Technical Q and A for classic motorcycle maintenance and repair.
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1977 Triumph T140 Bonneville Oil Pressure Problem
Q: I had the engine on my 1977 Triumph T140 Bonneville rebuilt, and it has about 300 miles on it since the rebuild. Oil pressure at cold start up is 40psi, and when the engine is warm at idle 20psi and at 3,000rpm up to 40psi. Is this the normal operating range? I also have a belt primary system that seals the primary from the crankcase. The crankcase breather has been moved to the camshaft inspection hole. The crankcase breather has a one-way valve. There is enough oil coming out of that hose to require an oil catch can. Is there a better way to vent the crankcase in this situation? Is the oil pump working properly scavenging oil and pumping oil? - Peter Perry/via email
A: Your idling pressure is OK, but your running pressure may be a little low. The latest manual I have is for the 650s, but it should be the same for the 750s. It calls for 20 to 25psi at idle and 65 to 80psi running. I’m not sure what you’re referring to as the camshaft inspection hole: Do you mean the timing hole on the back of the engine behind the right cylinder? If that’s what you’re talking about, there’s no wonder you’re getting a lot of oil out of the breather. That hole runs right into the flywheel assembly and it will fling a lot of oil. About the best you can do to minimize it would be to run your breather tube as close to straight up from there as far as you can before turning toward the rear of the Triumph T140 Bonneville. You may want to find a better place to connect your breather. By design the flywheel flings a lot of oil. Since your breather is there, too, it’s getting over-oiled. I have seen breathers relocated to the valve inspection covers on the head. It’s easy enough to get a spare inspection cover and drill it for a pipe connector. It’s not as efficient as venting the crankcase since the crankcase is where all the air movement occurs, but if you have no other option you may have to use it. It’s not as easy to get a vertical run from the cover, but whatever you can get will help reduce the oil coming out into the catch can. A one-way valve is usually a good idea to help reduce pumping losses by creating a partial vacuum in the crankcase. In this case it may be acting as a check valve allowing oil to be pushed past the valve but not allowing it to drain back into the case. Normally you’d put the valve as close to the case as possible, but you should experiment with putting it farther along, after the hose turns from vertical to make its way to the rear of the bike. In any case, for an engine breather you should try to get as vertical a run from the engine as possible before turning the breather back to the catch can. That will allow some of the oil to drain back into the crankcase. MC