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Question with regards the necessity of changing crank seals. TY250A


johnnyjazz
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Dear exalted shamans of mechanical knowledge and expertise,

Thank you so much for all the insights and advice on this forum thus far.  My little bike is coming on a treat thanks to you guys helping me learn how.

So i was wondering, as the bike had been sitting for 30 years, is it fair to assume its a given that one must change the crank seals as a matter of course?

They don't seem to be leaking and i did 'feetupfun's suggestion of measuring the oil and none had gone from the right side (i think my oily plug is telling me to be a little more sparing with the pre mix ratios)-but is it inevitable that its upcoming and i should muster up the courage and determination to get them done?

I've found some great posts on the forum about how to do it but any 'top tips' would be welcome as i am a newbie to this stuff and don't want to mess it up.

also, even though the bike runs fine should i do the points/condenser/ignition too or is it a case of if it ain't broke don't fix it? they have been on the bike perhaps for 30 years too.

and finally ( i do hope i'm not outstaying my welcome) is it a big deal i noticed i don't  have any vent pipes coming of my carb? does it make a big difference or is it ok just to leave it?

i enclose a pic of the mellow machine before when i got it and where i'm at now with it, and again thank you so much for all your help as you guys have walked me through alot of stuff thus far!

sending best vibes and bright moments from the land of trumptonian dystopia!

johnny :)

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If the bike is running fine I wouldn't bother changing parts 

I suspect when you need to replace the seals you'll also need to do the main bearings 

Vent hoses aren't a necessity for your riding situation but fitting a set wouldn't be the worse idea

 

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I once worked in a place that used lots of electrical relays, and we had an instrument that would check the performance of these contact points.

To get the best results we would have to burnish the contacts to a mirror finish with a special stone.   So the same applies to CB sets on motor vehicles.  What actually happens is that the negative contact looses some of its metal to form a whisker on the positive contact.   When this builds up, the circuit will not open and close instantaneously, and you get a reduced voltage for the spark.

So you need to remove this whisker, and grind down the hole that occurs on the other contact.   Maybe stop short of burnishing with an agate, but at least file the points up reasonably well, and make sure they are parallel.

 

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Changing crank seals is a pretty major job and as already written, if you don't needed to, don't.

Also, if you've never done them before, find out what's actually involved, the process sounds simple enough until you realise the crank is stuck on a main bearing or the crank isn't centred after you've rebuilt it!

If you're going to go to the effort of splitting the engine, it's probably wise to do a complete engine overhaul.

The obvious signs the seals are leaking is when it's a pig to start and won't idle properly when/if it does start (but they are signs of a lot of other issues as well)

I don't know the history of your bike, but I've bought, ran and rebuilt lots of similar stuff, (a couple of these too) and I find it's always a good idea to know what state everything you already have is in and to start collecting parts for when things do give up, sooner rather than later as some parts get harder to find as the years roll on.

In the meantime, enjoy it.

 

 

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Thanks so much dear trials friends, your advice is, as always, invaluable.

Methinks I am going to go with the if it ain't broke, don't fix it route!  For now anyway.

I do have all the new parts on hand and access to a workshop where everything one would need tool wise is available.

i guess its all down to confidence.  i changed the front fork seals with fantastic lessons from feet up fun and want to learn more about mechanics.

Im thinking the only way to learn is to jump in?....though i understand fully that time spent on reconnaissance is seldom wasted (hence my coming here).

@tony27 thanks always and Kia Ora!  gee i miss NZ, best to john lawton please.  will the bearings definitely be shot you think after 30 years? am i correct in thinking that's much more involved and you have to split the cases?

@scifi thanks for awesome electrical maintenance suggestions.

and @ Goudrons thanks for the great and sagacious advice! i wasn't planning on splitting the cases, but rather drilling the seals and extracting them with a drywall screw....well, that's if i can zip up my man suit and muster the enthusiasm, but as tony says if i should do the bearings too then i wonder if i'm getting in over my head?! ;) 

as always, really appreciate your time and responses. thank you.

best wishes from brooklyn.

Edited by johnnyjazz
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When I rebuilt my motor you could move the crank up & down when the motor was still assembled.

Bearings had never been done & motor was on standard size with so much end gap on the rings that I could turn the motor over by hand grabbing the end of the crank, with new bearings, seals & fresh bore there is noticeably more compression already 

To do the bearings is a full stripdown of the motor, if you need to do that due to play or rumbling then inspect every part in the motor & replace anything that isn't perfect along with all seals

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I've got an A model like yours and the motor has never even had the cylinder head off, but a few years ago when I was servicing the points, I did notice a small oil witness around the magneto side crank seal. The main bearings were still quiet and with no detectable free play, so I assumed that the seal had just started leaking a bit because it was so old. The motor still ran fine and if I hadn't looked would not have suspected that there was any leakage there. The crank seals on both sides can be replaced without splitting the cases.

If that was my bike I would inspect the magneto side crank seal for any evidence of oil residue, feel for any up-and-down play in the LH main bearing and have a good listen with the motor running for any rumbling noises, which are a tell-tale for failing main bearings. They are an incredibly sturdy motor so can do an amazing number of hours of regular use with little effect on the bearings, but like any two stroke, any rolling element bearings on the crankshaft assembly can be damaged fairly quickly from the bike being stored in damp conditions.

If there is any up and down play or rumbling, it is not worth changing the seals unless you also replace the main bearings, because the movement of the crankshaft will probably cause even new seals to leak

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Thank you as always so much for your response 'feetupfun'!  Thats where i'll start, with the magneto side and check play before i do anything else.

so grateful to be able to receive all your knowledge and guidance.  Snowy day in NYC so good excuse to be in the workshop :)

i'm in awe of all you mechanical masterminds! have a great weekend,

humbly, johnny

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I recently replaced crank seals in a team member's TY80.  The bottom end of that bike had been one that I had in the shed for over 10 years, the rod and crank bearings were nice and tight, the oil was clean.  We put one of our top ends on it and it ran good!  For half of one Trial anyway.  By the end of the first day it was smoking profusely and stunk of transmission fluid indicating a wet side seal had failed.  By the end of the second day it would barely run.  The crank seals I removed were so dried out that I was surprised the bike ran at all let alone started.  Crank bearings were still tight (I would hope so after only running 2 days!) but after 40 years the seals were toast.  The bike runs phenomenal now, and stopped stinking of tranny fluid after 5 minutes of running time.

We have done seals on a couple of the bikes that we knew had a bit of radial crank play due to worn bearings, and they are now in the shop to get new mains and seals before the Vintage season starts up next spring because the play knackered the seals within a few events.  If your crank moves, it's split the cases time.  If they are tight, I would certainly not be afraid to replace 40-year old crank seals.  It sure beats having them fail in a Trial because the bike ran fine in the shop and "they'll be OK, leave them alone".  Now while "runners" are easier on seals than "sitters", 40 years is still 40 years and that amount of time dries out rubber, period.

Just my 2 cents.  :)

Edited by brewtus
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Thank you so much Brewtus! great advice and really appreciated.  fingers crossed the crank is tight,

though  if i have the left side off (and have the parts already), i'm guessing i might as well do the points, condenser and coil too?

i just want to try and make it nice :)

 

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6 hours ago, johnnyjazz said:

Thank you so much Brewtus! great advice and really appreciated.  fingers crossed the crank is tight,

though  if i have the left side off (and have the parts already), i'm guessing i might as well do the points, condenser and coil too?

i just want to try and make it nice :)

 

Coil should be fine, points and condenser could not hurt.  Making it nice enough to horse-beat in competition always makes me smile!  B)

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sorry, slightly off topic from original post, but could anyone tell me the size or generic part# of the o-ring i need for the needle jet set screw in the TK carb bowl.  i got an 'automotive o-ring set' but none fit properly and the part# on my yamaha list brings up nothing on ebay.  many thanks! :)

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hi scifi, i got the metric (mm) set but they are all skinny.  i need one like a dougnut.  i think ive found some on eebs for a few $ so i'll take a gamble...but i realised i gotta change it as im running rich and im pretty certain its that! (again thanks to this awesome forum)  btw, totally noob question so please excuse my ignorance...but in my manual it says the clip on the needle needs to be in position 4.  Does one measure from the top so the clip is one notch from the bottom or vice versa with it being one notch from the top?  many thanks always :)

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yes count grooves starting at the top.

When you say the "needle jet set screw", I figure you are meaning the air screw.

As well as the diameter, measure the groove width and depth to size the o-ring.

A leak past the air screw o-ring will not make the bike run rich.

In a TY250A TK carby that has had a fair amount of use, a likely cause of the bike running rich is:

Wear in the needle jet and the needle (where they rub against each other) or a passing float needle valve

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