Mild To Wild At Bargain Prices Making the It250 Perform

MILD TO WILD AT BARGAIN PRICES MAKING THE IT250 PERFORM

Yamaha's 1983 IT250 has many well-thought-out parts but its power is weak and the powerband is genuinely lumpy. It has little low-end power, a brief mid-range surge and it's flat at the top. We really liked the IT250 except for the weird power characteristics. About the time we decided to jump in and modify some of iti parts, Yamaha came to the rescue via its Competition Support Wrench Retort (CSWR).

The CSWR is a bulletin that Yamaha sends to its support racers and dealers, detailing modifications that improve performance on specific models. Report number 40, dated July 22, 1983 deals with the ’83 IT250. The report is broken into three parts; general handling modifications and tips for longer headlight life, modifications for improving the power for enduro competition, and boosting power for cross-country racing.

GENERAL MODIFICATIONS (CSWR) FORKS

The IT’s ride height can be altered by lowering the stanchion tubes in the clamps until the tops are flush with the clamps. This slows steering response and improves highspeed stability. Raising the fork oil level will stiffen the forks’ action at the end of its travel and reduce front end dive when braking hard. Heavier riders might want stiffer fork springs. Standard YZ250K forks springs (23X23141-Lo-OO) are slightly stiffer and fit perfectly.

FRONT BRAKE CABLE

The IT’s front brake cable blocks the view of the odometer for some riders (depending on one’s riding position, height and style). If it bothers you, replace the top cable guide with one from an XT550K. The part number is 5Y1-23317-00-00.

HEADLIGHT FAILURES

Hard riders, or those who wind the engine high, blow headlight bulbs faster than they can be replaced. Voltage spikes are the problem: The IT’s electrical output is

is

is greater than the bulb’s rating. Installing a Yamaha voltage regulator (4F4-81910-60-00) will bring the output under control.

MODIFICATIONS FOR ENDURO RIDING

EXHAUST PIPE AND SILENCER

An Answer Products silencer will increase engine response and trim off 2.5 lb. It’s also a little louder than stock and may not pass strict sound tests.

Power and response can also be increased by shortening the headpipe 10mm. Remove the pipe, cut 10mm from the headpipe where it’s welded to the first cone, reinstall and tack weld in three spots then remove and complete the weld. (The gas tank should be removed while welding.)

REEDS

The stock reeds are steel and last a long time, but they don’t react as quickly as fiber reeds. Boyesen makes excellent fiber reeds that improve engine response considerably. The Boyesen reeds have dualstage petals (a smaller petal positioned over a window in each main petal). The smaller petals require less pressure drop to open and thus quicken response time.

AIRBOX COVER

If the IT is being used in muddy or dusty conditions, leave the airbox top on. If not, airflow (power also), can be increased by removing it.

MODIFICATIONS FOR CROSS-COUNTRY RIDING

Less torque, more power, is normally beneficial for opencountry riding or desert racing. The following modifications are suggested for those uses.

EXHAUST PIPE AND SILENCER

Installing a YZ250H pipe (4V3-14610-00-00) and silencer (4 V 3-1475 3-00-00) will let the engine rev higher with some loss of low-end power and torque. The pipe and silencer require modification to fit: the top mount of the silencer has to be cut off and rewelded 45mm forward. The pipe has to be dented where it touches the frame downtube, and the top mount repositioned 5mm higher.

CYLINDER

Top-end power can be increased by raising the top of the exhaust port 1mm. Use a hand grinder and carefully raise the top of the port, maintaining the stock curvature. When finished, the top of the port should be 36mm below the top of the cylinder.

PISTON

Another power boost is available by replacing the IT piston with one from a YZ250K. The cylinder should be measured for roundness. Rebore if it isn’t (or if it’s worn past the specs in your owner’s manual). If it doesn’t need a rebore, break the cylinder glaze with a cylinder hone, or have your dealer do it for you.

CARBURETOR

Serious racers might want even more power. If so, a 38mm Mikuni carburetor (4V3-14101 -00-00) will fit. It should be rejetted to the following specs:

Main jet; #370

Pilot jet; #50

Needle jet; Q-O

Jet needle-clip piston; 6F16-2

Slide cutaway; 3.0

Airscrew adjustment; 1.5

turns out (from fully seated).

These setting are a starting point; minor adjustments may be required for different elevations and riding conditions.

OUR MODIFICATIONS

We had already raised the fork oil level during our regular testing. We like it V2 in. higher than stock. We also like the stock fork spring rate. We haven’t been using the IT for competition so the brake cable blocking the odometer doesn’t bother us. And, we haven’t blown any headlight bulbs on the 250, although we instantly blew the one in our IT490 test bike (the regulator will work on the 490 too), so we also skipped this modification.

The lack of power and the weird powerband did bother us. We decided to do one modification at a time so we could evaluate each. Because the bike is used for trail riding we chose the enduro route.

The silencer was the easiest to change and therefore came first. The beautiful aluminum Answer silencer was lighter and fit perfectly, but we couldn’t tell any difference in power.

Next we removed the headpipe and cut V2 in. from it. The bulletin said 10mm but V2 in. is close. After tacking it in place on the bike, it was removed and the welding completed. The difference in performance is amazing; the power is stronger everyplace. Shifting from first to second will easily raise the front end off the ground. And the lag in the first quarter of the rev range is reduced slightly.

The Boyesen reeds were next. A local Yamaha dealer had them in stock. It took about 20 minutes to install them. Engine response improved dramatically and again, the lag at low revs was further reduced. The transition from the lag to useable power changed from smooth to violent. In fact, the bike became a handful to ride for all but expert riders. If the rider isn't ready for the power, he’ll end up in a tree when the power hits!

We had power but it was less than ideal. We replaced the stock 46 tooth rear sprocket with a 48 tooth. This lets the engine get into the powerband quicker and almost eliminates the lag, making it easier to ride.

CONCLUSIONS

We’re happy with the way the IT performs with these changes, but knowing what we know now . . . we’d make changes in a different order: first we’d change to the larger rear sprocket, then shorten the headpipe. These two changes will probably suit 80 percent of the woods riders. Experienced riders will benefit from these modifications and the Boyesen reeds. We wouldn’t change the silencer unless we had some extra money and wanted the bike slightly lighter and prettier. Expert desert riders might want to go whole hog and do all of the cross-country work. We didn’t try them but doubt many riders would like the radical changes. If you decide to modify your ’83 IT, do so in steps. It’ll save money. 0