95' XP nonlinear acceleration

[arlen2]

The RPM on my '95 XP does not correspond linearly to the handlebar throttle position from idle to top speed. When the RPM gets to somewhere between 4500 - 4800 it "jumps" up to somewhere in the mid 5-6000 range. No matter how slowly and carefully I increase the throttle it still happens. It's like when the second two barrels on a four barrel carburetor kick in or when a two stroke motocross bike suddenly hits its peak power band or when the afterburners kick in on a jet fighter. I assume carburetor butterfly opening and handlebar throttle position should be a direct linear relationship to one another, not a geometric relationship. In other words there shouldn't be a throttle plate opening that suddenly jumps ahead of the handlebar throttle position, unless my assumption is wrong. What is causing this?? I have a 1992 SP and it has "fairly smooth" throttle response from idle to top speed; there is no significant jump in RPM while accelerating. One thought I have is that perhaps something in the physics of water flow through the pump suddenly changes at 4500-4800 RPM thus reducing engine load and causing the RPM increase. I'd like to hear anyone's thoughts on this.

 

[bigJake]

The RAVE valves open around 4500 RPM giving you the burst in power.

 

[bigJake]

 

[Grim]

Might want to pull the raves and see if they need cleaning, they could be sticking and causing the abrupt power increase.

 

[arlen2]

The RAVE valves open around 4500 RPM giving you the burst in power.

There is no RAVE valve on my engine. The 1995 XP shop manual says nothing about it. My engine is a 717. What's your next best guess to explain the sudden increase in RPM?

 

[mikidymac]

Well, it is a 2-stroke so there will be a powerband. It has to do with the expansion chamber and porting so yes, they do come alive at a certain rpm. Known as coming onto the pipe in 2-stroke terms and yes, these do start to sing at about 5,000 rpm.

You are also transitioning fully into the main jet of the carb at that point too. Since these are a diaphragm carb, it really is less affected by the throttle lever and more by the vacuum the engine is pulling to open the full fuel flow so that can contribute a little too.

 

[bigJake]

There is no RAVE valve on my engine. The 1995 XP shop manual says nothing about it. My engine is a 717. What's your next best guess to explain the sudden increase in RPM?

There is no RAVE valve on my engine. The 1995 XP shop manual says nothing about it. My engine is a 717. What's your next best guess to explain the sudden increase in RPM?

Sorry, assumed you had the 787 model since the raves open at that same range. When mine are running optimally, it feels like my arms are being jerked out of the shoulder socket!