CB900C exhaust valve failure

Mikemo

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Greetings to all my Honda Twins friends. I was looking for a forum for the 900C but didn't find one, then I thought to look here. Hopefully the brain-trust can guide me on this repair.

The bike belongs to a friend of mine, a 1981 CB900C with 23k miles, totally stock. He bought the bike 10 years ago and only rode occasionally. He called me recently, frustrated with the bike. The rear brake locked up on a short ride and he had to get towed home. Then, he couldn't find a shop to work on the bike. Even the local Honda dealer said it was too old for them to touch. He was ready to junk it, but I told him that I'd have a look.

The rear brake was an easy fix. I rebuilt the master and caliper without any issues. It was on my test drive that I really noticed the issues. Just starting it was a chore. You have to pump the throttle to prime it with the accelerator pump just to get it to start. It would idle once warmed a bit, but the bike ran like garbage. Terrible throttle response, low power, etc. I was thinking that the carburetors needed to be cleaned/adjusted. A quick compression test showed differently, with 150psi on cylinders 1 through 3, but only 50psi on cylinder 4. A valve clearance check showed everything fine except one of the exhaust valves on cylinder 4 that had zero clearance. A leakdown test showed major leakage out of the exhaust on cylinder 4.

Getting the engine out of the bike was a pain! That thing is massive. You have to disassemble everything to get it out (rear tire, drive shaft, swing arm). Here's a picture of the valves on #4. The valve is toast, but the seat doesn't look melted. I ordered a new valve (Vesrah) and will lap it and see how the seat looks. I'm hoping I don't need any machine work done.

So finally to my questions. I'm trying to keep the cost down, but don't want to ever have to pull this engine out again. Head gaskets are no longer available from Honda, but can be had at a dear price of over $100 from csml or David Silver Spares. Can I get away with an aftermarket gasket set? Any suggestions for a source of aftermarket gaskets? If I can get away with it, I don't want to touch the other 15 valves. Is this a mistake? Should I completely disassemble the head and get all the valve seats cut?

Thanks for reading. I hope this wasn't too long and boring.
Mike M.cb900 exhaust valve.jpg
 
The tight valve clearance caused the valve to burn, and when a valve gets burned like that it usually damages the seat.
 
Hi Tom. It's been a long time. I hope you are doing well.

I took the valve out of the head and snapped a few pics of the seat. It isn't horrible, but it certainly need attention. The valve is obviously toast. I'll wait until I get the new valve, then try to find a machine shop that will look at it. Thanks.

It seems weird that only one valve was out and the rest of the clearances were in-spec. I don't think this engine was ignored. There was evidence that the valves were adjusted at least once before. (sealant evidence in valve cover). I wonder if the exhaust valve failed because of carb issues (too lean and hot), or maybe just chipped, then sunk in the head as it burned away. Who knows.

cb900 valve.jpgcb900 valve seat 1.jpgcb900 valve seat 2.jpg
 
I was a specialist on these bikes and had parts until a year or so ago. As Tom noted the valves recede into the head with mileage, since no one does the factory maintenance recommendations on the bikes. There is an older forum that specialized in the DOHC series which you can find here with more knowledgeable members for that DOHC series.

Website: https://www.cb750c.com/modules.php?name=Forums

I would suggest you pull all the valves and check the seats and do a light lapping of the valves while you have the head apart. I will check if I still have an OEM head gasket for the bike in my gasket sets. You can use an aftermarket gasket kit and the CB750 forum will have a best one to use based on their experience. Replace all the valve stem seals while your in there and a new gasket kit will include those in the kit.
The shims clearance will need to be checked afterwards and I suspect you will have some tight clearances on a number of the valves. The shims should be set at 004 on the intake and 005 exhaust. The 003 spec Honda recommends is not a good one to use these days. The shims are still available aftermarket at around $10 each last time I looked. Z1 Enterprises used to sell the shims years ago.

Very easy to go down a rabbit hole on this motor, as it has lots of areas the wear items such as the cam chain tensioners and slippers on the top end of the motor.

Good luck.
 
Mikemo said:
Hi Tom. It's been a long time. I hope you are doing well.
Click to expand...
Hi Mike, we're doing as well as can be expected at our age, thanks.

I wish I could recommend someone in the general area to do that valve seat for you, but United Speed World's owner - the guy you met when we went there a few years ago - passed away and his widow sold the business. The new owners are not overly trustworthy, though they kept the same machine shop guys. I just don't know if I'd trust getting it done there (though these valves and seats are significantly larger than your VF500F) mostly because of the ownership at this point.
Mikemo said:
It seems weird that only one valve was out and the rest of the clearances were in-spec. I don't think this engine was ignored. There was evidence that the valves were adjusted at least once before. (sealant evidence in valve cover). I wonder if the exhaust valve failed because of carb issues (too lean and hot), or maybe just chipped, then sunk in the head as it burned away. Who knows.
Click to expand...
As Flyin900 notes, these models were known for valve recession into the head and doing regular valve adjustments on them is critical. Mike Nixon knows these engines well and has mentioned it on his website too.
 
For head gaskets cometic probably has them but you'll have to call and specifically ask. You can use CMSNL to find what the part number is for "gasket kit a" for that year and search for it on ebay. I find NOS Honda gasket sets this way and usually cheaper than an aftermarket kit!

There was someone who could do valve jobs, and hes been mentioned on this site before but unsure his name. Pretty sure Tom knows. I found there is a local NAPA around me that amazingly still had a machine shop. and the guy is near retirement age but knows and respects the tight clearances for these old bikes. Worh asking around, but if you find someone local make sure to ask questions. Regular automotive machinist may not be up to to the task.
 
Depending on what your friend wishes to spend. Cutting 16 new valve seats will be spendy. I would pull all the valves and determine how they look after you clean everything up. You will not find NOS Honda gasket kits as suggested above. They were an A and B gasket set and long gone now. Finding just a NOS head gasket will be tougher I suspect today. Once you released the tension on the jugs with the head removal, it's a crap shoot on not removing the barrels too. I used to use some fabricated sleeves and washers to keep compression on the barrels to avoid pulling them too.

As I noted not many on here have worked on a DOHC motor and lack the familiarity with them.
 
Just out of curiosity I looked at CMSNL and, as David suggested, the A and B kits are not available currently. They do offer cylinder head gaskets and other individual gaskets, so worth considering. I defer to David or others as to the quality of the gaskets available there.
 
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