This blog is chronological description of my 78 CB750 chopper project which I began in April 05. SOHC (Single Over Head Cam) CB750 motors were produced by Honda from 1969-1978.

Monday, May 23, 2005

Day 53 - Its Alive!!!

Very good weekend, with slightly mixed results. First things first, I got the motor running and put about 50 miles on it this weekend. Got the motor in on Wednesday, got everything hooked up Thursday/Friday, and Friday night, she came to life. Was farely easy to get the motor running. After everything was hooked up and ready to go I was not getting any spark at the plugs. I think this was due to miswiring of the points. I swapped out the points from the F model engine and she fired right up. Couple of concerns/issues:

#1 - Compression is horrible. Its no secret why though. If you remember I put an F model head on a K motor. The F model has domed pistons, and a larger compression chamber in the head to compensate for the domed pistons. The K model has regular/flat pistons, combine this with the F head which makes for more space in the compression chamber. In the end what this means is lower compression, about half what it should be. I did a compression check yesterday and got about 80psi, it should be 150+ on a freshly rebuit topend.

#2 - Leaky valve cover. Not sure about this, but I think this also has to do with the F model head. I cranked the valve cover down, even pounded on it with the empact wrench, and it still leaks. I think there are slight differences in the valve covers that causing them to leak.

So whats the verdict? All in all I'd give the motor 1 thump up and one thumb down. I think the motor is solid, and I could go put 20,000 miles on it, but the lose of power is significant enough for me to conclude the F head is simply not a good idea. I'll find a K head and install that in place of the F head.

All is not lost though. This was my first rebuild and an excellent experience. Other than a few problems I didn't run into anything I can't handle. From here as I said I'll replace the head on the donor motor. I'm also going to put new rings/gaskets in the F motor. Right now my time frame for purchasing my chopper frame is after the July 4th holiday...then the real fun begins!!!
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