686cc of China Muscle

Well let's start from the top. I mentioned to a family member I was wanting to pick up a few more 4 wheelers. He mentioned he might let loose of a 700cc that he had just purchased due to back problems and not being able to ride anymore. I asked what he was looking to get for it. He said $1500. So, naturally I loaded the trailer up and went to look at it. I took it for a test ride and it rode really well for about 15 minutes then overheated. I checked the dipstick and the crankcase was filled to the brim with milky oil. He said the shop he bought it from replaced the head gasket and it was supposed to be fixed. He said forget the money just take it with me and see if I could get it fixed. So, she came home with me.

686cc single cylinder, CVT transmission, 75 miles on the odo, 750lbs dry, 33.5hp, liquid cooled, electronic 4wd with electronic locking front diff, 4 wheel independent adjustable suspension, winch etc. etc..

This bike is modeled very closely with a Yamaha Grizzly 700.

The teardown begins

Winch, horns, and radiator

Looking from the passenger side, header, cylinder head and water outlet. The black cover at the bottom covers the CVT section of the trans.

From the rear...interesting, it has a single pinion brake on the rear where the front has brakes mounted at each wheel.

From the driver side, oil fill cover, cylinder and head, coolant overflow, and pull start

The front diff

Pulled the valve cover. The contraption bolted on the front of the cam sprocket is the compression release. How it works is simple: The weights on the front pivot and are held in place with spring tension. They are connected to a shaft that runs inside the hollow cam. When they are in a "resting" position, the shaft activates a knob on the backside of the exhaust lobe. When this knob is raised, it blips the exhaust valve open during the compression stroke relieving some of the cylinder pressure. At the point the rpms raise above cranking, the spring force is overcome allowing the weights to pivot which pulls the knob back flush with the exhaust lobe's smooth side thereby preventing the compression to be released.

You can see the holes in the cam for oiling here. The pressurized oil is carried to the cam journals by a hard line that is on the exterior of the engine. The brass lines at the top are EGR. It has a frame mounted pump.

With the camshaft removed, you can see the freeze plugs. Typically these would exit the engine but these exit into what is essentially the crankcase. These were replaced with new ones in-case these leaked.

I pulled the cylinder head, the black thing sticking out is the adjustable timing chain tensioner. In this pic you can also see the oil filter, starter, and front driveshaft.

I pulled the cylinder and it got a new base gasket. I wanted to fully inspect it for cracks/issues.

5 valve cylinder head. 3 intake, 2 exhaust. It has an MLS head gasket.

Each exhaust valve gets its own port and header pipe. They merge into one later down the line.

One carb, splits into two runners, then the larger of the two splits again to feed the 3 intake valves.

The bottom side of the rocker cover. The rockers are shaft mounted to the cover. One of the intake rockers actually opens two of the valves and it is controlled by one really wide cam lobe. So, 4 lobes total to open 5 valves. Interesting.

MLS headgaskets getting the copper treatment

Reassembled, fresh oil, cleaned the entire cooling system and whaddya know. Same exact problem. Open her back up and what do we see:

So she ended up getting a new cylinder head.

To ensure I can keep close tabs on the coolant temp I ordered a Summit gauge, and used a Glowshift universal pod.

The original sending unit fed a dummy light. So I exchanged it with the sending unit for the Summit gauge. It's located in the thermostat housing. Ironically they do not fit these engines with thermostats. This engine likes to run 170-180 from what I've seen.

I've put a few miles on it since being back together the last couple of weeks and I can say without a doubt, this is the nicest and fastest chinaman 4 wheeler I've ridden. With the CVT trans, it's like having a big as* torque converter. From a dead stop, it stalls up quickly to 4200 rpms, and that seems to be the "variable" section where the gearing expands, you accelerate quickly and the top end really comes alive. It tops out around 55mph and probably 6500rpms.

Back up....

as long as you can get parts- my kid had two Chinese junkers- broker every two rides