Honda Twins banner

1 - 14 of 14 Posts

·
Registered
Joined
·
58 Posts
Discussion Starter · #1 ·
I noticed on my 1970 CB450 some air bubbles coming out around the oil dipstick and I can feel a little puff of air on the left timing cover blowing past the gasket.
Bad rings?
How much pressure is too much? :confused: Is even the slightest amount a bad sign?

Thanks,
Todd
 

·
Registered
Joined
·
2,561 Posts
I found out the hard way (blown seal) that even a partial pinch in the breather tube can be a problem.
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #4 ·
The only breather tube I know of is on top of the engine, and it's clear. No kinks or anything. That was one of the things I checked already.
The tube doesn't go into the airbox as far as I know. It just vents down between the swing arm and the rear tire.
 

·
Registered
Joined
·
2,561 Posts
That is correct - just an open tube to the ground. Mine was squeezed a little on the left side of the transmission; have since moved it to the other side, where it hangs free.
 

·
Registered
Joined
·
74 Posts
If the breather tube is clear then bad rings are a strong possibility. Have you run a compression test?
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #7 ·
No, no compression test - almost afraid to. :???:
But, Hazard Fright has one with my name on it. I will run by there after work tomorrow. Then by this weekend I'll run a test.

So the breather tube only works for the head of the engine? I'm not sure how the breather tube works and what its function actually is?
Does someone have a diagram of what is supposed to happen vs what is possible going on with my engine?

Thanks,
Todd
 

·
Sensei
Joined
·
27,173 Posts
The breather is ultimately connected to most of the "empty" volume within the main cases ...as the oil heats up, it requires more room...as the pistons rise and fall they "pump" waves of pressure underneath them as well as above...
The breather being open to the atmosphere gives these pressure changes and pressure shock waves some way to escape and dissipate...Plus, it tends to push hot/heated air out and then when the wave rebounds, it draws cooler air in......


That being said, I personally am a proponent of crankcase vacuum theory........;)
 

·
Registered
Joined
·
2,561 Posts
That being said, I personally am a proponent of crankcase vacuum theory........;)
That was what PCV valves were supposed to do: release excess crankcase pressure, but stop air returning when the pressure dropped, creating a small, and temporary, vacuum. Unfortunately, the valve itself is a restriction, so I wouldn't try on on this engine.:geek:
 

·
Sensei
Joined
·
27,173 Posts
I have been considering a larger escape port(s).... (current is only a single 8 mm ID tube) and light actuation reed-type valve(s)... (perhaps as simple as rubber flaps)....
For those not aware the basic theory is: Less pressure within = less air molecules = less weight to push/move around = less HP wasted doing so.....
 

·
Registered
Joined
·
2,561 Posts
So, fit a valve directly into the valve cover, with a larger ID tube? Interesting :idea:
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #12 · (Edited)
$30.00 poorer but one compression tester richer!

I get dead on 120psi on both cylinders. (throttle wide open)
Hmmmm....

So, this weekend I may take off the breather or rear head cover (what ever you call it) and check the "reed valves" (I think it's just a passage way to keep oil from just flowing out the breather tube) in there and see if they are plugged.

Todd
 

·
Registered
Joined
·
2,561 Posts
$30.00 poorer but one compression tester richer!

I get dead on 120psi on both cylinders. (throttle wide open)
Hmmmm....

So, this weekend I may take off the breather or rear head cover (what ever you call it) and check the "reed valves" (I think it's just a passage way to keep oil from just flowing out the breather tube) in there and see if they are plugged.

Todd
No valves, just a series of cast vanes in the cover to cause any oil droplets to return to the crankcase. You will a need good sealant if you divide the plate from the oil separator. I've never seen a separator clogged, but suppose it is possible.
 

·
Registered
Joined
·
167 Posts
Compression is on the low side at 120 PSI and a figure in the 150-160 PSI would be more in line with a healthy motor. Other guy's more familiar with the CB450 motor may chime in, or if you have a service manual the spec will be in there. You can try the old teaspoon of oil in each cylinder and retest and if the compression comes up quite a bit then the rings are suspect.
A leak down tester is the gold standard for identifying the worn areas in the motor, yet not in everyone's tool box unless you do lots of repairs on bikes.
 
1 - 14 of 14 Posts
Top