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Hey everyone, I've been researching this for days and can not for the life of me figure this one out. Thanks in advance.

2004 Sportsman 600, Not the Ducati coil and carbureted, 880 miles

Bike will start perfectly, idle and rev until 1/2 throttle where it starts to bog and spit. This happens both in park or under load.

I can maintain about 20-25 mph by feathering the throttle but anything above that and it starts sputtering.

So I did the following maintenance and repairs:
New spark plugs as the old ones were black so running rich
Cleaned carb
Did a Carb rebuild kit
Checked the emulsion tube / needle jet and seems fine at position 3
Diaphragm and slide looked good from what I could tell
Choke seems fine and not stuck open
New air filter
New air box to carb boot (it was split)
Float height is at 14mm
Jetted to 152.5 as I'm always at higher altitude, stock is 155

Tried with the air box removed and the same result as when its fully sealed up.
 

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If an exhaust clogs, it will cause the back pressure to limit engine performance - it will only run so fast - if the exhaust can't get out, a fresh charge can't get in.

The manufacturers use this method to limit speed on some youth models. The old Suzuki LT and RM 50 and 80 has a removable exhaust restrictor to limit performance until the rider gained experience and confidence to ride more aggressively and on a faster machine. The adult supervisor could assess the riders ability and determine if the restrictor could be removed for added performance without the need to upgrade to a different model.

I even ran into a restricted exhaust on a 650 Yamaha twin - on the Yamaha, the exhaust was double walled to lower surface temperature and reduce the bluing of the chrome head pipe. This particular exhaust (unlike the majority of double walled head pipes used on the majority of Japanese street four stroke engines) had vent holes near the head flange. In this unique instance, water had gotten in the head pipe and settled in the lowest point. I speculate that either over the course of several mid-west winters the water froze and pressed the inner pipe away from the outer pipe eventually reducing the inner pipe diameter to less than 1/4 of it's original dimension. The engine idled normally and accelerated well, but at a certain point one cylinder ceased to fire. After all the checks, tests and normal remedies failed to restore performance, I tested exhaust pressure - one cylinder was normal and progressive - the other cylinder increased pressure to a certain point and leveled out, increasing slightly as the other cylinder struggled to increase engine speed. I suspected a restriction (mouse nest, walnuts, rusted and distorted baffle) as the cause. I removed the one muffler and restarted the engine. It was somewhat improved, but still not producing pressure. It was at this point I shined a light inside the pipe and discovered the restriction. Removing the head pipe fixed it! I ordered and installed a new head pipe, the old muffler and the problem was solved.
 
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