1970 cb350k low compression left cylinder

L

lhemrick

Active Member
AT MY WITS END WITH THIS ONE!!! I bought a 1970 CB350 with a stripped plug on the left side cylinder and only 8000 or so miles on it.

Pulled the engine out took the top apart and installed inserts on both sides of the head. put new gaskets on and put it back together.

Had an oiling issue with the pump and ruined the top end. Had to replace the cam and rockers. Another new set of gaskets. Put it back together and had good compression with oiling issue solved.

Noticed a few days later that something wasn't right and checked the compression. Found the left cylinder down to around 60lbs of compression. Pulled it out and took it apart carried the head and had the valves and cylinder checked. Found no fault with either. Very little wear and cylinders were good and round. The guy at the shop did lap one exhaust valve on that left side and suggested that I check my rings when I got home. He also honed both cylinders and put a light cross hatch pattern back in both. I found a comperession ring on the left side worn past the limit so I ordered and replaced the rings on both cylinders. ANOTHER NEW SET OF GASKETS!

I have gone through the proper ring break in procedure and the right cylinder is where it should be but the left is only at 80!

WHAT GIVES????????

I AM ABOUT TO LOOSE MY MIND AND DROP A MATCH IN THE GAS TANK SO CAN ANYONE HELP PLEASE!
 
I know the feeling man!

Low compression can be lots of things.
First go through set the clearances on your valves, make it PERFECT, and check compression again. If an exhaust valve is opening just a fraction too early there goes your compression.
 
Valves were my thought when I first noticed the loss of comperssion and checked that before I broke it down this last time. I will recheck them tomorrow and recheck the compression tomorrow. I'll post the results.

Thanks for the time and advice!
Lynn
 
Then do a leakdown test and listen for where it's leaking out of.

Usually a leakdown test is to determine the percentage that leaks past rings or seals or valves. In you case you know it's not holding air and want to know where it is leaking from.

If your compression tester has swappable hoses, thread one into place and snap a compressor hose on the other and crank it up and listen for leaks.
 
teazer: I'm sure I follow what your trying to tell me to do with my compressor. My tester threads in and the guage disconnects with an air fitting. So can you explain (I can be kinda slow sometime) a little better. Are you telling me to connect this hose to my compressor and pressurize the cylinder?
 
lhemrick said:
teazer: I'm sure I follow what your trying to tell me to do with my compressor. My tester threads in and the guage disconnects with an air fitting. So can you explain (I can be kinda slow sometime) a little better. Are you telling me to connect this hose to my compressor and pressurize the cylinder?
Click to expand...

thats what im getting out of it as well. Make sure valves are all closed ;)
 
lhemrick said:
teazer: I'm sure I follow what your trying to tell me to do with my compressor. My tester threads in and the guage disconnects with an air fitting. So can you explain (I can be kinda slow sometime) a little better. Are you telling me to connect this hose to my compressor and pressurize the cylinder?
Click to expand...

Thats exactly what hes telling you

Make sure that your engine is at top dead center so that both valves are closed, then slowly pressurize the cylinder to 100 psi or so and listen for leaks.
A note on this you'll have to come up with a way to keep the engine from turning over, once you hit the air it wants to force that piston down! so either strap the rear wheel down or put a clamp on the sprocket or whatever it is you have to do


Oh, and I had this exact problem, leakdown test told me that my exhaust valve was leaking, i had just done a rebuild and lapped the valves so I couldn't figure it out. SO in hopes of not having to tear the engine down AGAIN i put some seafoam in the gas, some in the oil, and left some sitting on the exhaust valve overnight. Turns out that did the trick and my compression was back to perfect, some sludge or a piece of debris had probably lodged itself in my exhaust valve and the seafoam took care of it. So its worth a shot!
 
For what its worth while I had the head off I propped it up so I could fill the inside on top of the valve with some wd40 and let it sit for a hour or so and then looked to see if any leaked out on the bottom of the valves. I did both the intake and exhaust with no leakage.
 
So the compression in the cylinders is fine! I forgot to open the bores when I did the checks. Now I have a another question. With the bores closed I have 60lb in the left and 120lb in the right. Bores open I have 180 in both. Why is that and is this why the left cylinder is fouling plugs running rich and not performing properly. At idle the exhaust out of the left side is not as hot as the right.

Thanks
 
Progress. The high number on one side suggests that the carb is partly open on that side. I would start there.
 
Yeah check that your slides are seating all the way In each carb and that your throttle cable is set right and not holding one carb open
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light

    • Spread the Love

    Back
    Top Bottom