MarkT - your comment about detonation under light load that goes away under hard load is sensible. I also wondered if this could be shockwaves making noise in the expansion chamber or some other mechanism. But when this occurs the bike kinda bucks and surges which made me think something with the flaky CDI ignition...has anyone replaced cdi with points or an aftermarket unit and if so did the surging pinging go away?

I didn’t see any centrifugal advance mechanism when replacing the crank seals during the resurrection process so I wonder if the advance mechanism- which I suspect is electronic - is the real issue?

Pinging or detonation is the rattling noise when the fuel burns to fast after the spark is enitiated and ends up exploading more than a controlled burn. Pre ignition from too long detonating is when you get so much heat generated from the pinging that the fuel air mix explodes before the ignition event this is when you hole pistons.
Some sounds of pinging can be the fins of the head or barrel vibrating at certain frequencies especially if there's no rubber dampners between the fins or a loose piston to barrel fit can just make the piston rock around slapping against the sides. I can make my dt250 ping with to much or too little timing if been to the extremes of both directions in search of power and or rpm

Pinging, knocking and detonation all describe the same phenomenon. Spontaneous ignition of pockets of air/fuel mix in the combustion chamber ahead of the flame front of normal combustion. The contents of the combustion chamber experience adiabatic heating during compression and absorb even more heat from the piston and head. In a marginal scenario this will raise the temperature of the contents if the combustion chamber close to the autoignition point of the fuel. When compressed further by the pressure of combustion, a pocket of this mixture detonates. This little explosion creates a rapid and completely useless spike in combustion chamber pressure that can be audibly heard like a little hammer striking the inside of the chamber.

It is distinctive because it is not cyclical like any other mechanical noise. Piston slap, rod knock, all of the usual ticks and clatter vary with RPM and load and occur in a repetitive and ryhtmic fashion. Detonation, especially when light, is irregular in both frequency and intensity. The sound is like a small pebble hitting the windshield in your car. A metallic tick. As it gets worse it increases in intensity to more of a knock, like hitting the piston with a hammer. It should get your attention at that point even if you don't know what it is.

It is unusual to see it at part throttle because combustion chamber pressures are significantly lower than they are at wide open throttle. That's where the DT is interesting. We know the stock head design sucks, the stock ignition produces fixed timing which doesn't help and the DT360A has a gigantic crankcase volume which is not effective at scavenging the cylinder at low throttle openings. It not really a stretch of the imagination to consider that this may be resulting in an irregular mix of hot exhaust gases and fresh/air fuel mix in the combustion chamber. In a hot, air cooled engine, it doesn't take as much. At full throttle you have more fresh and cool air/fuel mix coming in and better scavenging.

So, following turbodan's theory, the reason that the pinging (assuming this is unintended ignition related and not mechanical) occurs at less than large throttle openings is because there is more exhaust gas than desired in the combustion chamber during the compression stroke. And, this causes higher than desired heat-of-mixture (fuel/air) and the additional heat of compression (adiabatic) raises the mixture above the auto ignition point before the spark even occurs and hence, pinging.

Now the crankcase volume. I can understand that having too much volume would not be beneficial. Too much volume would be like adding a storage (aka boost bottle but not exactly) area for the incoming fuel/air charge. I would expect that stored charge would: gain heat and drop some of the suspended fuel. I would also expect that the fuel air mixture in an "oversized" crankcase to lag that of the carburetor and that it would take a few cycles to catch up.

I can also see how having a smaller volume in the crankcase with a given cylinder displacement would create a stronger vacuum or "signal" at the carburetor.

My questions would be: Does this also create higher velocity flow at the transfer ports? Wouldn't the net volume of the charge be close to the same since it is dictated by the cylinder displacement? Is it the velocity increase (if this is the case) that improves the pinging issue or is it the carburetor signal?

I have lots of questions! I am finding this issue with pinging very intriguing, mainly because it is occurring at a load point and throttle opening that is counter to what I have experience in the past with cars from the 1980s that were being weaned off of leaded fuel on to low octane unleaded. And one of the engineering changes was on vehicles was to add EGR valves so that a small amount of exhaust gas was added to the incoming charge under certain conditions to act as "artificial octane". I would expect that his is a much small amount that could be drawn back into the cylinder on a two stroke and a "leaner" chemistry.

My head is throbbing. It's simpler to ride with ear plugs.

EGR is an emissions thing. Doesn't do anything beneficial other than that.

Higher primary compression from lower crankcase volume does increase transfer flow and velocity. The net volume of charge depends on the pressure ratio between the cylinder and crankcase. The exhaust system will result in some back pressure so with the engine running the pressure in the cylinder will always be above atmospheric. Higher primary compression will push more of the charge into cylinder before pressure drops down to the point where the crankcase and cylinder at equal and flow stops. A larger crankcase will develop less pressure before the transfer ports open, producing less transfer velocity and less flow with longer duration.

At part throttle is really where the crankcase volume hurts. The crankcase doesn't get a full charge of air and fuel, it operates under a vacuum until well below TDC as the throttle is only cracked open. This partial charge is compressed into a large crankcase to a relatively low pressure which results in weak and inefficient gas exchange. A small crankcase volume will make the most of this same partial charge, compressing it to a greater extent and more efficiently transferring it into the cylinder.

Like I said earlier, I don't know if this is an issue with scavenging due to the large crankcase volume or an issue with intake signal due to crankcase volume. Intake signal seems more plausible as far as the detonation goes. I do know the bike is running really well with significantly less crankcase volume.

I agree that some of theory is counter to my experience. EGR absolutely significantly reduces combustion temps and in my real-life experience with removing EGR on an engine it often results in severe pinging. I find it hard to imagine under any circumstances that exhaust gas dilution would create pinging.

I'll stop there... Everyone is entitled to their theory.

One thing that was mentioned that I theorized years ago after experiencing "pinging" at low load/light throttle on a Yamaha two stroke was that the mixture was not igniting every time in the combustion chamber and the unburned mixture would then ignite in the head pipe... that's what it sounded like to me on a friend's bike... like popcorn popping in the head pipe.

If I had a bike that was doing it I'd probably invest in some sort of knock sensor electronics and try to see if it's really "pinging".

Have you ever owned a 74 DT360, Mark?

I actually have. I currently have a DT400. I will qualify that with I have infinitely more experience with the 175's as they are my favorite... very little experience by comparison with the 250/360/400. But enough experience to know that not all DT360's have the same "pinging" for whatever reason.

I also was lucky enough in my youth to get to experiment with testing fuels and studied detonation using a Waukesha CFR test engine... as well as a lot of experience with all sorts of engines.

Light load, low throttle, exhaust gas dilution, and richening the mixture or running 100 octane fuel... are all things that reduce detonation/pinging... yet people state richening the mixture to blubbery rich or higher octane fuel has no effect? That and my own experience has me wondering if the light throttle, low load "pinging noise" is something else?

I wish I knew for sure.

here's a question out of left field
I took a ride yesterday, motivated by this thread, out into the foothills in S OC
my 360 hauls ass up to redline in each gear, but when I am in the wind on a road with a 55 MPH speed limit I seem to top out around 50. I weight 255 and am 6'1" and have quite the cross-sectional area, like a big fat sail.

Any thoughts on why I am topping out appreciated...I have no issue going past 60 on less open roads. Wondering if a size up on main jet might help, or this is simple physics of being 4" too short for my weight

thx

I've studied the subject of det a bit myself, more than I would have liked. It's cost me a few head gaskets and ring lands over the years.

It sounds like you are pretty familiar with it. You know the sound, the irregularity and variability of it. It's really unmistakable. The only question is why it is happening. I'm leaning toward intake signal and carburetion myself. Mine has never exhibited four stroking and detonation simultaneously, it has run pig rich at idle and low RPM and lean enough to ping at a moderate RPM cruise.

A sort of EGR is a constant in any two stroke engine. At low throttle there will never be enough air and fuel mixture to completely displace the exhaust gases in the cylinder. Even at full throttle it is difficult to achieve 100% delivery from the crankcase into the cylinder. At part throttle a substantial portion of the contents if the combustion chamber are going to be left overs from the previous power stroke. This gas is inert but also hot. Heat is the whole problem when it comes to detonation.

I would like to know for certain why Yamaha only used the six bolt pattern bottom end for one year. Early bikes used the four bolt bottom end pattern with its small case volume and transfer tunnels. One year later in 75 they went right back to it. Like a failed experiment. No doubt, mine was not close to my expectations in stock form. Its better now but it shouldn't have taken this much effort to get it to where it should have been stock.