The more I ride my Sportsman 400, the power seems to be decreasing. Getting used to it is part of the problem, but you would think a 400 would do a little better though. Bogging way down on hills, even in low gear does not inspire a lot of confidence. It's been a good dependable ride, but I've been trying to decide what I might do to kick it in the rear a little without spending a lot of money, or just selling it and moving up to a larger engine.

It doesn't miss or anything, it's just pretty weak on acceleration. Thinking that a lower restriction air filter might help, I wondered what would happen if I took the top off the air filter box just to let in a little more air. It seemed to torque up a little more than normal when I first hit the throttle, then it just bogged down and started missing and backfiring. Keeping the throttle below 1/2 would allow it to take off from a stop fairly well, but when you gave it more throttle, it started bogging down severely and backfiring again.

I had no idea that the airbox cover and snorkel were so restrictive. The standard airbox cover and airbox opening must be designed that way for some reason. It seems that taking the cover off was causing a really lean condition, and the carb was not providing enough gas to keep the mixture balanced.

Now i'm wondering if you enlarged the airbox and snorkel opening somehow, and did some rejetting, would that give it more power? I just got the feeling that it would have made a real difference if the carb had been pumping in more gas to balance with the additional airflow. The carb vents are not blocked, and it runs smooth but weak with the airbox cover on. Does anyone have any ideas on this.

 

yes, I will try later sorry out of time (wife).

later

 

Its probably running too lean.

 

Hate to tell you this but your 425 motr wont ever be a powerhouse.......but a clutch kit (or just a spring change)will liven it up.I have an older 425 Magnum and it is a tank (almost bullet proof)Go with a UNI or K&N filter and a clutch kit and you will get about as much as you can exxpect out of it.The pipe I put on didnt help as much as the spring changes.

 

There are some things you can do. First, all these CV carburators use vacuum to work. They simply will not open or work properly without some restriction in the airflow. We had a 425 Expedition that we were looking for more power out of. The 400 Sp is the same motor (top end) as both the Expedition and the 500 SP. An H.O. cam will slide right in. We also put the intake manifold from a 500 H.O. on and a 38 mm flatslide Mikuni Carb, which does not require a restriction in the airflow to work properly. Then we opened up the exhaust. We got considerably more performance, but the EXP has a built in speed limiter which we were not able to defeat, so even though it had more power and acceleration, top speed remained the same at 55 mph.

Bang for the buck, you would be better off selling the 400 and getting a 500 H.O. or 600 twin.