Compression Test Results

[Myumi-2]

I have a 1972 CB 750 with 60,000 miles on the clock which is returning to life after 20 years in a shed.

I have had the engine running and decided to perform a compression test.  The results were:

150
90
150
150

Wet test results were:
160
110
170
160

Whilst 150 is at the bottom end of the text book range (150-170) I think it is still acceptable.

Clearly cylinder 2 has a serious problem.

Can anyone interpret these results and the possible problem???

Thanks in advance.
Tim

[gonebiking-2]

Number two cylinder could be one of two things. A) A burnt valve, or worst case scenario B)worn rings on that cylinder. I hope for your sake its just a burnt valve

[Rick V]

Gonebiking is on the right track with the burned valve .It is possible that a valve may be adjusted too tight and staying slightly open,or could be sticky from sitting.  I don't believe the the change with the wet test was large enough to indicate bad rings as all the cylinders all changed by roughly the same amount.

[Tech78]

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[Myumi-2]

Thank you for the input.  I value the advice on this site highly as Im very new to mechanics.  

To answer the question, is their any noise.  

At start up there can be a brief knocking noise that seems to originate from the front and centre of the engine.  It is brief, perhaps three or four knocks in quick sucession and then it disappears.  It only occurs on a cold morning.  I can not identify the noise any further, as the choke is on full at this stage and the engine is not running smoothly and makes a numer of other noises at the same time.

The engine does have a loud exhaust pop when decelerating, which is not a consistent noise.  This I now suspect is a symptom of the low compression in Cylinder #2.

One thing I do notice is that whilst kick starting that the kicker sometimes reaches a point where it is extremly hard to push down.  It in fact kicks back.  This is particually noticable if you are kicking it over slowly whilst undertaking a compression test.  I am at a loss to explain this one.

Ive not had the valve cover off and will need to do so at some stage, so Ill attempt to remove it tonight and see what I find.  I probably shoudl wait until I have a replacement gasget, but ... I'm impatient.

A few questions: is the valve cover the same as the Cylinder head cover (the lasrge piece that rund across the top of the engine)?

Whilst Im in there, is there anything else I should be looking at?

From looking in through the Tappet Hole Cap (the 8 caps across the top of the engine) it seems to be multiple layers of carbon.  It this normal?  Should I remove all build up?  

Cheers
Tim

[Tech78]

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[Myumi-2]

Update.

Given the ease of Ridzc8's suggestion of putting some penetrating fluid in the valve, I gave it a try.  
My compression on Cylinder 2 has risen from 90 to 140.  Not particually good, but allot better than 90.  Ill test again in a couple of weeks and leave this problem for the time being until I really need to take the engine out.

I also made some adjustments to the valves, however Cylinder 2's valves were adjusted correctly.

The combined effect is that the idling speed has dropped from 1900 RPM to 1200 RPM.

Thanks for the help.

Tim