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What's goin on folks? Lookin for a little help if possible!
I finally got all of my wiring done on my 75 CB750f. Got her wetted down and started cranking. It actually started up very easily and sounds great, which was a relief since I had never heard the engine run until now. Anyways, it took a few seconds, but the oil pressure did build and show on my manual gauge that I put on the right side (60lbs.). Here's my issue: After about 15-20 seconds at 60lbs, it will drop down to 0. then pick right back up to 60 again. Just an FYI, the motor has been dry for the 2 years that I have had it (never upside down) ,but who knows how long it sat or how it sat before I got it. And I'm not going to lie, this is my first 750, so I had no idea about priming the oil pump until I ran into this issue and started reading. Therefor, I didnt prime it. Could an air bubble be causing the fluctuation? I've only let the bike idle for a total of maybe 5 minutes. Sounds great and idles smooth. Thanks in advance.
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Administrator
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Welcome, and congratulations on getting your bike running.
If the bike's been sitting for a while, the air bubble diagnosis is possible. The SOHC units use a dry sump system, which runs a bit differently than the DOHC's wet sump system. Priming would have helped, but in this case I think you're OK. IF it were low on oil and oil pressure for a long while, I would expect lots of dry knocking rattling noises in the engine, which could be catastrophic. If yours was running relatively quietly for 5 minutes, and the pressure stabilized at about 60 psi, you're good.
Other SOHC riders, feel free to chime in. My experience with the SOHC motor pales in comparison.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Thats the problem, It doesn't stabilize. It jumps from 60 to 0, then back to 60, then back to 0 over and over. Again, I only let it run for about 5 minutes TOTAL (probably more like 3-4 min.) so I don't know if it might work itself out or not. I just didnt want to take a chance.
Also, Thanks for the congrats!
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You stated, "...manual gauge that I put on the right side..." Do you mean that the oil pressure gauge is a mechanical, Bourdain Tube type, as opposed to an electrical gauge, with a powered sensor? If it were an electrical gauge, I would suspect a bad connection between the sensor, and the gauge, or else a poor connection between the gauge and the battery.
Momentary drops in oil pressure, likely will not damage the engine, as long as the pressure rises immediately (oil leaves a film on surfaces, by design, to provide lubrication on start-up, when there is no oil pressure, preventing excessive wear). The other potential cause, would be a missing tooth on the gears that drive the oil pump: when it hits the flat spot (missing tooth), pressure will momentarily drop, as the pump will briefly stop turning. Does it happen in a rhythmic pattern, repetitively (assuming constant engine velocity -- idle)?
I would suggest pulling the oil pump, and test it as per the FSM: check the gears for wear, test the gaps in the rotor with a feeler gauge, check the O-rings, etc. Better to be sure, than to risk turning that running engine into scrap metal. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
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Naw, Its just a screw-in, round, oil filled dial type gauge.
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Administrator
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It probably has gummed up from sitting, so long. Try running some "Sea Foam" through it. It should break up any deposits, and flush them out.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Will do. ANY idea how long I should wait before changing the oil out after I run it in the bike? Week, Month? Thanks
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Administrator
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Good point: maybe something is blocking the oil pump pickup? I second the recommendation to pull the oil pump and see. Not sure: did you change the oil yet? Maybe it's gotten thick/chunky over time. That would block oil flow as well.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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After adding the SeaFoam, run it on idle, centerstand, for five minutes (no fan necessary -- the engine will reach around 180 F, which is when the oil will begin suspending particles in it, for the filter to catch them), shifting through all gears, letting the transmission spin the wheel freely (at idle, only!). This will help flush out the transmission, as well as the rest of the engine. It will likely turn your oil black as coal -- this means it is dissolving all that sludge, removing it from the engine and the transmission. Tools is correct: start with the simplest, and work your way up to the most difficult, only if you have to.
 After that, the owner's manual probably specifies oil changes every 1,800 - 3,000 miles. You can follow that, or, if you use synthetic oils, you can go up to 7,000 miles safely. Always change out the filter with the oil, though. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
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Ya, brand new oil in it, but I'm sure that helped break free any sludge or chunks that were lying in the bottom of the pan or whatever as TOOLS said. Im gonna put the seafoam in and let it do some work, then clean the pump regardless. Put MORE FRESH oil in it, change filter and be done, hopefully.
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You guys rock!!! I'll do this tonight. Honestly, THANKS!!
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Administrator
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When I moved my mechanical gauge saw the same thing.I had it on the side of the motor,like yours,then I moved it up to the handle bars. I have a 4 foot long piece of AN line connecting it.It acted just like you said and eventually settled in. I think you could be right that it was air trapped in there. The way the pressure relief setup works on these bikes makes for a fairly sporadic needle anyway.
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Ok guys. I know its been a few days, but the wifes been out of town and I havent had a chance to mess with it. Here's where I'm at. I bought some seafoam and put it in. 1 1/2oz. per quart. Ran it for a bit and it seems like the oil gauge is less erratic, but still not very comforting. Now, it will read 0 at idle, but it definitely responds and picks up when I throttle it. If I barely tap the throttle it will climb to 20-30 lbs. The more I throttle it, the more it climbs, which is good, but at idle, it falls to 0. Kinda wondering if I got some sludge or something blocking my gauge in that galley?
Other worry that I have is with the oil level. I looked in the tank before I started it and the oil is very low (like an inch off of the bottom of the tank). As soon as I start the bike, it climbs and climbs and climbs. It will eventually start leaking out of the vent tube which I have going to the ground (just for now while i'm working out all of these bugs). I must be getting oil since my gauge is reading as stated above. I did have to size my lines down a bit (3/8" id) for the aftermarket oil tank so I'm not sure if thats causing issues or not. Ok, I'm done rambling. Any thoughts would be great. Thanks in advance!!
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