Author Topic: A10 plunger oil wet sumping  (Read 408 times)

Offline maximus79

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A10 plunger oil wet sumping
« on: 13.05. 2020 17:28 »
HI, i have a 51 plunger thats has been restored, tho when it sits for more than a day the bottom of engine fills with oil, is this the norm and does anyone else have this problem/ a solution?

i have to drain it every time to start it...could i put a tap/valve on the oil tank feed maybe?


Offline a10gf

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Re: A10 plunger oil wet sumping
« Reply #1 on: 13.05. 2020 18:10 »
Inside the engine (behind oil pump) there's an anti-siphon \ oil retaning ball + spring, something must have gone wrong there, as it should definitely not wetsump in such a short time.

The spring may be broken or weakened, so ball not seating\closing up properly.

Sadly, to access it one must split crankcase, so it's a job that may be done while fixing other things (like cleaning sludge traps).

Many, many posts on the forum, do some searches, examples,3066.msg20796.html#msg20796

About external 'oil blocking' device, also tons of posts, with some warnings about it's use.

A10 GF '53 My A10 website
"Success only gets you a ticket to a much more difficult task"

Offline muskrat

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Re: A10 plunger oil wet sumping
« Reply #2 on: 13.05. 2020 20:46 »
G'day Max.
Wet sumping is a common problem. The culprit is usually the ball & spring in a10gf's second link. Unfortunately it means a motor strip down to attend as its inside the cases.
My 51 A7 will empty it's oil tank into the sump in 2-3 weeks. I put on a sump with a drain plug and empty the sump before a ride, takes just a few minutes.
If your bike fills the sump over night it may also be a loose (or very worn) oil pump. If you go in there to check it a little job I do is with the pump off is to use carb clean with the thin plastic tube. Put it into the feed hole (top right) and feel for a light spring pressure. Give it a good blast of carb clean. This may help slow it down a bit.
Another side problem with the pre 53 plungers is there is no oil seal on the drive side of the crank. When the sump fills it drains off into the primary, over filling it. Which then leaks out from behind the clutch.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Muskys Plunger A7

Online chaterlea25

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Re: A10 plunger oil wet sumping
« Reply #3 on: 13.05. 2020 22:53 »
Another possibility is that someone put a thick fibre washer under the front pump stud
lifting that side of the pump and the gasket will not seal on the feed from the tank
I use a piece of the same gasket material as the pump gasket instead of a fibre washer (or one piece SRM gasket)
Pump body screws loose is another source of unwanted oil

1961 Super Rocket
1963 RGS (ongoing)

Online Ted_Flash

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Re: A10 plunger oil wet sumping
« Reply #4 on: 14.05. 2020 08:03 »
If I'm going to leave the bike unused for a while I syphon the oil out into a can.  This has to be done when hot, but can be left to its own devices if the pipe is secured. No tools needed, and quicker than draining via the sump.
Ted Wilkinson, Ramsbottom, Lancashire
1950 Golden Flash

Offline owain

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Re: A10 plunger oil wet sumping
« Reply #5 on: 14.05. 2020 22:51 »
Hi Maximus (dwyt ti'n cofio macsen?) - a Welsh pun there ;P

I had a similar problem with me 1950 A10. I agree fully with muskrat and others. Although the most functional solution that I have found is a) install an anti-wet sump valve (make your own or purchase from and b) install a new sump plate that allows you to release the oil easily. Removing the original sump plate over and over is an unnecessary burden if the bike is wet-sumping. Just remember to open the anti-wet sump valve when riding!

Sweden & North Wales
'50 BSA A10
'69 BSA A75R

Online RDfella

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Re: A10 plunger oil wet sumping
« Reply #6 on: 15.05. 2020 10:36 »
It can be a real pain. I rarely ride any one of my bikes more than once a week. In fact, I haven't ridden any as yet this year. My A10 is the worst for wet sumping - half the tank in the sump in a week. Trying to start it with the sump half full of oil is difficult due to drag on the crankshaft slowing it down below reliable spark levels with the kickstart. And if it does start, then oil pours out of the engine breather making an almighty mess. So far I've been draining it before every ride. I have a sump plate with drain plug, but even then it's an annoyance. Being fairly tidily dressed ready for a ride is not the best time to find a drain tray and clean it, get a spanner, get on one's knees to drain the oil and then proceed to filter it back into the oil tank. I got so fed up with this I've fitted a valve in the oil delivery pipe. I appreciate there's always the risk I could forget to turn it on, but as I always check return to tank on my bikes before setting off, hopefully I'd catch my mistake in time. I realise there are options for connecting a valve to the magneto, but couldn't be bothered with the extra work. To me, the risk outweigs the frustration it was causing me.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline AdrianJ

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Re: A10 plunger oil wet sumping
« Reply #7 on: 21.05. 2020 16:17 »
SRM pump stopped my 53 plunger doing it.
'53 Plunger Flash, Steib S500.

Offline hdawson

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Re: A10 plunger oil wet sumping
« Reply #8 on: 25.05. 2020 11:13 »
An in line oil tap to feed is the end of the problem. Just put in a fail safe such as a ignition cut off or my scoffed at silk scarf throttle  wrap that always goes on after the post ride put-to-bed routine when I turn the tap off. Others furiously disagree but each to their own.
Cheers Hadrian

61 BSA Super Rocket (cafe).
51 Matchless G9 Clubman.
81 Suzuki GSX 750 ES.
02 Triumph Sprint.