Author Topic: Longterm TY250C  (Read 2300 times)

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Offline tony27

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Longterm TY250C
« on: June 17, 2018, 07:00:35 PM »
Finally got my 250C back in a ridable state. We worked out it had been about 5 years since I'd ridden it last when the main bearings & rings were that worn that the motor could be turned over by hand by gripping the end of the crankshaft with the flywheel off



While the motor got a full rebuild with the cylinder being bored to 1st oversize I made a few changes both to looks of the motor & had the swingarm extended with shedworks axle plates
The axle plates extend the swingarm by 38mm or so which puts a bit more weight to the front. As it turns out even with the lengthened swingarm the wheelbase is standard which confirmed that the steering head angle had been modified, the welds are far better than normal & the front down tube are completely smooth. We stood the bike in front of a Beta Techno & the fork angle is the same which makes sense as the Techno was the current bike when the mods would have been done
The original shock mounts were deleted & a gusset fitted to help strengthen the tubes above the repositioned mounts that were done before it's last ride
I fitted a DT250C head that Ian Cameron supplied me for looks, I race a 73 CZ & owned a YZ465H when I built the motor so radial heads are my preference. The combustion chamber is meant to measure the same. There was minor grinding of a couple of fins to clear the exhaust & I cut about 10mm from the rear to match it to the cylinder
An electrexworld ignition is fitted now with the band removed from the flywheel, I had to put marks on the flywheel & stator plate for timing while the head was off. Unfortunately we didn't get a chance to strobe the timing as per the fitting instructions & had to run it set at the middle of the slots but it seems to work well

I'll see what other detail photos I have & try to put them up at some stage

Offline David Lahey

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Re: Longterm TY250C
« Reply #1 on: June 18, 2018, 09:49:39 AM »
Lovely bike. I'm interested in what you think about the motor performance with the weight band removed
previous pseudonym feetupfun

Offline tony27

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Re: Longterm TY250C
« Reply #2 on: June 18, 2018, 10:16:11 AM »
Have only ever ridden it without the weight so I can't say what it was like with the heavier flywheel. It's quite a nice motor, almost feels modern. I'm still running the standard carb but the airbox has been altered & I have a v-force reed block for a yz85 fitted
Biggest set up issues I've had so far is no freeplay in the clutch & needing to be quite deliberate in changing gears. I may also need to find a richer needle as I ended up on the lowest position
The full majesty forks work well, didn't get to try them on big rocks but I couldn't fault them on roots & the medium sized rocks in a river bed that we had in a few sections

Offline David Lahey

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Re: Longterm TY250C
« Reply #3 on: June 18, 2018, 01:02:07 PM »
Yes that gear shifting description is fairly standard for those motors. You wouldn't want to be in a race.
The reason I asked about the flywheel band is that many years ago I took the band off one and liked it for a while then went to using a flywheel with a lightened band. I removed a bit at a time from the width of the band and ended up with about 2/3 of the band left. Since then I have lent the flywheel with no band to people who are trying to decide what to do with theirs. In each case they have ended up using a lightened band. That's why I'm interested in what you find and do with yours.
Yours being a C model would have come with a "heavy" band, same as B, D and E models. The A model came with a lighter band which is pretty much the same weight band as what mine ended up with.
As for your jetting, it may be worth trying a bigger main jet before trying to find an alternative needle or reshaping the needle, because the OEM Yamaha Mikuni is an odd design and alternative parts are not available from Mikuni parts suppliers. The slide and needle are shorter than a standard VM26 and were made that way to minimise the height and weight of the carby. The B model TY250 carby does have a different needle shape and main jet size to the CDE models and I suspect that they needed to change the jetting when they changed the design of the airbox.
Yes the Majesty damper rods are a good thing
previous pseudonym feetupfun