CR125 Rework

So with the spark plug theory only being partially correct it would seem that hunting down the real cause of my horrid engine performance is back in the spotlight.

Then what is it? Well after noticing that I lose about 50% of my cooling fluid in 10 minutes of racing, evidence would point to a couple of possibilities. This "water loss" was found to be coming out of the over-pressure blow off valve (that is what the cap on your car's radiator is for).

So the possible theories are as follows....

1) Over heating causes the rapid water loss, and the poor motor sputtering is something else

2) The water loss is due to a faulty radiator cap (spring loaded mechanism), and the poor performance is due to over-heating/detonation due to over-heating

3) Leaking head-gasket is allowing water to enter the cylinder
3b) Leaking head-gasket is allowing high pressure gases to enter cooling system and causing over pressure blow off. Loss of pressure in power stroke (ever stroke for a 2-cycle) leads to poor and unreliable engine performance


As you can see by the detail I put into 3b, I hope it is 3b. Since I could solve two known problems in one 'go'. So why does this seem like the most likely candidate to me? Well if the piston is pushing this compressed gas into the cooling system, the motor is going to lose some power (observable one), it will definitely cause an over pressure in the cooling system and cause a leak (observable two), and since we determined that the water was exiting from the blow off valve (observable three). Thus, the leaking head-gasket theory would match the available evidence. Since water entering into the chamber wouldn't account for why we found the over-flow can full over water when we emptied it right before a test run. However, the pictures below would suggest that this does indeed happen. So perhaps it occasionally does happen in combination with the above theory. Both would require a leaking head-gasket.

That being said please bear in mind that the final solution could be a combination of any/all of these.
I'm going to replace the radiator cap, since that is a common part to replace and this one is old and crappy regardless.

Now time for lots and lots of pictures:

The motor with the hoses removed from the cooling tubes and the spark plug wire removed

Nuts removed and stored

Head removed and a view of the head-gasket (black), the studs (bolts), cooling ducts (brown), and piston (black in center)

Bottom view of the head

Close up view of piston shows what seems to be a calcium deposit. That could have only come from water (non-distilled) leaking into the combustion chamber.

Same view only zoomed out

Close-up of the head shows a rather clean head. The carbon might have been washed off from water blasting it off. The head looks pretty good over all. The ducts will be cleaned vigorously with mineral-spirits and scotch bright pads.

Cooling duct of the cylinder (the part that attaches to the head)

A scrape is visually evident, though I couldn't feel any groove with my finger so it isn't as bad as it looks. In the future I might have this area re-nickled and resurfaced.

So what does all this mean? Well I need to take the head to a friend's shop to check how level and straight the surfaces are. If they are not level enough, I will simply lap them level. For those that don't know what lapping is. It is taking a VERY flat surface like a machined piece of aluminum or a granite block, putting a piece of sand paper (some-what fine grain) on that flat surface, and methodically rubbing/turning the surface you wish to flatten in a figure-8 pattern. This will sand the surfaces back to a very level surface. The head will need to mate flawlessly to the cylinder and the head-gasket.

After that I will clean and re-die the head studs because they seem too dirty to reliably torque the head on to. The cleanliness of these are important because when you tighten something like a head back onto the cylinder you need to have all the bolts apply the same pressure to the head. By each bolt applying the same pressure to the head, you allow for the mating surface to be as flat and even as possible. To accomplish this you use what is known as a torque wrench. The wrench tells you how tightly you are pushing the nut onto the bolt. The problem is that if the bolt you are tightening the nut on-to is rough or dirty, the torque readings will be thrown off. You might actually be torquing the nut to the correct value, but the force that nut is actually applying to the head will be less than you expect.

So that's a rather technical update. I have no videos of me driving like mad or running into bushes.