Technical Tips
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Many Racers ask us for technical tips so we thought we would provide a section on our website for both our customers to access as well as provide this service to the general community.

General tips that span multiple engine combinations:
  1. Not running an air cleaner in the air box does not mean more power! A filter actually creates more power because in a piston port motor application it cuts down on the amount of reversion from the motor through the carburetor throat. This calming of the intake charge makes the intake charge more efficient.
  2. We at MCR typically tell our customers not to gap "iridium" tipped spark plugs. The reason being is if you use a typical gap tool and push the iridium tip too hard it will fracture and break inside the engine potentially causing expensive damage. If you are very careful you can gap them but we suggest against it. All our motors are tested with the standard factory gap settings.
  3. Make sure the pedal stop on the kart behind the gas pedal stops at the same time as the carburetor. If not you will bend the throttle shaft and make the butterfly go over-center causing much worse performance.
  4. We recommend using a copper based anti-sieze compound on the spark plug threads.
  5. We suggest the use of 108 octane leaded fuel with 8oz Burris Castor per gallon.
  6. We recommend replacing carburetor kits every 2-3 races.
  7. Change spark plugs every qualifying.
  8. Do not power wash your motor, we have seen that the water has a tendency to get into the engine, around the flywheel and create problems with the ignition systems.
  9. Do not spray brake-clean on any rubber seals, it dries them out and shortens the life greatly!
  10. There is typically no reason to have a gasket between the carburetor and the filter cup. We o-ring the filter cup to carburetor seal so there is no reason for this.
  11. Be aware of the plug boots that have the brass inserts in them. Brass is a great conductor but wears easily and will lose contact quicker with the top of the plug potentially causing a miss fire.
  12. When using the any Castor based oil in cold conditions some care needs to be taken because the oil may separate from the fuel at approximately 40 degrees. The mixture will appear "milky". You will notice this when fueling the kart, the clear tube from the jug will turn "white". There are several things that can be done to help this situation:
    • Do Not store fuel directly on concrete, elevate it off the floor.
    • Always shake your jug before fueling kart.
    • Mix fuel and oil after the temp has been elevated. The only way to make sure this doesn't happen, is to make sure the fuel is above 50 degrees.
    • From our experience it is best if you notice that the fuel coming out of the fuel line is milky to drain it back into the jug and try to elevate the temp of the the fuel out of the kart.

Comer K80 tips and tricks:

  1. The standard plug we use is the NGK BR10HIX. You can use the NGK BP9HVX plugs, we have them in stock.
  2. Carburetor settings are High=1/8 Low=2 1/2, with normal head temperature being approximately 440 degrees Fahrenheit. Head temperature is only a starting point, plug reading is still the only "true" indicator of the engines mixture. If you need more help, call the MCR boys and we will describe how to read the plug on your Comer K80.
  3. On the K80, the single one major issue on the carburetor is the plastic top. After a period of time, the nipple (where the fuel line attaches) will develop a crack underneath from the inlet to the screw hole. Some indicators are air bubbles in the fuel line near the fuel inlet top. Always inspect you line for air bubbles at this point. Changing the plastic top should also include installing a new cork gasket.
  4. When changing the gasket between the carburetor and the motor, be sure to place gasket on correctly not covering the pulse passage. Covering the small pulse hole will prevent the fuel pump from working.
  5. K-80 ignition systems have shown to be troublesome. Motors that pop, sputter or simply shut-off may need the module replaced. This is the black box under the flywheel, the one toward the front of the engine. This is a VERY common problem in the Comer K80 motors. This might be due to power washing karts/motors as well, don't power wash your motor!
  6. Always make sure your chain is aligned and adjusted. If the chain comes off the engine, damage may occur. Especially on the Comer motors, spinning at 12,000 RPM if the chain gets wrapped around the crankshaft it will stop the crank abruptly and potentially change the index. The crank is a three piece crank and it can get out of alignment.
  7. Every 4 race weekends you should send your K80 in for an MCR check up, where we change springs in the clutch, put a new kit in the carburetor, clean the muffler, and clean carbon of the piston and head.

Yamaha Sportsman with Walbro WA55B carburetor tips and tricks:

  1. The standard plug we suggest is the Autolite AR51 plug gapped at .040. Make sure you put the gapping tool or feeler gauge in at an angle.
  2. The carburetor on these engines have very touchy adjustments. MCR engines are made to run at Low=2 1/4 - 2 3/8 and High=1/2. Other engine builders are probably about the same.
  3. Be very careful about the high adjustment on the WA55B carburetor. A very little amount makes a big difference due to the taper of the needle. A "hair" is a big deal! This is one of the major tuning mishaps on these engines. Again, MCR engines are made to run the high at 1/2 turn on the dot.
  4. The low adjustment can be moved, but again we at MCR find that 2 1/4 works very well as a starting point and most of the time.
  5. With this carburetor and this setup, only use the head temperature as a gross measurement of mixture. Head temperature is not as exact as the other carburetor configurations. We see good operation between 360 and 390 on the head temperature.
  6. Ensure that the carburetor is lined up with the opening in the adapter block. Look down the throat of the carburetor and make sure there is no overlap. Since the carburetor throat is very tiny even a small amount of offset can cause flow issues and decrease horsepower.
  7. Change the pump in the carburetor often, it is a small diaphragm. MCR suggests every two days, and we send out our engines with new pumpers with Dyno time only.
  8. Do not run a fuel filter, and ensure there are no air bubbles in the fuel line. The small pump in the carburetor cannot pump the fuel against an air bubble and will cause a lean condition. When running without a filter it is very important to strain your fuel and clean the screen in the carburetor on a regular basis.
  9. Ensure your clutch is tuned and comes in between 7,300 and 7,500 for most situations. Blueprinting the clutch in this class is more important than others due to the lower horsepower. MCR provides blueprinting services for Horstman clutches and suggests you use the single disk HDC-5A clutch.
  10. Check and clean the clutch often.
  11. MCR has designed it's motors for Sportsman (WA55B) to provide the most horsepower down low in the RPM range. Run a gear that gives a maximum RPM of 12,000. Try smaller gears during practice as our Sportsman motors do very well with lower RPM's. Testing for your track and driver style is the only thing that will allow you to find your best setup.
  12. Ensure the chain and clutch are all aligned and functioning properly. With low horsepower classes this is very important. Any drag is bad!

Yamaha with SSX can and Walbro WB-3A:

  1. The standard plug we suggest is the Autolite AR51 plug gapped at .040. Make sure you put the gapping tool or feeler gauge in at an angle. Even though we have found the AR-51 is the best, we have also found that the NGK BPR9-EIX is also a good choice.
  2. Carburetor settings are 3/16 high and 2 1/4 - 2 1/2 low. The high speed needles in the Walbro WB3A are very sensitive due to the needle taper.
  3. The high speed has a greater effect on the head temperature than the low speed.
  4. At the beginning of the race start 3/16 and 2 1/4 and by the end of the race you should have opened the low to 2 1/2. If you need to adjust the low any more than that then adjust the high from 3/16 to 1/4 and the low back to 2 1/4 at the start of the race. You should now be between 2 1/4 and 2 1/2 at the end of the race.
  5. If you get the low speed out further than about 2 3/4 the fuel curve does not change and you will not get a richer setting. If this happens there is an issue somewhere else in the fuel system. Check for a dirty screen, bad pump, pulse line, etc.
  6. Set the clutch at 8,600 - 8,800 RPM. We use a two disk Horstman HDC-5B dry clutch.
  7. Max RPM is about 14,000 - 14,400 depending on track. Shoot for 14,000 without being in the draft.
  8. Run a high quality fuel filter about 3-4 inches below the brass fitting on the carburetor This will typically put the filter straight up and down to aid in air purging.
  9. Clean the screen and replace carburetor kit every 3 races. Do not pull directly up on the metering diaphragm, it has a hook and fork and if you pull it up you raise the metering level. This is a common problem we see. The metering diaphragm is #20 on our WB-3A diagram and you need to slide it out to not cause damage.
  10. We want to run the head temp between 420 to 440. Plug reading is still the only "true" indicator of the engines mixture. If you need more help, call the MCR boys and we will describe how to read the plug on your Yamaha KT-100
  11. You need to lube your clutch bushing every time out with this class.
  12. If the motor cuts out or experiences a sputtering, 90% of the time it is ignition related and more than likely it is a bad TCI box.
  13. When you tighten the can, make sure you do so evenly, it will not seal again if you "squish" the gasket unevenly.
  14. We use the standard Yamaha throttle return spring part number 90504-09027. Call us if you need one.
  15. We use thick wall tubing for the pulse line. Always make sure your pulse line is tight.
  16. If you use an anodized chain guard, you need to clean the anodize off all the bolt surfaces because the TCI box will not ground properly and you will damage the ignition.
  17. We prefer the Odenthal motor mount because they are stronger. The degree of the mount really does not matter to us as engine designers and engineers.

Q: What spark plug should I run and at what gap?

Yamaha CAN Classes JR/SR Autolite AR51 @ .040
Comer K-80 NGK BR10HIX @ stock gap
Yamaha Pipe Classes/FY Autolite ARZ592 - stock gap
HPV - All Autolite AR51 @ .040
Shifter 80/125 NGK BRIOEIX - stock gap
KidKart 50cc Bosch R0971 #WS5F

Q: What kind of maintenance is expected of my MCR Engine between races?

  1. We recommend replacing carburetor kits every 2-3 races.
  2. Change spark plugs every qualifying.
  3. K-80 - every 4 races - MCR check up, change springs in clutch, kitcarb, clean ex. muffler, and clean carbon off head.
Q: How often should I have my motor rebuilt?
A: Usually our customers get between 4-6 races on Yamaha and Komet KPV's (HPV) and 7-8 races on Comers. These estimates vary from motor to motor, depending on use/abuse and casting imperfections in parts. When calculating race time, figure in test days as well. (Example - 2 races with 4 complete days of practice equals 6 races.)

MCR Engine Development
3161 Pegasus Drive
Bakersfield, CA. 93308
Phone: (661) 399-3580
Fax: (661) 399-3584