Chird Bobbitt Copyright © 1997- All Rights Reserved
Email: firstname.lastname@example.org Searcy, Arkansas
When the manufacturers set a bike up for the masses, they design a certain amount of
safety margin into some areas. One is jetting. All other things being equal, leaner
jetting gives better performance. That said, leaner jetting can also destroy motors. But
they tend to give us jetting that is far from optimum. In fact, it can border on being a
problem..... like what you're experiencing. First thing I'd do is try a pilot jet
one step leaner. This can help reduce/eliminate the fouling and increase the throttle
response. Adjust the air screw for best throttle response with the leaner pilot. Same
with the needle once you're confident the pilot circuit is as good as it can get.
Starting procedure can affect plug fouling. Try this. Once your bike cranks, let it idle with the choke knob pulled up. As soon as the rpms begin to drop down (normally within 30 seconds), push the choke knob shut and rev it up to clean the bottom end out. The oil that lubricates a two stroke top end will run down to the bottom of the crankcase and collect in a substantial pool. This pool of oil needs to be "blown out" once the bike is running.
Another thing that can affect plug fouling and the amount of spooge coming out the exhaust is the spark plug heat range. The fuel/oil ratio has little to do with this. 20:1 is fine but you still might want to switch to 30:1 just to try to keep internal engine deposits to a minimum. The plug heat range is another place the manufacturers design in a little margin of error. Plug heat range helps determine the combustion temps. Going to a hotter plug will help burn off the excess oil that is finding its way out the exhaust. The only time we run the recommended heat range is when we're running at high rpms continually. This would be a semi- or a pro level motocrosser on an outdoor track or anyone who runs high rpms in sand or mud. These conditions can escalate the engine temps where you need to keep the factory recommended plug. Otherwise, you can easily and safely go a heat range hotter. Your Cr250M takes a B8 series plug. I'd go to a B7 series plug. The lower the number, the hotter the plug. This will also give you better throttle response at lower rpms.
On very hot days, place a wet white towel over the fuel tank so the sun doesnt
heat the fuel. Cool fuel gives a definite advantage on the start of a race.
You can use a non-synthetic automotive oil with no problems whatsoever. You need to stay away from oils with the "energy conserving" rating in the circle. They add friction modifiers to make them energy conserving. These additives can contribute to a wet clutch failing prematurely. All the Mobil 1 synthetics have the energy conserving rating except for the 15w50. The motorcycle oils do NOT lubricate any better or stand up any better to shear from transmission gears than auto oils.
Jetting is basically trial and error. The only real worry you should have with
trying different jetting is going too lean on the main jet. Otherwise, just remember
the golden rule of jetting - only make one change at a time. Start with the pilot
jet, move to the needle position and then the main jet once you're satisfied the lower
speed circuits are okay. On a bike that old, you need to be aware some carb parts can wear
which will affect jetting. The needle jet and jet needle can both wear richening up the
mixture. Trying to jet with these worn parts can get you chasing your tail.
The most accurate method for checking the main jet (where seizures occur) is to do a "plug chop". This involves finding a long straight that you can run at full throttle for about 1/8 mile or so. Use an almost new spark plug for this test. Carry a plug wrench with you. Once you have reached the end of your straight, pull in the clutch and hit the kill button. Remove the plug and inspect it's porcelain color. Ideally, it'll be a tan to light brown color. Anything approaching white is too lean to be safe and anything darker than chocolate brown is too rich to give good performance.
Remember, the main jet only affects the jetting on the upper throttle settings. You're describing an off-idle jetting problem. I'd try leaning the pilot jet and see if that gives a boost in throttle response. The needle affects the middle of the throttle settings. Fine tune the needle once you get the pilot circuit where it needs to be.
Start with the pilot jet, Once you have that sorted out, adjust the needle. Then go
on to do the main jet. The lower speed circuits overlap with the higher circuits but not
vice versa. Adjust the low speed circuits for the best throttle response and the main jet
to get a safe insulator color using the plug chop method.
The most basic method of determining correct carburetor jetting is "ride and feel." This method requires you to determine if the carburetor tuning is too rich or too lean by the sound and feel of the engine. The first step is to mark the throttle body in 1/4-throttle increments, from closed to full open. Then, this method requires that you ride the motorcycle on a flat, circular course. To check the carb jetting for throttle positions up to 1/2 throttle, ride the motorcycle in second or third gear. Roll on the throttle slowly from 1/4 to 1/2 open. If the engine is slow torespond and bogs (engine makes a booooowah sound) then the carb jetting is too lean. You can verify lean jetting by engaging the carbs choke to the halfway position. This will make the air-fuel mixture richer and the engine should respond better. If the carb jetting is too rich, then the engine will make a crackling sound; the exhaust smoke will be excessive and the engine will run as if the choke is engaged. Careful engagement of the choke can help you determine if the jetting is rich or lean. Another important tip is to just change the jets one increment at a time, either rich or lean, until the engine runs better. Most people are afraid to change a jet because they think that the engine will be in danger of seizing. Believe me, one jet size won't make your engine seize but it could be the difference between running bad and running acceptable.
To check the jetting for throttle positions from 1/2 to full open, ride the motorcycle in third and fourth gear. (You may need to increase the diameter of the circular riding course for riding in the higher gears.) Check the jetting in the same manner as listed above. The carb jets that affect the jetting from 1/2 to full throttle are the jet-needle, main jet, power jet (electronic carbs) and the air jet (on four-strokes).
If you want to take this technique out to the racetrack, you can test the pilot/slow jet when accelerating out of tight hairpin turns, the needle clip position on sweeper turns and short straits, and test the main jet on the big uphill or long straits. Of course be careful if you try to use the choke technique because you could lose control when riding one handed
This is a perfect plug. The color is mocha brown so the carb jetting is optimum. The first three threads are black signifying the plugs heat range is matched to the application. There are relatively low deposits considering that this engine was run on regular pump petrol.
Changing your oil frequently is as important as choosing a good oil. This is one
thing you can do to help determine the life of your motor. My Yamaha oil comes out looking
just like it goes in. You would do better using a non-synthetic auto oil and changing it
more frequently than using the motorcycle oil and trying to stretch the intervals out.
Muddy conditions: front 8 psi, rear 6 psi Dry conditions: front 14 psi, rear 12 psi. Always use heavy-duty inner tubes because they allow you to run lower tire pressure without the threat of a puncture. There are some goods tools available for the purpose of restoring the edges on the knobs between motos.
The best way to tighten the spokes is to start at the valve stem and tighten every third spoke 1/8 turn. When you get to the stem again, start with the next spoke and repeat the procedure. Once you have tightened three spokes from the stem you will have tightened every spoke equally. Check the spokes for loose ones, then tighten them to the same tension as all the others. Dont use the "tap the spoke and listen to the pitch of the sound method." It doesnt work! You have to develop a sense of feel for spoke tension. If the spoke is too tight, youll hear stress-relief sounds.