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Author Topic: 1981 CB750C Engine Rebuild - Valve Health  (Read 283 times)

Offline SLVRGHST

  • Posts: 3
  • Once a tinkerer, always a tinkerer...
1981 CB750C Engine Rebuild - Valve Health
« on: Jun 22, 2017, 14:17:53 »
Hello Everyone,

This is my first post of maybe a few that I will add in this area of discussion. Hoping to get some support from experienced mechanics about the rebuild work I am doing with the CB750C engine I am working on.

I am doing a complete tear-down of the engine to rebuild it. A compression check of each cylinder revealed to me that there was need of work (90 psi avg.), not to mention engine noise that sounded quite unhealthy. I am hoping to re-use certain parts and replace others.

On the subject of valves, I have pulled all exhaust and intake valves out of this engine. There seems to be the usual wear and tear. The exhaust valves had a lot of carbon build up on the stems and the fillet, all the way down to the valve seats. Not tons but enough that it took a few minutes of careful cleaning with a wire wheel. Once I got them clean, I noticed that on a few, there appears to be some rather concentric bulging in the valve stem. Like a ring that bulges at a height of about 1/16 of an inch and is about 1/4 to 3/8 inch in length along the stem. The bulge does not travel up to where the stem guide starts when the valve is seated. Now, I don't see any damage in the valve faces at all and the rest of the stem is quite smooth. Is this a result of the machining originally done when the valves were made back in the late 70's or early 80's? Is this the result of the engine overheating at some point in its life and somehow the valve stem compressed due to high revving of the engine? Bottom line, should I replace those valves and the guides? Do I just bite the bullet and replace all the guides and valves to get better longevity out of the engine rebuild?? I mean, I have come this far...

Anyhow, I will certainly post some pictures of these valves when I get home today. That should help I hope. Thanks guys!  :)
 


Offline Cookie

  • Posts: 538
  • God help the child who grows out of everything
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #1 on: Jun 22, 2017, 16:28:45 »
Hey, Welcome to DTT!

Your valve concern does sound a bit odd...do you have any pictures?

If in doubt, replacements can be had for not too much $$

Also, as far as bad noises go, these engines are bad for timing chains and guides wearing; clutch baskets rattling due to wear, isolator rubbers or springs being old/hard;  as well as carburetor synchronisation causing problems that can act/sound like noise.
« Last Edit: Jun 22, 2017, 16:31:34 by Cookie »
* Insert something witty here *

81 CB750C
84 Tempter - well, most of it...

Offline SLVRGHST

  • Posts: 3
  • Once a tinkerer, always a tinkerer...
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #2 on: Jun 22, 2017, 22:40:58 »
Hi Cookie,

Thanks for the reply. As I have gone through tearing down this engine, I have found out all those lovely thinks you mentioned. The timing chains appear to be in good condition but I haven't got them completely out of the engine yet to measure for any stretching. The rear guide was completely torn up. If you have a look at the two Youtube sessions I posted privately (links below), you can see what I found. LoL






As for some pictures of the valves, have a look at these samples. They are the first exhaust and intake valves of the first cylinder. You can see what I was trying to describe. The exhaust valve is in bad shape. After taking the macro shots, I can see where it looks like the hump in the stem may be extra material that needs to be removed. Looks pretty rough even after the initial wire wheel cleaning I gave it.

Cylinder 1 Exhaust Valve 1

https://drive.google.com/open?id=0B0EuSCXsmLUHdkQtTVkzZnlUR3c

https://drive.google.com/open?id=0B0EuSCXsmLUHVDdsdmtycnRRR0E

https://drive.google.com/open?id=0B0EuSCXsmLUHVTdKenBDTjc3NjQ

Cylinder 1 Intake Valve 1

https://drive.google.com/open?id=0B0EuSCXsmLUHRjR6TzBrZG9OR3c

https://drive.google.com/open?id=0B0EuSCXsmLUHcTRGbzlqX0ZlTVE

Forgive me for dumping a lot of links to pictures and such here. My technical savvy on forum composition is a bit rusty.

And again, thanks for your feedback and moral support on my quest for getting this old engine back into shape. Cheers!  ;D


Offline demitrix

  • Posts: 7
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #3 on: Jun 23, 2017, 16:33:26 »
Will be following this thread as I am getting ready to start the same process. Seeing the condition of your motor does not build any excitement for me though  :-X

Offline TheCoffeeGuy

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Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #4 on: Jun 24, 2017, 08:28:27 »
The hump looks like a manufacturing aspect.   There is no way that your engine got hot enough to soften the metal in your valves.  Your pistons would be soup at that point.   however the seating edge is done.  Valves are cheap compared to the damage they can do it they break up.  I dont think even an engine on fire would get nearly hot enough to melt them.  Buy new valves.  If your valves are that bad you may have to get a machine shop to re grind your seats too.
www.thecoffeeguy.ca Not your ordinary Blog site.

1968 Harley Aermacchi ML 125 Rapido, restored
1972 Honda Cl 450 engine rebuild, a day rider soon, now sold
1980 Suzuki GS850GL Street Brat  My other day bike
2000 Harley 1200 Sport S My day bike
1982 Honda CB650 Chopper light

Offline esmoojee

  • Posts: 127
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #5 on: Jun 24, 2017, 12:44:30 »
I agree with TheCoffeeGuy.  I think it’s just a manufacturing flaw.  I have a 750f and I rebuilt my engine a year and a half ago.  I would suggest using Viton valve seals when you start getting it back together.  The old rubber seals are notorious for cracking an allowing oil to get past them into the combustion chamber.  Did you find any bent valves?  I would bet that your shims might’ve been out of adjustment along with the cam chain.  Probably what was making all that noise. 

Offline Ryan Stecken

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Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #6 on: Jun 24, 2017, 17:11:09 »
Watching.

Sent from my SM-A310F using DO THE TON mobile app


Offline SLVRGHST

  • Posts: 3
  • Once a tinkerer, always a tinkerer...
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #7 on: Jun 26, 2017, 13:43:09 »
Thank you esmoojee and TheCoffeeGuy.

After reading up a bit on how some valves are manufactured, looks like the stem is welded to the rest of the valve and machined down. The seats for the exhaust on this cylinder head are in pretty rough shape too. Looks like they were never really seating properly given that the seating areas look dull and textured like the valves. Grinding will be in order. No bent valves at all. I will definitely be replacing the valve seals. I have a complete engine gasket/seal kit that includes these. Purchased from CMSNL. Given your combined experience with replacing valves, is it common to replace the valve guides as well? The ones in this cylinder head appear to be in good shape. The valves just slid right out without issue. And when I get to the point of purchasing new valves, other than CMSNL or vintageCB750.com, any other good suggestions on where to pick up new valves?

Offline esmoojee

  • Posts: 127
Re: 1981 CB750C Engine Rebuild - Valve Health
« Reply #8 on: Jun 26, 2017, 13:53:17 »
If the valve seats aren’t cracked I wouldn’t worry about replacing them.  If you do, you should take it to a machine shop that specializes in it.  You don’t want to fuck up a valve seats.  I bought mine from a guy on the cb1100f.net site.  He sells a lot of parts for our older cb’s.  He gave me a pretty good price on them and I believe they were K & L.

Ed