ty250A compression

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bikecrazy2010
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ty250A compression

Postby bikecrazy2010 » Mon Feb 20, 2012 7:21 pm

Hi all, im new to this forum, i have aquired a 74 model ty250a it does run but as soon as you try and apply throttle it dies but if you wind up the throttle screw it will slowly increase in revs from everything ive read these things should be fairly hard to kick over this one doesnt seem to be does anyone know what a standard one should read if you give it a compression test ?



pop
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Re: ty250A compression

Postby pop » Mon Feb 20, 2012 7:51 pm

Going from memory 100psi or over, but in 2 strokes its the bottom end leaks that are the worry, if you have spark, fuel, compression, but it does not run then something is leaking below, most 2 strokes die this way. :twisted:



David Lahey
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Re: ty250A compression

Postby David Lahey » Mon Feb 20, 2012 8:01 pm

If the compression is low it might be worn rings, broken ring/s or maybe just the rings stuck in their grooves with gunk. Before you pull it apart, try putting some carby cleaner down the plug hole with the piston at the top. If the rings are stuck, that might release them enough to get some compression and get the thing going.
The way it dies as you open the throttle could be caused by a blocked pilot jet. Thats pretty common with bikes that have not been run for a while.


relax, nothing is under control

timlangham
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Re: ty250A compression

Postby timlangham » Thu Aug 25, 2016 3:34 am

The manual says 6:1 compression I read this as 6 times 14psi (atmosphere) = 94 psi. But I doubt this accurate. Have just pulled mine apart had 100 psi if I kicked it a few times. The piston rattles in the bore and there is almost 1/2 a mil of clearance. Have ordered a 71.0 Piston & will get it bored to size. I am the same as you I assumed there would be a compression spec somewhere.
I can't find it



David Lahey
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Re: ty250A compression

Postby David Lahey » Thu Aug 25, 2016 12:46 pm

timlangham wrote:The manual says 6:1 compression I read this as 6 times 14psi (atmosphere) = 94 psi. But I doubt this accurate. Have just pulled mine apart had 100 psi if I kicked it a few times. The piston rattles in the bore and there is almost 1/2 a mil of clearance. Have ordered a 71.0 Piston & will get it bored to size. I am the same as you I assumed there would be a compression spec somewhere.
I can't find it

Finding out what the normal pressure generated is in a TY250 motor is more complicated than multiplying the quoted CR by atmospheric pressure because:
1
6:1 compression quoted may be "corrected" or "not corrected" which means it may be calculated taking the effect of the flowing gases into account, or may not. This is important for two strokes because there is an effect due to the flowing of the gases into and out of the cylinder
2
When you compress the gas in the cylinder it generates heat, which causes the measured pressure to be higher than it would be if heat was not generated

A much better way to know of your rings are working or not is to look at the surfaces of the bore and the piston, looking for evidence of combustion product blowby, check for the rings being stuck in their grooves with goo, and to measure the ring end gaps in the most worn part of the section of the bore where they run
Kicking an engine over, or spinning it somehow and measuring the peak pressure generated will give highly variable results depending on the amount of oil on the rings at the time of the test


relax, nothing is under control

TriCub
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Re: ty250A compression

Postby TriCub » Mon Aug 29, 2016 9:46 am

timlangham wrote:The manual says 6:1 compression I read this as 6 times 14psi (atmosphere) = 94 psi. But I doubt this accurate. Have just pulled mine apart had 100 psi if I kicked it a few times. The piston rattles in the bore and there is almost 1/2 a mil of clearance. Have ordered a 71.0 Piston & will get it bored to size. I am the same as you I assumed there would be a compression spec somewhere.
I can't find it


I would have thought that 100psi was quite good for a TY. 6:1 is the corrected CR which is calculated from the closing of the exhaust port to TDC. How did you measure tha 1/2mil clearance, I've never seen a TY with that much clearance that was still running and able to pump a 100 PSI.



TriCub
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Re: ty250A compression

Postby TriCub » Mon Aug 29, 2016 9:50 am

bikecrazy2010 wrote:Hi all, im new to this forum, i have aquired a 74 model ty250a it does run but as soon as you try and apply throttle it dies but if you wind up the throttle screw it will slowly increase in revs from everything ive read these things should be fairly hard to kick over this one doesnt seem to be does anyone know what a standard one should read if you give it a compression test ?


As David says, but more likely to be a blocked main circuit.



timlangham
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Re: ty250A compression

Postby timlangham » Sun Oct 09, 2016 12:45 am

Ty250 bored out to 71mm with a good wossner piston, slicked up with two stroke oil 125 psi. cold with throttle wide open without ever being started. I rode it down the end of the driveway & back it . Now what do I do ???
I tried to set the ignition timing 3mm before top dead centre + or minus .15mm. There is .3mm difference on the dial gauge off the top of the piston between pushing up or down on the fly wheel
There should be a better way



Tony27
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Re: ty250A compression

Postby Tony27 » Sun Oct 09, 2016 9:46 am

Your main bearings are worn if you are getting movement at the flywheel, mine was still on standard bore a few years ago & started running erratic on the second day of a 3 day trial so I decided to have a look what was going on
I was able to remove the flywheel without a puller - first clue, grabbing the end of the crank I could turn the motor over by hand with the plug still in showing how little compression it had, there was also noticeable up & down movement at the crank due to the bearings being worn which also affected the jetting due to the crank not sitting in the centre of the seals
Unfortunately you will have to do a full engine strip to replace the main bearings & it's a good time to replace all the seals & generally check on the condition of the clutch & gearbox parts while you're in there. I think I ended up replacing all the bearings just because I could & had to replace a piece in the shift mechanism but the rest was still perfect
I was also able to find a genuine first oversize (70.25mm) piston kit & get it bored to factory clearances



David Lahey
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Re: ty250A compression

Postby David Lahey » Sun Oct 09, 2016 12:04 pm

timlangham wrote:Ty250 bored out to 71mm with a good wossner piston, slicked up with two stroke oil 125 psi. cold with throttle wide open without ever being started. I rode it down the end of the driveway & back it . Now what do I do ???
I tried to set the ignition timing 3mm before top dead centre + or minus .15mm. There is .3mm difference on the dial gauge off the top of the piston between pushing up or down on the fly wheel
There should be a better way

Yes it sounds like your mains are worn or are a loose fit on the crankshaft or in the casings. In this condition, the timing will be all over the place due to the relative movement of the crankshaft to the stator plate, so there is no way to achieve constant timing with the points ignition. Fitting electronic ignition would get the timing working, but would only be treating the symptom, not the disease
If it has 0.3mm at the piston crown, then the radial movement at the LH main bearing will be at least double that amount, which is too much for even a new crank seal to be happy with.
I'm still riding a TY250A with original bore, rings and mains. It is nice and quiet and runs like a dream and has no play in the mains, but the LH crank seal was replaced a few years ago when I noticed some oily residue in the magneto casing, which means the seal was leaking a bit even with no movement in the crankshaft so if you have movement in the crankshaft and an old seal, you will be sucking in air big-time


relax, nothing is under control


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