SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
MrDavo:
I've left the gallery plug out for now, so I'll get plenty of warning before the oil goes upwards, just seeing some in the gallery would be an improvement.
Triggers 20 percent rebuild rate due to no oil pressure on rebuild worries me. I'm going to drop the pump again at the weekend, I want to double check the feed side is primed ok, I may take the stopper valve out while it's submerged in oil, then replace. I'm going into town soon, a tub of Vaseline may be on the shopping list
K2-K6:
I hate that feeling if the pressure does not come up as expected, leaving you trying to decide if to investigate further or still try to get the light out.
Vaseline is not a bad method, I have always worked on V8 Rover engines on which it's a prescribed method of priming the oil pump after assembly. It's odd on that as the pump is a paired gear type which builds no suction when empty so absolutely requires this method. You even have to be careful during oil changes to make sure the oil filter is not taken off with the sump empty as the oil in the pump head will run back down the pickup tube to the sump and require dismantling the pump to get it going again was a Vaseline repack.
Oil pumps generally aren't total displacement devices and don't really start working well until dynamic sealing with oil when spinning gets them to deliver at the expected volume. One way of priming any type of pump is to fill via the outlet then spin it so the vacuum starts to pull the feed through. I appreciate that you can't get to that side of the pump in this case, the nearest I could get to that was as I wrote previously of trying to fill the oil filter first and reassemble to see if any of that oil would make that sort of arrangement.
Further thoughts are to point a fan heater at the oil tank for half an hour to get the oil less resistant to pumping. Put a piece of tube onto the tank breather with a tire valve in it to pressurise the tank by a couple of psi and force the oil to go through the pump as you turn the motor over.
The design criteria of an oil pump requires that you, within reason, turn it continuously to get the following system to prime from empty (it's that oil within the total system that prevents a run back of oil when you stop any motor) so you may need a team of kickstarters to work in relay to get the pressure up, or spin it on the starter motor.
The killer on these is that the oil restrictors to the head are blocked (covered in other posts on here) and prevent one or both sides of the cam being oiled. That won't show on the oil pressure light anyway as the gallery pressure still builds with them blocked. Don't think you've even got oil near there because your gallery is empty.
Just a further thought, the motor running while shredding the original bearings.......all that scrap will have passed through first the scavenge side of the pump as it was recovered from the sump....... and then again through the pressure side of the pump on its way to the filter BEFORE it was caught in the filter material. Can you see any effects of that inside the pump?
Just remembered that pump was giving you a low / flickering oil light during running before you stripped it if I recall correctly, you need to be certain that's not going to catch you out now.
MrDavo:
There was some marking of the oil pump end plates, but it wasn't bad, and the wear on the rotors was low, I don't think it got to run sick for very long at all.
I did some Googling of priming oil pumps with Vaseline, and got several results from US V8 forums, apparently old Buicks and the like can only be primed this way, though there were still posts from good old boys who thought Vaseline was for sissies, and had never used a torque wrench because pappy said so.
Pump off, gloves on, pump apart and I packed the stuff into the inlet until it came out of the port in the pump, same from the engine side after undoing the spring to the stopper valve. Put it back together and packed it between the rotor and the outer, end covers back on, and slightly turning the pump caused Vaseline to rise from either port. In case I got an air lock in the oil line, I got the pump mostly on but not all the way, I put a bit of oil in the tank, once it started coming out all over the oil pump I pushed it up and tightened the bolts.
Sump plate back on, then I put a pint of oil in, some into the gallery (bike was on the side stand), some into the cam box to drain back down. A few kicks and oil started coming back out of the gallery - result! Put the gallery plug on, lots more kicks. Checked the oil light, well and truly out.
I could hear gurgles from the cam box, and saw a weep of oil around the rocker spindles. There was a trace of Vaseline floating on the oil in the tank, which can only have come via the return, so that side works too.
At the start of this thread, the oil light was staying on up to about 1200 rpm, albeit with hot oil. I can now make it go out with 3 kicks, using cold 20/50. Eventually I tried the starter (coz my leg hurt), it struggles to get going, then spins happily, if slowly. I hate to think how it would cope with the plugs in. Trigger will be along soon to tell me off, I didn't do it for long though.
I'm considering changing the oil to 10/40 - I used 'Classic' 20/50 because it was what I had, bought when I had low pressure. Now I'm worrying I may have too much oil pressure, leading to leaks or hard starting ::) It would be kinder to use a thinner oil I'm sure, particularly while it is as tight as a tight thing. It span over much easier when I had no oil circulation, so I reckon its the cold thick oil being a bugger to pump round, not an over tight engine trying to seize! (that reminds me, a line on Westworld last week that made me laugh, when someone asked if the glass was half full or empty - 'We are engineers, the glass was designed to the wrong specification!')
If I do drop the 20/50 you can be sure wont be letting the oil pump run dry. I haven't seen a ton of oil upstairs but I'm guessing I wont, at least until its been turned over a lot. The fact that there is any sign of oil coming from the spindles can only be good, though.
Trigger:
So you did not believe me about the Vaseline and had to question my advise :(
20/50 is OK if you are running the engine in, in the summer months. You still need to see loads of oil at the cam ;)
MrDavo:
Hmm, that didn't go too well. :(
I left the battery on trickle charge overnight, just went out to the garage, 12.5 volts, barely turns the engine over at all with the cold thick 20/50 in it, no spark plugs. That can't be right.
I'll try it with 10/40 when I get some, it span over quite happily when it didn't have oil pressure, but I am worried. One extreme to the other $@#~!
The O rings and carb top seals arrived this morning, trigger, thanks! :)
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