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Difference between revisions of "Yamaha TZ250"
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The '''TZ250A''' came out in June 1973 and, like the 350, was virtually unchanged porting wise from the previous aircooled racer apart from a slight widening of the main transfers, also, the exhausts were unchanged. | The '''TZ250A''' came out in June 1973 and, like the 350, was virtually unchanged porting wise from the previous aircooled racer apart from a slight widening of the main transfers, also, the exhausts were unchanged. | ||
Major differences from the TZ350 included barrels and pistons, cylinder head and primary transmission which also required a different water pump gear. | |||
Yamaha also chose to "beef up" the main and small-end crank bearings as well as a few other minor mods . Output was up slightly from the TD3 to 51bhp @ 10,500rpm. | Yamaha also chose to "beef up" the main and small-end crank bearings as well as a few other minor mods . Output was up slightly from the TD3 to 51bhp @ 10,500rpm. | ||
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[[Image:Yamaha-TZ250H.jpg|right|thumb|Yamaha TZ250H]] | [[Image:Yamaha-TZ250H.jpg|right|thumb|Yamaha TZ250H]] | ||
The 1981 "H" represented the factory's first departure from the popular 54mm bore x 54mm stroke (essentially) configuration of the previous 9 years along with a switch to an individual cylinder casting configuration. In an effort to further boost horsepower, the bore was increased to 56mm and stroke reduced to 50.7mm, achieving the full 249cc in the process and mechanical exhaust power valves were fitted, as was a transmission oil pump to deliver the reduced oil capacity of the gearbox to crucial points. The "H" had a 38mm shorter motor than the "G" and 36mm ZC series Mikuni carbs replaced the usual 34mm SC's, which had been in use on the 250's since the TD3 way back in 1972. Intake port was again widened, as were the secondary transfers for the second year running. | The 1981 "H" represented the factory's first departure from the popular 54mm bore x 54mm stroke (essentially) configuration of the previous 9 years along with a switch to an individual cylinder casting configuration. In an effort to further boost horsepower, the bore was increased to 56mm and stroke reduced to 50.7mm, achieving the full 249cc in the process and mechanical exhaust power valves were fitted, as was a transmission oil pump to deliver the reduced oil capacity of the gearbox to crucial points. The "H" had a 38mm shorter motor than the "G" and 36mm ZC series Mikuni carbs replaced the usual 34mm SC's, which had been in use on the 250's since the TD3 way back in 1972. Intake port was again widened, as were the secondary transfers for the second year running. | ||
Unlike the [[Yamaha TZ350|TZ350]], and earlier Yamaha TZ250, which shared many components including ignition, Yamaha chose to have built-in automatically variable ignition advance in it's Hitachi TIA02-16 CDI unit. It was programmed to advance linearly from the initial mark alignment static to a maximum advance of typically 14 degrees at 3000rpm. It then returned slowly and linearly as the revs increased, reaching the original alignment set point again at 10,000 rpm. Beyond 10,000 rpm it continued to retard at the same rate at which it approached 10,000 rpm. (i.e. 2 degrees per 1000 rpm). | |||
The motor spun in reverse direction as the factory tried to further reduce the stress on the rear of the pistons. The crank was no longer a single unit, it was now a pair of cranks with a gear on the end to deliver the power to the gearbox. Despite the frame shedding 3kg, the overall weight was reduced by just 1kg. The steering rake was altered and the swingarm also lengthened to compensate for the shorter frame length of this model. The engine was now mounted slightly further towards the front of the bike. The new rear shock became fully adjustable for both compression and rebound damping as well as pre-load and the shock picked up a rear rubber mounting. The fork diameter went up by 1mm and brake wise the factory reverted back to the old cast iron calipers which didn't bend under pressure and also didn't fade when hot. The new ignition dropped an igniter and pulser coil, there was now just one of each and the inner clutch hub now became a single piece unit to eliminate one reliability problem from previous years. The un-necessary neutral switch mounting disappeared | The motor spun in reverse direction as the factory tried to further reduce the stress on the rear of the pistons. The crank was no longer a single unit, it was now a pair of cranks with a gear on the end to deliver the power to the gearbox. Despite the frame shedding 3kg, the overall weight was reduced by just 1kg. The steering rake was altered and the swingarm also lengthened to compensate for the shorter frame length of this model. The engine was now mounted slightly further towards the front of the bike. The new rear shock became fully adjustable for both compression and rebound damping as well as pre-load and the shock picked up a rear rubber mounting. The fork diameter went up by 1mm and brake wise the factory reverted back to the old cast iron calipers which didn't bend under pressure and also didn't fade when hot. The new ignition dropped an igniter and pulser coil, there was now just one of each and the inner clutch hub now became a single piece unit to eliminate one reliability problem from previous years. The un-necessary neutral switch mounting disappeared | ||