Valve/Piston clearance question

Hi All; I'm working on a '79 KZ650-C3. We fitted new OEM 1st over pistons/rings and sent the head to a pro in Florida for a street valve job. The shop sunk the valves too far into the head and as a result we have a brand new engine that runs like a tired one! The compression measures 140-145 across the board. My intent is to get the compression back by machining the head and this leads to the question, "How much is safe". I understand the head volume is to be 24.7 cc's and that should yield the 156-185 psi. If it takes .030 to get the correct volume, will I risk bending valves? All motor components are stock factory. The motor has about 5K on the new parts and runs like a watch, just no power. I'm trying to save the work of using modeling clay to check clearance myself. Thanks in advance and ride safe, Steve

if i recall on a kz900 u whant .080" intake and .060" exht.dont forget to check piston to head..040"should be min. including compressed head gasket.u can use solder to test.take 2 pieses and bend then into an L (sort of) then run them into the spark plug hole.try to get 1 on each side of the cyl.paralell to the crank and turn the eng over by hand.the piston will squash the solder and u can measure it with a mic.u will have to experiment with diff. thicknesses.probably .060" will work.if u use only 1 ,the piston will rock and give u inacurate results.as it is, u will likly get diff measurements on each and u can average.u can do the same with piston to valve(just do 1 at a time).once u know what u have it will be easy to figure out how much u can take off the head.u should probably stay below 180 psi if u want to use pump gas.can u still get shims for these sunken valves?

I don't know what compression is new on your engine, but something else may be wrong here. 140 is decent compression isn't it? And What's the timing set at? And if there's barely any valve clearance now, that's going to mess with compression also.

Yeah! nads.com I would kill for 140 compression. My did a compression check on my 1980 ltd kz1000. 80 is all I could get. Come to find out I have 0 ZER0 0 valve clearance. My intake in not closing and neither is my exhaust valve. I just ordered new shims so I can fix my valve clearance! Cant wait to replace them and do another compression check?

140 is great last I checked, put some playdoh or other putty on the cylinder domes and crank the bike over. take the thinnest piece and calculate for sine. ORRR....


take off whatever you need and make up for the space with a thicker base gasket.

-J

ps: dumb question but have you doublechecked your cyl firing order?

edited: now that I think of it I wouldnt go much over that without some serious investigation as to why it isnt running right. 140 is great and if you dont detect any binds then look in the fuel or ignition system...petcock filling the bowls fast enough?

Post edited by: A2-Cow-A-Socki, at: 2007/08/21 15:57

Only thing I haven't seen mentioned so far it the centrifugal advance. It you take the points (ignition) cover off and shine a light on it (don't need a timing light for this step) as you accelerate the engine you should see the weights smoothly swing out as you accelerate and smoothly close and you decelerate. If they work ok, then you check the timing with the timing light or use the static method.

Wow! Lots of good comments and advice. I'm a weekend warrior when wrenching, so here goes:

mark1122, The head shop ground off the ends of the valves. On a couple of the cylinders, I had to grind the exposed ends of the keepers to keep them from hitting the shims!

nads.com, The timing is set to spec, with a Dyna S system. The valve lash is correct.

A2-Cow-A-Socki, Yep the play doh is the way to go, I was trying to save that exercise by asking if anyone knew from memory. The firing order is correct. No popping or anything. The combustion chambers and tops of all pistons are coated with black "soot", so I'm sure I'm getting too much fuel (float level, maybe). The plugs all look the same (rich). The petcock is OK.

The Milkman, I have not checked the advance yet. The bike just didn't have the "hit" when above 5000 RPM. I sure hope bumping up the compression to spec cures the problem. I figure I'll degree the cams and go through the carburetors (again). It's such a sweet little motor. . . Thanks to all and ride safe, Steve

mark1122 wrote:

if i recall on a kz900 u whant .080" intake and .060" exht.

That is backwards. The larger clearance is always on the exhaust, as the piston is always chasing the exhaust valve.

If you miss a shift, or over rev it enought to float the valves, the exhaust valve gets "late", and the piston catches it.

Jay

Post edited by: APE Jay, at: 2007/08/25 15:12

isnt the extra clearance in exhuast also to allow for expansion of exhuast valve compared to intake valve ?

Post edited by: PLUMMEN, at: 2007/08/25 15:47

you do get a bit more for the ex, heat does play into the clearance factor, cam profiles can also effect the amount of clearance you need