Topic: Carbon build up on valves & valve discoloration

[Chad]

So I'm pretty new to working on bikes and want to get some advice on this. I currently have my top-end apart (this is on a '77 CB750F2) and the combustion chambers, valves, and the top of the pistons have quite a bit of carbon build up on them. Also, the exhaust valves are discolored (a very light brown).

My thinking is my bike is running rich and the exhaust valves are discolored because the exhaust is not properly sealing onto the head. Would this be correct?

The worst exhaust valve:

[brandEn]

The 77 and 78 f bikes have valve issues. They wear out guides prematurely. Looks like your a head there is a victim. Time for a head rebuild.

[Pat_at_APE]

We can fix you up and have everything needed...  Send it in to us and we'll check it all out.

http://cbrzone.com/sohc.html

Pat at APE

[Chad]

It's not possible to be certain of huge cause of the discoloration simply by looking at the head. It could be rich, it could be poor timing, it could be.... The exhaust valves often tend to look more "lean" due to the fact they are post-combustion. Did you happen to look at your plugs prior to disassembly? Was there a compression check? A leakdown test? If not, best to start taking micrometer measurements of the cylinders and pistons, ring gaps, clearances, etc. That way you know what condition your block is in and check the valves, guides and seals in the head.

Ya, thats what I figured. Last compression check was about a year ago and everything was perfect. (Done by local shop). I have to keep investigating.

The 77 and 78 f bikes have valve issues. They wear out guides prematurely. Looks like your a head there is a victim. Time for a head rebuild.

This bike had a whole top end rebuild in 2011 (by forum member Mandic). Got all new valves, springs, 836 kit, etc all from Cycle X so it no longer has the stock F2 valves that cause issues, but there is obviously something not quite right.

One thing I do know is when I'm on the highway (going about 70 mph) it seems there is a vacuum that gets created and the carbs have a harder time getting air and the bike can run rich. (The carbs have velocity stacks). I saw another post where a guy had the same issue on his hot-rodded 750. The only solution for him was to get new carbs. (Unless of course he just put the stock air box back on.)

Found the thread: http://forums.sohc4.net/index.php?topic=40895.0

[flybox1]

.....and the bike can run rich. (The carbs have velocity stacks).

maybe if you have 150 mains 
I would think really lean.  Verify your main jet size.

[Vinhead1957]

I think the 2:3 cylinders are a little lean but otherwise it looks good. Were there any problems while it was running.

[Chad]

.....and the bike can run rich. (The carbs have velocity stacks).

maybe if you have 150 mains 
I would think really lean.  Verify your main jet size.

I believe they're 145 mains actually.  These carbs were set up by a local mechanic who is reputable for working on these bikes so I never messed with them or thought to change anything on them. The bike runs good except for when I'm going 70 or so and it seems like I can't get air. A lot of the time I can actually move my leg or put my hand down by the intake to force air into the carb and the problem clears up.

I think the 2:3 cylinders are a little lean but otherwise it looks good. Were there any problems while it was running.

No problems another than whats stated above. Bike runs great.

[lucky]

The 77 and 78 f bikes have valve issues. They wear out guides prematurely. Looks like your a head there is a victim. Time for a head rebuild.

Branden ....How can you tell if valve guides are worn beyond limits by a internet photo???

[lucky]

It does not look that unusual. Looks like normal stuff.
But check your valve guides for wear, and put some liquid in each combustion chamber to see if it leaks.

[Muckinfuss]

Lucky's right on both posts.."how" and "looks normal"...my question.....why did you take it apart?  Your condition reports for the past indicate you had faith in the work done and the results.

[flybox1]

The 77 and 78 f bikes have valve issues. They wear out guides prematurely. Looks like your a head there is a victim. Time for a head rebuild.

Branden ....How can you tell if valve guides are worn beyond limits by a internet photo???

Its a known issue with the F engines, guys
larger valves/different valve guide angle/premature valve guide wear.
brandEn's statement is in reference to this.

[Chad]

It does not look that unusual. Looks like normal stuff.
But check your valve guides for wear, and put some liquid in each combustion chamber to see if it leaks.

This is good to hear. I just don't have enough experience to know whats normal and whats not. I'm going to just clean it up and not worry about it. Like I said, the bike runs great.

Lucky's right on both posts.."how" and "looks normal"...my question.....why did you take it apart?  Your condition reports for the past indicate you had faith in the work done and the results.

Wellll.... it's kind of a long story.  I was having a leak around the head. I suspected the rubber pucks were leaking. Had oil coming from the spark plug openings. While I head the valve cover and cam out, I wanted to retorque the head and make sure everything was good. Ended up breaking a bolt off (not a stud, one of the small positioning bolts....I made another thread on this). To get the bolt out, I had to remove the entire top end so now I am just doing a full top end rebuild. I have all new gaskets and seals coming in from Cycle X tomorrow! I'm also painting everything while I have it apart.

[Muckinfuss]

That would be correctly identified as fixing it 'til it breaks.  We've all been there.

[MRieck]

The 77 and 78 f bikes have valve issues. They wear out guides prematurely. Looks like your a head there is a victim. Time for a head rebuild.

Branden ....How can you tell if valve guides are worn beyond limits by a internet photo???

Its a known issue with the F engines, guys
larger valves/different valve guide angle/premature valve guide wear.
brandEn's statement is in reference to this.

I am not nit picking but the guide angle is the same....Honda just sunk the chambers and consequently the seats. F2 valves are shorter (and much, much heavier which causes the wear)