Honda's HPP system started as a butterfly operated canister mounted between the cylinder and pipe. It served to control the volume and length of the exhaust pipe. It had little effect on the power and most aftermarket pipes eliminated the canister. The butterfly was prone to carbon seizure and required frequent maintenance. The next generation HPP was used on the 1986-91 CR250. This system featured two sliding valves that operated within close proximity of the piston and effectively varied the exhaust port time-area in accordance with rpm. The square valves moved horizontally through a valve guide. The system was plagued with a mixture of design problems and misinformation on how to service and re-time this complicated exhaust valve arrangement. This section lists some common problems and some tips for timing the system, installing the cylinder, and engaging the HPP mechanism.
Common HPP Problems
Two main problems plague the HPP system: carbon fouling and rack-and-cam-spindle damage. The square shape of the valves contributes to the accumulation of carbon on one corner of the valve guide (stationary part), in the corner of the guide that is directly in the exhaust gas stream and this causes the valve to become carbon seized. Chamfering the corresponding edge (one-millimeter) of the valve will eliminate this problem. The rack and cam spindles are easily damaged when the cylinder is installed incorrectly, or the HPP mechanism is engaged incorrectly. See the photos for examples of damaged rack and cam spindle parts.
HPP Timing Procedure
Use the following procedure to time the HPP system:
1. Install the HPP valves and levers and tighten the pivot nuts. Place the washer on the stud first, then the lever (marked left and right), and then the flanged center bushing with the flange side facing up.
2. Turn the cylinder upside down. To position the rack correctly, slide it to the left until it stops; then move it right 2mm. Rotate the rack so the square notch faces you. Now the rack is in the correct position so you can install the pinion shafts. Carefully turn the cylinder right side up without changing the position of the rack.
3. Close the valves and install the left pinion shaft with the screwdriver slot facing the one o'clock position. Install the right pinion shaft with the screwdriver slot facing the eleven o'clock position (see photo for correct positions). A simple way to determine if the pinions are mis-timed to the rack is to look at the screwdriver slots. The wrong position is with both slots facing twelve o'clock.
Installing the Cylinder and Engaging the HPP Drive
After timing the HPP mechanism, the cylinder is ready to be installed on the crankcases. Here are some tips for installing the cylinder and engaging the HPP drive mechanism:
1. Make sure the reed valve is removed from the cylinder. CR250s have such large intake ports that the rings tend to slip out of the ring grooves during installation of the cylinder. This takes the spring pressure off the cam spindle. Now turn the engagement bolt 1/4 turn clockwise. You should feel it positively lock into a groove and stop. Remember that the HPP engagement bolt is a spring-loaded detent not a threaded bolt. Slide the cylinder down onto the piston and rings, use a screwdriver to push the rings back in the grooves until the rings clear the intake port.
2. The HPP mechanism should be engaged while the cylinder is being installed, just to keep the cam spindle in position. The cylinder will stop about 3mm from the crankcases because the cam spindle and the rack are misaligned. Now disengage the HPP mechanism by turning the engage bolt 1/4 turn counter-clockwise. Grasp the right-side valve lever and wiggle it; the cylinder should then drop evenly onto the crankcases.
3. Bolt the cylinder down tight. The best way to engage the HPP mechanism is to insert a screwdriver in the right-side pinion shaft and turn it counterclockwise. Now turn the engagement bolt clockwise. You should feel the engagement bolt lock positively in position. If you try to rotate it too far, you will bend the cam spindle and the system won't work at all, so don't be a hammer-head! The best way to check the HPP system is to remove the left-side valve cover from the cylinder, start the engine and warm it up, then rev the engine. The valves should be fully closed at idle and fully open when the engine is revved.
In 1992 Honda introduced the HPP system currently used on the CR250. This system features a center valve for the main exhaust port and two rotating drum valves to control the flow of the sub exhaust ports. This system also features a return of the old resonator as used on the mid-eighties model. The resonator improves the throttle response and mellows the powerband at low rpm. A thin rod links the valves together and the whole system is mostly self-scraping to prevent carbon build-up. The inside of the center valve has an elongated passage where the tie rod travels. This elongated passage is prone to carbon build-up over time (1-2 years). The carbon limits the range of movement in the valves. The carbon is easily removed by using a small diameter rat-tail file. The sides of the center valve and the drum valves interface, and that area is prone to carbon build-up too. A wire brush or file is an effective tool in cleaning the exhaust valves. Here is a simple way to check the operation of this system. On the left side of the cylinder there is a 17mm cap bolt that exposes a straight line mark in the left drum valve. There is a corresponding mark on the cylinder. The "L" mark denotes the low speed position of the valve and the "H" denotes the high-speed position. To check the HPP, start the engine. At idle the valve should align with the "L" mark. Then rev the engine, the valve should align with the "H" mark. If the angle of the mark on the valve is slightly off, then the valve probably needs to be de-carboned. This system is very easy to disassemble and can only fit together one obvious way so we won't waste space on that procedure. There is some aftermarket parts to adjust the performance of this system for different types of dirt biking. Pro-Racing in England makes a spacer for the right side valve cover. It serves to add volume and length to the resonator part of the system. This is especially suited for enduro riding where a smooth transition to the mid-range is important for better traction. ESR (Eddie Sanders Racing) in California makes a replacement HPP system that holds the valves wide-open. The center exhaust valve is thinner which enables tuners to raise the exhaust port. The ESR system is primarily used for dirt track or kart applications where low-end power is of no consequence.
Whenever the cylinder is installed on the bottom end after top end rebuilding, the valves need to be put in the closed position. Otherwise the HPP cam spindle that connects the actuator in the cases to the cylinder will get damaged when you tighten down the cylinder. That will also make the valves inoperable. Always check the HPP valve operation after you assemble the top end by using the inspection cap on the left side of the cylinder.
The CR125 HPP system was redesigned in 1990. Honda chose to use a system similar to the 1986-91 CR250, featuring horizontally sliding valves. This system was plagued with problems over the years. The valves are prone to carbon seizure because the critical square edges face the exhaust stream. If the clips that fit on the ends of the valves vibrate off or if the valve wears too much then the valve can tilt on an angle and strike the piston. Another common related problem happen when tuners widen the exhaust port during porting and neglect to grind the valves at the outer corners for piston clearance. There again the piston strikes the valves because they protrude into the bore. In 1998 Honda made a modification to the valves, they added an L-shaped rib that prevented the valves from angling in and contacting the piston. The other problem of clearance between the top of the valve and the guide was eliminated so the new style valves provide more low-end power. These valve and guide sets from the 1998-99 models fit the CR125 models back to 1990.
In 2000 Honda redesigned the CR125 engine and adapted the exhaust valve system used on the RS250 roadracer. Honda also used this system on several dual sport and street bikes sold in Asia and Europe. The new system is so simple and effective. It is a wedge-shaped valve that pivots at one end, similar to the CR250. The valve is much thicker and can vary the exhaust port's effective stroke, time-area, and duration over a wider rpm range. It's a self-scraping set up so maintenance should be greatly reduced over previous models.
