Piston Slap? (Pics)

[76static]

Okay, I finally decided to break down my engine to find out what this pesky knocking sound was that kept getting worse and after believing it to be piston slap, now I'm pretty certain. Here are some pics. Give me your opinions on whether or not this could be the situation and also if I do need to simply overbore and replace the pistons and rings, how big should I go?

Also, the piston that was in a better shape looks a little oil where the scuffed piston looks really dry.

 

[76static]

Heres pics of the chambers.

 

[jd750ace]

Looking like a tired top end there partner. How big you go is going to be determined by how worn the bore is, unless you go after a size for gain. I'm sure Gary, 5twins, and Leo have plenty of experience and can guide you.

 

[52Chevy]

Yeah definitely some wear there. I can't advise as to what size to go with, except why not drop on a big bore kit?

I understand the reasons for not doing that, but whatever you do, good luck on your build. Glad you explored before you ran into more problems!

 

[76static]

Yeah, I would like to go with the 750 big bore but I was wanting to fix it with least expense. Yeah, I was hoping one of the gurus on the forum could tell me if they think that piston wear looks like it would deffinetly cause the knocking.

 

[sseres]

Here's my piston, which seemed to be the cause of low compression. There were matching scratching in the jug:

$125 later and I have 1st oversized honed and bored cylinders (and $100 for new pistons and $60 for new rings) and I'll be putting the beast back together!

 

[Tomterrific]

That is sometimes called a 4 corner seizure. The piston top looks like it may have suffered some detonation and over heating before the piston seized. Only one cylinder appears to have been over heated. The other is heavily carboned.
Have you been using 87 octane? Use higher octane. One of my bitches is guys cheaping out on octane when premium is only 20 cents more.
Is the timing set at full advance? Use a light and blip the throttle to check.
Was the spark plug fouled on the dirty side? This would push the load to the side with the seizure.
Lean mixture? If the mixture was too lean the cylinder would run hot and expand the piston.

The heavy carboned cylinder might have suffered from fuel washing due to a leaky float/needle valve or improper float heigth. There seem to be multiple issues that you should correct before the new rebuild gets ruined.

Tom

 

[5twins]

Take the parts to the machine shop or cycle shop and let them examine them. They can usually tell you just by looking how big you'll need to go. Then you can order the pistons and take them to them. They need the pistons in hand so they can match the bore to them.

 

[XSLeo]

I agree with Tom on this, correcting the problems that caused this in the first place should be high on your list. You don't want it to happen again.
Leo

 

[76static]

I appreciate the observations Tom. I gotta believe that the issues you mentioned we're from the previous owner. The bike had about 17k on it when I bought it and was pretty neglected. It was running without any obvious engine issues though so I just went through the normal maintainence (cam chain tention, valve clearance, timing with a light, and carbs) and got got it running pretty well. Also, I always use 92 octain when I go to a normal gas station but usually I would go the a station a couple miles from my house that has 0 ethanol gas.

I also tweeted the carbs quite a bit and made sure that if anything I wasn't running lean. But I noticed that it seemed like the 2 sides never could get on the same page if you know what I mean. I installed new manifolds and used the best parts from 2 sets of carbs, cleaned and with new jets to make sure they were as close to identical condition as possible. But even after syncing them they just seemed to run different. Both sides had good compression though and I installed new exhaust gaskets to make sure that I didn't have a leak on that side to cause them to be different. But if that had already started before I got it, it makes sense now.

I put about 2,000 miles on it total and about 200 of that since the knocking showed up. I was pretty diligent about making sure that everything that can be handled without opening the engine up was where it need to be.

 

[5twins]

Exactly how big did you go on your jetting? Too rich is no good either. You run the risk of washing the oil out of the cylinders and, well, something like this can happen. My buddy did that to his. Two years of running with fucked up carbs set way too rich and he needed a 2nd size overbore.

 

[76static]

Now that I think about it, the kid I bought it from had been running it with leaking petcocks. He also didn't seem to know much about tweeting the bike so I bet he was running it super lean on that side because a ton of gas was leaking out before it got to the carb.

 

[76static]

Exactly how big did you go on your jetting? Too rich is no good either. You run the risk of washing the oil out of the cylinders and, well, something like this can happen. My buddy did that to his. Two years of running with fucked up carbs set way too rich and he needed a 2nd size overbore.

I was running with pod filters and a 2into1 with an emgo short reverse megaphone. I was running 1 over on the pilot and mains but I was getting some hesitation at wide open so I went to 2 over on the mains and that seemed to work pretty well.

 

[76static]

Was the spark plug fouled on the dirty side? This would push the load to the side with the seizure.

The heavy carboned cylinder might have suffered from fuel washing due to a leaky float/needle valve or improper float heigth. There seem to be multiple issues that you should correct before the new rebuild gets ruined.

Tom

Also Tom, the plugs were fine and I did have a float go bad and leak on the dirty side recently but I replaced it immediately and changed the oil. That has been since the knocking started. Float heights are both set within spec and needle valves are a year old with new o-rings.

 

[GreasyC]

That piston looks very familiar. My left one was like that when I redid my top end. The machine shop figured it got too hot at some point. The left exhaust valve was pitted too deeply to machine out as well. Not sure what caused what but you should definitely check your valves out while you've got it open. I also heard occasional knocking when accelerating through the midrange which led me to tear it down. On tearing it down I found a PO had used a thin copper head gasket. Maybe searching for more compression/power, maybe they just couldn't find an NOS one (its a 256 engine so until recently not even mikes sold them).

 

[1974jh5]

Aluminum expands at a different rate than steel, meaning it's possible for the piston to expand more than the cylinder under the right (wrong) conditions. Hard acceleration too lean is probably the #1 cause.

The 'wet' carbon on the left piston is from oil burning, the 'dry' carbon on the right side is gasoline carbon. That oil can come from two places: rings or valve seals. Since you are doing rings already, I would certainly do the valve seals as well, it's not that much more work.

Also, oil consumption will lower the effective octane rating. That means if someone decides to run 87 octane in an engine that's using oil, the octane will be lower thus the engine will run hotter, leading to seizure.

 

[XSLeo]

Oil consumption can also come from a leaky head gasket. If it leaks from the cam chain tunnel into the combustion chamber, oil gets sucked in during the intake stroke. Just as leaky valve seals will.
Oil changing the octane rating is something I have wondered about. A fellow I know has an older Cadillac, He dumps in a quart of deisel fuel with every fill up, claims it increases octane rating. Claims it keeps the engine from knocking.
And on two strokes does the octane rating change from a 20-1, 50-1 or 100-1 oil ratio?
Leo

 

[76static]

Aluminum expands at a different rate than steel, meaning it's possible for the piston to expand more than the cylinder under the right (wrong) conditions. Hard acceleration too lean is probably the #1 cause.

The 'wet' carbon on the left piston is from oil burning, the 'dry' carbon on the right side is gasoline carbon. That oil can come from two places: rings or valve seals. Since you are doing rings already, I would certainly do the valve seals as well, it's not that much more work.

Also, oil consumption will lower the effective octane rating. That means if someone decides to run 87 octane in an engine that's using oil, the octane will be lower thus the engine will run hotter, leading to seizure.

That's interesting. There wasn't an obvious leak but it does look too me like oil as well.

While I have the topend torn down, I'm going to clean the head, lap the valves and seats, clean the valves and install all new gaskets, seals and o-rings. The only thing I'm not sure on is the cam chain. It looks to be in pretty decent shape. Should I replace it anyway?

 

[1974jh5]

Carbon soaked with fuel will dry up and look 'fluffy' pretty quick, oil won't. If the chain wasn't slapping before it shouldn't now. But I would sure look into replacing that front cam chain guide, those things are notorious for failure and the engine has to come back out to replace it.

In reply to XSLeo: yes the octane changes with various mix ratios (many brands of premix are compunded to partially offset that) and two strokes will detonate much more easily than a comparable four stroke. In fact, they can and will run without a spark if conditions are right, er, wrong. Honda spent a bundle on developing a clean burning two stroke called the EXP2 which capitalized on that idea. It won its class in both Baja and Paris-Dakar but after that pretty much got shelved. http://www.motorcycle.com/manufacturer/honda/quick-take-honda-exp2-15170.html

 

[XSLeo]

Your chain may look ok, but as the wear, they get longer, stretch as most refer to it. This longer chain will let the cam timing be a bit retarded. As much as 4 or 5 degrees.
With all you fresh work, why run with a worn chain?
Thanks for the link. I'll get to reading it soon.
Leo