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1980 CB750F Oil light On/High and Erratic Idle

ThiccCake

CB750 Member
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Hey Guys looking at for some insight on some issues that I am having with the bike I just bought.

So I bought the bike running took it for a spin upon purchasing it runs great the idle does sit high at 2,000 RPM when warm. The bike does not idle when cold, the bike starts right up though but I have to hold the throttle open till is warm then it idle just fine the idle being a little high. Knowing that I was gonna be putting a some work into the bike I started doing some general Diagnosis to kinda see where she stands. The The previous owner said that they cleaned the carbs and that they just needed to be synced. Taking his word for what it is I was poised to pull off the carbs and inspect them. They seemed fine but by all means I am not a carburetor expert. Playing around with the pilot screw I re adjusted them to the factory spec as they had seemed to be leaned out all the way. Anyways with these adjustments made and a simple bench synced on the carbs the Idle seemed to have come down to about 1500RPM with the tachometer bouncing up and down around this reading alas I thought we were making progress. So I hope on the bike and rode it around and to find that the bike seem to be pulling all and well at the High RPM range with some minimal sputtering at the lower ranges. Now giving it some time and riding around some parking lots near my house I have found that when the bike is in gear and I pull in the clutch the RPM shoot up to 3000RPM with the bike lurching forward at low speeds while in gear. I don't think to much of it take it home and park it for the night. The next day my buddy want to go out for a ride so and see the new bike so i think nothing of it or running somewhat well I ride it. So I go and i start her up hold the throttle for a few minutes to let her warm up so she can idle not letting her idle for more than 3-5 to minutes after getting her warm enough to idle I drive down to my friends house and turn her off. He gets his bike out of the garage gets it started and start to roll out. I sit on my bike turn her on start backing her up to the road and when I take off and look down the oil light is on. Thinking it might turn off I continue with my ride no more that a half mile we drive down to the factory's near our houses. We get to the factories and stop for a few minutes and the rpm on the bike fire up to 4,000 RPM concerned that the engine is about to explode I tell my friend whats happening and we drive back to my house no more than a mile back.

So to make some bullet points of what Ive done to the bike and what I see from riding it.

  • When Warm idle sits High Around 3k-4kRPM
  • Engine Does not Idle When Cold
  • Engine Sound to be Healthy
  • All plugs make good spark and seem to be in like new condition
  • Timing Seems like it may be off but this could be attested to the carbs calibration
  • Oil is fresh and Clean and at proper level on center-stand 10w-40
  • Oil Light on
  • Good Spark and clean plugs
Things I am yet to do
  • Compression test
  • Actually check timing (Cover bolt aren't stripped but I did break a screwdriver trying to get them off)
  • Change filter
  • Properly clean and sync carbs
  • Check valve clearance
  • chenck for holes in carb rubbers
 
I got rear ended at a traffic light one time on my '79. Of course, the taillight was broken. When I restarted the bike, the oil light was on. While reading the manual I found that (in '79 at least) this was an indication of a burnt-out brake light bulb.
 
I got rear ended at a traffic light one time on my '79. Of course, the taillight was broken. When I restarted the bike, the oil light was on. While reading the manual I found that (in '79 at least) this was an indication of a burnt-out brake light bulb.
Interesting! The service manual I have says nothing about troubleshooting oil pressure light besides the obvious of no oil pressure.
 
As you can imagine, it kinda freaked me out at the time. It was the first day after I bought it that I rode it to work. As I recal(been 28 years) it was in the owners manual.
I just bought another 79 that comes with a manual, but I don't take delivery until Dec. If you haven't figured it out by then, I'll check the book.
 
If oil light continues to stay on after all bullet points are checked off. Drop the pan and see what the pickup looks like.
 
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