'74 CB360 Possible Piston Ring Replacement

Hey all,
I have a CB360 which I had just gotten running really well (did timing, cam chain tension, valve adjustment, inspected and replaced carb. diaphragm, rebuilt clutch etc.) and just bought insurance for. I rode it early last week and it ran great, I went to ride it 2 days later and it started and ran on one cylinder. I checked the plugs and they were fouled and have since done a compression test. I found that the left cylinder is compressing slowly and is getting up to between 15 or 20psi less than the right cylinder. Also, when it does start the left cylinder barely heats up and smells of raw fuel. I fear that I may need to tear into the engine and replace the piston rings. Is there anything I can check to decide whether or not this is necessary, or should I just resign myself to the fact that the motor needs to be pulled. Furthermore are there any specialty tools besides a piston ring compressor that will make this job easier. This will be my first time really tearing into a small bike motor and while I have a fairly good understanding what needs to be done and will have help if necessary, any advice and suggestions would be greatly appreciated!!!! My hope is that I can do what needs to be done and salvage at least some of the quickly shortening Rhode Island riding season.
Thanks

Actually, piston ring compressor isn't necessary. I would recommend a torque wrench, a valve spring compressor (if you plan on checking your valves), and something for removing gasket material... I can't really think of any other specialty tools besides maybe a rubber mallet and some brewskis.

Digital calipers

Awesome so sounds like all I need is the valve spring compressor and a twelve pack. How do you compress the piston rings without a compressor, I know some people use sheet steel and hose clamps is this what you would suggest?

Since reading the low compression, have you checked the valves again? Maybe a valve tightened up.

Just use your fingers. Not much tension on these rings.

I haven't rechecked the valves yet but i plan to double check the valves and recheck the compression, hopefully tonight. Furthermore, the bike has very low miles, around 3000, is it essential that I have a machinist check my cylinders when replacing piston rings? Am I running the risk of low compression after replacement if i don't? I am a college student and don't have a ton of money to sink into the bike on top of what I already have but I figure it will be worth it to do it right the first time!

I'm no expert but I've heard you can determine bad rings or valves not sealing by squirting in about a teaspoon or so of oil down the spark plug hole so it coats the top of the piston and the sides. Run the compression test again and if it doesn't increase it's not the rings and it's most likely the valves. If the compression does increase then that means the oil is helping the rings seal so they are the problem.

Knowing that you might be able to just lap the valves and not pull the cylinders. But once you're in there you might decide to go deeper just because you're already there and it's not really hard to change what you're talking about. Just be thorough and careful and slow. Get a good gasket set and a good German lager.


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1978 Honda CB550K

did you remove carbs to check compression?
sounds like the carbs need proper tuning and a valve adjustment to me.
also, if the new diaphragms you installed are jbm, in my own personal experience, thats probably got you screwed from the start.

On top of what he ^ said. Make sure you're checking compression with throttle wide open. Also, what are your compression readings? Make sure all of your electrical is on point, especially the charge. Low charge can cause them to run on one cylinder. Are your spark plugs wet? Could have more to do with spark than compression.

Can charge on the battery really have that big of an effect on the performance?! I had heard that but I figured if my compression was low there was no reason to suspect it was electrical.

Well I'll be damned battery was a low charged it up fired up and ran like a peach!!! I can't believe it was something so simple thank god i tried that before committing to pulling the bike apart! Thanks guys!!!!! still a little confused about the low compression though

Always check the things that are easy to check first. It'll keep ya from feeling like a dummy later

Compression readings around 90~120 psi are usually because throttle wasn't open or choke was on (or both)
As Trek said, it sounds like you need carb sync. You'll need vacuum adapters, I make them for 360's

Took the bike for a long ride today and it ran great. I tested the compression following the correct procedure, throttle wide open choke off. After riding for a while today I did notice a little leakage on the head. I think my head gasket might have a bit of a leak, which could lead to slightly low compression? I'm just glad the old girl is running and rideable, the Rhode Island riding season is quickly coming to an end but at least I have a few cold weeks of riding left!!!

with the cv carbs you may need to manually lift the slides to get an accurate comp test reading,if it is running well and not smoking, enjoy it wjhile it lasts is what i would do
i am assuming oil,is oil seepage coming from the rocker cap or cyl/head joint ? and no it won't cause low compression

Check to see if oil is coming from the breather in the cap at the top of the rocker cover. It can come from there. Regardless, like XB says run it.

Check the torque of 6mm bolts under the spark plugs. You won't get low compression if they are 'loose' but you will get an oil leak.
It's more common on left side