Topic: CB400F compression check

[Scott S]

 What sort of numbers should I see on a compression test on a CB400F?

 I have an old swap meet gauge and this is the first time I've used it. I'm not very confident in its accuracy.
 With the engine semi-warm, plugs out, choke and throttle open, I'm getting an even 95 PSI across all four cylinders.

 That obviously concerned me.....until I put the gauge on two of my CB550's. Those engines were cold, but on one cylinder (plugs in the rest), choke open and throttle open, I got 105 PSI.
 One engine has a fresh rebuild. The other has 5,000 original miles.

 I'm thinking the gauge is off, not the bikes.

 The CB550 should have around 140 PSI, right? Will the smaller CB400 have something a little lower?
 Again, I don't believe it's an actual 95 PSI, but an inaccurate gauge. The important thing is that they're even. Would be down only ~10 PSI versus the larger, fresher 550's indicate that the 400 is reasonably healthy, as far as compression is concerned?

[thirsty 1]

I did mine last week with 35.5k w/ full tune up and came out at 120 across the board. 9.0 cr / 14.6 atmos. = 131.4 compression.  I cut my hose off of my gauge and shortend it. You'll lose some compression from the long hose.

[Deltarider]

Why do you want to check the compression? Do you suspect something is wrong? Symptoms?

[Scott S]

Why do you want to check the compression? Do you suspect something is wrong? Symptoms?

 Finally got around to doing the 3K tune-up after this thread: http://forums.sohc4.net/index.php?topic=126726.0

 Bike smoked a little on start up and wanted to check things out. I think we've determined that this bike just doesn't like a full oil level. It didn't smoke after that day until I changed the oil, then it smoked on start up. A few miles later, it's nearly all cleared up. I've also noticed that the oil level on the dip stick changes by as much as a pint (assuming that there's a quart between the high and low marks) when I let the bike sit for a while.

 I'm about to read up on the cam chain, as there's a little noise there, but otherwise the bike runs great. I think the compression readings are symptomatic of my gauge.

[Scott S]

You'll lose some compression from the long hose.

 The hose on mine is at LEAST a foot long, too!

[thirsty 1]

This was about as short as I could cut it and still get to #'s 2,3 cyl.  Everyone has there own way and this is how mine is.