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Do Series A singles have sumping problems?


Well Known and Active Forum Website User
VOC Member
“I've been a Vin rider for many a year,
I spent all me money on black leather gear.” - I'll let youwork out what the music should be.

When I got my first Comet it was only twenty five years old, so I have now owned it for longer than all of its previous owners put together. I've also owned my Knight it for longer than all its previous owners put together.

My '39 Meteor was seventy three years old when I got it so I've got to live to over a hundred and thirty for the same to be true.

It has so many differences from the postwar bikes that I have a lot of learning to do. Today I managed to damage a piece of the oil feed pipe close to the tank causing an oil leak. I decided to drain the tank so I could replace the pipe and disconnected the other end from the pump but and found no oil flow from the end of the pipe. I had to remove it from the tank and quickly fit a drain hose. Once I'd got the pipe off I found it had been assembled from at least four different pieces which appear to be a rubber connector from tank banjo to a metal unit, a once clear plastic pipe and a rubber pipe to pump banjo.

The lack of flow suggests to me that the metal part is an anti sumping valve of some sort with the clear section below it acting as a sight glass.

This raises the question of whether the anti sumping valve is needed with the A oil pump. I thought that gear pumps didn't have a problem.

Accepting that the machine is old and likely to have some wear is there a better way to prevent sumping?

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
Mine would dump the entire tank into the sump over the course of a week. There is an article on the club site about checking end clearance on the pump gears. None of this helped and I concluded that it could be passing through the pump gear spindles. A Conways sumping valve sorted it. Just make sure you bleed the air out above it and prime the pipe below.


Well Known and Active Forum Website User
Non-VOC Member
I'm not familiar with the A but if my bikes are anything to go by, my pre war Speed Twin with plunger pumps, does not sump whereas my 37 Empire Star with gear pumps does, so I think gear pumps are more prone to sumping. During the riding season, when the bike is ridden regularly, this is not and issue, however, after a lay up of a month or so I have to drain the sump. After this length of time, I get about 300cc of oil. I did look at anti sumping valves as well as installing a tap in the feed pipe but decided these weren't a good idea, the tap in case I forgot:( and the valve because suction on gear pumps is low and if for this reason and/or a mechanical issue with the valve, it would be too late to save the motor before I realised. Of course Conways are the experts on these matters so it's personal choice.


Well Known and Active Forum Website User
VOC Member
I have a tap on the feed side of my 1938 Comet. I also have a sticker saying OIL in my damper knob. So far so good. I have a spare set of pump gears (thick and thin) in case there are issues later on. I always check the return pipe flow before riding away and when I bought the bike I checked the oil flow from the rocker feed and cylinder feed lines.


Well Known and Active Forum Website User
VOC Member
My crude understanding was that the anti-sump valve that Conway sold was designed for the Series A and that it was a definite problem. When Carleton was racing the Eric Oliver TT Rep he had to install a tap or all the oil would go to the sump. However, a check valve seemed like a good thing on the A as it has a relatively powerful oil pump. It seemed to me at the time that the check valve was not recommended for the C motor as there was some question about its ability to overcome the initial spring pressure of the check valve, which is less of an issue in the A. I was under the impression that the check valve has been used by some C owners with no issues.

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