TOPIC:

I just got my cylinders bored out to around 700cc from my KZ 650 CSR and found that i have almost no compression in cylinders 2 and 4. The valves are the cause, and I figure they have to be replaced because they are probably too small for the new bore. I have no idea what I'm doing in this build, it's just kinda by the seat of my pants that I've been able to get this far. So anyway, what should the measurements of the valves be, or should I just use stock, or what?

did you have compression before?

Emberblade wrote:

The valves are the cause, and I figure they have to be replaced because they are probably too small for the new bore.

This dont make sense... They are probly just too long maybe, or not shimmed properly.

Who did the head work? Were the vavles lapped/changed at all?

Your sure its the valves right?

Have you adjusted the shims correctly? Whats the gaps?
Probly have lobes holding valves open.

Need more info here for a better evaluation of the situation.

before the rebuild, all 4 cylinders were stuck with rust, the engine having been left outdoors for a while (I think without sparkplugs...) anyway, I never did a compression check because of that, and I know that the cylinders are nice and tight now with a weiseco kit, so my only other option is to replace the valves.

I honestly know nothing about how valves work. I'm really just a novice, and I don't have the dough to turn this over to a mechanic. I don't mind getting my hands dirty, but rather than take a dremel and start stripping rust and whatnot, I'd like to know what i'm doing.

As far as I know, my valves are stock for an '81 KZ 650, and they've survived the worst of the rust that overtook the piston heads. However, I'm not sure if bigger displacement necessarily calls for bigger valves. Sorry for the confusion, I meant mixture wise, not fitment. If they are the cause of my compression issue then they definitely need replacement.

Is there some material I can read to better understand how all this works?

factory service manual

As suggested, would first check valve clearances to assure gaps being to factory specs

Clearances are quick and easy to measure, using a thickness gauge.

The checking and measuring -- That's the quick and easy part.

Where adjustment is necessary, the procedure is more complex, as cam shafts must be removed to access, remove and replace the under-bucket shims. And then camshafts properly reinstalled in strict compliance with step-by-step procedures shown in the FSM (official Kawasaki factory service manual).
As it's really easy to bend a valve during reassembly, or strip cam bolt threads, etc.

Would respectfully suggest that a novice may be well-advised to seek competent assistance in the adjustment process.

Good Fortune!

You don't need bigger valves for what you're doing.

Since you say you have no compression in those two holes, it's assumed you already assembled the motor to discover that.

With 2 & 4 dead on compression, my hunch is you installed the cams 180 degrees out of time. Check that first.

I just bought a 1981 Kz650 CSR (the 'H1' model of the Kz650) myself, and you have made a fine choice, these machines are cool, plus you can probably get all parts and tools you need from vendors in the U.S, no need to order stuff from the UK.

Here is a step-by-step to get you to know if valves are an issue:

1) get the shop manual for your bike -- at
jarlef.no/Kawasaki/Series/Z/650/z650files.htm

Once at this page scroll to the bottom and on the right bottom of the page, click on the picture of the factory 'Kawasaki KZ650' shop manual. Now you have the shop manual on your computer.

2) do what larrycavan says -- go to page 6-23 in the manual, which in your .PDF reader will be page 153 of 226. Read the section that says 'Cylinder Head Installation' and follow the instructions there. Pay attention to what Larry recommended, cams installation.

3) now go to page 2-5 in the manual, which is page 35 of 226 in your .PDF reader, and do the valve adjustment process you'll find under the 'VALVE CLEARANCE' section found there.

After you make sure the cams are installed right and the valves are adjusted, re-check compression again. Make sure you're doing the compression check right: all 4 spark plugs removed, throttle held wide open, keep turning the motor over until the compression guage stops increasing.

The one observation I have -- with so much oxidation/rust due to being left outside, maybe the valve stems are rusted and the valves are not moving freely in the valve guides. If rust on the valve stems is preventing the valves from moving freely, it is possible they will not close all the way. If the valves are not closing all the way, that could explain low compression.

If that's the case, you will need to pull the head and follow the procedure in the manual for addressing valve issues.

With the cylinders and pistons work out of the way, you should be nearly there, so good luck, and keep us informed.