Loss of compression. CB175.

Fund Man

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Hi. I'm new to the group and this thread seems appropriate. CB175 with 90psi in both cylinders. Throttle open. Drop of oil didn't seem to make much difference but when I do a leak down tester I get a considerable hissing out of the oil filler cap hole. I cant hear anything discernable from the valves.

I think I'm looking at pistons or a head gasket failure which on a recently restored bike by the PO is dissapointing.

Engine out I guess......

Any thoughts anyone?

Ian
 
It can be a lot of things. The first I would check is the valve clearances and if you have access to a borescope to run inside on of the spark plug holes to get a look at cylinder walls.

Good thing the engine is pretty easy to pull solo.
 
Thanks. I have checked the timing a couple of times and can't hear anything from either side on test.

I did look with my borescope and nothing obvious.

Last week I had an umbilical hernia repair and I'm sure the surgeon said something about lifting anything heavy....

Ian
 
Yeah, the 175 engine isn't overly heavy as these twins go but it does have to come out of the frame for any top end repairs.
 
Unfortunately, you never know with a PO rebuild what exactly was done - and how that was accomplished. The 175's aren't as bad as the 160's in this regard, but the valve guides are often overlooked when these rebuilds. They can get ovaled out or even just get sticky from corrosion/carbon buildup. If the valve springs are on the weaker side this issue can be exacerbated.

Have you been able to run the engine at all? Were it a bad head gasket or valve guides, you'd likely be burning some oil. Bad rings would have shown a pretty decent difference in compression between wet and dry.
 
Was running not too badly but I couldn't get it to idle nicely and it wouldn't run on a single cylinder. I am resigned to pulling it apart but I might just check the torque on the head bolts first.
 
Fund Man said:
Was running not too badly but I couldn't get it to idle nicely and it wouldn't run on a single cylinder. I am resigned to pulling it apart but I might just check the torque on the head bolts first.
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These engines weren't designed to run on one cylinder when within serviceable limits and the idle drop method was only briefly recommended by Honda during the early Gold Wing years IIRC. Considering the low compression (if truly accurate) I'd suspect it would be hard to get to idle well at all.
 
Before pulling the motor, I would want to make sure those compression numbers are legit.
  • Have you used compression gauges much in the past?
  • Were you using the electric starter or kickstarter during the tests? How many cycles/kicks? It should take 6+ kicks to get close to the max.
  • Has the gauge been verified to be reasonably accurate, say, on another motor?
  • A drop of oil shouldn't do much. Does a capful make a difference?
I would definitely allow some time to heal up before pulling the motor!
 
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Careful of too much oil though. The compressed volume of combustion chamber is only ~10cc - you can really shoot the number to the moon
 
I used an inexpensive cable hoist rigged to the rafters in my garage to hoist the engine in and out. Worked great!
 
stl360+450 said:
Before pulling the motor, I would want to make sure those compression numbers are legit.
  • Have you used compression gauges much in the past?
  • Were you using the electric starter or kickstarter during the tests? How many cycles/kicks? It should take 6+ kicks to get close to the max.
  • Has the gauge been verified to be reasonably accurate, say, on another motor?
  • A drop of oil shouldn't do much. Does a capful make a difference?
I would definitely allow some time to heal up before pulling the motor!
Click to expand...
This is a really good point PO - make sure you know what you are measuring before you start trying to fix stuff that may not be broken. The compressed volume will be less than 10cc per cylinder, so if you are using a car type compression gauge with a long, whippy rubber hose that might have an internal volume of 4 or 5cc, then 90psi will be perfectly normal.

Alan :)
 
Thanks everyone. I'm using a proper but admittedly cheap leak down tester by "US PRO" and will check using another one I have as suggested. The manual says it should be at least 135 psi and the amount of air hissing out of the oil filler seems pretty irrefutable that there is an issue.

Lots of amazing help from everyone, thank you.

I will update you with my progress.

Ian
Othen said:
This is a really good point PO - make sure you know what you are measuring before you start trying to fix stuff that may not be broken. The compressed volume will be less than 10cc per cylinder, so if you are using a car type compression gauge with a long, whippy rubber hose that might have an internal volume of 4 or 5cc, then 90psi will be perfectly normal.

Alan :)
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Othen said:
This is a really good point PO - make sure you know what you are measuring before you start trying to fix stuff that may not be broken. The compressed volume will be less than 10cc per cylinder, so if you are using a car type compression gauge with a long, whippy rubber hose that might have an internal volume of 4 or 5cc, then 90psi will be perfectly normal.

Alan :)
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Long hose, even short hose, is a problem on small engines unless there’s a one way valve in the threaded end of the hose.
 
However, regardless of apparent compression readings, how does the bike run ? It ought to get up to an indicated 70mph on a still day on the flat, average sized rider in a slight crouch. ( UK bike with flat bars ? ), redlined in 3rd and 4th before 5th.
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Apologies for repeating my earlier post, but have you actually ridden the bike and checked the performance that way ?

It might just need a bit of Italian tuning, to free off sticky rings if the engine has been stood around for a few / many years.
 
Valve position in the hose shouldn't matter as when on compression stroke the entire length of hose is joined to the combustion chamber volume.
 
ericfreeman said:
Valve position in the hose shouldn't matter as when on compression stroke the entire length of hose is joined to the combustion chamber volume.
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If it's a check valve, the hose is only exposed to the combustion chamber for the portion of the stroke corresponding to greater pressure than is in the hose. This is the reason compression readings come up with each kick/cycle and eventually max out.
 
Ok, here is my update. I was only a few bolts away from pulling the engine. I couldn't do it immediately and so I thought I would give it one more try to get it tuned.
I had previously looked around the bores and valves with my borescope and nothing seemed unusual. I also realised my compression of 90 psi on each side was done with the engine cold.

I stripped the carbs and set the float heights. I synced the throttle slides.

I'm very happy to say that the bike is now running very nicely and I was able to go for a ride out on Saturday. One of those rides that takes ever increasing circles around the home address just incase I have to push!

All went well. I had done all the above previously but was too focused on the compression numbers. The bike is pulling nicely and ticking over fine.
Thanks all for your comments.

I hope we can all enjoy our special relationship across the pond for many years to come.
Kind regards.

Ian
 
Fund Man said:
I was only a few bolts away from pulling the engine.
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Now that it seems to be running decently, I hope you remembered to replace all engine mount bolts and re-tighten them.
Fund Man said:
I hope we can all enjoy our special relationship across the pond for many years to come.
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We'll be here, and do come back to update your progress as the ride circles get larger. :giggle:
 
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