Rebuild

[shootrr]

I bought a 1977 CB750K a month ago, had the same bike 45 years ago so I'm stoked to have it. I've been going through it as rubber anything hardens and cracks. I also did a valve clearance test .002 intake, .003 exhaust, and a compression test 92-120 PSI across the 4. I have 2 questions and I've read everything I could on this forum in regards to low compression.
1.It has been said that the stock shim thickness from Honda is too tight? some people were talking .005-.007?
2. Compression being low and so uneven across the 4 I'm thinking that I need to pull the motor and do at least the top end? Where is a reputable place to purchase what is needed for a top end rebuild? A kit or something?
3. I am not aware of any other checks I can do other than clearance and compression, so any education is extremely welcomed.

 

[dirtdigger]

There are no shims in the sohc motor. They use an adjuster screw and jam nut to adjust clearance. Stay with stock clearance if it has a stock cam. Did you have the throttle wide open when doing the compression test? If so put like half a teaspoon of engine oil in each cylinder crank it over for a bit then rerun compression test and see if it goes up. If it goes up then it has ring sealing issues if it doesnt go up then it has some valves leaking. Have cyclex rebuild the head and bore the cylinders if you end up going that far. More than likely the cam chain tensioner wheels are shot, cam chain probably has some stretch and the primary chains are for sure stretched but then you need to split the case. Honestly if you are not very mechanically inclined I would send the engine off to have done as there are a couple things you can do that can really mess up the sohc motor if you dont know what your doing. Cyclex is the go too for machine work and engine parts. They do engine rebuilds too.

 

[UK K6]

Has it been standing? If so, it can sort itself out with a few runs on the road.

If it were me (and it was, 3 years ago!), I'd get a workshop manual. The Honda one is the best in my opinion and can be downloaded free of charge, just Google it. Then do the safety checks (tyres, brakes, chain, lights work etc., clean the carbs and make sure the air filter lets them breathe, give it a tune (points gap, static timing and valve gaps). A basic service, all nicely covered in the manual. Check the oil level (and colour), charge the battery and give it a go.

If you ever watch YouTube, the guy on the Brickhouse Builds channel bought a seized '75 750K. He un-seized it and ran it, initially with low compression. Several years and a few oil changes later, it's still going, stronger than ever. These things just want to run!

 

[grewth]

What he said.
Low/uneven compression could be down to worn bores, pistons, rings, valve seats etc.
Or it could just be the rings gummed up in their grooves, or a little rust on the valve faces and seats.
Set the valve clearance and then ride it for a few hundred miles, see what that does for the compression