Solving the oil line problem?

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Solving the oil line problem?

Post by The1970s »


I have been a long time lurker of the forum as I slowly piece my 75 GT750 back together but I've finally registered and thought I would share some ideas and welcome feedback for a plan I've come up with. It seems the most common problem for a stock GT is the oil pump, lines, and check valves. My originals had a few cracks so I thought i'd tackle the repair while I have the top end at the machine shop. I found a good looking used set on Ebay but found they where also cracked when they showed up. So i've since set out to solve the problem once and for all.

Digging into memory from my days as an engineering student I put together a CAD file for the oil spider/manifold that sits below the pump, and have located some appropriately sized fittings that will allow me to ditch the fragile factory nylon pipes and fit polyurethane tube via a compression fitting. My plan is to have this machined out of aluminum which will last much longer and allow oil line replacement as they get older. The only thing that has me stumped here are the check valves. I have seen one or two other designs that incorporate the check valves into this base ring but this raises some questions for me. If the check valves are moved to the pump end of the line, would this allow air bubbles to be introduced from the pumping action in the crankcase? If so, how is this not the case on the left-most line on the factory assembly, which has the in-line check valve? My thoughts are that if the crankcase pressure does not effect the oil flow in the lines, what is stopping me from getting rid of them all together and instead installing a shutoff valve on the bottom of the oil tank? I'm sure this could all be figured out with some testing, but seeing as my pistons and cylinders are on the other side of town I thought I would instead pose the questions here.

As a side note I have also frequented the Suzuki 2 strokes forum and attempted to register with them, but have been unable to get an activation email. Anyone know the reason?
1975 GT750, 1978 GS750, 1976 TC185, 1982 XJ650 Turbo
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Re: Solving the oil line problem?

Post by PhilGT750 »

I believe the Kawasaki non-return check valve is still available, at least I hope so.
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Re: Solving the oil line problem?

Post by barney01 »

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Re: Solving the oil line problem?

Post by xr738 »

We had long discussions on the oil lines on a german forum.

In my opinion the check valves work in two ways, primarily to prevent air of getting into the line and secondly to prevent oil from leaking through the pump into the crankcase.
Having an larger amount of oil (10cc and more) in the crankcase is a sure indication of a malfunction. Normally weak springs or something preventing it from closing properly like gummy oil residues or the like.
To install a shut off valve at the oil tank only does not work. As the pumping frequncy that goes with precompression in the crankcase is roughly 3x higher than the one of the oilpump oil is prevented from getting out of the line in a sufficent amount. Having the check valves further up in the line doesn't make a difference as the oil which has passed by the valve is when pushed back being held in the line before the valve. It doesn't matter whether the valve is closed by air pressure or by oil.So it doesn't really matter where in the line the valve is situated as long as there is one. Look how far the oil still has to travel through the motor.

Two years ago I had a nasty seizure on the RH cylinder while cruising onthe autobahn. So far I had most of the failures a kettle can produce experienced but never a real piston seizure. Holed pistons, broken piston skirts you named it but I never spent any thought on having a seizure unless running out of oil. We had a discussion among the highly experienced afficionados on our annual rallye and came to the conclusion after having discarded all other possibilities that the only culprits left are the checkvalves.

I wouldn't dismiss the nylons ones on the grounds of its material. If a look at the envirement in which they do there duty and for how long that material wasn't such bad choice in the first place because they have exceeded their original life expectancy by far.
But having said this creating new ones or at least repair kits would be something really worthwhile.
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Re: Solving the oil line problem?

Post by oldjapanesebikes »

The1970s wrote:... So i've since set out to solve the problem once and for all.
I was curious whether you had made any further progress on this ? I personally just change out the check valves, replacing them with the Kawasaki ones (see this link on my web site) but I recognise there will come a time when a replacement of the lines will be required. What did you find out ?
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