Compression dead... 78 CB400 t-1

Bird76Mojo said:

It'd probably be a good idea to lap the valves but I'm new to certain parts of these bikes myself.. Others with more knowledge will hopefully chime in soon.. :?:

I've heard of people turning the head upsidedown and filling them with acetone to determine how bad or if the valves leak. Acetone has less surface tension or viscosity than water. I'm assuming that's why they use it.

GB :mrgreen: :?:

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That's exactly right, Gerald.
I turn the head on it's front (or back), so the ports are facing straight up, then pour acetone into the ports - first the intakes, then the exhausts.
If there's a leak, acetone will find it.
If the valves don't leak at all, don't touch 'em !!!!!
Lapping valves well is partly an art, takes a lot of technique. It's the kind of thing that, if you don't do it very often, you can really make a mess of things. A good machine shop can do it perfectly every time - my guy only charges about $10 per valve, and I happily paid him for it on my 450.
Now, on Wifey's CB200 I lapped them by hand myself, but I was sweatin' like a rat at a Jack Russell Terrier convention. I had a spare head and valves just in case I screwed it up too badly. I hadn't done it in years and it took a while to get it right.

As for the cylinders, take them to a machine shop and have them miked - they won't charge much for this, a few bucks. If they're in spec just hone 'em and throw in some new rings.
One thing to keep in mind is the cylinder "taper", the difference in the bore from top to bottom. The allowable spec on this is very small, way less than 0.001". And it matters.
It's why I bored out my 450 project, the taper was about 0.0015".

Disclaimer: my experience is limited to mower engines(many), VW(3), chevy small block(1), and my honda 100.

Regarding the valves - if you are going to have a machine shop check the cylinders, you may want to ask them to check and lap the valves in, and check the valve guides. I recommend you check the exhaust valves especially to see if any of the edges are "burnt" from the heat. The ideal valve wear will be a uniform ring around the center of the face of the valve corresponding with a similiar ring in the center of the seat of the valve.. Do the valves have rubber O-ring seals? if so they probably need to be replaced. Buy a valve spring compressor if you are going to do it yourself.

Rather than hand lap small engine valves, I have used a simple tool made by taking a shock absorber bushing, squeezing it between two washers with a nut on a 3/8" x 3" bolt. I attach this to a portable drill then add a little water and some valve grinding compound to the face/seat of the valve. Power the drill and let the bushing tool spin the valve from the edge of the top of the valve. Do not apply pressure in just one place and you can reverse the drill also to get a good seat. With a little practice on how much down pressure to apply, this has worked great for me. Be sure to clean out any and all compound when finished. Check the wear pattern when done. If you can get a drill on the stem of the valve, put some copper between the chuck and the valve and just use the drill to seat them. [If this is a bad way to do this for some reason let me know]


Make sure the grooves in the pistons are completely clean before you put in the rings. Look for any nicks in the piston that will cause the ring to stick. Be sure to stagger the rings (3 rings = 120degrees apart). Use a ring compressor if you can. I have used a metal can and a hose clamp but the right tool works best.

Larry