Well, here is what it looks like. If you look closely you can see the top ring and a smaller chunk out on the other side. I thought this might just be carbon, but after this photo I scraped the piston with my knife and the carbon is barely more than a dusting. No folks, it's missing metal. There are no dings in the head, so whatever came off must have it's way out the exhaust port.
Carbon build up is uniform. All 3 pistons look the same and so does the head.
So what made it break up like that? I would think if something was dropped in the hole it would beat the hell out the head but there are no marks. I'll try to get e pic if the cylinder wall after a while. Getting light to the subject is difficult.
I was born with nothing and still have most of it left.
Coyote wrote:Well, here is what it looks like. If you look closely you can see the top ring and a smaller chunk out on the other side. I thought this might just be carbon, but after this photo I scraped the piston with my knife and the carbon is barely more than a dusting. No folks, it's missing metal. There are no dings in the head, so whatever came off must have it's way out the exhaust port.
Carbon build up is uniform. All 3 pistons look the same and so does the head.
So what made it break up like that? I would think if something was dropped in the hole it would beat the hell out the head but there are no marks. I'll try to get e pic if the cylinder wall after a while. Getting light to the subject is difficult.
Sorry, to hear about this. I would say your left seal is shot. It pains, to find something like this I know. But, that's the chance we take buying an old bike. You'll get over it, in a couple of weeks. Then accept what needs to be done. Get the crank rebuilt. Don't ask me how it got there. On my 750. I found what I thought was a pea sized piece of crank seal jammed into the tip of my left spark plug when removing it for a reading. That was my sign, the crank needed new seals. Even though, I had no other signs prior to finding that. A month later it started puking crank oil out the breather at moderate rpms only. That was a week before I planned to tear it apart for the restoration. I feel good now knowing the rebuilt crank should last a long time. I was totally sick at the situation for about 2 weeks, that I couldn't even look at starting to tear it apart. But, it got finished once I got over it. There is good news to your situation. If you decide to go another route, you didn't tie a lot of money up into this bike buying it or beyond that to this point. If I remember ? Look on the bright side and decide which way you want to go.
I realize the piston situation is not good, and I realize that could not be responsible for the compression to be 100+ low. I'll be 68 next week and don't know that I have it in me to go the whole gamut.
Hard to justify the cost of rebore, new pistons and a crank rebuild and seals and no right carb etc'. By the time I am finished, Ill probably be finished riding. I would never get my money back out of it I would be in well over $2000. Things add up really fast and part prices are getting increasingly stupid by the hour. For instance, my little air screws are missing the tiny o-ring. Alpha sports price is $4.88 each. So that's 15 bucks for 3 minuscule pieces of rubber. Talk about the tip of the iceberg.
I was born with nothing and still have most of it left.
Sorry to hear this. Is there a motorcycle junkyard near you? Maybe you can get a cylinder and matching piston for cheap. Swap it in, if that doesn't fix it, then you can decide whether to fix, sell, or part it out. BTW: I spy some nice bikes in our for sale section.
OK. Someone needs to explain this to me. The compression tester only tells me how much compression is developed between the piston rings and the head. I don't understand how low compression would indicate a bad seal. I'm checking the top - not the bottom. The checker is only telling me the developed pressure between the piston and the head. Nothing else. What do the crank seals have to do with developed pressure between the piston rings and head?
I was born with nothing and still have most of it left.
It would take a leak down test to test the seals .. i am with you that a compression test only starts after the piston rings cover the exhaust port on the way up . What does the cylinder look like ? Can you get a look at the piston through the intake and exhaust ports ?
As clean as the top of the piston is i wonder if wasnt recently rebuilt ? if the port wernt chamfered i suppose a ring could get hung up on the port popping the top of the piston off .
I am having a difficult time removing the outer exhaust pipe from that side. Its really stuck on the crossover. 30 minutes of yanking and twisting, still no go. When I get it off I'm going to pull that jug. I'm hoping to find a broken ring or some other obvious reason for the compression loss, I doubt there has been any kind of rebuild as the bores are stock size. The cylinder won't be usable without a re bore. There is a lot of heavy deep scoring on the exhaust port side. hopefully that's where the loss is. Where else could it go? I still need to remove the oil pump and lines and the now petrified SRIS line.
I won't really know any more till I get the jug off. I thought maybe someone had installed keystone rings upside down, but there is no evidence of this ever being opened before. Keep you posted as I get there. Thanks Dave for reassuring me that I'm not nuts in my compression check interpretation.
I was born with nothing and still have most of it left.
I'm half afraid of removing the 6 banjo fittings on the oil lines. Everything is imbedded in a 1/4" layer of muck. Need to pick up some short bolts to install immediately after each is removed. I swear this thing hasn't had a bath since rolling out of the show room in 76. Thick glop everywhere. You can get dirty just looking at it
I was born with nothing and still have most of it left.
Chris and Dave, you are correct that the piston doesn't really start to compress gas until the exhaust port has closed but even at cranking speed there are pressure waves that make it not so simple.
Think about things. The piston falls and pressurizes the bottom end which fills the top end through the transfer ports. Some of that goes out the exhaust port at that speed and some gets pushed back in. Let's think about this.
If atmosphere is 14.7 PSI we would expect to see compression pressure of say 14.7*say 6.5 (corrected compression ratio) or = 95psi at 6.5:1 after the exhaust port closes. My motor has around 165psi and there's no way the corrected CR is 11.2: a. It's around 7.1:1. So we know that the transfer ports and exhaust play some role in the pressure even at that low speed.
But that said, we'd still expect to see 90 - 100 psi with good rings and no seals and no exhaust and yours is way less than that. So the probability is that pressure loss past the rings/piston is significant and probably the major problem. That doesn't mean the bottom end is OK though. Just means that the top end is a real problem.
If you cranked the engine extremely slow the gauge pressure = compression ratio * atmospheric pressure… if most didn’t leak passed ring gap that is.
But when you kick it over it is not that slow and the change in volume results in an increase in temperature that result in further increase in pressure… this part is significant.
I don’t think good seals have as much effect on pressure… for new charge from crankcase is more just an exchange as gas in cylinder is pushed out exhaust port pretty easy at kick starter RPM and most of new charge is lost out Ex port before Ex port closes.
I do think the idea ring snagged or something got caught in top end chipped piston and then pinched ring so it could not expand and seal. Usually a holed piston is near 0 psi at kick rate and stuck ring is low but not 0.
Does the head have any pitting marks? That may be a clue as well
Mine had a hole in a piston and a torn crank seal - so zero compression.
That piston is interesting. There's the missing chunk on the exhaust side plus a nick on the inlet side. What's even more interesting is the fact that both appear to be old and covered in carbon indicating that the motor continued to run with that damage
It will be interesting to see what Chris finds when that pot comes off.
To answer Vintageman, there are no tell tale signs in/on the head All 3 chambers are smooth. Carbon black but slick. Whatever the object or piston piece was either made it out the exhaust port or jammed into the top ring as the piston traveled up -- snagging the particle. Speculation should be over tomorrow. Right now I am faced with this Note all the oil line are empty except for one.
I was born with nothing and still have most of it left.
Hopefully that means that the bottom end is full of oil protecting the crank. I opened a 750 motor recently that was full of oil and I'm hoping that the rebuilt crank in there is as good as gold. Hope yours is too.
What do the crank seals have to do with developed pressure between the piston rings and head? 
Note all the oil line are empty except for one.
