'73 CB500F K2 motor rebuild questions

Hey guys, I joined a while back but haven't posted in a while. I do have a few questions about doing a rebuild of my 500F motor. I started this on Friday night last week after giving some consideration to it. I've been restoring the bike from a mediocre state that I bought it in a year back; new electrical, paint, clutch, lights, wheels and tires, chain, rebuilt carbs, and others. But I had to send the electrical loom back to Matthew Sparck over at SparkMoto to add a few items to it (great guy, very professional). While I had the bike partially disassembled, I realized the frame needed to be refinished or powdercoated, which meant removing the engine and front end.

I'm very mechanically inclined, having restored a golf cart from scratch and my 2002 Ducati M750. The SOHC motor of the 500F is incredibly simple, and I only have a few questions for opinions. Since I have the top end apart, do you recommend replacing the piston rings regardless of the condition? They look fine, but its a stock motor, so there is going to be blowby. I haven't done a compression test, other than pour water into the intakes and see if any leaked out through the valves (nope).

As for removing carbon buildup on the valves and pistons, what is the best way? I'm using a wooden spoon and trying a combination of WD-40, Seafoam, and chlorinated brake cleaner (the former recommended by my neighbor who owns a German auto shop, the latter recommended by a a guy who works at CARQUEST and used to work as a mechanic at a dealership).
Lastly, if I have the top end off, do you recommend I check the crankcase and clean it as well, or leave it? I hadn't planned to, but I am curious about seeing whats in it and cleaning it out. Thanks in advance, I'll post some photos soon.

Personally I have found that unless you are having bottom end problems, the bottom end of these machines is remarkably pristine unless of course someone has been racing or doing something they shouldn't with the bike.

As far as the rings go, for the cost I would replace them. There are ring wear specs in the manuals.

JMO

Ok this is helpful. Definitely hasnt been raced, and the top end wasnt in bad shape, just 40 years of carbon build up on the valves and tops of the pistons. I'll check out ordering a set of rings for the pistons since I'm already in there. Thanks for the reply!

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clean with spray can carb cleaner and toothbrush size steel or brass brush . . finish with brake cleaner.

then spray carb cleaner in the ports one side at a time and see if it weeps out around the valves . . if it does, they are leaking.
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Sure thing. Got the pistons fairly clean last night, and a lot of the caked on hard carbon deposits on the valves removed just by soaking and cleaning with a nylon brush. I'd like to do a compression test though too, but ill go with your recommendation first. It looks like the cam chain tensioner may be tricky to put back in though, yeah?

Oven cleaner can help to break up carbon.

I see that you plan to order rings. In the future, check the ring gaps against the factory specs.

Be sure to check the gap on the new ones, file as neccessary, and properly install them with the correct orientation and gap stagger.

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DohcBikes said:

Oven cleaner can help to break up carbon.

Click to expand...

It also works well on ovens for cleaning up the mess left by overcooking Blueberry Scones.