BREAK IN PROCEDURES

Sprint Instructions:

• The engine should be broken in with starting with part throttle runs through the gears with light loading and low rpm. Gradually increase the length and loading of the pulls through the gears over a period of 5 minutes. Do not apply full throttle or rpm above 11,000 during this period.
• Cool down for 3 minutes.
• For the next 5 minutes apply short bursts of full throttle in all the gears. Gradually increase the length of the full throttle bursts until full throttle is constantly applied in 1st thru 4th gear and intermittently thru 5th and 6th gear.
• Go Racing!

Road Race Instructions:

• The engine should be broken in with starting with part throttle runs through the gears with light loading and low rpm. Gradually increase the length and loading of the pulls through the gears over a period of 5 minutes. Do not apply full throttle or rpm above 11,000 during this period.
• Cool down for 3 minutes.
• For the next 5 minutes apply short bursts of full throttle in all the gears. Limit the rpm to 11,000.
• Cool down for 3 minutes.
• For the next 5 minutes apply full race power and rpm in 1st thru 4th gear out of each turn. Use part throttle and limit rpm to 11,500 in 5th and 6th gear until the next turn.
• Cool down for 3 minutes.
• Go Racing!

CARB JETS
Incorrect jetting can cause piston failure. Use only genuine Mikuni or Keihin jets. Aftermarket jets are not calibrated as accurately as the OEM jets. Read the plug, piston, tail pipe and EGT for best jet selection.

The relative air density (ADR) is the % of oxygen available relative to 60 degrees F, sea level pressure, 0% humidity. Any calculation of ADR should be made with current temperature, barometric pressure, and humidity considered. Main jets are spaced about 2.7% apart in fuel delivery. In general if an engine is jetted correctly at a particular air density, then change to the next jet size if relative air density changes 2.7%.

SLIDE NEEDLE:  Start with the clip in the center position (3). Adjust as necessary to give the best throttle response when the throttle is snapped open from very fast idle. The needle in the  Keihin carb is a DGH. 

AIR SCREW and PILOT JET:  Adjust the low speed air screw and  pilot jet size to give the best throttle response when the throttle is snapped open from idle.

RACE FUEL & OIL
Maxima 927 castor oil mixed @ 6.0 oz per gallon of fuel gives excellent lubrication, low deposit formation, and eliminates ring sticking. Use only quality race fuel such as Tracktek 111 or VP C12.

IGNITION COIL MOUNT
The ignition coil should be mounted in a way that isolates it from engine vibrations. There are several anti vibration mounts available or that have been improvised by karters. Usually the two wires leading from the CDI module must be lengthened to reach from the CDI to the coil.

Connect a grounding wire between the laminated steel core of the coil and the engine crankcase

IGNITION TIMING
If you have a programmable ignition, the ignition timing can be varied by selecting curves A, B, C or D. The curves are separated by 0.5 degree. Thus the “A” curve is 1.5 degree more advanced than the “D” curve.
The “B” curve is the best dyno curve. The engine will probably run best on the “B” or “C” curve.

EXHAUST GAS TEMP
For road racing the EGT should be about 1160 degrees at 11900 rpm and peak in the high 1200’s at 12700 rpm at the end of the longest straight. This is for an engine that is jetted correctly with programmable ignition using a Mychron2 temp probe placed in the pipe 6” from the piston face. Temp readings can vary depending on the type probe used and its condition. The best indication of correct jetting and temp is to look for a brownish gray tailpipe color and a piston that has black carbon deposits on top. If the piston top is grey or has erosion at the outer edge of the dome, increase the main jet size and or retard the timing.

PISTON
THIS CYLINDER USES THE LATE MODEL RS125 PISTON. DO NOT USE OTHER PISTON/ RING COMBINATIONS BECAUSE OF DECK HEIGHT AND RING PIN VARIATIONS. Piston life is about 2-3 hours. Rings may be replaced more frequently if desired. Ring gap should be .010-.012”.

SQUISH CHECK
After replacing the piston, check the clearance between the piston and head to insure proper port timing and compression. It should be .028 - .030”. Measure the squish using .050” radio solder (available at Radio Shack). Insert the solder through the spark plug hole and butt it against the cylinder wall above the wrist pin position in the piston. Rotate the crankshaft so that the piston mashes the solder when it goes through top dead center. Measure the minimum thickness of the mashed solder. Repeat this procedure on the opposite side of the piston. Take the average of the two measurements. This is your squish clearance. Adjust the clearance to the proper setting with various thickness cylinder base gaskets.

Tighten the cylinder base hold down nuts to 185 inch pounds.

SPARK PLUG
ALWAYS USE A RESISTOR IN THE SPARK PLUG CONNECTOR OR SPARK PLUG. FOR BEST PERFORMANCE USE A RESISTOR IN THE SPARK PLUG CONNECTOR OR A RESISTOR PLUG, BUT NOT BOTH.

Modified engines using the RS125 piston should use a cold plug such an NGK BR10ECS (resistor) or B10EG (non–resistor) because of the higher compression these pistons provide. Do not attempt to gap these plugs.  The small ground electrode is welded to the plug body and will break off if it is bent. Other plugs to consider are NGK  R7376-10 (non resistor), NGK R6385P-10P (non resistor), NGK R5184-105 (non resistor) or iridium BR10EIX (resistor).

HEAD GASKET
Your modified CR125 has O-rings to seal the head instead of the stock head gasket.

To install the head, clean the head and cylinder deck with acetone. Lay the O-rings in the grooves in the head. Be sure that they remain in the grooves when the head is installed on the cylinder. If they won’t remain in place, apply heavy grease to hold them in place. Place the head on the studs and lower it onto the cylinder. While holding the head firmly in place, rotate it clockwise against the studs to center it over the piston. Torque the head nuts in a crisscross pattern to 200 inch pounds in 3 steps.

LOSING WATER
Water loss usually indicates a leaking head O-ring. If the cooling system is allowed to run with a large water loss, the engine will overheat and damage to the piston and
cylinder will occur. USE REDLINE WATER WETTER IN THE COOLING SYSTEM TO PREVENT CORROSION.

TRANSMISSION OIL
Use Redline Shockproof Light weight Gear Oil. Do not use petroleum based ATF’s or gear oils. Change the transmission oil after initial break in and practice on the first race weekend for a new engine and every 2 race weekends thereafter.