You can see the stock DT360A has a ton of volume in the transfer tunnels. The bottom end has a considerable amount of room on top of that. The MX360 main transfer tunnels have a substantially different taper to them, whereas the DT transfers taper like a funnel, the MX transfers taper like a trumpet. The forward wall has a convex shape to it. It can be seen somewhat in the pics but it is easily discernible in person.

The SC500 is stuffing an extra 150cc's or so into the same size cases and transfer tunnels. I imagine the overall primary compression ratio is appropriate on this engine, all things considered.

The TS400 is significantly larger in displacement than the 351cc DT and still has less volume in the tunnels. Less volume in the cases too. The Kawasaki Bighorn 350 has the smallest case volume of the bunch. The rotary valve design maximizes crankcase filling and produces a bunch of primary compression. These were known for stump pulling torque, a broad powerband and serious HP potential.

These are good comparisons and I see the differences you are describing. Even the "trumpet" shape in the MX jug is clearly visible.

I'd like to ask, why the two different mods to the pistons? Was it to create two extremes in intake duration to evauate the epoxied transfer ports effectiveness at both a long (reed controlled) and short (piston controlled) duration?

The effect of the crankcase/transfer ports volume on the vacuum level is easy to understand. Given the same size pump (the piston/stroke/port timing) a larger volume will see a smaller change in pressure. Negative pressure in this case.

I am impressed by what you have observed, learned and applied with a positive outcome!

I can't buy the "primary compression" theory especially when the early 360's you're saying had higher primary compression than the DT360 had the same exact surging and part throttle running issues from the very first model!

Also not sure the DT360 was designed as the SC500... My guess is the MX/YZ 360 came first and then Yamaha made a 500 out of that design for "bragging rights". But I don't know for sure.

I am happy that whatever mods you've done seem to be working for you! Whatever works, works. Just not buying into the "theory".

The first cylinder with the Boyesen ports was modified intending to increase intake port area, in the early stages of my tinkering with this thing. At that point, I had yet to exhaust all of my options to get this thing running properly so I hadn't resorted to the strange stuff yet. The skirt without windows was just an easy way to increase primary compression and intake signal.

That one worked well so I tried a similar approach on the ported jug. That piston was already drilled and the additional ports already done so all I could do is go all in with epoxy filler to achieve a similar effect.

I was somewhat interested in the effect of cutting back the intake timing with the stock ported cylinder. My goal was cleaner carburetion and low end power so a short intake period doesn't inhibit that. Top end power didnt suffer at all interestingly enough.

Hard to say if the early bikes exhibit the exact same symptoms. When an engine doesn't want to run cleanly at a steady speed and throttle position there are many potential causes. The pinging seems unique to the DT360A, and these bikes do have a reputation for holing pistons.

I admit I am taking my best guess that the 500 design came first. The SC500 and MX360 came out in 1973. The DT followed a year later. The SC500 also uses the lowest numerical series of part numbers, the SC prefix is 363, the MX is 364 and the DT is 365/445. Then there's the cases. The DT does not utilize the balance shaft or any of the associated components but the casting still has all of the provisions.

I wish I could say for sure that it is what I think it is. I do know this particular 360 has a hell of a lot more case volume than the early RT's and later 400's. Reducing it has damn sure helped.

I wish you could ride the bike. Nothing ambiguous or subjective about the results. It runs great, no pinging, clean carburetion, better idle and a substantial bump in low-mid power.

There is much about this condition that I certainly don't understand.

The perplexing part to me is that (in the case of my 360A) is that the time frame from a 5 minute warm up at idle (with the starter circuit open and a cold engine) until the engine is fully heated (from cruising) there is no signs of pinging at that one, very narrow throttle opening. Of course the starter circuit is closed before I take off. My riding is typically on secondary roadways with speed limits of 55 mph or less and gentle inclines. If I was riding mostly off road and using the throttle on and off more it would probably not be so noticeable. But, with my "commuting" style of riding I find myself at that sensitive throttle point very often.

With the throttle open the bike pulls strong even in 5th from a low RPM. If held there it easily surpasses 70 mph and is surprising smooth for a single at doing it. The wheels are straightened and balanced which certainly helps with the smoothness. And I am certain the Sleeper Pipe #74 and the high flow air box helps with the strong top end.

I am tempted to purchase a spare jug just to try the port stuffing trick but I know that can get expensive with a jug, piston and rings.

In the meantime I will be getting a TrailTech spark plug base temperature meter (from Santa, of course) so that I can look for a correlation between cylinder head temp and the commencement of this condition.

One thing about these Enduros is that they are never lacking for your attention.

Same for me. A one way ride, ten minutes or less, runs great. Longer trip or another short trip on an already warm engine, it starts acting up. Seems like the engine has to get thoroughly warmed up, top and bottom, before the part throttle pinging sets in.

I wondered at one point if the CDI box was heating up and the timing was becoming erratic. Another box does the same thing though, and neither box is prone to pinging at higher load. Only at a cruise. I never did put a timing light on it but I don't think I'll find anything there anyway.

I recently re-did the cylinder used in the original experiment. I wanted to see how well the epoxy had held up to the chemicals and conditions inside the engine. It was still very solidly in place and it took a great deal of chiseling and grinding to remove. Just to be safe though, I cut some grooves and notches into the transfer port tunnels that would give the epoxy more to grab onto than the cast finish. The one on the bike is running strong so I don't know when I'll get around to swapping it out again. Next on the to do list is to install the Rex's ignition, then I'll go from there.

You might try just the piston without windows to see if that brings a significant improvement. You would need to make a spacer to boost the cylinder up. Not expensive but somewhat labor intensive. Between the two cylinders I have with the epoxy treatment, the one with the stock intake port and no windows drilled in the piston runs cleanest. The one with full open intake timing and Boyesen ports makes more midrange but it just doesn't run as nicely at 0-1/8 throttle.

I'm curious at what cruising RPM the "pinging" happens?

For me it came on around 3500-4000 RPM, not too bad, just a light, low level but continuous tickling of det. Cruising at 40-50 MPH once warm it just did it, all the time. At higher revs it worsened, the first time I took this thing for a cruise at highway speeds, 60-65 MPH I was shocked how bad it pinged. This was all stock, down to the carb jetting, which is rich at my 5500 feet elevation. Trying to cruise at steady throttle at those speeds resulted in constant and very severe detonation. Rolling onto the throttle would alleviate it and completely closing the throttle would snuff it out as well. To get to my destination I rolled onto and off of the throttle to minimize the problem. If I held the throttle in the sweet spot I am certain it would have burned a hole through the piston or stuck a ring.

Its amazing to me how much this motor likes to ping considering the compression ratio is so low. Mostly at part throttle too. Never had a two stroke that behaved like that, and I've had at least 20 of them over the years. Old and new, big bores and small bores, air cooled, water cooled, high performance and not. The 74 DT360A seems to be uniquely flawed.

The current setup in the bike now will cruise up to 70 with no issue, though I discovered recently going across town that it still pings cruising over 70. This is with the ported jug and 360 degree intake timing, in conjunction with the MX360 pipe, KX reed block and 35mm PWK. It's got a lot of epoxy in there but the porting reduces the effectiveness of this modification compared to the stock intake port and un-drilled piston.

Setting up my DT360s with a tight squishband has solved the detonation 100% on all 3 I have done it to. My latest one I was working on had a factory squish of nearly 3mm...and the casting of the head was crooked by .012..so one side of the head was more than 10 thou closer to the piston than the other. I set it to .050" and all is well.