ETZ 250 burning gearbox oil

[Rory]

I bought my 1983 ETZ 250 as a non runner. When I got it running it was burning gearbox oil. I have since given it a rebore and a full strip and rebuild, including new rod kit and bearings. It is still burning gearbox oil (about 50mL per 20km). I have put in new seals twice and I have checked that the breather plug is working. Any ideas what is causing it?

[Agronski]

Thick white smoke out the exhaust? If so, some things to check:

- Bad seal seating / installed wrong way round (spring should face inward toward crank, not outward)
- Case-halves not sealing / jointing surface damaged / mis-applied jointing compound.
- Case-half screws not tightened to correct torque
- Excessive compression (blowing past crank seals?)
- Leaking clutch-side gasket (not burning oil at all, but leaking maybe?)
- Leaking/damaged/missing rotor-side paper gasket.
- Magnetic drain plug not sealing

See also: https://www.youtube.com/watch?v=yhBuD4IJiss

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[arry_b]

"Bad seal seating / installed wrong way round (spring should face inward toward crank, not outward)"


...is only 50% right - they should face the oiliest side. i.e. The spring should face inwards toward the crank on the alternator side, but outwards toward the clutch on the clutch side. That gives you a better chance of keeping your lips lubed and unworn for longer.

However, losing it at that rate suggests something more than bad seals. Are you really burning it, or have you a leak somewhere? I'd be looking intently at the core plug under the neutral switch, checking for leaks.

[Rory]

Thanks for the replies. When I first put the cases back together I used jointing compound and I thought that I had used too much or not enough, so I split the cases again and put a gasket in instead. I also put a straight edge over the cases jointing surfaces and they seemed ok. It was still burning gear oil. The oil is getting into the chamber and then leaking out the seal at the dynamo side (right hand side). But it is only slight compared to the amount burnt, I can smell the gear oil burning and the engine is very sluggish, if you let it idle for a minute or so then rev it, there is a lot of white smoke produced. I have actually put 3 sets of seals in it all up, it still continues to burn gear oil. Where is the paper gasket that you mentioned?

[den]

i had the same problem with a honda 125 , its totally different but the mistake could still be the same,The Gasket i used for the two halves of the engine was for the right engine but wrong export model , it looked identical but had one little difference,,it left an oil gallery wide open and oil would be pumped into the airbox,

ive looked at your engine split and can see where there may be a problem for you..apart from the seals the only other place is here,, circled,,could be a idea,
also just check your airbox , oil could be flowing back through the carbs,thats how i found out in my case

[Rory]

I'm not sure of the mileage on this bike, could it be that the crankshaft has worn down and oil is leaking between the seal and the crankshaft? I replace the points ignition on the bike because the cam lobe was not opening the points enough, it was worn down.

[thomaskwscott]

Have you checked the seal behind the front chain sprocket? My 251 leaked gearbox oil for ages from that seal and, because it was only slight I didn't notice it on the fully enclosed chain system. Then I took it motocrossing one weekend and the seal went completely, covering my back wheel in oil. The ride home was a little hairy...

[gdmorris]

Hi Rory
Did you ever solve the missing gearbox oil problem ?
I am keen to know as I have an ETZ 250 with exactly the same problem.
I know WHAT is happening (The gearbox oil ends up in the crankcase, causing the clouds of white smoke),
I don't know WHY it is happening.

Like you I bought the bike as a non runner. I fully restored the rest of the bike, thinking the engine was OK. Its not.
I pulled the clutch and was able to access and replace the crankcase oil seal from the outside (without splitting the crankcase).
My thinking was that this is the only non metallic interface between gear oil and crankcase and therefore the weakest link.

As you can imagine, I was not well pleased when this made absolutely NO DIFFERENCE !!!!

Does anyone else have a solution ? Its driving me nutz.

[Rory]

I think that I finally found the problem, the o rings around the one way valve that the 2 stroke oil goes through. The 1983 model has the 2 stroke oil going through the cases not over the top.

[tigcraft]

I know it’s an old thread but I seem to have the same hair pulling problem.

[Zoeypenelope]

The oil is getting into the chamber and then leaking out the seal at the dynamo side (right hand side). But it is only slight compared to the ...As I understand it, the 250 motors went from 33:1 to 50:1 at the time the Supa5 was introduced. prior to that the main bearings were lubricated by gearbox oil ...Which oil should you use for your MZ/MuZ ETZ ETZ 250 (1981-1988)? Complete advice for all components, such as the engine, transmission, brake system and cooling ... Your engine appears to be the type with the 2 stroke oil going through the cases. I have one of those and it burns more gearbox oil than a small ...

[jannickz]

Double-check that the piston rings were gapped correctly and installed in the proper order during the rebuild. Improper gapping or installation can cause oil to pass between the piston and cylinder wall, leaking into the combustion chamber and burning with the fuel. Even new rings can wear prematurely if the cylinder wall is not honed properly during the rebore process. Worn rings won't seal effectively, allowing oil to pass through.

[Guesi]

This might be right on 4 stroke engines, but not for 2 stroke engines...
First: there is no gearbox oilunder the piston that can pass through the piston rings...
And second :the oil is mixed with the fuel, so it is everywhere where the fuel is...
on the bottom and on the top of the piston.
Burning gearbox oil is always caused by the oil seal on the left side of the crankshaft.
Often it is followed by oil coming out of the oil filling hole or the clutch cable.

And the oil coming out on the right side is no gearbox oil (because there is no connection to the gearbox) but oil from the fuel/oil mixture..