Oil venting from gearbox

[SDDaytona]

I'm getting oil venting from the gearbox vent on the back, righthand side of the engine when running near redline on my RD400. Bike runs fine, gearbox oil is right on spec per the dipstick.  Was thinking of connecting a vacuum gauge to the vent and see if I'm getting pressure from the crankcase into the gearbox.  Thoughts?

[Kawtriplefreak]

It will puke a little running hard. I take it you have a hose connected to it? If not put one on it. I always direct mine over top of the chain. If you don't have a hose on it you can end up with gearbox oil on the rear tire, which can be unpleasant.

[SDDaytona]

Hopefully that's it.  It spit probably 1tbsp twice after wide open runs. I do have the stock vent tube routed over the chain but it still made quite a mess.  Think I'll do a leakdown test if it worsens

[paul1478]

A few years ago I had a issue with gear oil coming out of the gear box. Took a little wile to find it but it was coming out of the tach cable. Has to replace the seal. Not seen anything since.
 

[RDnuTZ]

coupla things to consider: too much crankcase pressure & dipstick not correct. I had 2 separate 1981 Yamaha YZ250s that would dribble out some oil from crankcase breather when revving it sitting in neutral and also after stopping a ride. Easiest thing to check is correct oil level to start with is measuring your drained out oil to see if it matches the amount cast into the case. Then see if the specified amount you put in registers correctly on dipstick.

In my situation, I found out Yamaha knew about too much pressure problem & put out a TSB with revised guidance for oil level and instructions to add a 2nd breather to case. Eventually changing location of the breather on later production units.

[IR8D8R]

I've got a bunch of dipsticks in my RD parts and they're all different lengths. They all screw into the side cover.
I have info saved from another thread. Poster was SoCal250. (thanks!)

RD250B: (in mm) Total length-111.2 High mark-107.1 Low mark-97.6

RD350: Total-110.7 High-107.6 Low-98.3

 My RD350 dipstick was not long enough to touch the oil when filled by measure, shorter than this info by 1/2" and filling to the full mark caused oil to spray all over my rear tire. My chain was well-lubed though.

IR8D8R

[scully]

Take oring off dip stick 👍