Last month, I highlighted some of the important areas of the Honda CR125 Stock Moto engine and racing program. This month I am going to focus on our approach to running the Stock Moto. Some topics in this article will be specific to SwedeTech Racing, but I will try to make this a broad as possible.
Let us make the assumption that you have a brand new engine that is properly installed on your chassis. If you do not own a data system that, at minimum, records water temperature and r.p.m.’s, stop reading NOW, call SwedeTech or your favorite parts supplier to purchase one now. YOU NEED DATA!!!!!
All your nuts and bolts are double checked and you have properly secured loose lines, wires, and cables with tape, zip ties, or other means. You have primed your fuel system and double checked that the fuel is flowing in the proper directions. The radiator water lines are filled with distilled water and you have checked all connections, routing, and have burped the water system. Make sure the temperature sensor is between the water outlet(s) on the head and the inlet on the radiator. If any of this sounds Greek and you live near some great restaurants, I am available for hire.
So you have taken all of your photos of the new package and posted them on Facebook to read all the comments from your jealous friends. Your face is permeated with a huge smile and you can’t wait to hear that first ping of the exhaust pipe. You can’t wait to fill the garage with two-stroke fumes and have a swig of beer.
As with any racing engine, it is the end users responsibility for tuning. When breaking in your engine, it is encouraged to richen your jetting by 1-2 sizes on the main jet. Do not add more oil to the gas for break in, this will only change your jetting and not properly increase the lubrication.
SwedeTech does not recommend heat cycling your engine on the kart stand, as this will do nothing for your engine. To properly break in the piston and ring, the engine requires a load that is similar to your application. Holding the brake pedal, while winding the shit out of your engine is not a proper load. At best, it pisses off everyone within a 3 mile radius. This also will not make your KT100 or Comer engine faster.
We do however recommend that you warm the engine before heading out on the track for your break in sessions, practice, or timed events. While the engine is warming up, double check your work. After about 15 -20 seconds of running, place your hand in a safe spot on the cylinder. If it is hot or warm to the touch, shut the engine down and double check your water plumbing and make sure you burp the system.
Warm the engine water temperature up to 115 degrees Fahrenheit. Make sure the temperature sensor is between the water outlet(s) on the head and the inlet on the radiator. Once the engine is at 115 degrees, strap on your helmet and hit the track. For break in, your engine water temperature should reach 150 -155 degrees during the first couple of laps. If it is not, come in the pits and add tape to the radiator.
On a 30 second track, slow the pace 4-5 seconds per lap. Run the engine at full throttle on the track, keeping rpm’s low by short shifting. Allow the engine to cycle through the RPM range from 6,500 – 10,500 for the first five laps. At 5 lap intervals, increase the rpm’s by 750 – 1000. Continue until maximum rpm’s are achieved. Run the engine at full throttle on the track, keeping rpm’s low by short shifting.
For the last couple of laps, your engine is ready to be flogged. Once the engine is properly broken in, it is time to tune for best conditions. At this time, you will want to keep the water temperature between 125 – 135 degrees.
If you find that you are driving the engine at 100% and it is not creating the proper water temperatures, double check the water circulation. If the water is circulating properly, lean your main jet down 1 size.
Now let’s fast forward a couple of race weekends and three cases of beer. Being a racer, you probably have A.D.H.D. and just have to work on something, and your preference is to screw with the kart. And since you have A.D.H.D., your track notes are flawless and you know that your engine now has exactly 4hrs:17min:32sec of run time on it. Perfect, now it is time to think about a top end rebuild. If you can properly use a wrench and have basic mechanical aptitude, you should be able to do your own top end.
SwedeTech recommends replacing the piston, ring, wrist pin, bearing, circlips, head gasket, and base gasket. If you are using a SwedeTech CR125 O-ringed exhaust manifold, we recommend inspecting the o-rings every hour of run time. When it is time for a lower end rebuild, we recommend that you send your engine to SwedeTech. If you do not like our recommendation, please send the engine to a trusted and reputable engine builder that is familiar with the CR125 and the Stock Moto rules.
Here are some quick notes that you will find helpful for your Honda CR125 Stock Moto engine package.
Break-in water temperature should be 150° – 155°
Normal running condition water temperature between 125° – 135° (Take the reading anywhere between the cyl. head outlets and the radiator) (Engine will lose performance after 135° water temp. Overheating can occur with temperatures past 160°)
New engine break-in 15-20 laps, New Top End break-in 12-15 laps, Increase main jet 1-2 sizes for break-in
Usable RPM Range – 7,000 – 12,400
Spark Plug – NGK BR10EG or R7376-10 Spark Plug Gap – 0.024” – 0.028”
Oil – Motul Kart Grand Prix or Maxima 927 – mixed at 24:1
Fuel – VP MS98 – Do Not Use Pump Fuel
Transmission Oil – Motul Trans Oil 10w30 or Motul Trans Oil Expert 10w40
New engine-650ml (21.9 oz)
Use 50ml less for routine oil changes (20.0 oz )
We recommend changing the gearbox oil every 4 hours. Oil is cheap compared to the cost of replacing clutch or gearbox parts. If possible, change the oil while the engine is warm.
Keihin 38mm PWM
160-180 main (center of the carb, 6mm hex ) Main jet supplies fuel from ½ throttle to full throttle
48-62 pilot (next to main jet, use 5mm flat screwdriver to remove) Pilot jet supplies fuel from 0 – ¼ throttle
DGK,DGJ, or DGH needle – 3rd from top. Experiment with 4th & 2nd clip from top. Needle regulates fuel from 1/8 throttle to ¾ throttle
Air Screw (side of carburetor body, above fuel inlet) – 1 Turn Air Screw regulates air from 0 – 1/8 throttle. Turning the air screw in will reduce air through circuit, effectively, increasing the fuel to air ratio.
Top End Maintenance /Top End Kit – Piston, ring, circlips, wrist pin, bearing, head gasket, exhaust o-rings, and base gasket.
For normal use – 6 – 8 hours
For optimum performance – 4 – 6 hours
Normal use – for the recreational racer that is looking for longevity and cost effectiveness. Typical normal use includes jetting the engine on the richer side.
Optimum Performance – for the avid racer that is looking for every performance advantage. Optimum performance includes jetting the engine for best performance.
Note that the above schedule of maintenance is based on our experience. You may find that these times may be adjusted based on your needs, driving conditions, and overall engine maintenance. Many factors affect wear and tear on components.
Clutch adjustment- you need about 3/8-½ inch free play at the end of the lever. You shouldn’t have more than 1 inch of travel when you engage the clutch. Any more than that and the outer plate could jump out of the basket. Make sure your cables have the nylon inner housing and keep them straight and well lubricated. You should not need more than two fingers to engage the clutch.