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'82 Honda CB900c Burnt Valve.... Info?

Eli69

Eli69

Forum Sage
Past Site Supporter
Does anyone have experience with these motors? We bought this bike cheap hoping for a quick flip but that's not gonna happen... Anyway I'm looking for any info I can get that might help get it back together and running.

Here's the facts: Low compression all the way across with number 2 at O psi. Checked valve adjustments and most were pretty close to spec and none were being held open. So we pulled the head. An exhaust valve on number 2 looks burnt and a few others are pretty gunked up with carbon sootiness. We're fashioning a valve compressor tool right now so haven't removed any valves yet.

The Clymer manual says there is some kind of coating on the valve seats and that they shouldn't be lapped or ground... so what do we do?

The bike itself is really nice. Looks like it's never been down and the only thing missing is the tool kit. Even has good tires. Any help would be greatly appreciated.
 
If it's like our Suzukis it's going to be easier and cheaper to find a good used head, slap some new valve stem seals in it and ride it another 100,000 miles or so.
 
I'd pull the valves and survey the damage. Then find a real Honda service manual and a Honda forum and get the low down on servicing the cylinder head. The course of action will come out of what you learn.
 
Clean the head, face the valves if needed, or if not just lap them. The manufacturers ALWAYS tell you these things to get you to BUY NEW PARTS, instead of servicing what you have. No issues with doing them this way, been doing it for years.
Ray.
 
While you are there, check the piston rings also.
Could you show us, the plugs?
 
Thanks GSR. I'll put up some pics soon. The plugs were pretty black and sooty. The plan is to do the regular stuff - top end overhaul, carb dip, electrical clean up, etc. We're gonna check the pistons and rings out, if they're in spec we can put them back in... fingers crossed. Cylinders look good so far.

Rhetorical question... Why the ffff does this bike need a hi/lo range transmission? Also, we're wondering if it was run in high gear and low rpm too much causing all the carbon build up. A previous owner may have felt uncomfortable winding it out.
 
Eli69 said:
A previous owner may have felt uncomfortable winding it out.
Click to expand...

That makes a lot of bikes and old cars with small engines run bad. Can't imagine riding a motorcycle that way, but I have known guys who did. Even knew a few who had no idea it made more power up near the redline. One guy wondered why he could never keep up even when we traded bikes.
 
Anyone who learned on or rode Harleys do. Lots of torque and a low (relatively) rev range teaches you to use the bottom of the tach.
 
Eli69 said:
Does anyone have experience with these motors? We bought this bike cheap hoping for a quick flip but that's not gonna happen... Anyway I'm looking for any info I can get that might help get it back together and running.

Here's the facts: Low compression all the way across with number 2 at O psi. Checked valve adjustments and most were pretty close to spec and none were being held open. So we pulled the head. An exhaust valve on number 2 looks burnt and a few others are pretty gunked up with carbon sootiness. We're fashioning a valve compressor tool right now so haven't removed any valves yet.

The Clymer manual says there is some kind of coating on the valve seats and that they shouldn't be lapped or ground... so what do we do?

The bike itself is really nice. Looks like it's never been down and the only thing missing is the tool kit. Even has good tires. Any help would be greatly appreciated.
Click to expand...

Another good information source for the 16 valve Honda engines of that era is www.cb1100f.net . Although dedicated to the chain drive bikes, most of the engine info is applicable to both the chain and shaft driven bikes. You can download the Honda shop manual for your bike there as well.

Eli69 said:
Thanks GSR. I'll put up some pics soon. The plugs were pretty black and sooty. The plan is to do the regular stuff - top end overhaul, carb dip, electrical clean up, etc. We're gonna check the pistons and rings out, if they're in spec we can put them back in... fingers crossed. Cylinders look good so far.

Rhetorical question... Why the ffff does this bike need a hi/lo range transmission? Also, we're wondering if it was run in high gear and low rpm too much causing all the carbon build up. A previous owner may have felt uncomfortable winding it out.
Click to expand...

When Honda decided to build the shaft driven, custom version of this bike, they used an existing engine (CB750F & CB900F SS) and an existing drive system (GL1100 Gold Wing). They had to get the engine output (left side) over to the right side (splines on rear wheel). Since they had to go through two 90[SUP]o[/SUP] turns with space in between them, in typical Honda fashion, they filled that space with a hi/lo linkage.

I was consulting/assisting a friend in his search for a large displacement, comfortable bike for two-up weekend trips a few years ago. We found a very nice, roadworthy '81 CB900C listed for $1500, and wound up getting it for $1,200. It had 19,000 miles on it, I believe.

I did all the usual stuff to it, and we mounted a windjammer on it I had removed from my GS1000G. Now, five years later, he still rides it regularly and has over 45,000 miles on it. It's smooth, comfortable for all day riding (swapped out the handlebars, of course) and he gets about 50 mpg cruising two laners at 60 mph in hi-5th gear, with his wife on the back and soft luggage.

Good bikes.
 
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Here's something I learned at a friend's BMW repair shop.
A customer brought in a relatively nice R1100RT that had always run great but had lost power and was running poorly "all of the sudden".
It had low compression on one cylinder and almost none on the other.
It turned out the owner had dumped a can of Seafoam in the tank "just to help as it had been sitting a while".
He was out riding, missed a shift, over revved the engine, knocked some carbon loose from the combustion chamber that ended up caught in the valve seat, hence, no compression. The heads were pulled, the valves were re-seated and no permanent damage was done.
Just a thought.......
 
We got a chance to pull a couple valves tonight. Here are pictures of the one that was clearly damaged. Exhaust cylinder 2, valve 2:

DSCF1184_zpsflu6igsq.jpg



DSCF1185_zpswjq3j4xf.jpg



DSCF1186_zpsnniklb8l.jpg


The other valve we pulled had a glassy looking seat and was very carbon encrusted. It seemed to clean up well though. I think the valve oil seals are pretty bad. I think it was burning a lot of oil.

I'll pull all the valves next chance I get and clean them up. Probably post another picture or two.
 
I liked the 900. It felt big, but not heavy to me, and it was softly sprung. That was great for me as canyon carving is alien to me. I liked the look. It didn't feel underpowered, but couldn't hope to run with my 82 1100EZ.
 
I got the valves out. Found another slightly damaged. You can see the little nick at about 1 o'clock.

DSCF1192_zpsod0416ty.jpg


Here's what the exhaust valve seats look like on cylinder #3. That's where the above damaged valve came from.

DSCF1194_zpsikgwxwjj.jpg


Typical valves, Intake on left, exhaust on right.

DSCF1187_zpsy1wuujhq.jpg
 
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