worn crank seals - symtoms

Hi, What are the symtoms of worn crank seals ?

Is it erratic idle, easy stalling, hard starting and poor low RPM power ?

The bike is a very old YZ50 with points ignition. It does have worn crank bearings and as a result likely the seals are leaking. Thing is it idles and starts just fine but I dont seem to be able to set the ignition correctly I am having to retard it allot just to get some low end power if I set it to 1.8mm BTDC you just cant ride it as it completly bogs down at anything less than screaming. I believe it is bored out to 65cc if that makes a difference. If you feed in the throttle gently with incresing RPM it is better but if you crack open the throttle at low RPM you just start slowing down and it dies.

The worn crank bearings could well be upsetting the ignition timing as the cam is on the flywheel and will in effect vary the points gap hence timing.

The symptoms you describe arte classic of worn crank case oil seals, erratic idling and poor starting when hot but usually starts OK when cold. Also if the piston rings are worn a 2t can be a pig to start when hot,

Tom R

Thanks Tom, thing is mine starts fine when hot and idles lovely which is why I am not sure it is the crank seals. The problem with the bike is bogging down at low and medium RPM ? especially if you just crack the throttle open rather than feed it in, can that be attributed to worn crank seals ? I am not so sure. cheers Ben.

How about testing your seals by opening the flywheel cover and spray some carb cleaner at the area? It surges and you know. That is the most dangerous side. Also I would wiggle on the flywhell if it moves the rollerbearings are shot which makes it impossible for the seals to seal.

Arnego is right the flywheel side seal is not in a bath of oil so it will wear quicker than the clutch side.

A YZ with points?? Sure it's not a DT motor? I was sure all YZs' had cdi.From 74 on..

Anywho....Try a timing light to see if it's jumping around.It very well could be a worn crank\bearing surface or worn bearings.

Also a bad condenser will act like this,look for a small amount of spark at the points when it runs.

It's a 50 and yes they had points, at least the early ones did.

Hi Guys thanks for all your responses. I took the cover off made a couple of white lines at TDC started her up - 1) A few small sparks from the points but not much, I think they are OK. 2) If I put the timing light on and rev her up the timing scatters all over the shop really badly. 3) Tried squirting carb cleaner behind the stator plate but couldn't notice a change in Idle, couldn't really get at the seal.

Turned the flywheel to get the points fully open and then tried moving the flywheel up and down as the crank bearing are badly worn and the points gap varied hugely.

So in short the crank bearings are shot (which I kind of knew) but I think it is the effect this is having on my ignition timing that is giving me the bogging at low and medium RPM problem rather than the seals.

So best get myself some new crank bearings, seals and gasket set then.

I am guessing it is quite straightforward spliting the crankcase and changing the bearings and seals on this old girl, anything I should watch out for ?

Bet the cases are well stuck together.

cheers Ben

Geeez,at first i thought you said '250'. That's why i was thinking "points on a YZ?"LOL!

You also might find the crank surface that rides on the I.D. of the bearing is worn. I see this a lot on old and new bikes.

In fact i saw it this week on a 09 CRF 450. Had a funny lower end knock..

If the cases are stuck,it most likley would be at the front and\or rear where the dowels are at.They like to get rusty.

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