Oil leaking onto rear mudguard then tyre

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RICHARD MOLYNEUX

RICHARD MOLYNEUX

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Jan 22, 2020
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Hi All

I have recently bought a 850 Commando which was converted to 920. It needed quite a lot doing to it considering it was newly built (replacement master cylinder, voltage regulator, inlet valve seals to name a few).

Each time I go for a ride there is oil on the outside of there are mudguard just above the swingarm pivot. this finds its way onto the rear tyre as it drips off the guard. This oil appears to be coming from high up the oil tank (above the main outlet banjo and the supply and return pipe connections). I can see no holes in the tank, and I can't see where the oil is coming from when running the bike when stationary. It is as if the filler cap is leaking when the bike is running but this is screwed down as hard as can be done. I have fitted new clips onto the supply and return pipes so it is not to do with these. Is it due to the crankcase pressure in the tank (no PCV valve fitted)? I have a non return valve to stop wet sumping but the oil was leaking onto the tyre before this was fitted.

Any help, thoughts ot advice greatly appreciated.

Richard
 
There is an outlet on the oil tank that originally fed a chain oiler, perhaps that has not been blanked off.

And... NRVs in the feed line kinda get a bad response on here...
 
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Have you checked the cap seal? Or could it be that the tank is over filled? Make sure you check the level after you have been riding for awhile. Just my thoughts, Chuck.
 
Agree with HN above... you'll be inundated with so much to worry about... but the tank cap seal, if cork, can compress over time and lose efficiency... cheap to replace.... always a good first try.
 
Bit of paper kitchen towl, elastic band, wrap around filler cap. Go for a ride, then inspect. Plus one on over filling the tank, I am a fairly recent convert to this theory (not over filling)
 
Check the bottom of the oil tank, the threaded insert is stressed and eventually the steel tank its welded to splits. When it first appears it only leaks when the engine is running, later the contents of the tank leak out when the bike it parked.
 
Have read through here:
Notice the shitful pinch weld my tank had.

Oil tank repairs... let's see 'em

it's a had a slight dampness on the nether region for a while now, I recently dove in for a closer look. The crack is small, tight, the liquid penetrant barely produced a red stain, but, periphery crack along the toe of the weld securing the bottom mount lug (pattern failure as described here...
www.accessnorton.com www.accessnorton.com
 
Had CNW do oil tank mod on mine. Does away with bottom bolt and weld a plate across bottom, strengthen top brackets with larger rubber mounts, rubber pad under tank.
 
Even with the tank level below full, when riding it will splash up to the cap and leak at the cork seal unless it's perfect. @Steves' suggestion of a paper towel around the filler neck is great troubleshooting advice.
 
Does it have an operable chain oiler? If so, that gets oil everywhere on the underside near the chain. My center stand, rear fork, chain guard underside, and rear fender underside are all caked in 40 yr oil crud. That should be capped off and the chain oiled with proper chain lube and not dirty engine oil
 
kommando said:
Check the bottom of the oil tank, the threaded insert is stressed and eventually the steel tank its welded to splits. When it first appears it only leaks when the engine is running, later the contents of the tank leak out when the bike it parked.
Click to expand...

This would be the first place I would look.
 
Have had the same symptoms with oil coming out of the cap & down the back of the oil tank - fitted a PCV & it was solved so I think you problem is crankcase pressure - because of the pressure, I also had oil being forced past the camshaft oil seal into the points housing
 
I had this problem after I fitted a reed valve sump kit , mainly on a long fast run .Cured it by putting a large o ring between cap and oil tank , bit stiff but no leaks , the kitchen roll is a good idea wish I had thought of it , took awhile to work it out.
 
Yes, if you have excessive crank case compression's, you have to make sure that cap is on tight. Guess how I found out. With new pistons etc and a Bradford breather, I have no issues now but I still crank that cap up tight.

Dereck

ps. before the new pistons, the engine pissed oil out everywhere around the crank case as well.
 
Check the oil level hot after a good run. Midpoint between the two marks is plenty. Anymore and it wants to puke out.
 
For you folks who added a breather, did you block off the oil tank breather too? Mine still goes to my airbox. I think. You have to let the oil tank breathe.
 
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Shit yes. The gases have gotta go somewhere. Mine still go into the air filter. [ it was an early anti pollution method. burn the fumes. ]

The breather valve's only job was to stop the return of the gases it passed out. A [ technically ] vacuum created keeps the engine oil tight, and the mass of air inside the crank case is reduced somewhat.
Dereck
 
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