Topic: CB750 K2 bent valve??

[disco]

Hi Guys,

I'm rebuilding a CB750 K2. It supposedly had 15,000 miles on it when laid up. I've given it new bearings throughout the transmission & new rings/hone, lapped the valves etc. I finished installing the cam & was about to set the valve clearances when I noticed the No 1 inlet valve seemed a bit off with its rocker. I put a straight edge across the valves & No 1 is definitely out compared to the others. All the valves fitted up with no problems at all & were snug in the guides. The cam towers have the 8 x 14mm dowels fitted underneath locating them. I guess I have to pull the valve to find out whats going on. Is the head gasket toast now? 

btw The rockers have an 'A' or 'B' stamp on them. I have installed 'B' & 'B' for No 1, 'A' & 'A' for No 2, 'B' & 'B' for No 3 & 'A' & 'A' for No 4 cylinder. That's how they were installed & looks right to me.   

[heyitsrama]

Looking at this image below, it would appear that "letters" are matching with the cylinder



I would take a look at this thread, at the minimum pull the value out and take a peak, might also be worth looking into the valve guide.
http://forums.sohc4.net/index.php?topic=132233.0

[disco]

Hi Rama, thanks for the reply. It seems certain the valve is bent slightly. I removed the rocker shaft & was able to rotate the valve 180 degrees with vice grips. Then I replaced the shaft & rocker. the rocker was now centred on the valve. Not sure how this occurred but I will have to strip it back down now.

[heyitsrama]

Thats good news to hear that it was "fixable", although running it would probably ruin the guides. I had to remove one on my bike I'm working on, I used a dremel to cut down the bent portion where the spring is, then pulled it out from the other side. In hind-sight I'm not sure if that was the best approach tho, if there is still a portion the bend above the guide, pulling it through can damage the guide as the stem travels. 

[Flyin900]

You can reuse the head gasket if the motor hasn't been fired. Which seems to be the case here. The gasket will still look new when you remove the head and is fully reusable.

[CycleRanger]

If the valve stem is bent it must be replaced.

[disco]

Hi Rama, 900, Cycleranger,
Thanks for your advice. I pulled the motor down today & sure enough I have a bent valve, as well as broken valve guide. I checked and the tappet screw adjuster for that valve was not fully backed out. It must have caught up on the edge of the valve as per that attached post. I do recall hearing a noise as the cam tower was tightened down. Oh well...I'm annoyed with myself for making a stupid error but glad it was caught now rather than later. I have a spare head with a decent valve that I can use & Yamiya have a valve guide on way to me.