Topic: '76 CB750 F compression issues

[blondepirate]

So I recently just got a new to me non-running 1976 CB750 F.  The bike is complete, stock but is mildly apart (harness is still on the bike but uncut, oil tank empty and brakes empty.  Engine will turn over by the kickstarter but when I attempted a compression test, #1 topped out at 30 psi and fell, #4 maxed at 90 psi and held. This isn't my first vintage bike but it is the first one I got that I have convertible skills and a will to dig into.  (Have experience with top end rebuilds on various marine diesels.)

For now, this bike is just to be a running daily driver. So reliability and runability are important.  Having said that, I can probably figure out how to adjust the values and figure out if it's the rings or valves to blame for low compression.  What my question is... is at what  point should I consider rebuilding the top end?  More specifically, I have a local pro-wrench who's willing to trade me a 77F motor with a good top end for some extra good used K pipes that came loose with the bike.  I've been told that the pipes I have are probably worth something despite not being perfect.  He also has other things to trade but a healthy motor is all this bike really needs (if at all) as well as maybe a new chain.  The only real issue with the motor he has is that the block is cracked where one of the long bolts mount the engine to the bike.  He says I can still strip my top end and bolt his on and I should be out of the words.  While I like the concept of a trade, I'm a little leery of doing a top end rebuild with used parts even if they all check out.

Any thoughts on this deal?  Anybody local to LA who has a good running F motor that might need some pipes or some other extra stuff I have.  Thanks in advance everybody. 

 The bike came with an extra set of OEM K pipes which I might sell or trade.

[ekpent]

Well look at it this way. If you do the 'trade' then  you will be tearing the top end of your engine apart anyway and the other. For one I think there may be a parts compatibilty issue if the 77 swapper is an F going to your 76,they are different. Think it would be best to see what is wrong with what you have FIRST and repair that. Could be something fairly easily rectified istead of building a frankenengine out of 2. So see whats wrong with yours first,adjust the valves ,make sure your doing the compression test correctly etc then remove the engine if needed and delve in.   Sell me your pipes for gasket cash 

[RpBet]

If those pipes are original 4 into 4 and are in excellent shape i.e clean and not rusted through the bottom of the tip where moisture collects, I have been offered $800 -1000 in canada

[RpBet]

Remove the plugs and pore oil in the cylinder. Crank it over and re-check compression. If it goes up then you know its the rings....................then decide. Rebuilding the top end is not that hard.