What am I doing wrong?

[Darrell]

Upon tearing down my 250SL to replace a sheared countershaft (another story) I discovered scoring or scratches on the exhaust side of the piston and cylinder.

The top end has about 450 miles since the .010 rebore and new Wiseco piston. I also took it apart at 150 miles because I was second guessing the port chamfering; the machine shop invoice said they "deburred" the ports, but I had later reservations that might not have been adequate and was conservative with the throttle up to that point. (As an aside, is it a given that factory chamfering still be in spec after a .010. .020 ... rebore?) The piston and cylinder looked fine at that point and I rechamfered the ports for additional piece of mind.

Anyhow, can anyone discern if the the scoring/scuffing likely results from a too lean air or oil mix and/or inadequate warm up?

As for sanding the scratches from the piston: is that best accomplished by sanding vertically with, or across, the scratches?

For those that read piston crowns, I'm attaching a picture of that too.

Many thanks again for your insight and input!

[thrownchain]

You realize that sanding the scratches out will change the diameter of the piston, increasing piston to bore clearances ?

[viclioce]

Though I don't work on the 250, the piston too sure does look like a lot of incomplete combustion has been going on. What does you spark plug ceramic look like? As dirty as the top of the piston? ; D Victor

[Bullfrog]

Would you indicate exhaust/intake sides for us on the piston crown photo? Comments after getting that information.
Ed

[Darrell]

Would you indicate exhaust/intake sides for us on the piston crown photo? Comments after getting that information.
Ed

Thanks Ed,
The exhaust side of the piston crown is straight down.
Additionally, the cylinder is a bit scratched too (visual and finger-nail inspected).

[Bill2001]

I'd like to see those comments. I've also read something about evaluating the color of the exhaust port or the inside of the exhaust pipe when doing the jetting tapdance.

[Bullfrog]

My lack of direct experience with 250 SL engines has me leaning back on 125cc Hodie experience . . . and that leads me to some questions which relate to two distinctly different trains of thought regarding what is happening with this engine. Hopefully the answers to the questions will help zero in on the real deal. Note that I can't help but think about the "pancake" head on the 250 and how it might be influencing things on your highway cruising - but even that doesn't nail anything down for me yet.

When I look at the piston photo, I immediately think detonation. It looks like detonation has been chipping grains of aluminum off the edge of the crown. The aluminum looks really bad above the ring. But the piston crown photo doesn't look anything at all like I would expect on the exhaust side. MISMATCH. What's going on? So, here are a few questions. 1) The bright areas on the piston crown - are they simply "clean" areas showing the factory piston surface? Or do those areas look like they have been sand blasted? 2) What does the cylinder head look like? Is it all carboned up too? Are there "clean" areas? Are those areas simply "clean" or do they look sandblasted? A good photo would be nice (be sure to indicate exhaust side).

Is there any chance you've sucked some gritty/dirty watery/air? Loose rubber boot at carb or air cleaner box? Any evidence of nasty stuff going through the air cleaner? The striations on the piston look like they were caused by foreign material. They don't look like "seizures" to me. I don't see any sign of "galled" or "smeared" aluminum . . . and I'm guessing there isn't any aluminum galled to the cylinder wall either. Is that correct?

Ed

[bchappy]

My first guess would be to check the air filter. Is it in place properly or maybe not oiled and just put in dry. It appears that dirty air is doing all this damage. The clean areas on top of the piston just looks like sand blasting coming from the transfer ports.

[dcooke007]

Looks like dirt / debris to me also. Maybe the cylinder was not throughly clean after boring / honing? I have found globs of abrasive mixed with honing oil stuck inside port openings that required getting at with a brush and detergent to remove. Can be easily missed since it is about the same color as the port interior.
Danny Cooke

[Bill2001]

Scrub-a-dub-dub. I tend scrub in a couple of solvents, soap & water and mineral spirits, with a last rinse of spray brake cleaner. Scrub with a old toothbrush and a small wire brush. And maybe go over it with a small wire wheel on a Dremel. Never can be too clean.

[michael_perrett]

The 250's have a number of defects in stock form as most of you already know. One area is the air filter. The stock filter has about as much filtering as panty hose. With some modifications, the stock air box can be changed to use a Balwin PA2050 element. The other is just as serious, the lack of engine cooling. I noticed that on this forum, the issue of engine cooling is of low priority. Looking at the stock 250 specifications, I believe they are set to compensate for lack of cooling, with the timing being retarded and fuel setting running rich.
With my 250SL, I am running, along with the Balwin filter, YZ 125 expansion chamber and a Suzuki TM 250 head, a 32mm Mukini carb with a timing of 25 degrees BTDC, ( compared to a stock setting of 17 degrees). The porting is stock. The jetting is: pilot 60, 2.5 slide, needle 5DP7 (set at the leanest position), 06 needle jet and a 160 main (stock main is 190). Also am running the stock oil injection pump using Pennzoil 2 cycle oil. Compression is 162 lbs.
If one is forced to use the "pancake" head as Ed calls it, I would adjust the oil pump to run a bit on the rich side. Running more oil increases the "forgiveness" factor when it come to lack of cooling. I do that on all my Hodakas using the pump. Eliminate the front high fender and replace it with a low one to increase the air flow over the head. KEEP the cylinder and head fins clean! Also keep a clean crankcase. When doing your jetting runs, ride at the specific throttle setting, up a slight hill for at least 2 miles. The engine needs to be at a 100% heat absorption for an accurate reading.
Mike Perrett

[Darrell]

OK, I'm back now with more pics and feedback.

Note to Ed:
The bright areas on the edges of the piston crown are clean and show like an out-of-the box piston without detonation damage.
I've included a pic of the combustion chamber too. The carbon pattern was symmetrical when the engine came apart, I got a start cleaning it up before taking the photo.
The grooves (Torque, or Singh, Grooves) in the combustion chamber have been discussed on the forum before. The benefits seems to be a noticeably smoother and steadier idle. And I think the little bit of aluminum displaced by the grooves lowered the compression about 10 psi (from 160 psi) and helps reduce detonation. The offset squish design of the head may be the cause of some of the piston issues with the 250.

The attachment IMG_1056.JPG is no longer available

Yellow arrow indicated exhaust.

Bottom of piston crown


The exhaust side of the cylinder shows some fine scratching, but no aluminum galling.

Dirt and Debris?

I'm starting to wonder...earlier in this post mentioned that I recently rebuilt the top end, and reassembled it -- hospital clean.

THEN, a hundred miles or so later I had second thoughts that the port chamfering was adequate, so I took it apart again and Dremelled the ports myself (the machine shop invoice reads that they "deburred" the ports after boring).

Am I the author of my own misfortune?

Second guessing myself here...I remember at least using the air nozzle to blow the ports clean after the interim chamfering procedure. But with interruptions (called for dinner, or something) and sub par short-term memory performance, I'm wondering if I followed up with the cylinder bath time with soap and water in the laundry room sink. If it's any encouragement, the ports still are almost white glove clean, with no residue for debris to cling to.

As for filtration, the intake side of the airbox and filter element show clean, but getting installing the foam element definitely takes some diligence there's consistent contact around the perimeter of the element.

I better go before this times out again Thanks for all the brainstorming...

[Darrell]

%*&#@!*!! Sorry about that -- photo dropped off the last posting:

250SL Yellow arrow indicates exhaust

Further to my previous reply, I think I should clean up the scoring anyhow. Is Scotchbrite the way to go in this day and age? Historically, we may have used 400 grit emery.

[Bullfrog]

Darrell,

Your cylinder head photo pretty much eliminates detonation as the cause of the nasty scoring on the piston - which leaves foreign material as the prime suspect. Do you have a 10X magnifying lens? You might be able to discern something by closely examining some of the larger black pits both above and below the rings. If there are embedded bits of steel that would be both a good thing and a bad thing. Good thing? Yes, because you would have a pretty strong idea about how they got there. Bad thing? Well yeah, there are bits of steel embedded in the piston!

If it were me, I'd do the abrasive clean up going across (or mayhaps 45 degree angle to the scoring) - definitely not parallel to the scoring. And I'd spend some time to assure that no hard bits (steel or trail grit) are left embedded in the piston. I guess I'd go with the Scotchbrite conditioning pad - it would help assure that you don't create any "hollows" or "flat spots" as you do the clean up.

Um, you might want to at least flush (or better, replace) the main bearings - they've taken a bit of a beating too.

Ed
That sure seems like a heavy coat of carbon on the piston crown for only 450 miles of riding.

[Darrell]

Flushing, and or replacing the main bearings won't be too onerous as I've already have the cases split to attend to this bit of nonsense:

This incident followed this piston melt down earlier this year:

For what it's worth the intake side of the piston has fine vertical scratches -- but no pitting. Out of curiosity, I'm trying to induce some rust in the piston pits to determine if they are iron flecks or trail grit.

Quite a quit of carbon built up since I inspected the top end at approx. 150 miles, it only had a couple of streaks. Since then I've been adding more oil to the premix and switched from Yamalube 2R to Lucas Semi-Synthetic. Spooge is way down using Lucas.

If time permits I hope to be riding the 250SL next weekend. I can't wait to see what's going to go bad next time In the meantime, the Wombat has been running like a champ.