Just developed a hole in piston

[meanstreakmark]

Hi all, it's been awhile since I've been on here, and wow... I feel like I've forgotten everything I've learned. (Mad cow disease!) So, I'm hoping to be pointed into a direction-

The issue is a hole in the right-hand piston, and what would have possibly caused it.

After sitting for a couple years with only intermittent usage, I loaned my xs to my son as temporary transportation for a couple days while he got his car fixed. He knows my love for xs's and that this engine has never been rebuilt, and to not 'get on it'. Well, it ran fine, no issues, until the third day he was riding to my house and lost power about 3 blocks away. Today, i started it; it was only running on 1 cylinder; it wasn't the carbs; I did a compression test... left cyl was 163 psi, the right was 0. I looked into the spark plug hole with a high intensity led flashlight and observed the hole, about 1/8 wide and 3/8 long. Rotating the crank slowly, it seemed that both valves were opening and closing correctly, meaning that they weren't hitting the piston. That's as far as I went. I know I'm in for a rebuild, but I'm just wanting to know why this happened. Any ideas?

1981, pma upgrade with pamco about 3 years ago, new coils/wires/boots/iridium spark plugs (Mike's), everything else original except filter pods on carbs, went up a couple sizes on main jets, original pipes/mufflers.

Thanks for any input-

 

[lakeview]

I have an engine with a similar condition, the hole is about the size of a pencil, I will be watching this thread.

 

[ippytattoo]

Generally speaking a holed piston with no valve interferance is due to either a lean condition or timing that is too far advanced or a combination of the two. Good luck with the rebuild and make sure to check everything when you tear it down as you may find that you have other issues from the loose pieces of metal going into the crankcase.

 

[meanstreakmark]

Thanks ippy- I am really looking forward to a complete re freshening of this engine. I just wasn't planning on doing now, but I just can't let it sit like this...

 

[XSLeo]

I agree with ippy's lean condition and timing can be involved. These along with too low octane gas is a common way to hole pistons.
You have a Pamco from three years ago, means you are still using a stock ATU. With 163 psi compression, A bit high but still not too high, indicates the need for more than regular gas. If your son was putting regular gas in it and the ATU was a bit worn and let the timing over advance, this can lead to detonation which can lead to holed pistons.
I think others may have other theories and they should add thiers to this.
Leo

 

[gggGary]

+1 lean or advanced if all is stock. The most common cause I have seen is aftermarket pipes with stock jetting. I have a 78 here with a holed piston. Guy bought with a 2 into 1 He rode it around town for a while with no issues took off on a 200 miler and never made it.
I have a 79 engine with a holed piston too. Same deal short pipes, guy had been riding around town just fine then made it about half way to Sturgis when it holed.

 

[meanstreakmark]

Thanks Leo and Gary. For what it's worth, the Pamco with the new atu I bought from mikes three years ago, probably have less than a 100 miles on them. Maladjusted, perhaps; but not worn out(?). When I got the bike, the air box was messed up, so I removed it with the air filtering and put on pods; but I did keep the stock exhaust and mufflers. I thought with that combination I would go up 2 sizes (iirc) on the main jets. I must admit, I looked at the working piston spark plug just now and it looks a bit "dry and grayish/white". Hmmm.

 

[xskid]

Its called denation, basicly the fuel inside the cylinder ingited. Before the piston was at the top of its stroke, the explosion had no wear to go but thro your piston. Sorry to hear man that sucks

 

[meanstreakmark]

Xskid- makes sense.

Soooo... After the rebuild, make sure timing is spot-on, and make sure carbs (jetting) are
rich enough?

Side question: anyone ever use the Gunson color tune?

 

[jd750ace]

The mistake a lot of folks make on jetting is they think just because they have a richer main that everything should be fine. The PILOT being lean will burn your shit up. It's where most of the fuel delivery is coming from in your lower throttle openings, like cruise anywhere from 35-50mph, depending on gearing, and gear selection. A big main or a high needle won't save it if it's not in the overlap, and even then you could be on the lean side still if you have freed up the intake and exhaust. COMPLETE tuning throughout the rev range is necessary to long life.

 

[650Skull]

To add to JD's post the air mixture screw has to be set to match as well.

Its called denation, basicly the fuel inside the cylinder ingited. Before the piston was at the top of its stroke, the explosion had no wear to go but thro your piston. Sorry to hear man that sucks

I agree with XSleo its Detonation. Or per-ignition or per-igniting

 

[ippytattoo]

There have been quite few posts on here about the quality control issues Mikes has had over the years. One of the things that has had a few bad posts is his replacement ATU. I would suggest looking into the e-advance for the pamco which when installed correctly greatly reduces the possibility of over advancing the timing. If I hadn't gotten my grubby little hands on a NOS ATU that would be the direction I would go.

 

[xskid]

Does anyone have a picture of a spark plug, off a correctly tuned bike?

 

[Br4denJ]

I know starter fluid can cause holes in your piston. I would agree with running lean as well.

 

[meanstreakmark]

Guys- much thanks for all your replies and advice, and it will be heeded. Until the mad cow sets in. I guess now it's reading and researching to do a complete rebuild.

And yes, I've been reading about the e-advance...

 

[grizld1]

Gents, the single most common cause of holed pistons in the XS650 is advanced ignition due to ATU failure. In Mikuni carbs, OE or aftermarket, leanness in the pilot circuit is going to cause a host of symptoms before it cooks the motor; it's not something subtle that can sneak up and bite you in the, uh, piston.

Mark, I like new toys, and bought a Color Tune plug some years back. I found it to be useless and gave it away, refusing to take money for it. Unless you're on a "rolling road" you can't read any steady state mixture above idle without blowing up your motor. Yeah, there are directions for reading off-idle transition mix, but I found that drill less useful than simply observing throttle response as I'd been doing for decades.

 

[jd750ace]

True statement. Surging, weak acceleration, and ping should be getting your attention long before you hole a piston if you have a clue what a properly tuned motor should do for you and sound like.
I guess it could be subtle if you are the "screw it, it runs, let's go" kind of guy. I've known a couple. Roam around the countryside on a lame running machine, "riding around" your poor running issues, and you'll be closely observing a section of the side of the road eventually.

 

[meanstreakmark]

Ace and Griz... I hear ya on that and agree. But this is what puzzles me in this instance: I'm not one of those types. Actually, I'm pretty anal about the details. Although not a trained mechanic, I own more tools than most of the mechanics I know- and I know how to use them. I never observed anything strange with this bike, but I never rode it hard either. (Big sigh) But this occurred while my son was riding it. (Again, big sigh)

 

[xjwmx]

^If it's due to knocking, the metal disappears a microscopic fleck at a time until the crown is thin enough to blow through. Takes longer than a couple of commutes. It was probably weak already and if it was due to anything he did then consider it a favor...

You can hear this knocking. It sounds exactly like a ring of keys faintly rattling. Timing and mixture and riding style have the most effect on it. I haven't noticed much effect myself going to higher octane gas. Retarding the timing as much as allowed will help you, and it won't affect performance.

 

[CDNTX650]

CV carbs are very good at masking problems. I ran my TX650 for quite a while with mikes Brit mufflers uni pods (bs34's) and only one size up on the mains with the Mix screws set out at 3.5. Always ran good a city, low rpm driver

Well I decided to finally check the main and gave er hard till red line. Well it was super surging before I even got there. Went up another on the main and now I have small shims on the needles. It runs so much better in mid rpm's with the shims. I could probably still do a bigger main as well. Still get a little surge up there by red. But the plugs look good and I don't ride it that high of revs ever. My mid range never even seemed that bad...now I couldn't go back.