Oil rising up through push rod guides!

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Oil rising up through push rod guides!

Postby tyajs » Thu Jun 14, 2018 12:10 pm

I had a friend help me strip and rebuilt a 59 model31. It all seemed to go back together well apart from one annoying issue (a big issue that stops me riding). Before i stripped the bike and post rebuild, the bike pushed and is pushing oil up the push rod guides and flooding the rockers which ultimately finds it's way in to the inlet valves and bore. This causes the bike to produce blue smoke and foul the plugs till the bike runs like it has bad timing after 2-3 minutes. I have checked oil return. The pump seems fine and has been set up the right way round. Wet sump is minimal after sitting. Am I missing push rod seals or O-rings?

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Re: Oil rising up through push rod guides!

Postby Mick D » Thu Jun 14, 2018 12:22 pm

Hi

Sounds like the crank case breather is not working correctly and crank case pressure is blowing oil up to / preventing the drain down from, the rockers - check the operation of the breather.

Regards Mick

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Re: Oil rising up through push rod guides!

Postby robcurrie » Thu Jun 14, 2018 12:33 pm

There are no pushrod seals or guides; the holes in the barrels for where the pushrods are, are also return holes for the oil in the rockers and this oil lubricates the cams and followers when it drops down. There are smaller holes in the barrels to direct oil up to the rockers; on later models, there are brass metering plugs in these holes to restrict the oil flow. There is also an oil distributor driven by the exhaust camshaft which meters the amount of oil to these oil holes.

It could be the oil distributor or the metering plugs are missing that causes over oiling.

It could also be something amiss with the pistons and rings and oil is getting past them to the combustion chamber.

Rob C

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Re: Oil rising up through push rod guides!

Postby tyajs » Thu Jun 14, 2018 3:02 pm

Thank mick, rob.. If I take the rocker cover off and start the bike I can see the oil rising up the push rods. The metering plugs are in place and oiling as they should. The rising oil level up the push rods are over oiling the rockers and ultimately the bore. The crank case doesnt have a breather other than the non return/breather nut on the end of the crank? The oil still rises when the rocker cover is removed. I would have thought the crank case would breath with the rocker cover removed? I have asked a couple of members in NZ but no clear idea of what the cause could be. Crank pressure sounds like the obvious cause but the removed rocker allowing the crank to breath would illuminate that?

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Re: Oil rising up through push rod guides!

Postby Mick D » Thu Jun 14, 2018 3:19 pm

Hi

I think if the crank case breather is stuck removing the rocker covers will make things even worse - ie. the crank case can breath through the open cover via the push rod tube oil drain galleries.

On the other hand, could these galleries be blocked and you seeing a rising level from oil not being able to drain to the crank case?

Regards Mick

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Re: Oil rising up through push rod guides!

Postby MalcW » Thu Jun 14, 2018 3:50 pm

Hi

I don't suppose it's possible that the oil pumps have been assembled incorrectly, with the feed gears larger than the scavenge gears, rather than the other way round?

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Re: Oil rising up through push rod guides!

Postby robcurrie » Thu Jun 14, 2018 5:19 pm

Please check if oil is returning to tank. Is the crankcase empty and does the breather function correctly? Is the primary chaincase oil level just touching the chain, and does it keep this level after running the engine? Don't overfill the oil tank, half full is ok for most short trips. Does the breather in the oil tank vent properly? Are the oil pipes fitted correctly, feed at front, return rear?

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Re: Oil rising up through push rod guides!

Postby tyajs » Thu Jun 14, 2018 7:17 pm

No malcw, oil pump is configured correctly. Good points Mick. By crank case breather, do you mean the vent nut on the end of the crank holding on the rotor in place? The engine has no other external vents from the carnk case that I know of. Good advise too, Rob.. i did put one of these together many years ago when I knew very little, miraculously it went well for years with the odd tweek. Now I know a bit more, these bike seems so much more complicated! In uk for another week and heading back to NZ. I'll check through the points suggested and report back.

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