Changing exsaust valve and valve guide on a Kohler 12 horse engine.how do I remove the guides.

First remove the valve cover to see if the camshaft is damaged. If not, maybe you can get away with changing the lifter. Remove the camshaft and find the bad lifter. It will be worn on the surface that the cam faces. The valve spring may also be broken. The oil is likely coming down the valve guide. Remove the valve spring and check to see how loose the valve is in the guide. If loose, you'll need to pull the head and have the valve guide replaced. Not sure if that is going to be worth tne cost.

Pull the plug.
With the piston up close to TDC, take a length of ~5/16 diameter cord and feed it into the sparkplug hole till the chamber is full (leave a length to pull out the cord.) The purpose of this is to keep the valves from dropping into the cylinder which would mean you have to pull the head.
Pull the valve cover, remove the valve keeper and springs, remove and replace the seals. Reassemble. Adjust the valves.
.006 int/ .008 exhaust. Install valve cover. Pull out cord.
Replace the plug.
Don't nut up over the cord thing. Kart racers use that technique all the time.

Here's a AutoZone repair guide on removing your valve cover gasket. Repair Guides Engine Mechanical Components Valve Covers AutoZone com

Follow the detailed procedure mentioned in the link below to remove replace VALVE cover:----
Click the link below:---
VALVE COVER GASKET REMOVAL GUIDE http://technoanswers.blogspot.in/2012/06/valve-cover-gasket-removal-guide.html

-----------
For more help on these issues,check the link below:---
Car oil leaks from valve cover gasket? http://schematicsdiagram.blogspot.com/2011/12/car-oil-leaks-from-valve-cover-gasket.html

--------
Lexus RX: Tapping noise under Valve Cover At Low RPM? http://technoanswers.blogspot.in/2012/04/lexus-rx-tapping-noise-under-valve.html
---------- How to replace Timing Belt,Water pump And Valve Guides on Mitsubishi Montero http://repairhelpcenter.blogspot.com/2012/10/how-to-replace-timing-beltwater-pump.html

----------
2000 Toyota Solara: Valve Cover Gasket Replacing? http://technoanswers.blogspot.in/2012/04/2000-toyota-solara-valve-cover-gasket.html

----------
Valve Cover Keeps Cracking? http://schematicsdiagram.blogspot.in/2012/04/valve-cover-keeps-cracking.html

----------
These details will help.
Thanks.

Blowing black smoke means fuel mixture is too rich, possible choke is stuck partically closed.
Blowing white smoke especially driving off the traffic lights indicates either worn valve guides or worn piston rings. Valve guides require the head to be removed to replace valve guides.
Replacing piston rings means a major engine repair. It maybe better to get a reconditioned motor than replace worn piston rings because other worn engine parts should be replaced at the same time.

it is most likely rings

Read this site before you go to the trouble of changing the valve guides.
http://www.saturnfans.com/forums/showthread.php?t=65966

Use castro GTX 10W30 I use it in my Saturn and it does not burn off or leak out till i remove it.

Timing Chain and Sprockets
REMOVAL & INSTALLATION

1. 
   
   
   
   Fig. 1: At 90 degrees past TDC, the crankshaft sprocket timing
   mark and keyway will align with the main bearing cap split line
   
   
   
   
   
   
   
   Fig. 2: Exploded view of the timing chain and sprocket
   assembly - SOHC engine
   
   
   
   
   
   
   
   Fig. 3: Insert the timing pin to ensure that the camshaft is at
   No. 1 TDC - SOHC engine
   
   
   
   
   
   
   
   Fig. 4: When the camshaft is at TDC, rotate the crankshaft
   counterclockwise 90 degrees to achieve TDC
   
   

DOHC Engine

1. Disconnect the negative battery cable.
   
2. Remove the timing chain front cover.
   NOTE: During timing chain and sprocket removal, position the
   crankshaft 90 degrees past Top Dead Center (TDC) to make sure the pistons will
   not contact the valves upon assembly.
   
   
   
3. Carefully rotate the crankshaft clockwise so the timing mark on the
   crankshaft sprocket and keyway align with the main bearing cap split line.
   
4. Remove the bolts, then remove the timing guides and tensioner.
   
5. Remove the camshaft sprocket bolts, using a 7?8 in. (21mm) wrench
   to hold the camshaft. Then remove the timing chain and camshaft sprocket. Remove
   the crankshaft sprocket, if necessary.
   To install:
   
   
6. Inspect the chain for wear and damage. Check the inside diameter of the
   chain, it should be no more than 23.15 in. (588mm). Inspect the chain guides for
   wear or cracks and the timing sprockets for teeth or key wear. Replace
   components as necessary.
   
7. Verify that the crankshaft is positioned 90 degrees clockwise past TDC. The
   crankshaft keyway should be at 3 o'clock aligned with the main bearing cap split
   line to prevent piston and valve damage.
   
8. Install the camshaft sprockets, retaining bolts and washers. Make sure the
   letters FRT on the sprockets face forward, away from the cylinder block. Use the
   wrench flats provided on the camshafts to hold the shaft and tighten the bolts
   to 75 ft. lbs. (102 Nm).
   
9. Bring the camshafts up to No. 1 TDC by rotating the camshafts and sprocket
   until the dowel pins are at 12 o'clock. Install a 1?6 in. drill bit
   into the hole in the sprocket about 9 o'clock.
   
10. If removed, install the crankshaft sprocket, then rotate the crankshaft
    counterclockwise 90 degree up to No. 1 TDC (keyway and sprocket timing mark at
    12 o'clock, in alignment with the block timing mark).
    
11. Position the timing chain under the crankshaft sprocket and over the
    camshaft sprockets so 2 silver link plates align with the reference marks on the
    camshaft sprockets and another 2 plates align with the downward tooth (at 6
    o'clock position) on the crankshaft sprocket. Excess chain slack should be
    located on the tensioner side of the cylinder block.
    
12. Verify that the crankshaft reference mark aligns with the cylinder block
    mark at 12 o'clock and that the timing pins are installed in the holes at about
    the 9 o'clock position. Remove the timing pins from the camshaft sprockets.
    
13. Install the timing chain fixed guide to the right of the block face toward
    the water pump. Tighten the bolts to 21 ft. lbs. (28 Nm) and verify the chain is
    snug against the guide.
    
14. Install the pivoting chain guide and check for clearance between the block
    and head. Tighten the bolt to 19 ft. lbs. (26 Nm) and verify the guide pivots
    freely.
    
15. Install the 2 forward camshaft bearing caps and the upper timing chain
    guide, then tighten the retaining bolts to 124 inch lbs. (14 Nm).
    
16. Retract the tensioner plunger and pin the ratchet lever using a 1?8 in. (3.18mm) No.
    31 drill bit inserted in the alignment hole at the lower front of the component.
    Install the tensioner and tighten the bolts to 14 ft. lbs. (19 Nm), then remove
    the drill bit.
    
17. Make one final check to verify all components are properly timed, then
    remove all timing pins.
    
18. Install the timing chain front cover.
    
19. Connect the negative battery cable, start the engine and check for leaks.
    
    
    
    
    
    Fig. 5: Rotate the crankshaft clockwise until the timing mark
    on the crankshaft sprocket and keyway align with the main bearing cap split line
    
    
    
    
    
    
    
    
    Fig. 6: Loosen the timing chain tensioner mounting bolt . . .
    
    
    
    
    
    
    
    Fig. 7: . . . and remove the tensioner from the engine block
    
    
    
    
    
    
    
    Fig. 8: Retract the tensioner plunger and pin the ratchet lever
    with a 1?8
    in. (3.18mm) drill bit inserted in the alignment hole
    
    
    
    
    
    
    
    Fig. 9: Remove the mounting fasteners from both timing guides .
    . .
    
    
    
    
    
    
    
    Fig. 10: . . . and remove the timing guides from the engine
    
    
    
    
    
    
    
    Fig. 11: Remove the camshaft sprocket bolts, using a 7?8 in. (21mm) wrench
    to hold the camshaft
    
    
    
    
    
    
    
    Fig. 12: Remove the timing chain and camshaft sprockets
    
    
    
    
    
    
    
    Fig. 13: Insert the timing pins to verify that the camshafts
    are at TDC
    
    
    
    
    
    
    
    Fig. 14: Make sure that the silver link plates and reference
    marks are all in alignment as shown
    
    

Yes, you can, but you need to have a special tool that screws into the spark plug hole, so that you can attach an air hose to it to apply pressure inside the cylinder against the back side of the valves to keep them from falling down into the cylinder. The problem is that if the valve seat is not sealing, such as is commonly the case when a valve is burned, then the tool will not help you and you will have to remove the head anyways. Ford makes special tools for removing the valve guides and to install the seals.

There can be several causes for low compression.
Chck the obvious first. the spark plugs must be checked that they are all in good condition. not burnt, the gap must be good( for your engine).Check the wires and coil so that good spark is obtained. Blue spark , not weak yellow spark. After everything is good. Check the cylinders to see if they have compression. To test the cylinders do the procedures shown below.

If the cylinder shows little leakdown or holds good compression when a little oil is squirted into the cylinder (wet compression test), it would tell you that the engine needs new valve guide seals and/or guide work. Most late model import engines have positive valve guide seals. Often, the guides are fine, but the seals are worn or cracked. The seals can be replaced on some engines without too much effort and without having to remove the head.

compression test will help you determine if the oil is getting past the valve guides or the rings. If the cylinder shows little leakdown or holds good compression when a little oil is squirted into the cylinder (wet compression test), it would tell you that the engine needs new valve guide seals and/or guide work. Most late model import engines have positive valve guide seals. Often, the guides are fine, but the seals are worn or cracked. The seals can be replaced on some engines without too much effort and without having to remove the head.

finally if the rings on one or more of the pistons can caus compression leak. It must be located and fixed.