Help: Zero compression, yes zero. 1979 CB750K

ceisen

Member
1979 CB750K

I'm at my wit's end. I've problem solved as much as possible, checked clearances, replaced gaskets, ensured timing marks are bang on (followed this for cam/timing installation as I found it much clearer than the Clymer manual... https://www.tapatalk.com/groups/deuceswild/installing-cams-t1144.html) but when I run a compression test I get zero across all cylinders.

With the same compression meter a cb450 nighthawk a friend just gave me that's been sitting for years will push 110 psi.

Not sure what I'm doing wrong or what is wrong with the engine.

If I put my hand over the intake there's plenty of air being sucked into the engine, great vacuum, and seemingly plenty of air being pushed out but again zero register of pressure in any cylinder.

Carbs are off, exhaust is off, I'm pulling one spark plug at a time.

What could be the cause of this? How can it be absolutely zero? Bent exhaust valves?

Any/all suggestions would be appreciated
 

kerryb

Member
Yes, valves are certainly suspect if cam is timed right and tappet clearances have been checked. So give us more background...what gaskets? Have you had it apart? I once tried to start an engine before checking for sticky valves. When the piston hit the sticky valve it promptly closed, and all the expensive fun began to replace the now bent exhaust valve!
 

ceisen

Member
I replaced all gaskets down to cylinder base, piston rings were good, no scarring or marking in cylinders, had to replace cam tensioner 'b' as it was broken. Not sure what else there is to disclose...
 

SONICJK

Reminds me of...me No, I'm sure of it. I hate him
Has to be valves in one way or another. Although it seems unlikely that all 4 valves would be leaking.
Did you rebuild it?
Open the rocker covers and watch as you turn it over slowly, if on the compression stroke the rocker is not free to move then something. Is wrong with cam timing.
if the rockers are free at tdc on compression and still no compression then it's likely bent or leaking valves. Although u find it unlikely that all 4 cylinders would be bad at once.

more likely an assembly issue imo
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
I suspect you're not setting your valves at the right point of the cam, and possibly 180° out. That might explain why you're not getting compression on any cylinder all at once.
 

cxman

Active Member
DTT SUPPORTER
on a dohc you can see if you are setting the valve at the wrong point as the lobes are visible

you put the feeler gauge between the cup and the lobe it wont fit at the wrong point afaik

unless you push real hard

high point of the lobe should be 180 to the cup when you set it
 

crazypj

Split personality, I fake being smart
To have ZERO compression your doing something fundamentally wrong. Remove all the plugs and disconnect coil primary leads. It's most likely engine isn't spinning fast enough to actually pressurise a cylinder. There is also the possibility cam timing was out and all the exhaust or intake valves are bent but even then you should get something (10~30psi) You have Schrader valve in line or plug adapter?
 

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