1972 Combat breather

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Mar 26, 2008
I have recently bought a 1972 Combat engined Interstate, engine number 207857. It does not appear to have the breather modification, but the original system from the lower rear of the crankcase. I've added a reed valve into the breather line to the tank, using a rather neat Ducati Paso unit from eBay (£4.95). The engine is now oil tight.

Experiments with running at various speeds, then stopping and checking the oil level have revealed that the level in the tank does go down at higher engine speeds. That said, it does appear to reach new equilibria, with lower levels in the tank the higher the rpm. Since I cannot hang on for any length of time at much above 4500 rpm (and it is in any case illegal, except in nearby Germany, and there are now many cameras round here), it is not proving to be too much of an inconvenience. One cannot go burning round the countryside at high rpm too much. The number of small villages round here provide the needed respite for the engine.

By the way, the foam filter in the breather line has been removed, so it is a clear path. I know oil goes up the breather line; if I inspect the reed valve, it always contains few millilitres of oil. Plus, when starting the bike after a couple of weeks rest, you can see the oil spitting back into the tank through the breather line until the crankcases are emptied.

At some point, I will get round to modifying the crankcases. The question is, is there any point in moving the Combat breather from the lower rear crankcase up to the timing chest? Why not just modify the scraper casting and oil pickup. If oil does return through the breather, then so be it. It's going where I want it to anyway.

Best regards, Phil.
The general rule is if you move the breather to the TS you have to mill off the scraper, drill 3 holes in the TS, block the front pickup (TS case) and open up the DS case so the original rear pickup is exposed. When I got my Combat in 98 the breather had been moved but nothing else done. Result was at 4000 + rpm the tank emptied in 10-12 mins. Some will say they have no issues with the 'Combat' breather at high speed but experiences seem to vary depending on how loose crank clearances are (more or less oil flow past the bearings). I even read where someone had narrowed the feed gears to reduce flow on a Combat!
The question is, does the breather have to be moved to the timing case?

Or can it be left in the original '72 position but with the oil pickup moved rearwards to catch better the oil that the crank has swept to the rear of the cases?

Thanks, Phil.
I misunderstood the question. I never heard of anyone wanting to do that. I think if you remove the scraper more oil could pool in front of the breather exit but just how much extra is anyone's guess. Personally I don't see the point in going to all that effort without adding the timing side outlet and holes which is done to keep the breather as high as possible and pass mostly air not oil. Is it for originality, keeping the original breather? I guess with the new (pre 72) pickup location there has to be an improvement and if you drilled the 3/8" holes while the cases are apart you would not have to strip the engine again to install/move the breather to the T/S if it didn't work well enough. That could be done with timing cover removal.
Thanks for the advice. I'm just curious to see whether the old breather position would work after moving the oil pickup. It would save a bit of effort machining a flat surface on the back of the timing chest to mount an outlet. But that could be added later if I have already drilled the air holes from the crankcase to the timing chest - good idea.

I'll probably get round to it during the winter layoff. Gets a bit icy round here Jan-Feb. May get the oilways sorted while it is apart as well, to prevent wet sumping during storage. Typical, loads of things to sort. Trouble with buying a one previous owner bike which, whilst in good original condition, has not had anything done to get rid of the '72 design faults.

Best regards, Phil.
Just been out for a ride. Found a quiet place away from villages (it is Sunday) and watched the return to the oil tank as I revved it to 4000 then 4500 then 5000 rpm. Oil stops returning to the tank as the revs go up, then, after a wait of 10-20 sec, all of a sudden, frothy oil comes spurting through the breather. The original service notes quote of oil returning through the '72 breather is spot on. Must take a fair pressure to push that much oil up the breather line.

On the way home, after a bit of fun with brief excursions to higher rpm, the clutch starts slipping.

Get home, remove primary chaincase level plug, discover rather too much oil in it.

Removed sump plug, and about a litre of oil came out. Explains why level in tank is below the dipstick.

Needless to say I'll modify the oil pickup and scraper in the crankcases at the earliest convenient time and move the breather to the timing case. Meanwhile, will keep the engine speed down and enjoy the view.

Best regards, Phil.
Doing the crankcase/breather mods and putting a decent inline breather valve solves a lot of problems. You will find the oil stays inside the engine. Nice, huh?
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