I purchased a 2001 4x4 Scrambler 400 and was told everything works, just needs the carb cleaned due to it sitting. The seller didn't want to invest anymore money into it so I purchased it cheap and with a title. I tore the carb off, cleaned it (like I have dozens in the past) put everything back together intricately, hit the start button and it fired up without the choke and idled like a dream. I was actually overly impressed. Dropped it in gear and at about 1/8 to 1/4 throttle it acts as if the carb is trying to blow off of the bike. It jumps, snorts and flexes the air box. I figured there was probably trash around the floats or jets so I tore it down again.

Long story short, I've had it off a few times and it is still running horrible if you give it much throttle. Idles perfect and smooth in gear and out of gear. I've adjusted the air screw, removed the air filter, added the air filter and restricted throttle, choked it, etc nothing seems to matter.

I was leaning towards a rev limiter box since I seen they cut a wire off of it - which reading the forums - many do. I've completely unplugged the rev limiter while running and it didn't phase it. Out of curiosity I was browsing eBay for rev limiter boxes and carb kits when I realized all the carbs for the 400 didn't look like my Mikuni. I started researching and realized my 400 has a 500 Scrambler carb on it...... I'm not sure if this is my issue or not? Any advice is greatly, greatly, greatly appreciated at this time as I've chased wire, troubleshooted, and done intermediate work on this bike.

It throws me for a loop that it idles like a dream up until about 15% power or so. Right about the amount of throttle it takes to engage the drivetrain is where it starts popping and spitting gas into the intake... I even put it up on jack stands to see if it would work in gear without as much load and it still falls on its face around 15% throttle or so.

 

Think you need to get the correct carb.. Before you get too deep into it,do a compression check to see if you're ok. Should be around 130 or so. Probably has the BST34 cv carb on it..Takes a VM34SS Mikuni carb OR plenty of 34 mm carbs on ebay. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse
http://www.ebay.com/itm/34MM-CARB-CABLE-CHOKE-S-M-/360814014966?pt=Motors_ATV_Parts_Accessories&hash=item540230f1f6&vxp=mtr

 

I would think it would have good compressions for the fact that it starts effortlessly. I know when atv's are low on compression they can be a nightmare to start, or start after numerous turn-overs until the rings swell from friction/heat causing a better seal and more compression. I don't have a compression gauge but could probably borrow one.

As far as the carb, here's an identical one on eBay: http://www.ebay.com/itm/05-POLARIS-SCRAMBLER-500-HO-4X4-04-06-MIKUNI-COMPLETE-CARB-CARBURETOR-W-CABLE-/131172613095?pt=Motors_ATV_Parts_Accessories&hash=item1e8a7f3be7&vxp=mtr Mine has the brass/copper top that is held on by 2 screws as well as only 2 screws holding on the float bowl. Most have the screw on throttle top with 4 screws holding the float bowl on the bottom. I'm not sure if they had to change out the complete throttle (thumb mechanism) as well when they swapped carbs.

 

The 400 engine's not designed to operate with a constant velocity diaphragm carb that works off a vaccum,but a manual lift carb slide such as the VM34.Get the correct carb for it..

 

In theory, I need a carb that works from the throttle actuating the carb slide, whereas in my situation, my carb throttle controls the butterfly which in return effects the slide based off of vacuum? I apologize for the questions, you definitely seem to know your carbs. There are so many 34mm carbs on eBay it is mindboggling. I'm not sure my throttle will work with a new one, so I may have to buy a used one in order to get the choke and throttle cables.

 

Plenty of used 400 parts on ebay to make this machine correct. Why the PO tried to install a cv carb.. Who knows? This carb would never work without a vacuum port on the intake and even if he altered or found an intake manifold with one,the vacuum is not strong enough (compared to a 4 stroke engine) to lift the slide up enough to even make it run properly. Just sell your carb setup on ebay to someone that needs a 4 stroke carb and get the parts you need.

 

Thanks for your advise this morning, I'll browse eBay and see if I can get one headed my direction today!

 

Another quick question, looking at my throttle it is a single cable from the thumb mechanism to the butterfly. Looking at the 400 online, the throttle cable is single from the thumb mechanism, then splits. One will have to go to the slide, the other I'm not sure. I don't see how it could go to the choke, any thoughts?
1998 Polaris SCRAMBLER 400 (W98BC38C) Controls - Throttle Asm./Cable Scrambler 400 | Babbitts Polaris Parts House

 

Yes it's a split cable that operated the carb and also the oil pump located at the front of the engine. PO probably blocked off oil injector at the intake manifold and premixed fuel I assume..You can keep premixing if you wish if the pump is frozen when(if) you remove the three screws on the pump plate to inspect.http://www.cyclepartswarehouse.com/f...2001&fveh=5541 40-1 is plenty enough. As far as a single cable operation there are ones available for the correct carb to eliminate the split cable if you continue premixing..

 

That makes perfect sense. The PO did in fact remove the oil injection and tank. I purchased some 2 stroke oil by Pure Polaris and mixed it 40:1 initially when I purchased it. Thanks for the information! I'll order that carb now!