I was hopeful the head mod would eliminate the mild pinging at light throttle/load when completely warm. Like you, Turbodan I had moved jetting (pilot, air screw, needle and main) up and down in search of a sweet spot. I could not locate a 2.5 cutaway slide but would entertain trying that to get one step closer.

I must admit I do not fully understand what causes the pinging.

One of my attempts to understand it better was to purchase 93 octane straight gas and boost the octane up to a calculated 100 points. The performance was good but still the same light pinging under light load. Knowing that hot spots (like glowing carbon) could be a cause of pinging I questioned the seemingly rapid build up of carbon being a contributor.

From your experience, Turbodan is it the lean mixture due to a poor vacuum signal that is the root cause of the pinging under those light load/throttle conditions? You have obviously put a lot of time and effort into working towards a solution.

The Pennzoil oil I was using was one that was preferred by an ultralight aircraft organization due to the low carbon build up. So I bought a case several years ago. I would say I have the reformulated lot from your comments MarkT.

I bought a 2.5 cutaway slide from Summitt Racing. It was slightly shorter so you need to cut an old slide to make a spacer so at full throttle the needle doesn't come out of the jet. This was on my 76 DT400 when I was fighting the same symptoms. The 2.5 slide DID help and along with upping the pilot and turning in the air screw nearly all the way in is where I ended up. Also, keep the idle as low as possible, 1400 should be doable. Timing is at spec, 2.9. It is still not perfect but if you do all that you'll enjoy the ride more. Also, I've found at low speeds and low loads just close the throttle all the way if you are coasting. A low idle speed really is critical. I ride where the motor likes it too.

I recall reading your post, Pedalcrazy about the results of using the 2.5 slide which sent me on a mission to locate one. The folks at Sudco had a part number for one but 6 month lead time.

At that time I considered creating a 0.5 mm extension (molding or attaching a skirt) to the existing slide but after reading how adding a device like the UFO to the bottom of the slide can drastically change the jetting requirements, I shyed away from perfoming a destructive mod to the existing slide. There would have been no going back.

As it is the bike is still a whole lot of fun. It just feels like I'm doing damage if I treat it gently! Like you, I try to avoid hanging out in the 3/16 - 1/4 open throttle position. There are times when I'm cruising on a paved secondary road where that is exactly where I want to be!

Your approach leaves the original slide unmolested.

With Turbodan's comments fresh in my mind it will be diffficult to not think crankcase and crankshaft while stuffing the turkey on Thanksgiving morning!

If you have access to Summit Racing on line that is where I got my slide. I don’t know what Sudco is? It’s been a couple years now so I don’t recall part number.

I can only speculate. Detonation can be caused by many things though. In this case we can rule out most of the usual suspects since the detonation only occurs at low load during cruising. If compression ratio, combustion chamber design, squish clearance, fuel quality or ignition timing were the cause, the detonation would be most likely to occur at full throttle. That leaves either air fuel ratio or poor scavenging. Lean jetting can always cause det but that doesn't seem to be the issue here since I have experienced the pinging while jetted significantly fat on the same low throttle circuits that are controlling fueling at a cruise.

As far as scavenging, it is possible that the low crankcase compression causes inefficient and incomplete scavenging during the transfer period. Though the intake signal is weak, some air and fuel will be drawn through and trapped as the piston tops out at TDC and begins to compress the contents of the crankcase. At part throttle, there simply won't be much to work with here as the entire intake period takes place under significant vacuum. At some point quite a ways down the bore, the crankcase will go from a vacuum into positive pressure. This is essential since the only thing motivating flow through the transfer tunnels into the cylinder is the pressure ratio between the crankcase and the cylinder bore.

The stock DT with what I consider excessive crankcase volume produces comparatively modest compression in the crankcase, especially at low throttle. When the transfer ports open and scavenging occurs, I think this lazy, weak transfer flow is likely ineffective at cleanly chasing out the spent exhaust gases from the cylinder bore. At part throttle, you may only have 80cc's or less to do the job. I suspect this results in an irregular mix of fresh air and fuel and old, burned gases that do not contribute to the next combustion event. Instead of a nice, controlled burn we have an irregular and uneven burn resulting in pockets of air/fuel mix that light off on their own. A smaller crankcase volume will compress the any given amount of air and fuel under the piston to a higher pressure and produce more effective gas exchange.

It could be that or it could be the weak intake signal causing carburetion problems. A large crankcase volume subdues the intake signal, taking longer to go into vacuum and producing a weaker intake vacuum altogether. A smaller crankcase volume more precisely responds to changes in volume as the piston rises from BDC and produces a stronger, longer draw on the carb. Again, during a part throttle cruise this is very important as there isn't as much to work with having the throttle just cracked open. The reed valve intake exacerbates this on the DT. The stock reed cage is small and the reeds are fairly stiff. You already don't have much draw from the crankcase and the reeds cut it down further.

Whatever it is, I'm finally happy with this thing. Better power, more torque, better manners at low throttle positions, cleaner overall carburetion and better idling. And no more pinging. I will get some video, I'm sure these wild claims are hard to believe.

The only thing about this theory is that Yamaha was known to have high primary compression ratios... especially on the big bores.

Modern theory uses comparatively low primary compression to achieve far more power... Would love to hear Rich's thoughts on this.

On most of them they did. The 74 360 is a one off motor. The older rt1/rt2/rt3 used a similar bottom end design with small case volume. The dt400 in 75 went right back to it.

The dt360a is a downsized sc500 engine. It's a 496cc crankcase with the 351cc top end. The massive transfer tunnels result in even more crankcase volume for the small displacement motor.

The current state of the art two strokes vary somewhat in this regard. All of the high performance two strokes built in the last 30 years have stuffed cranks and minimal dead space in the cases. My KX250 even has plugs in the crank pin.

The idea behind larger case volume is to provide more volume for the pipe to suck on, as I understand it. Lower primary compression produces, in theory, a less energetic and more orderly gas flow into the cylinder. High transfer velocity is supposed to cause a peaky power delivery and less efficient scavenging. In general terms, this is correct. In practice, it is all about finding the sweet spot or at least getting close to it.

In a reed valve motor it can draw in as much mixture as it wants at any time, so a large case volume is not particularly useful. It definitely has a negative effect on pumping efficiency at low throttle openings. That much is evident in the dt360.

So, the large crankcase volume/weak signal is only an issue when the carb is operating in a range where the intake vacuum is the primary force behind fuel delivery to the intake track. Such as when the pilot and idle air circuit are the main delivery components.

When there is ample air flow through the carb (due to a wider throttle slide opening) the signal is generated by the venturi effect at the jet needle and the vacuum (crankcase signal) is not the primary force.

So, with a weak signal the low speed circuits of the carb become less effective when the throttle is open slightly (dropping the vacuum level) and unless or until the throttle is opened further to create venturi generated vacuum.

"Stuffing" the crankcase allows for a higher initial crankcase vacuum level which in turn allows the carb circuits dependent on intake vacuum to delivery fuel with wider throttle openings and provide a wider overlap from one circuit to the next.

I may be following your train of thought. Not that I understand all the complexities or dynamics of this but to at least form a cause and effect understanding in my own mind!

Stuffing ... what an appropriate pre-Thanksgiving topic.

Sudco is a California based Mikuni Keihin carburetor dealer.

I think that's more or less what I'm rambling on about.

The low throttle jetting circuits have to handle all potential low throttle operating conditions. Idle, low RPM cruising, high RPM cruising. The carb doesn't know the difference, it meters fuel at any given throttle position the best it can.

Think of a large volume in the crankcase like a damper. The larger the volume, the greater the effect. The piston is trying to pump air in on the way to TDC and compress it on the way to BDC. An unnecessarily large volume in the cases will dampen and diminish the pumping effect. The crankcase will take longer to go into vacuum and the overall vacuum will be less for a large volume than a small volume.

In practice, considering the jetting problems I observed both with the stock carb and the Keihin PWK, it seems to run rich at idle when there is a longer duration intake pulse. At higher revs, the intake pulse is significantly shorter and the engine ran lean while cruising. With a smaller crankcase volume, the signal at the carb should be more consistent allowing for better carburetion.

I put about 20 miles on the bike this morning puttering around and it is a pleasure to ride. I am no longer trying different jets, needles and slides several times per week. I can't tell you for certain why the epoxy filler worked so well but the results are unmistakable. I've done two cylinders so far with the specific goal of reducing crankcase volume, both have achieved the same results. One is using a piston with no windows in the intake skirt, isolating the intake port from the crankcase like a piston port engine about 80 degrees after TDC. This cylinder got less epoxy, but still a substantial amount, in the transfer tunnels in consideration of the effect of the piston. Didn't want to go too crazy on the first attempt. The next cylinder was my Boyesen ported 81mm jug, running with the piston modified to provide a 360 degree inlet period and relying solely on the reeds to control the inlet. Since the Boyesen ports and piston modification connect the intake port to the crankcase at all times, I applied the maximum amount of epoxy to the main transfer tunnels. That's the jug I have on the bike now.

For the sake of comparison, here are some other comparable big bore cylinders from the era.
First, a stock 1974 DT360A:

Next, a 73/74 MX360:

Now the SC500:

For something completely different, an early TS400:

And lastly, a 1970 Bighorn 350: