Honda CB750 (and CB450) Compression Test Results

[750K8]

So I'm at the point where I'm questioning if my bikes are in need of a rebuild, or if my methods of testing are poor.

I have a 1978 CB750k with 12,000 miles. Bought it used off eBay. Runs really good. Idles awesome. Kickstarts really easy. It even feels like it has good power.

This was with a compression meter that screwed in the spark plug hole:

Cyl #1: 125
Cyl #2: 110
Cyl #3: 110
Cyl #4: 110

* Wide open throttle
* Didn't open the choke
* Can't remember if the engine was cold or hot during testing.
* Dry numbers.

You can hear the bike here:
http://www.youtube.com/watch?v=HTrkFKirg-E

I just acquired a 1972 CB450 with 22,000 miles. It was parked for several years, and ran a couple times last year by the PO. I did some rudimentary tuning (adjusted the idle, points). It now idles pretty good, but still needs additional tuning.

This was with a compression meter that you press up to the spark plug hole:

Right cyl: 120 PSI
Left cyl: 110 PSI

* Wide open throttle
* Opened chock all the way
* Cold
* Wet numbers

So I'm starting to question if both engines are tired. I've read in a few places that if it's under 150PSI, then it's time for a rebuild. That would mean that my engines are long overdue.

[TOOLS1]

I really doubt that your 750 needs rebuilt. I have seen these engines go 100,000 miles without any problem. Also your compression test results are probably off because the choke was closed. However since the readings are not far off from each other that is a very good sign that the engine is sound.
As to the 450, dry numbers are needed. The only thing wet numbers are good for is telling if it is rings, or valves causing low compression.
TOOLS

[750K8]

Thanks Tools. I was definitely thinking about leaving the CB750 alone anyhow: I'm just finishing up the paint work and really want to get out driving it. Probably will rebuild it either this winter or next winter, as long as the CB450 is ready for me to drive while I do it.

As for the CB450, I do plan on redoing the compression test (both dry and wet). after I get some miles on it. I've heard that if a motorcycle has been sitting for some time, then the compression will continue to open up after some miles have been put on it. I've put about 2 miles on it since I've purchased it, and have ran it off and on for about 3 days.

I did do a dry test for the CB450, but I wasn't used to the tool I was using (nor was I doing wide open throttle). I definitely want to say that the numbers did not go up more than 10-15 PSI from going dry to wet, if that means much.

I plan on changing the oil in both machines in the next couple days. Don't know when or what is in there.

[seestheday]

If it were a DOHC I'd say you need at least a top end job, but I believe the SOHC's (which is what you have) still run reasonably well at a lower compression.

I don't think I have ever disagreed with TOOLS as he definitely knows the SOHC's better than I do (I only know DOHC's), but I just checked the service manual for the K8 and the compression spec is 185 +/- 14 psi.

If TOOLS hadn't weighed in, I would have said that your K8 engine will need a rebuild now or soon.  I guess you could double check by re-taking the readings with the choke on.

[750K8]

When my father comes up (this guy: http://honda-cb750-s.456789.n3.nabble.com/MY-1979-CB750-DOHC-BUILD-td4035619.html) to visit for spring break, then I may have him take the CB450 engine back home with him and do a rebuild since he's more experienced with DOHCs than I am (even though the CB750 and CB450 DOHCs are different motors) while I drive the CB750 and enjoy it.

The CB450 is still awaiting a brand new petcock, and then I'm going to run it some more and perform some more tuning, get it hot, and re-run a compression test. If the compression is still low (which I would guess it would still be), then I'd go ahead and begin tearing the engine out for him to bring back with him...only because I have another CB to drive. Still plenty to do on the CB450 without the engine being here, too!

As far as the CB750, it really runs fine, so I'm really not worried about the compression numbers. Plus, those spark plugs are not the easiest to get out, especially with a hot motor, and I don't plan on turning it on to get it hot for probably another few weeks because it's undergoing paint work. The plan is to just drive and enjoy it. When the CB450 is rebuilt and is ready to be a main driver, then It would be time to tear down the CB750.

Sounds like a good plan to me!

[robs750]

Before you do a compression test or leakdown test ,you should check the valve clearance.You want to be sure none of the valves are opened.

[Piute]

    Your #'s are about the same as mine,("77"F2)not knowing what I was doing ,My thought was so long as there all close , whith the miles of 9k Not worried about it ,Just wonted to ride also,
     Since then I've doubled the milage (well another 400 miles will be at 19k ).Same stk # ngk plugs.
   lots of oil changes with auto oil has caused the clutch to slip & gettitng worse.
  Ready for a real good tune up.
                                       Took a while to find were I wrote #'s

[750K8]

Thanks Plute.

That's kind of my idea and looks like we have similar numbers. Why tear down an engine at 9-12k miles, even if the compression isn't very good? If it starts easy (relative to a CB750 that is), idles great, and runs great, what's the harm in just driving it out further?

Make sure you are using oil that is for a motorcycle, though. I plan on using Shell Rotella, which is actually an oil designed for diesel engines. It has some substitutes that are ideal for motorcycle clutches, etc. That could be the reason for your clutch/slip issues is that auto oil isn't really designed to protect or lubricate that.

[Piute]

                                                 HooterMan warned me a lil late ,
                          You both are right But going to try to stick with cycle oil after this
                             next experiment(Rotell Syn.)After some of the magic juice

[TOOLS1]

When these motorcycles were designed there was not any oil specifically designed for motorcycles (I have used auto oil in my motorcycles for 42 years, and never had a problem.) Today unless the label says that it contains "friction reducers" it will not affect a clutch in any way. However unlike metal parts; clutch plates are subject to aging. After 35 years the plates can become pretty hard, and wear quickly. Especially if the bike is loaded down with things like a fairing, bags, trailer hitch...  
TOOLS

[750K8]

I know this is primarily a 750 forum, but I did play with the 450 some more today. Here's a rundown:

Picked up 1972 CB450. It runs and drives. It was stored for many years. Previous owner last drove it 6 years ago. He ran it 1 year ago. Stock exhaust. Stock air box.

Things I have noticed:

1) Right exhaust is producing significantly more pressure than left exhaust.
2) Choke must be at least half-way on for the bike to run.
3) Didn't notice anything wrong with the carb acceleration on either side.
4) It could use an oil change

Things I have done:

1) Set point gap
2) Adjusted idle screws
3) Adjust air/fuel mixture screws
4) Tightened hoses. Did not see an air leak.

Compression test results (2/24/2013):

* Engine ran for 10 minutes prior
* Choke all the way on.
* Wide open throttle

Dry:

L: 118 PSI
R: 123 PSI

Wet:

L: 122 PSI
R: 127 PSI

[TOOLS1]

Your engine, and compression are just fine!!! You can not do a compression test with the choke on/closed! Your bike is running weak on one cylinder because the pilot jet, and probably main also are more plugged up then on the other cylinder. The fact that it will not run without the choke on is telling you that the pilot jets are plugged! Now go clean those carburetors, and enjoy that bike!!!!!!!!!
TOOLS

[LukeM]

I just did an oil change on my Shadow this weekend.  Shell Rotella T6 5W40 oil in the big blue jug.
If you're going that route, find it at your local Wal-Mart.  All the car parts stores sell the gallon jug for $27.99 US.  Wally's sells it for $21.  That's almost enough to cover the cost of the filter.

Be prepared for a few drips of oil if you're switching from conventional to full synthetic.  The synthetic oils tend to dissolve grit and grime, including "dirt seals" inside your engine. Mine drips a few drops/week, depending on how much I ride it.  

Luke M