When I first got my 1976 R75 /6 it smoked on start up, but not terrible. I seldom, if ever, use the side stand. When I purchased the bike from the previous owner, he had bought the "barn find" at an auction and told me it had 12K miles on it which was indicated on the speedo. Shortly after taking it home the speedo started spinning like a Vegas slot machine and I had it refurbished. He said he did replace all the fluids in the bike.
Now, a few months later when I start the bike, it fills the neighborhood with white smoke, and I am worried someone is going to call the fire department. After it warms up, there is considerably less white smoke. I can’t really tell if it is completely white or has a bluish tint. The smoke made me think I should do a compression test. I got these results:
Left Side compression, (carbs off) 135
Right Side compression, (carbs off) 110
Left Side compression, (carbs off) - added oil into the head 150
Right Side compression, (carbs off) - added oil into the head 135
Normal Ranges:
Good - 147psi
Normal - 125 to 147psi
Poor - 125psi
Today, I did a leak-down test on the right side, and so much air was escaping out of the oil fill hole that I couldn’t tell if either valve had an issue. I didn’t bother doing the left side because I figured I would replace the rings on both sides anyway. When running, the left exhaust is 100 degrees hotter than the right; I am not sure if that is significant or means anything.
Is it safe to assume that the bike has a LOT more miles on it than the previous owner told me?
Also, anything I should consider doing before I replace the rings?
I figure I should do an additional leak-down test after replacing the rings to see if the valves need work.
If it has enough miles for the rings to be bad, what other components need a serious look at?
Thank you
Hello Rodney, sorry you got such an issue. Yeah, the milage could be anything from sub 100K to 2 or 3 hundred K. Also, barn finds are sometimes stored in barns where moisture does a job on the cast iron cylinders.
So if it were my bike, I would pull the heads off, check the valves seats and guides. Check the jugs, to see if they are not oblong or pitted. Check ring gap. As for just installing new rings, I would only do that if tolerances are good enough in the piston size and actual bore size and shape. You may find you might have to go one over what ever pistons are in the jugs now.
Make sense? LOL, I have the same issue now with a bike I am restoring, unknown miles and perhaps in need of one over bore. Good luck, St.
Beware! I do not suffer fools gladly! St.
> Your compression numbers without carbs are the ones to believe.
> However, compression has nothing to do with oil control. On the pistons are compression rings and an oil control ring. Two different ring types; two different jobs.
> IMHO, due to the difference in piston ring job descriptions, the "leak down test" actually tells the mechanic very, very little. The compression stroke that the engine runs on happens in a thousandth of a second. So how is a pressurization test that lasts minutes supposed to represent any type of equivalence or hold any relevant meaning ?
> Physical inspection of the cylinder and valve seats will tell you if the speedo is lying about the bike's mileage. The cylinder needs to be mapped for taper AND ovality, after being honed with a bead-type glaze breaker hone in hot soapy water. Taking these multiple readings requires a right-angle, thousandth-reading, dial indicator, an instrument you are not likely to own.
> Actually, I believe what you will find is a ring of rust pitting circling 1 or both cylinders. The only cure for this is a bore job and 2 new pistons. This is the most common cause of extreme oil smoke on vehicles that have experienced long storage.
> If you don't find extreme cylinder wall pitting, then I would jump directly to replacing the sump breather flapper valve with the later model Airhead reed valve. This is located in the top of the engine, under the starter motor. Snowbum can tell you more about why this is a needed update. In fact, you may wish to make this worthwhile update regardless.
> Your write-up can be read in a way that makes it sound like you may be doing an extended warm-up ?? With multi-grade engine oils this can be more harmful than helpful. My suggested cranking sequence is: start the bike, put your helmet on, turn OFF the choke, and then take off. Stationary warm-ups longer than 1 minute are really not necessary. In the shop, you might find it helpful to run a bike for extended times. In those cases a box fan should be employed.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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