Topic: '78 CB550, where to go from here... opinions wanted!

[pwright]

Greetings,

Just picked up a nice '78 CB550 which, if the mileage is correct, has 24K on the clock. I have a running 350f, but wanted a bike for a cafe racer build. I intend to tear it down to bare frame and work from there.

The bike didn't come with any exhaust, other than the old stock one with a bunch of rust holes. The guy I bought it from had a Kerker on there, but I didn't want to pay extra for it as I didn't really like the looks of it on there. I plan to buy a new (probably MAC) exhaust for the project anyways, so the Kerker wasn't very appealing. I have an old '76 550k exhaust that I could thrown on there also... but it's an aftermarket 4-2 megaphone type. A bit of history on the bike: It hasn't run in a couple years. The PO mentioned that he drove it for a couple years, and started having carb troubles. It was more than he wanted to tackle, so parked it two summers ago.

I did a compression test today, using the kickstarter as I haven't gotten a new battery for it (no headers on it). The compressions are all over the place! First, I did a dry test, and found the #2 and #3 cylinder to be very low... like under 110psi. 1 and 4 were at 140 and 128. Then I did a wet test by syringing some oil in there, and the compressions came up on all but #3, but still differed by a LOT. I'd think that means either #3 is really worn or it has a stuck ring. I tried all the tests at least twice in each case. The original plugs that I pulled were pretty black and carbon coated.

OK, so I wasn't originally going to get this running before I tore it apart. But, I'd like to have a clue as to what kind of engine work I want to do while it's out, so I have been rethinking that strategy. Should I get it running, which will mean spending a lot of time rebuilding the carbs, throwing a hole filled exhaust on there (which will probably affect how it runs), and tinkering with valves, timing, etc.. and it has a peeling liner in the tank which doesn't look good.

OR, should I just pull the engine so I can pull the head and look for stuck rings, bad valve seats, etc... as I'm going to pull the engine anyways? Can I diagnose these problems without having a running motor?

I really need some direction here... as I want to get working on this soon one way or another. Any help/advice/criticism/jokes would be appreciated.

Thanks,
-Pat-

[Paul]

Hey Pat,
I got a completley junked 500 a few years back, Didn't know my arse from my elbow, but decided to start "Cleaning".....
Rocker cover off to polish, Didn't like to look underneath, another bit..off, and before I knew it I had the whole bike stripped to the bone. I was happy enough 'cos this is what I wanted...a project rather than a rider....hadn't even thought of ridin' it
...Just a well, New pistons - required, Rings, Gaskets, Chains etc...
but now rather than just painting the exterior, I now have a 100% motor that runs nice, and I'll probably never have to keep constantly tinkering with it.

IMH...it would be a let down to spend time on your cafe project only to find the engine still needs to be pulled.
On the other hand, maybe just a set of rings / gaskets and you're on your way.
depends on how much you want to take on board.

good luck

[TwoTired]

A bit of history on the bike: It hasn't run in a couple years. The PO mentioned that he drove it for a couple years, and started having carb troubles. It was more than he wanted to tackle, so parked it two summers ago.

I did a compression test today, using the kickstarter as I haven't gotten a new battery for it (no headers on it). The compressions are all over the place! First, I did a dry test, and found the #2 and #3 cylinder to be very low... like under 110psi. 1 and 4 were at 140 and 128. Then I did a wet test by syringing some oil in there, and the compressions came up on all but #3, but still differed by a LOT. I'd think that means either #3 is really worn or it has a stuck ring. I tried all the tests at least twice in each case. The original plugs that I pulled were pretty black and carbon coated.

OK, so I wasn't originally going to get this running before I tore it apart. But, I'd like to have a clue as to what kind of engine work I want to do while it's out, so I have been rethinking that strategy. Should I get it running, which will mean spending a lot of time rebuilding the carbs, throwing a hole filled exhaust on there (which will probably affect how it runs), and tinkering with valves, timing, etc.. and it has a peeling liner in the tank which doesn't look good.

A compression test on a cold engine that hasn't run in a couple years and wasn't properly stored is unlikely to be stellar.  Cylinder walls could have some rust, rings could stick on the pistons lands, valve mating surfaces could have rust/crud on them, guides sticky, etc.  I suspect you didn't check the valve lash either.

The fact that the added oil didn't help seal up the piston on Number three points AWAY from a ring issue on that cylinder.

If money is no object and you don't mind spending $500 or more on the engine up front, then yes, rebuild the top end now.

 The failed tank liner probably has a lot to do with the carbs getting plugged up.  Regardless of what happens to the engine, this has to be addressed before even a completely rebuilt motor is fired up.  Were it me, I'd get those parts working and see how the motor runs.  Do put an exhaust on it, if not the new MAC then the holy ones.  If you could put a few miles on the engine with clean oil so much the better.  Then do another compression check sequence and compare numbers.  Most likely things will have improved quite a bit.  If still not to your liking, well then the top end rebuild was in the cards all along.  Either way, a running engine has an incentive effect that's undeniable.

By the way you can do a complete top end job on the motor with it sitting in the frame.  The lower ends on these are so robust that 24K miles is hardly any worry.

One more point is that the engine probably ran sour before it was parked and ignored.  This can effect how the rings seat in the lands and build up crud in the cylinder workings.  Having all cylinders working at combustion temps and pressures, can put things back in more even working order across all cylinders.

Best of luck!

[pwright]

Thanks Paul and TwoTired, I was hoping you'd chime in! Those are all good thing to think about, and I think I will follow TT's advice and see if I can get it running this weekend. However, I should have been more clear about my apprehensions... if indeed there is some rust/crud buildup, or other issues from being stagnant for a couple years... could running the bike do any further harm before these issues are addressed? I assume from your reply that it's not a big concern and most likely it'll somewhat help to resolve itself...

I wouldn't be opposed to getting some cylinder work done as you did Paul, and if need be I'll do so. I guess it's probably a good idea to make sure it even runs before I start paying people to bore/hone etc... but I hope to have a niiiiiice cafe racer when I'm done..

Thanks much,
-Pat-

[TwoTired]

There is no doubt that rust is abrasive.  Surface rust will be scraped off and chucked out the exhaust port in short order.  Possibly the rings, if they aren't already stuck in the piston grooves could get moreso with a bit of rust packed into them.   Running the engine will either free them, or not.  If not, top job.  If the rust had gotten bad enough that cylinder wall pitting has occured, again a top job.  Most of the issues would be increased wear on cylinder walls, piston, and rings, that all get replaced or bored if a top job becomes required.
A burned intake valve will likely show up during carb sync.  A burned exhaust valve will never recover full compression.  Addressed when the head is off during a top job.

What else?
I suppose, blowby could push some of that junk into the crankcase where the oil pump would then encounter the abrasives for one pass.  But, the oil filter should keep it from getting to any lower end journals or recirculating through the engine.  I would expect no great issue there.

You know there may not be rust inside at all, I'm just speculating.  A dry climate or storage in low humidity may have protected the cylinders where the valves were open for 2 years.  If you could borrow a borescope, you could look inside the cylinder to observe conditions.

Anyway, after you have 50 miles on it or so, tail pipe blue smoke ought to subside, if it was present on the intial running.  If it doesn't, do your compression test again, if it shows rings seal gone or a leaking valve.  You'll know what needs to happen.  I think risk to the lower end is minimal, though.  Perhaps the experts know better.
As much as I hate trashing an engine, I have seen whole engines sold for $100.  So, if the worst DOES happen, recovery is less costly than a top job up front.

One last thought before you run it.  Put a teaspon of Marvel Mystery Oil into each bore the night before.  It been known to loosen stuck rings.  And, it won't hurt the ones already free.  It'll be smoky when it starts up, though.

Cheers,

Cheers,