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Discussion Starter · #1 ·
I have a Prairie 650 that I acquired from my new homes previous owner. He said he has always taken good care of it and I believe it based on the condition of everything else around the house. It fires right up on the first pull and will run fine with the choke all the way out until it starts to warm up, 30 seconds or so. Once warm there is no way to keep it running outside of holding the throttle in a bit.

He mentioned that he's never gotten the thing to idle correctly. He had it jetted for altitude but that didn't seem to ddo anything. If I hold the throttle in just a hair it will idle all day long but when released it sputters out after 5-10 seconds. The idle adjustment was already all the way up (clockwise) if I turn it at all the other way it sputters out instantly. Any thoughts? Next step will be a carb rebuild and plugs but it runs great otherwise. I'm thinking of trying to tighten the idle adjust unless that's a bad idea. I'm not sure what speed I'm idling at with the knob turned all the way up ( no tach) but it's low.
 

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If the idle control is all the way up its more like at an 1/16th+ throttle already...way over or past any idle position. Says the slow or pilot circuit is starving. Check that the circuits are clear and open, that they have the correct pilot jets and air jets, check that the float levels are correct...very important, and that the A/F screws are set somewhere between 2.25 and 2.75 turns out from fully closed. Bet they are either shut off or plugged. Also check for any vacuum leaks and fuel supply problems as well as the tank vent check valve. Common problem.
 

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Discussion Starter · #3 ·
Thanks for the info, I'll try these out. I've checked the A/F they were close enough, set them to 2.5. I'll start digging, I'll start at the floats after checking vacuum hoses, probably wouldn't hurt to pull the jets and blast it with some cleaner.
 

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I have a 04 700 prairie and have had some issues here at 7400 +- Stock atv save a UNI foam filter, factory jetting and yes, its a tad rich. Ive farted around a bit and always come back to the idle mix screws out 1.5 turns. When I had then at 2 or 2.5 out it would just shut off at idle sometimes after a good run unless I worked the throttle. My complaint is its a SOB to start when its teens, but once going it runs good and pulls hard. Ive hit 62mph on flat dirt FR roads so no complaints there. Im curious what the PO had it jetted to. Thru logic to lean mine out I think fatter needles are in order but have not really looked for them. being I dont ride that hard I cant see smaller mains doing the trick entirely. On mine I can remove the bowls so changing the jets would be possible without yanking the barbs which is a PITA.
 

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Pineco, do yourself a favor, get a service manual, pull those carbs out of there and do a proper inspection to see whats going on. Like NM said, these carbs are pretty finicky and require immaculate cleanliness to operate properly. If she rips on the top end, your mains are probably fine, but if shes not idling properly, there are problems with your pilots, or the pilot circuit. If you need jets, get Keihin jets, as most of the aftermarket ones are not the greatest in my experience. They are harder to find (in Canada anyways) and cost way more, but they are worth it. Trust me on this one. Float height is critical on these carbs. It's gotta be bang on, and you have to set them correctly. When you set the height, there is a spring setup inside the needles (needle and seat) that control fuel intake into the carbs. You have to set the height when the float arm barely touches that needle, not when the full weight of the float is on the needle, as it compresses the spring in the needle entirely and gives you an incorrect float height. Not a hard job, but a fiddly one as the only way to set the height is to bend the arm a bit, check it, rebend the arm, check it.
So, pull them, gut them and make sure you distinguish parts between the front and rear carbs as the mains will more than likely be different between the two, clean them with brake clean or carb cleaner, blow out all the little air and fuel passages with compressed air, check all your parts before reassembly and you should be good to go.

P.S. SERVICE MANUAL!!!!!!
 

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Discussion Starter · #7 ·
Thanks everyone. I do have the service manual and have started messing around over the past week. Qtown, I found that 1.5 turns works for me as well, thanks for the advise. I can now get it to idle but only in a very thin sliver of the choke, about 1/3 on. A hair forward or back and it will stall but it's a start. Should I try 1.0-1.25 turns or something else?

mastrblastr, thanks for the detailed response. Next is pulling the carb and going over everything you mentioned. I have some experience messing with carbs but not ATVs. This will be fun. I'll look into Keihin jets, I probably would have wasted a lot of time otherwise. Do you know if they have recommendation chart based on altitude? I couldn't find one in a quick search. I don't want to use what PO has in there as a reference, who knows what he did trying to chase down a problem.
 

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I still mess with my 700 occasionally. I pulled the air box lid off for the heck of it the other day. made lots of noise and farted thru the carbs when it was cold and I gassed it. never rode it like that figuring there was no point. Ive yet to see a "altitude" guide for jets and Im kinda leaning toward needles making a bigger improvement but havent really tried digging to deep for part numbers. I cant recall if I posted Sudco or Jets R us for parts but they may have a idea of things. Again ,plug reads at different rpms after a decent run will tell lots toward getting things right. It just takes time and patients which Im short of apparently.
 

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When I was jetting my Brute after the CDI I went with what the manual stated for above 3900 feet. That was good to about 9000 then it would be rich again and needed a little more. I then installed two TPI slide vents in my air box which when opened would lean it about as much as another jet size down. They took me to 11,000 feet no sweat. Just had to remember to shut them on the way back down below 9K or she would be so lean it would start backfiring. This is what they looked like. Had to pop the top to open them but they were sure nice to have.



 

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Discussion Starter · #10 ·
I tried running mine as well with the cover off, smoother for sure. I think I was achieving exactly what NMKAWWIE's vents are doing, getting a leaner supply. I have new plugs and will tear down the carb in coming weeks. I'll let you guys know how it goes.
 

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I tried running mine as well with the cover off, smoother for sure. I think I was achieving exactly what NMKAWWIE's vents are doing, getting a leaner supply. I have new plugs and will tear down the carb in coming weeks. I'll let you guys know how it goes.
Can't run them with the covers off..it's too much.
 

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Pineco, your manual should have a baseline for jets for the altitude you are running at. The manual for my Brute does so I am assuming the manual for the 700 should as well. Remember, these are just basically a suggestion by the manufacturer but every machine runs differently. Get to know these carbs before you begin jetting. Something as simple as a shim or two under the needles can make a big difference.
 

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Discussion Starter · #13 ·
Thanks, I'll have to dig into it and get to know this carb. I'm sure it's a lot different than the Holleys I'm used to messing with.
 
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