Topic: Low Compression CB750 - Advice Before Tearing Engine down?

[andyvclifford]

Hi All,

I have a 1978 F3 Cb750 with 24,000 Miles on it.  The engine turns over and all cylinders have 120 PSI cold compression test except for Cylinder #4 which is about 50 PSI.  There is an audible “PFFFT” sound coming from somewhere near it when i have the compression tester hooked up but I can’t quite locate it by ear.  What are the steps I should do before i take the Engine off the bike and try to rebuild.  What easy things should I Check which could be causing this leak?  Any pictures, videos, sound files that you guys might need please say so and I will post ASAP.

Bike Is currently almost 100% apart, Engine is sitting in frame but carbs and exhaust are currently off the bike.

Thanks!
Andy

[BomberMann650]

Did ya run it before tear down?

[andyvclifford]

Yes!  I started it up - was not able to drive it as the chain was rusted and needed new wheel bearings however I was able to get it running setting idle / all of that. 

[Mr Freeze]

You should run it a bit. Sometimes compression will come up after a bike has run that hasn't run in a while.

[BomberMann650]

You should run it a bit. Sometimes compression will come up after a bike has run that hasn't run in a while.

+1

Inspect valves too

[CB750F2]

You can do a leak down test to find the cause of the compression leak in No. 4. Search "leak down test" on this site for instructions on how to do this test. The "PFFFT" sound you hear is air escaping from somewhere - most likely the head gasket. Squirt some soapy water around the head gasket while performing the test. You could even try the soapy water trick while repeating the compression check on No. 4 - you will probably need an assistant to help with the soapy water. Pat

[andyvclifford]

Thanks pat that's exactly the kind of advice I'm looking for. I'll have my gf help me tomorrow

[BomberMann650]

Thanks pat that's exactly the kind of advice I'm looking for. I'll have my gf help me tomorrow

She'll be so happy about that.  Especially if you use the good towels to clean up the mess afterwards 

[Don R]

Double check the valve lash on that cylinder,

[scottly]

Use a piece of tubing as a stethoscope to locate the source of the "PFFFT". If it comes from either the intake or exhaust ports, check the lash as Don says.

[andyvclifford]

Tomorrow -

-Valve Lash
-Play doctor with the Engine
-leak down test

[scottly]

1. Play doctor first, before performing any operations.