ebay.com/itm/382125244104

I changed the main jet from a 175 back to a 145, replace the slow jet as it was nasty with the one from the kit, replaced the plunger with a new one from the kit, and replaced the pilot screw. In doing so it looked like the only things that were different from the kit were the pilot screw and the main jet, which makes sense since I had upgraded to a K&N air filter and Speedwerx Exhaust.

After a lot of youtubing, tutorials etc I got it dialed in close to running well.

I am having issues getting the pilot screw exactly on point. It starts and idles fine, and the top end is fine and roars, but somewhere around 2/8ths to about 5/8ths it bogs down, but not all the time. I can start it and rev it from 0 up through 100% no problem, then I let it idle back down, then I do the same thing 0% - 100% and around 20% or so it bogs down till about %80 where the main kicks it and it recovers. I only have issues when coming back up after going down to idle.

I am able to get the pilot screw dialed in to a point where it's almost not noticible and rarely happens, but not gone completely. Additionally, after two plug tests with brand new plugs, it's running really rich, black soot all over the plugs.

I don't think it's the pilot screw being the culprit, I am getting it dialed in somewhere around 2-2.25 turns. Additionally I am doing the plug tests fully assembled, air box on, filter on, cover on. Not sure what's making it run rich, but I suspect I need to fix this before I get the pilot screw dialed in perfectly and am chasing my own tail.
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Old 01-22-2018, 10:23 AM
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have you changed clip positions on the needle?
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Old 01-22-2018, 10:28 AM
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The stock needle doesn't have a clip, it just sits in there. The plunger under the float does have a clip, but I replaced it with a stock one. I also made sure the air hole in the vacuum piston isn't covered up by the needle holder. I already checked the main jet and jet needle, they're basically brand new, no clogs or deposits.
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Old 01-22-2018, 04:26 PM
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So, essentially you are back to stock? If so, you might want to get a Dynojet kit, especially for the tunable jet needle. As a rule, just follow the recommendations in the kit. You can fine tune low end-midrange by moving the c-clips on the needle. Main jet is easy to tune, just bracket the suggested jet2 sizes lean to 2 sizes rich. Check top speed, highest top speed is the optimum main jet. Play with the c-clip position for normal driving. Also make sure the choke is working and not sticking. My old 2002 AC 400 really responded with a Dynojet kit. Top speed went from 50 to almost 60 mph on 27" tires!
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Old 01-22-2018, 05:39 PM
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Thanks, upon doing some more video watching and talking to others, I suspect the needle is not sealing in the needle jet, allowing fuel in at idle. I think this because I can close the Air Flow/Idle screw all the way, blocking that circuit, and the quad will still idle. If the throttle is off and the slide has sealed the needle in the needle jet all the way and I have that pilot air screw closed all the way, the quad should die right?

So I suspect it's running rich in the mid range and it's because fuel is coming in when it shouldn't be. Based on my analysis of having had it apart, I think the needle comes up with the slide, but doesn't fall back into the jet all the way when the slide comes down. Because my first rev is always good, it's only when the slide is coming back down (returning to idle) that it starts to act up, like the needle just isn't coming home after being brought up. Or maybe the vacuum is not releasing and the slides not coming down all the way?
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Old 01-22-2018, 06:33 PM
Weekend Warrior
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Just pulled the carb back off the quad. First thing I decided to revist is the Vent Hose. The vent hose melted through in one place, I thought it was melted in a way where it was still open, but I pulled it off and I cannot get air through it, so the melt closed it off. I figure the hose came loose from the steering column when I rolled it in November, then probably melted on the exhaust pipe or cylinder and was just laying down below the steering shaft. I found a piece of the vent hose laying down there to (the other end that melted off).

The vent hose should be open no? Maybe that's effecting the vacuum and making the needle stick open longer?
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Old 01-22-2018, 08:34 PM
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Update: I fixed it!!

It was the vent hose. I cleared the vent hose, put everything back together, and she fired right up like a champ and then rev's all the way through without issues now, no bog no back fire! TIL!

This video helped me a lot, let me understand my carburetor MUCH better.

So if your bike runs with the pilot screw closed all the way, you've got a problem, check the vent hose!

" target="_blank">https://www.youtube.com/watch?v=-B-fLDOBqzo
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Old 01-22-2018, 08:38 PM
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The thing I am confused about now is the Pilot Screw and the Idle Screw.

The way I understand it, I should be able to have the idle screw all the way out (butterfly 100% closed) and still start the bike, if I can't, then I think I would need to open the pilot airflow screw until I can, or am I wrong there?
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Old 01-23-2018, 08:24 PM
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I think you have the cart before the horse (but I'm dyslexic). Typically the pms screw is turned about 2 turns out, no more than 3 turns. Then the idle screw is turned to give a nice idle, not too fast, not too slow. Anyway, makes sense, the vent hose being plugged.
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Old 01-24-2018, 09:46 AM
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Thanks, I have it all straightened out now. PMS screw is at 1.5, stock setting. I was able to find the stock spec for it. Then turned idle screw in until it starts on the electric start nicely and idles well. Then adjusted the throttle cable so it's at 0 at rest and 100% at max with no slack in it so the throttle isn't jiggly. Quad's running great now, feels better than it has since I've had it in my opinion. I've got a clean plug in it now, riding it around for a while and will plug test it again when I get home, should be tan I hope and good to go.
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