Very low compression, what to look for?

[deafanddiabetic]

Well, after tuning my 66' CA72 (points, plugs, valves, float height), while it ran (roughly) I found that the left exhaust was barely putting anything out. I should've checked compression first. Albeit it's a cheap Harbor Freight tool, but I got 100psi cold in my right cylinder and 30psi in my left. There was lots of smoking from the breather tubes and it sounded awful while running.

I set all the valves to .1mm (.004in), double and triple checked them but it didn't effect the compression at all.

I spent the night pulling the engine out of the frame and setting it on the table.

I have a rough understanding of what to do next, but not really what to look for. From what I understand, worn piston rings usually lead to oil out of the breather tubes? Valves could not be seating correctly, but I don't know what to "look" for to know that. I have a few manuals for the bike to help, but to be honest I'm kind of scared to pull her open and definitely don't know enough to know how to fix it.

Any thoughts?

[G-Man]

Hi

It's a simple thing really. If you are happy that you have got the correct valve clearances then you either have worn or broken piston rings on the left side or you have a damaged valve / seat.

The smoke from the breather suggests piston / ring damage.

You now have two choices.

a) Find someone else to fix it but be prepared to pay for parts and time
b) get the manuals and learn enough from the books and generous people here to gain enough knowledge to fix it yourself.

Neither way is a guarantee of success because you could wither run out of money or skill / patience.

There's nothing to be scared of in dismantling the bike. Its broken now and won't get any less broken. You may as well proceed at a pace you are comfortable with and see what knowledge you can build up. Th feeling of satisfaction if you complete the repair is truly immense and the knowledge you gain will never be lost.

First step on the way is to get the books then start dismantling at a pace that you can record what you've done so that you can put it all back together again.

Feel free to ask plenty of questions but try and figure things out as you go. The knowledge that you get by doing things is so much more 'sticky ' than the stuff you get from asking...

Remember - experience is the thing you get just after you needed it most...... :-)

Here's the inside of my 1961 CB72 - pretty much the same as yours - nothing to it.....

Engine stripped.jpg by graham.curtis, on Flickr



G

[deafanddiabetic]

I appreciate it, G-Man. Poking around this forum the past few weeks I've noticed you're the forum-father.

That picture is awesome, very helpful. I know mechanically it isn't terribly complicated, I guess my anxiety comes from not knowing how to diagnose if a valve seat or valve itself is bad besides common sense (cracking, bending, etc). I ordered some piston rings ($15, not a bad deal) and that should help.

Thankfully, I've got enough resources to try to do this myself. I've been apprenticing at a shop for the past year, but it's mostly oil/tire changes and tune ups which I've gotten pretty good at. This will really carve me out as a mechanic, I'm hoping.

[G-Man]

D&D

You're too flattering. I am far from being the "father" of anything, especially here where there are many more knowledgeable people than me about these bikes. I have had the good fortune to be trained as a Mechanical Engineer which helps when you are trying to figure out why things were done in a particular way. Engineering is based on Science and Science is about finding the truth. Facts don't usually lie....

I like these bikes but I do not hold Honda in any special place other than being able to strike that important balance between quality cost and reliability. The 'perfect' balance is not fixed - it changes with time, knowledge and fashion.

I'm curious about the piston rings. Do you know what size you need before stripping the engine. If the bike has never been apart you might get lucky with standard size rings but after 50+ years the cylinders may have been bored out to take oversize pistons and their matching rings.

Once you have it apart, you will know but before you've seen inside it's guesswork.

As for how to tell what's wrong. You have a half-decent cylinder on the right and a poor one on the left. What you have to do is look at the valves, pistons, rings and bores on one side and compare them with the other side. Make notes of which part came from where and you will build up a picture of what's not right.

Take pictures, post them here and we'll help you. You'll get plenty of opinions and it's your choice which ones best fit what you see. You'll become the motorbike equivalent of Columbo in no time.

Above all, have fun!

G

[Hoosier Tom]

I got 100psi cold in my right cylinder and 30psi in my left. There was lots of smoking from the breather tubes and it sounded awful while running.

You noted on your first post you just got the bike and it was running last year then put away. Do you know the history of the bike or were you sold a bill of goods?

HT

[deafanddiabetic]

There's a chance of that, Tom. It has plates and the title are both from summer of 2013. It's got brand new tires, and a bunch of other things that are indicitave of having been used not that long ago. The guy I bought it from has had it in a shed since then, he bought it from the guy who had it running with plans of restoring but bought a Harley bagger instead (surprise). It had no major red flags, otherwise I wouldnt've bought it. Although I got it for what I consider a fair price despite it's shortcomings.

[Hoosier Tom]

If you're fine with it that's all that matters. Rebuilds on these motors are not difficult. Lots of help here on the forum if you run into issues.

HT