1979 CB750K with problems

[SoonerRon]

You take a shot and read the gauge, then take another shot and read the gauge, just to be sure! Works every time.

As far as the compression test on the cylinders that had pressure, it is my understanding that the correct interpretation of an observed pressure increase upon addition of oil to the cylinder indicates a worn ring situation. Is that true? I thing you're right about the dead cylinder. It must have a serious valve problem as well as other potential issues. I will know this wkend when I tear the engine down.

Thanks for your input!

[TOOLS1]

You are right about the rise being a ring issue.
TOOLS

[shinyribs]

All cylinders,even a healthy one,will give a higher reading wet vs. dry. But yes,you are right on that.

[SoonerRon]

OK, I finally got the motor out of the bike and head removed. Here is the cause of the zero compression on #3...

[shinyribs]

I wouldn't worry about that too much.It'll lap out ok

[SoonerRon]

I hope so. So much carbon build up... is this common?

[TOOLS1]

Did something take a bite out of that valve? That carbon looks like if has some oil in it.
TOOLS

[shinyribs]

LOL,I was just kidding.I'm fairly certain that valve will not lap into the seat.lol.The seat may have survived,though.Strange,usually when a valve burns to that extreme it is because of an extremely lean fuel mixture.With all of the build up on that head that doesn't seem to be the case.I have seen valve heads crack,but usually they burn.Thats a tough one to diagnose,really.BUT,that "carbon" does seem awfully wet.I'm with Tools on that carbon looking more like oil.Looks like a little of both carbon and oil actually.A good cleaning and disassembly will be the only thing to shed more light on the subject at this point.Also,excessive carbon build up usually is a result of an excessively rich fuel mixture.There is more fuel in the engine than it can burn so it just builds up like so.That will also create fuel-wash which is a sure killer of piston rings,especially in high rpm motors such as these.Considering your previous compression test numbers and these pics,looks like you are due for a set of rings.Just my $0.02 of course.Sorry to see you have this problem.Good luck on you repairs.

[SoonerRon]

Sorry, I knew you were kidding, my sense of humor is lacking at the moment... I was just thinking about the seat. Obviously the valve is toast. I was hoping to be able to just buy a new valve and lap it in. In view of the damage to the valve, I wager the seat will have to be recut, something beyond my technical expertise.

As to your conjecture about the fuel mixture, I will say, during rebuild, I found the air screws on all four carbs turned out 3-4 turns. I don't think the previous owner had any idea how to properly adjust the mixture.

[SoonerRon]

Just FYI... here is pic of the bike I'm attempting to resuscitate.

[sgtslag]

I have a '79 K, with 22k miles on it.  Mine runs perfectly.  Unfortunately, your's has been abused, which explains the trouble.  If properly maintained, these engines should go 70k+ without issues, other than normal maintenance.  The '79 is my favorite, as it is the classic tear-dop design -- a beautiful bike, IMNSHO (in-my-not-so-humble-opinion).  I hope you can rebuild it.  It's a fantastic bike, especially after upgrading the suspension with Progressive fork springs, and shocks -- it will ride like a 21st century bike, even two-up, touring with luggage.  Best of luck!  Cheers!

[seestheday]

I have an 81 750K with 84K miles on it and still running very strong. Mine was taken care of and I plan on riding it for much longer.

Sent from my BlackBerry device on the Rogers Wireless Network

---

From: "sgtslag [via Honda CB750'S]" <[hidden email]>

Date: Tue, 7 Feb 2012 08:58:05 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: 1979 CB750K with problems

I have a '79 K, with 22k miles on it.  Mine runs perfectly.  Unfortunately, your's has been abused, which explains the trouble.  If properly maintained, these engines should go 70k+ without issues, other than normal maintenance.  The '79 is my favorite, as it is the classic tear-dop design -- a beautiful bike, IMNSHO (in-my-not-so-humble-opinion).  I hope you can rebuild it.  It's a fantastic bike, especially after upgrading the suspension with Progressive fork springs, and shocks -- it will ride like a 21st century bike, even two-up, touring with luggage.  Best of luck!  Cheers!

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1979-CB750K-with-problems-tp3691671p3723109.html

To unsubscribe from 1979 CB750K with problems, click here.
NAML

[shinyribs]

Alot of humor/sarcasm gets lost in these typed messages That is such a beautiful motorcycle you would never suspect anything to be wrong with it internally. It looks like brand new!

[SoonerRon]

No sweat brother. I appreciate you takin the time the offer advice.

[SoonerRon]

Thanks for the encourging words. I fully intend to restore it to as close to original as possible, with maybe some upgrades as you suggested.

[SoonerRon]

After pulling the jug I was able to get good inside micrometer readings on all four cylinders. I measured top, middle and bottom of each cylinder in two perpendicular directions as described in the Clymer manual. The smallest value I obtained for any reading was 61.99mm and the largest 62.02mm. The specs say the nominal values are 62.00 - 62.01mm with a service limit of 62.10mm. I'd say these cylinders have a lot of life left in them! I think I now believe the odometer reading of 12K is accurate. Unfortunately, the pistons did not fare so well. All four pistons skirts measured at the service limit of 61.90mm. I'd like to replace the pistons with new std pistons, but their availability is limited and the cost is high. I see on eBay a vendor in Japan (CruzinImage_co) that sells pistons, rings and wrist pins for a +1.0mm overbore at a very reasonable price. Does anyone have any experience using these pistons? I would hate to punch out this good jug but that may be my best option.  

[TOOLS1]

Red at Cycle one was telling me he was going to check out some ebay pistons like, you are describing. Give him a call, or e-mail. He will be happy to talk to, you about them. His number is on the web site.
TOOLS

[SoonerRon]

After working on my bike some more I discovered the #1 rod has play in it consistent with a spun (as in gone) rod bearing. Alas, I must go into the bottom end of the engine, something I did not want to do. The service manual states removal of the clutch assembly is not required to split the crankcase, but it is recommended. Can I get away with leaving the clutch on or will this cause me undo heartache later?

[SoonerRon]

Well i split the crankcases on my engine to have a look at the #1 rod. I found the rod bearing was still in place but worn so thin you could shave with it. Not sure what was holding it in place, but there it was. The other rod bearings are shot as well but not like that. Pulled the crank and all main bearings are so worn the copper is showing through. Mic'ed the #1 rod journal 65.992 mm in the horizontal direction and 65.757 in the vertical direction... so i bought a used crank from ebay from the same guy in Nevada I got rods and pistons from! Unfortunately, can't read the rod and main codes on the counterbalancers. Gonna have to mic the main journals to determine main bearing color. Bearings that came with the rods are all brown and in good shape. Guess i'll plastigauge those to be sure brown is correct and buy new ones. This engine was rode hard and put away wet. Once I have the new bearings i'll be going back together with it. It better be soon my wife wants her parking spot back.

[shinyribs]

Well,I hate you ran into problems,but I'm glad you were able to source everything you need. Sounds like you are on the right path. Good luck to you! I've never seen inside one of these engines. I wish I could stand there and look over your shoulder while you do this.