2005 Arctic Cat 400 4x4 FIS Manual.

I bought a new carburetor recently and the quad now runs great, but there is a huge power loss on the top end. I used to be able to open it up and get up to 55-60mph out of it, but now I struggle to get it over 30 mph. When I got the carubertor I just put it on, didn't take it apart. Can't really figure out what's wrong. It could be clutch slippage and coincidental to the installation of the new carb, but I'm not sure.

Or maybe the stock jets in the new carb are lower than stock jets in the states for my altitude? I bought the carb from the Ukraine and had it shipped to West Virginia.

Going off the parts diagram, the main jet ranged from a 140 to a 148. So maybe I need to take it apart and see what jet's in it, and try bumping up?

Edit:

I had to replace the fuel line recently. The stock fuel line is 7/32" and I couldn't find it anywhere, so I put one on slightly smaller (barely, maybe .5 mm smaller). Could that cause me to run lean?

I'm going to do an oil change on it too, to an oil I know is wet clutch rated.

 

I'd ensure the slide is moving all the way up before doing anything else...push the throttle wide open and verify the slide is all the way up/open (visually or stick your finger in there). If that's good then I'd do a plug chop...usually running lean means more power...but you'll know right away after a plug chop...

 

Slide only opens about half way and flutters there, won't open any more than that. I can push it up but it won't go up at full throttle and I'm moving the trottle arm manually so I know it's 100% throttle and not a cable issue.

 

I opened the top of the carb to inspect the vacuum piston. The diaphragm is in perfect condition, seated completely and moves up and down as expected. I also checked my carb vent hose to make sure it wasn't clogged, it's not. I suspect the main jet the new carb came with is just not big enough and I don't have a bigger main jet, left old carb with correct main jet at mechanics, I'll see if I can pick it up tomorrow.

Worth noting the spring the vacuum piston came with is way springier than the last spring I had (but it was old), and I have that spring here and the new spring has tighter spacing and coils than the spring I have. Could the new spring be too strong?

 

Sure, try the old spring. You can buy a few different size main jets from <jetsrus.com>. The correct main jet will give you the highest top speed. Another option is a Dynojet kit. The adjustable jet needle is nice to fine tune midrange. I had an Arctic Cat 400 with the Suzuki motor. Fine tuning with a Dynojet kit , was able to get top speed from 52 mph to 60 mph! Throttle response was much better. I also drilled out the hole in the slide to 1/8". My motor was the water cooled version (2002). Was a great ATV!

 

So not knowing if you bought an OEM carb or a replacement...the spring could be the issue. It's a CV carb and the vacuum controls the travel of the needle that opens up the main jet...usually the diaphragm is the culprit...I hold the rubber up to the light to ensure there are no tiny pin holes...but if that "new" spring is significantly harder than the stocker, then yeah, I'd put the stocker back in and give it a go. If it still feels like it's not hitting WOT, then you may have a crankcase vacuum issue...but let's hope not...

 

Thanks, I'll pull the carb again when I get home and take a closer look at the piston vacuum diaphram. I'll try swapping out with a spare I have laying around too (have 3 total now, lol). As well as swapping springs back and forth, and see if there is any difference.

 

Ok, I pulled the slide out again and compared it to my old one. The new slide looks perfect, no holes or anything and it seats perfectly in the carb body, can't see any thing wrong with it. However the old slides vent hole appears to be just barely bigger than the one in the new slide.

Just for kicks I put the old slide in the new carburetor and and started the quad, and boom WOT slide opens all the way, power restored.....

It's weird, the hole is barely bigger than the old one, maybe mircrons bigger, they are damn near exactly the same size.

Only other thing I can figure is maybe the new carb slide diaphram needs to be worn in?

 

Just took it for a test drive... Yep, she's back, faster than ever. I hit 30 in 2nd gear and was hauling ***. Don't have room to see top speed here, will take it out on the trail next weekend. Still weird why the new slide is ****. I'll get it under magnification and see if it's defective.

Right now though I think this quads running like new or better.

Thanks for the help everyone!

Kind of impressive that I road Taskers Gap twice with 50% power though, lol.

 

I had asked earlier if it was "replacement" or "OEM"...there are differences...glad to hear you're up & running WOT!