YZ80A 1990 Model Disassembly

Taking The YZ 80 Apart
Basically all I am doing here is taking the bike down to it's separate components.

I like to use a camera to document how and where parts go because I know I will forget. Below is a picture of how the overflow hose/tube (the yellowed one) from the radiator is routed. The bottom hose has been taken loose and is draining into a funnel back into the coolant jug. The fluid was new and only in the engine for break in period. 

In this picture the routing of the throttle cable and overflow radiator hose.

Here shows electronics from right side and throttle cable as it went behind the radiator itself. This picture will also aid in other items placement as can be seen for future reference.

Left side of the electronics and engine wires going to the stator/flywheel side in the engine. The exhaust pipe is gone but the hanger is still present. 
                                     

The air box has been removed and disassembled for cleaning. Using several ziplock bags and a sharpie marker to label and contain all the bits and bolts and screws etc as it comes off the bike.  Plastic containers are also used to put assemblies in with bagged items to keep together. I bet this box has never ever been cleaned form when the bike was new. It was damned nasty inside and out.  

Here the screwdriver is used as a pointer to show where the ground wire goes.

The front brake and the throttle assembly. See all those white speckles from someone's previous project overspray? That will clean up with some Goof Off spray. Shouldn't give away secrets of the trade but some things are just to great not to.  

Just showing the direction of one of the engine mount bolts (lower). It goes in from left side of bike.

This is the front mount from left side of engine/bike.

Removing A Handle Bar Grip for Reuse
Should have shown this when showing the throttle control and front brake lever. Anyway, to remove a handgrip, that you might want to save, if it's in good shape to reuse after refinishing your bars or putting on a different set of bars. Works on bicycles also. Take a vey small flathead screwdriver and carefully insert it between the grip and the handlebar...

next take some WD-40 with the straw, lever up the end of the grip just enough to get the tip of the straw in there and spray. Insert the driver a quarter way around and do the same. Let it sit for a minute. You should now be able to remove the grip with a twisting motion.

You can see the WD-40 swirls on the bars from the twisting of the grip. Comes off like buttah.

Now back from the commercial break. Lastly, this is the rear mount which also doubles as the swing arm pivot mount, it comes across from the right side of the bike. The other two bolts to the right are for the chain guide that protects the swing arm, this one will have to be replaced as it was worn through.

YZ 80 Engine Test Run

Test Run The YZ80

After getting the top end re-assembled we gave the nasty carburetor a bowl cleaning. This is how it looked before.





Filled the gear case with 10W-40 Racing oil, the proper amount is found on the engine case.

We put on the exhaust pipe, filled the radiator,

and the fuel tank and put in a couple of quarts of a slightly heavier fuel/oil mix to be sure top end was adequately lubed for break in.

The bike fired up after around fifteen kicks and run for five minutes at various RPM levels. No over revving here, just slowly bring up the engine revs and hold for a few seconds then let it idle down.  Cut it off to cool for fifteen to twenty minutes and did it again. After the first run and each cranking after she fires right off on two kicks or less once I figured out to give just a little bit of throttle while kicking it over. We did this several times to begin with. Finally it was taken for a little ride without putting it under a heavy load but also not lugging it around. After a couple times we got on it moderately going through the gears to be sure transmission was in working order. The next day I couldn't resist one good blast around on it before disassembly of the whole motorcycle, the little YZ 80 pulled the front wheel off the ground with no effort at all...much to my surprise as I didn't mean to do it. Back in the shop with it to begin the tear down process.

1990 YZ80 Top End Assembly

Assembly of the Piston and Cylinder of the YZ80

Laying out the top end assembly components.
4 brass head stud washers, 4 head stud nuts, connecting rod needle bearings, gasket set (top head gasket, cylinder base gasket, reed valve gasket and a new exhaust O-ring),  New wrist pin (or piston pin), 2 c-clips for wrist pin, piston, piston ring, Warren 2 cycle oil for assembly lubrication, cylinder head, reed valves, carb base, vent hose.



The cylinder has been bored and honed to size and decked (decking makes sure that cylinder head mounting surface is flat for good sealing). I failed to show the bottom of the cylinder so you can see the chamfering which allows easier installation of the piston and ring into the cylinder. Even though the cylinder looks good and clean you need to wash it thoroughly with dishwashing soap to make sure all the honing grit and minute metal shavings are gone. You can be sure it's clean when a clean dry rag wiped in the cylinder comes out clean.

Next, install the piston ring into the top of the cylinder before installing it on the piston. You need to do this to check your ring end gap.

To square the ring in the cylinder use the top of the piston to carefully push it (ring) down a quarter of an inch to half and inch in the bore.

Remove the piston and use feeler gauges to check the ring gap. Minimum ring gap is .004" thousandths of an inch per inch of bore. (A one inch bore would be .004" a 2 inch bore would be .008" and a 3 inch bore would be .012". In other words multiply the size of bore X .004" to get your ring end gap.
In our case the bore is right around 1.9288 so we round it up to 2 inches X .004" and we get .008" ring gap.

If you need to open up the ring gap, place a small metal file in a vise to hold it secure. File in one direction only (don't double file which is staying in contact and rubbing the ring up and down the file without losing contact with file). Take your time and check often.

Using a ring expander tool install the ring on the piston in the groove making sure ring gap is at the small pin in the ring groove. You can install a ring without using the tool but you can break the ring if not careful, twisting the ring during installation will more likely break it. Use the tool. The Wiseco racing piston for our YZ80 only has one ring. Some pistons have two or three rings.

Take some  2 cycle oil and coat the cylinder walls using your finger. Barely visible in the photo can be seen the cross hatch pattern left from honing cylinder walls. Set the cylinder aside.

You don't have to but generally I like to install one of the piston pin clips in the piston before final installation. These clips can be a bugger to wrestle with so having one already done aids in the installation of final assembly of piston onto the rod.

Coat with 2 cycle oil and install the needle bearing race into the rod (not shown) carefully place the piston over the rod (making sure piston is in correct orientation). Coat piston pin with 2 cycle oil and insert through piston and rod until it stops against the clip on other side. This is where the one clip already installed in piston helps out. When pin is seated all the way in you should see the other groove for the second clip. Now install the other c-clip into the groove by putting one open end inside the groove and hold it tight against the piston while using a small flat blade screw driver to gently pry the remaining part of clip into the groove. Make sure it is seated completely in the groove using the screw driver to push it snug up to the piston pin, you should hear it snap in.

Below you can see the base gasket sticking out from under the blue shop towel. You can put it on after the piston is on the rod but I like to do it before so I don't forget as I did by not showing it earlier to you. The shop towel is to catch any parts down in engine during assembly of the piston to the rod. Those little clips like to spring off sometimes when you try to get them in the groove. you don't want that bugger going down in the engine.

Carefully slide the cylinder over the studs and down on the piston. Pinch the ring into the groove and slide cylinder down. The chamfer on underside of bore aids in guiding the ring in the cylinder. Don't force it, take your time. As soon as the ring enters the cylinder the piston will glide right in.
Next install the head gasket noting in the repair manual the correct orientation to insure coolant can flow properly. Also note the small arrow engraved in the piston top at left side of piston in picture. This is so you know the piston has to be installed with arrow pointing towards front of engine. If not then your ports in cylinder and ports in piston itself will not line up, you will quickly burn the engine up if not installed in correct orientation. More over the engine probably won't even start.

The cylinder head has two (On this YZ) hollow locating pins, they do not allow coolant to flow. The gasket holes where the locating pins go are not large enough on this set at least. The paper was cut away from hole circumference where the pins go to allow head to seat using a slightly larger sharp drill bit that is the same size as the outside diameter of those locating pins.

Install the head on ( you may need to tap it with a rubber or plastic tipped hammer. Do not use a metal hammer please). Install the four copper washers on the studs and then the nuts of course. Use a torque wrench to tighten nuts in two stages. Tighten all in a crisscross pattern to 9 ft lbs, then tighten all four again to 18 ft lbs in the second stage. If the head is not going down evenly stop and check to see what might be causing it to bind.  Check the operation of the piston for binding using the kick starter by hand. It should move with ease without a spark plug installed. It should move up and down in cylinder nice and smooth. if it does then your good to go.