Topic: Pulled the top half of the engine and this is what I found

[Lunchbox]

I bought this bike (76 CB750K) from a guy who rode it twice. He went to change the spark plugs and broke the #2 plug. He didn't have the resources or skills to pull to repair it so I bought it off him for next to nothing with the intentions of doing a complete cafe rebuild. The PO know next to nothing on the history of this bike. It has 18,000 on the odometer.

So... after getting the bike stripped down, I get the engine prepped for a tear down. I pull the head and jugs off and this is what I find. The top of the piston on cylinder 1 is completly clean! No carbon buildup at all. Pistons 2, 3 and 4 have more carbon buildup on them as you go down the line. The valve have some secent carbon buildup on them. remember I have no history on the bike so I don't know what type of work or mods were done on this bike in the past.

Any idea on what caused this? Pics attached. The first is cylinders 1 and 2. the next one is cylinders 3 and 4.

 http://imgur.com/8lo45

http://imgur.com/kAwj4

[chewbacca5000]

The clean piston may have not really ever fired.  No spark or fuel would mean no fire.  No fire no carbon deposits.  3 and 4 look like burnt oil to me.

[grcamna2]

Did you save the spark plugs? & do you remember which ones came out of the different # cylinders ? Try to post them with the #'s and it will be easier to give a good understanding of the way the engine was running...looks a little like it was burning oil on the #3 & 4 to me & maybe #2 also,but I can't see that piston too well.

[tlbranth]

I'll side with Chewbacca on this one

[Gonzowerke]

What does #1's pipe look like? The carb on that jug? Did you do a compression check before ripping it apart? That could have helped point you to either rings or valve seals for the oil burning. What did the head gasket look like? any signs of blowing oil from the studs to the cylinder?

[Lunchbox]

Unfortunately in my stupidity I didn't anticipate these issues so I didn't run a compression test. I attached pics of the plugs but I didn't keep them in order so I don't know which is which, Looking into the exhaust port, there is definitely carbon build up in there and also in the tail pipe. The head gasket appears to be in good shape but I did notice that the jugs came off really easily and I didn't have to give it any force or prying to remove it.

Sorry for the crappy pics. My camera is giving me fits.

http://imgur.com/LrQUP

http://imgur.com/DX2HE

http://imgur.com/0SdfL

http://imgur.com/rrNS6

http://imgur.com/9VtdV

[ekpent]

I would be more concerned with the clean piston/cylinder at this point as the others clearly show they were doing their job.Looking into the cylinder wall of the clean piston does it have a different appearance or is it scuffed up etc. What I would be worried is that maybe that cy
ylinder was still getting some gas but no spark and was run that way for a prolonged period of time. The gas would wash clean the piston and all of the lubrication from the rings and cylinder and possibly accelerate the wear.
   Check that carb over well for that piston also and see if anything looks askew in there that could cause a problem.

[Grabcon]

Number 1 is interesting. I would agree that that cylinder never fired. But it also looks like someone dropped something in that cylinder and it bounced around in there marking up the top of the piston. I would suspect a small nut or screw. Does the choke plate on number carb have both screws in place?

[ekpent]

The newer piston theory sounds plausible,or a cleaned up piston because of a broken ring or other such issue.Report back sometime on the appearance of that cylinder compared to the rest,like maybe there is still signs of a recent honing or something. If the valves for that hole have some good build up it must have fired at one time.
   Anyway the guy you bought it from can hook you up with the person he aquired it from ?.Might be helpful to track him down.

[grcamna2]

If it was mine I would bore it out w/ new pistons & rings & do the valves,then put it together after going through the carbs.

[Gearheadgreg]

I bought this bike (76 CB750K) from a guy who rode it twice. He went to change the spark plugs and broke the #2 plug. He didn't have the resources or skills to pull to repair it so I bought it off him for next to nothing with the intentions of doing a complete cafe rebuild. The PO know next to nothing on the history of this bike. It has 18,000 on the odometer.

So... after getting the bike stripped down, I get the engine prepped for a tear down. I pull the head and jugs off and this is what I find. The top of the piston on cylinder 1 is completly clean! No carbon buildup at all. Pistons 2, 3 and 4 have more carbon buildup on them as you go down the line. The valve have some secent carbon buildup on them. remember I have no history on the bike so I don't know what type of work or mods were done on this bike in the past.

Any idea on what caused this? Pics attached. The first is cylinders 1 and 2. the next one is cylinders 3 and 4.

 http://imgur.com/8lo45

http://imgur.com/kAwj4

I clicked on your picture 3 times and got a super closeup, Something was pounding around on top of that piston you can see marks.

[CycleRanger]

I clicked on your picture 3 times and got a super closeup, Something was pounding around on top of that piston you can see marks.

I would agree. What does the head and valves for that cylinder look like?

[Lunchbox]

I bought this bike (76 CB750K) from a guy who rode it twice. He went to change the spark plugs and broke the #2 plug. He didn't have the resources or skills to pull to repair it so I bought it off him for next to nothing with the intentions of doing a complete cafe rebuild. The PO know next to nothing on the history of this bike. It has 18,000 on the odometer.

So... after getting the bike stripped down, I get the engine prepped for a tear down. I pull the head and jugs off and this is what I find. The top of the piston on cylinder 1 is completly clean! No carbon buildup at all. Pistons 2, 3 and 4 have more carbon buildup on them as you go down the line. The valve have some secent carbon buildup on them. remember I have no history on the bike so I don't know what type of work or mods were done on this bike in the past.

Any idea on what caused this? Pics attached. The first is cylinders 1 and 2. the next one is cylinders 3 and 4.

 http://imgur.com/8lo45

http://imgur.com/kAwj4

I clicked on your picture 3 times and got a super closeup, Something was pounding around on top of that piston you can see marks.

Great info from everyone here. Good catch on the marks on the top of the piston. sure enough, there are small gouges all over the top of the piston, but oddly enough the cylinder walls are clean and smooth as are the bottom surfaces of the valves. There are some small gouges around on the cylinder head around the valves but none that are too deep. I was also able to clean off the oil/ carbon buildup on the other pistons very easily with a wire brush.

I'm thinking that the PO or the owner prior to that had some sort of nut drop into the cylinder and caused the damage.  They pulled the head to remove the hardware that dropped into the motor and made the bare minimum of repairs to remedy the situation but reused the piston. They put everything back together and it didnt run right from the get go so they sold it. At this point I'm thinking about taking the head to a machine shop for a once over and make some decisions from there.

[grcamna2]

Give the cylinders to them too & have them carefully measure them at different heights and across to see if they are still round bores.

[Hinomaru]

I clicked on your picture 3 times and got a super closeup, Something was pounding around on top of that piston you can see marks.

OK, I’ll bite and offer my theory ...  

The fine peppering marks on the #1 piston look to be made from a disintegrating and/or pieces of the spark plug core nose ceramic insulator. The insulator is a hard material and can easily damage aluminum. Probably happened from a improper gapping procedure which fractured or split the nose of the insulator and the plug stopped firing. Then fuel wash took over and cleaned off the top of the piston. The top surface of the compression ring looks to be undamaged and the evidence of carbon in #1 exhaust port would suggest a short term problem occurred. Did a fresh plug correct the non-firing problem?

[Gearheadgreg]

Take your cylinder head and flip it upside down fill the chambers with solvent and see what happens, may have a bent valve or damaged seat.

[Really?]

Thanks for showing me imgur, the gallery is FANTASTIC!

[Hondawggie]

The score marks on the piston top look like detonation or something.  IMO they are way too small for a piece of hardware like a nut or screw.  If you had even a tiny screw in there you'd have scoring on the barrel walls and chunks, not little scratches, on the head of the piston.

It's too late to know what the spark plug looks like that came out of the #1 cylinder -- the PO changed the plugs along the way to trying to solve his engine problem and you do not have the spark plug whose 'dismemberment' during detonation caused the tiny dings in the piston head.   But I suspect the spark plug was missing some metal -- here's a photo of a spark plug that has disintegrated during detonation:



If that motor was detonating on #1 you would expect the piston crown to be clean due to the heat. 

If you have an air leak on #1 (due to a burned valve seat, or a crack/hole in the intake manifold, etc.) you could run lean and then the cylinder will get real hot and then you get detonation. 

- do a leakdown test
- do a compression test

I suspect you have an air leak and/or burned valve seat on #1 there.

[Gearheadgreg]

Could be all of the above comments, Detonation may have caused the electrode to break off inside the cylinder. Most detonation Hot spots tend to be at the edge of the piston or at the ring lands or electrodes. but one thing is for sure i can tell you these old CB's that have sat all have clogged slow Jets and some Main Jet issues. Some Piston death pictures>http://kenoconnorracing.com/Piston%20Death.html

[Lunchbox]

Thanks for showing me imgur, the gallery is FANTASTIC!

Imgur is in fact the #$%*. Thank reddit.

The score marks on the piston top look like detonation or something.  IMO they are way too small for a piece of hardware like a nut or screw.  If you had even a tiny screw in there you'd have scoring on the barrel walls and chunks, not little scratches, on the head of the piston.

It's too late to know what the spark plug looks like that came out of the #1 cylinder -- the PO changed the plugs along the way to trying to solve his engine problem and you do not have the spark plug whose 'dismemberment' during detonation caused the tiny dings in the piston head.   But I suspect the spark plug was missing some metal -- here's a photo of a spark plug that has disintegrated during detonation:

If that motor was detonating on #1 you would expect the piston crown to be clean due to the heat. 

If you have an air leak on #1 (due to a burned valve seat, or a crack/hole in the intake manifold, etc.) you could run lean and then the cylinder will get real hot and then you get detonation. 

- do a leakdown test
- do a compression test

I suspect you have an air leak and/or burned valve seat on #1 there.

The broken spark plug came out of cylinder 3- 1 was still in tact. I have some pics of the three existing plugs somewhere in this thread. The guy I bought the bike from knew very little about it and had only ridden it twice before he decided to pull the plugs and broke number 3. I am going to do a leakdown test this weekend so i'll report back on what I find.