1340 Evo, OEM (+.005) pistons, with 1000 miles. Stock heads and gaskets, break in properly done, timing correct, 93 fuel, verified by vacuum pump to be zero intake leaks. Rear cylinder only and on the squish side, but moving on around the diameter as miles are added. Irregular pinging intervals, but usually at 55mph with slight throttle added. I've seen SE hi-comp pistons do this on the entire top, with too much timing.

I know what it is, but what's causing it? I'm guessing a sharp edge at the top of the bore glowing and causing detonation / pre-ignition, but I'm totally open to ideas and opinions. This is the rear only, front looks good with color covering the entire top of piston, unlike this one.

 

Detonation - check for intake leak on rear cyl if front looks good. Vacuum pump isn't a good test. Gently use little shots of carb cleaner around the flange to head. Just little squirts. If you bomb it, it will get into the front of the carb and simulate an intake leak. Also check at carb to manifold gasket. Intakes can leak when cold and not hot and vice versa. Watching it grow is watching it fail.



Timing could be an issue but if you are certain it is correct, I won't go there.

 

Thank you Ed.

I'm rather well versed with vacuum leaks on Evo 1340s. In fact, every time I see someone on here "guarantee" they don't have an intake leak, I just roll my eyes. Over the years I've found most often, that if an intake leak is bad enough to actually find with the traditional, accepted methods at idle, hot or cold, it's running so crappy there wasn't much doubt anyway. I totally agree they can, and often will leak hot, not cold and visa-versa. And, I've learned that spraying fluid or even the propane method on an idling engine really won't guarantee a leak free manifold at speed and full operating temps.

On suspect "problem children" I get both intake valves closed and use my HVAC pump thru manifold gauges hooked to a rubber coated plate that butts against the manifold/carb grommet. Spray around the flanges then look inside the manifold for fluid. Do it cold and again right after run while still hot. Believe me, 2 minutes from a high volume pump pulling about 28.8 hg on the manifold will suck some fluid if it's not sealed. So with that, I'll humbly repeat my claim of no intake leaks.

PS, nose cone ignition, timed static and verified with timing light.

 

Not fully agreeing on your intake leak test method but it’s your party. Did you verify VOES is working?

 

That's cool. What works for one doesn't work for all and I'm big on whatever works. I don't go to those lengths often btw.

Voes set to 8hg and checked for in/out not sticking. It doesn't flicker the pilot light on the module until idle is under 500 rpm. All external temps during break-in were consistent with every one I've done before, per mileage and ambient (cyls, heads, etc) by an infrared thermometer. Did the heads myself. Valve/seat contact concentric, proper width and position on the valve.

Whatever it is, it's got to come apart. Won't tear it down till late next week when I get a ship notice on parts and will post back what I find. Thanks

 

Got a god picture of both spark plugs?

 

No I don't right now hotrod. Not sure my old hands can hold still enough anymore to take a good one.

Front runs almost perfectly clean and the rear shows some faint (fuel) color on a 60 mile ride, installed with engine hot. (Reading a new plug after been on a cold start with CV is a waste of time). Anneal line center of bend on strap and full turn of color on the base. Did get black oil specks when it first started pinging, but not since that first time. Never did find any silver specks but didn't look too hard. They either there or out the pipe I figure.

 

For a cylinder with only 1000 miles on it, I'd expect that there would be plenty of cross hatch. I don't see any. In fact the pitting from the piston is reflected in the cylinder. I'd suspect something wrong with the bore. Did this bike sit? Maybe it's not detonation but corrosion. Generally when I see detonation, it's in the carbon on the top of the piston. Looking at the rings can tell. Top ring may have chunks of the moly steel blown out.

 

Max, surely you been around here long enough to have read some good rants about panicking over screen shots of a cylinder made with a cheap borescope. Yeah, it'll look way different with the head off.

I did 3 thou with 70 grit, 1.5 thou with 150 and finished with 280 wet to a .0012 clearance. The front I finished a little rougher so it'd hold oil better since the front doesn't get as much. Less than 5% leak down and pumping 165 with an EV13, flat tops and stock heads so not too worried about that part of my work. But yes, I'm curious to get the rings under a glass for a look see too.

I didn't even look at the top edge of the bore when I took them out of the plates and still think there's a sharp edge on the rear one. That's why I was guessing an edge glowing is what started all this. Could be wrong. Maybe that one didn't have much if any chamfer from the factory to start with. Only going +005 I didn't even think to look at that end.

Or some gas I think was bad that I couldn't get out for about 60 miles. It was 10 miles after that fuel stop when it started all this. Dumped some octane boost and dropped the timing to 30 to get home. The pits in the piston you see are 10x worse than just 200 miles before, so it's not getting run anymore till fixed.

 

You could be right but when I see vertical stripes and no cross hatch, I get suspicious. The reflection is so good it kind of makes me wonder. Maybe there is a light coat of oil masking the cross hatch. Doing stone is like old guys arguing about which bourbon they like best. IMO Your honing would be good if you went real close to finish with the 150s and only knocked the peaks off with the 280s. I finish with 220s but do a quick brush hone (plateau). It leaves a cross hatch that won't go away. Good if there is too much timing and fuel..

I'd almost believe that what ever you got in the fuel might have been corrosive. (Hydrazine )

I might have been a sharp edge on the top of the bore but I doubt it.

Anyway..