RD400C transmission oil disappeared?

[Eugene Haymes]

I just finished having my bike partially repainted (frame, engine cases) and some new parts (no engine work done). I added 10W30 oil to the transmission to the proper level. On my first ride of 20 miles, I started hearing a hissing sound in 6th gear. When I got home, it would not downshift below 5th. There was no oil in the transmission that I could measure. I put oil in it today and it is not leaking onto the ground. Where did the oil go? Can it be sucked from the engine through the crank center seals? I would think that the crankcase would have a positive pressure. I am broken hearted. 

[bitzz]

The pistons go up AND down. Half the time the crankcase has positive pressure, half the time it has negative pressure.
If the thing sucked back 1.5L of oil in 20 miles the plugs will show and the thing would smoke like a bear.
A leak down test is in your future.

[robrd]

Try putting a catch tank, (bottle), on your gearbox breather pipe and take it for a run. Like said before, after just twenty miles loosing that much oil you would be lucky if your motor run if it was leaking into the crank and would surely oil your plug up. After a ride, check the catch tank and if it's empty it's a mystery.

[Gr8uncleal]

It's probably not this, so only really to satisfy my curiosity, but is the hollow cam bolt and all of the fitments present and correct (28 to 30 on the attached)? It is located on the underside of the engine and is often mistaken for the sump plug.

https://www.cmsnl.com/yamaha-rd400-1976-usa_model8584/partslist/B-05.html#.YyX6so_MKUk

[RDRD]

Maybe the oil level was not correct to begin with?
I do the following:
re-fill transmission oil with about 1.1 litres
run the engine for a couple of minutes
wait a few minutes
check oil level
re-fill as necessary
run the engine again, etc

The oil level after a drain and re-fill is notoriously misleading without following this or some similar process.

[Eugene Haymes]

I did not wait after checking the oil level initially to double check. It was possibly low. If the dipstick touched a gear or something, it could have left a drop of oil on the end of dipstick. And, the dipstick is shiny with no oil or with oil. I recall pouring all the oil from a quart bottle. As it is now, it cannot be ridden unless I can get going from a standstill in 5th gear. Before painting the frame and engine covers 3 months ago, it ran fine. Everything points to not enough oil. Thanks for the excellent inputs. I do like the idea of a leak-down test because the bike lacks power for some reason.

[murp]

Put the crankcase breather pipe into a small jar of water.... If you see air bubbles when bike is running, crank seals gone..... This a good easy alternative to a full leakdown test

Sent from my Pixel 4a using Tapatalk

[Eugene Haymes]

Murp, that is a great idea! I never thought of doing that. And, I found that a kit to do my leak test is hundreds of dollars. It will check all seals except the one behind the ignition. If it leaks there, there will be oil on the points.

[pidjones]

I made up a lead-down test rig for not a lot of money. Rubber stoppers from ACE hardware and rubber gasket material (thick, soft orange rubber) from there also. Teed in a low pressure fuel pump gauge from Harbor freight a,d used a small valve that I had lying around to meter in a small amount of air from my compressor that I had already vented down to low pressure. You could probably use a tire pump. One of the stoppers need to be a one-hole to connect the pressure and gauge to. Pull the carbs and connect stoppers to the rubber insulators. if any leak-down, use light soap solution to look for the bubbles.

[Eugene Haymes]

I like the idea of a home made leak down tester.

The clutch cover is removed. I found the one tiny hole (3/8 inch) that carries the oil from where it is poured in (clutch area) and realized that it takes a while for the oil to fill up in the transmission. I should have waited to measure the level or poured in a measured 1.5 liters. My bad.
So, I am waiting for my clutch holding tool and 29mm socket. Then I will learn more about the transmission damage.
Thanks for good suggestions.

[Eugene Haymes]

I am going to split the cases and replace the gears. Recall that I added oil to clutch chamber and checked the dipstick quickly and the dipstick indicated full (it was not). So, I ruined the transmission during a ride. I just did  a leak down test and found a bad leak at the seal on the clutch side. The leak prevents me from knowing or testing other leaks including the center seals. My question is that after I split the cases, is there a way to test the center seals? Suppose I pour gasoline into the bottom of the crankcase and crankshaft and see if the gas leaks to the center bearing? Otherwise, I will need to replace the outside crank bearings and then reassemble the engine and do another leak down test.

[Eugene Haymes]

Forget the above question. I figured it out. I adjusted my air compressor to a very low setting and held the air hose on the fixture. I had the crankcase breather hose on water. There were no bubbles. I repeated it for the other side (I had to put the clutch back on) No bubbles were seen. My center seals are O.K.
I assume that I will need a dial indicator to reset the timing on my Vape ignition since I will need to remove the flywheel.