• Enter the June CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Physical Exam just became an autopsy!


CB750 Member
Reaction score
Carrollton, OH
I have been examining the motor on a CB750K (1981) with an eye to getting it to run. It's a bike that was left in a storage locker with a license plate that expired in 2001. I found the electrics had a dead short and other apparent issues but today I did a "thumb" test and found no compression on cylinder #1. So I pulled the oil pan and filter to prep for tossing it on the bench and found this on the oil pickup screen. Maybe the location of the "toss" has changed...


  • IMG_3316.jpg
    461.9 KB · Views: 74
You will not know for sure until you crack open the engine. I would take the head off first and have a look at what you have. If the cam was in fact out by 180 degrees then you could easily have a broken piston/bent valve, Having the aluminum in the oil filter is not necessarily the end of the world but it is not great either. If the Valves are all good and the pistons are intact, then flip the engine over and take off the bottom cover. This will expose the transmission and the lower end of the connecting rods. If there are no glaring areas that are marked up, then you can pull the main bearing caps and check for wear and the clearances, followed by checking each connecting rod for wear and clearances. Something along this path will tell you what has caused the aluminum particles in the oil filter. With any luck somebody just put something together in the bottom end and missed a thrust washer and you got some engine casing wear. If that's all then rebuild the engine and you are good to go. Have fun with the project. Oh and make sure you have a good shop manual to refer to.
This motor is from an abandoned bike left in a storage locker. It had a license plate that expired in 2001. I never had unrealistic hope, but thought it would be cool to get it running (so long as it didn't suck money from my other toys). I pulled the cam cover and found the particles were from the cam chain guide. Apparently it was running against the sides and wore into the metal. My curiosity drew me to pull the head and I found an even more curious condition. The pistons number 2,3,&4 had a black carbon crud on top BUT piston number 1 looked as though it was new. It was apparent that the motor had not been apart before (the odo showed about 30k) so I have assumed that the bike was run on three cylinders for some time and by not firing that cylinder the cleaning action of the fuel mix worked its detergent magic.
A clean piston is an indication that the cylinder has not been firing. Could be all kinds of things, like carb not working, coil not working, bad high tension lines, even a short in the high tension line. First thing I would look at is the spark and make sure it has one. after that I would be looking at fuel.. Remeber start simple and don't get to complicated, usually it is something simple that is the problem.