Ok so I don't have the lingo down yet. Several of you guys have tons of experience...I would have loved to work in a Yamaha shop back in the day! What is the squishband and how do you modify it as said above?

Squish on mine is right on. Tom Morgan cut the head. With the FL350 piston I had to correct deck height anyway so I was able to account for the head gasket thickness and the relief cut into the head.

The squish band is the relatively flat area around the dome inside the combustion chamber. The primary purpose is to concentrate the volume of the chamber to a small, central dome. This greatly increases detonation resistance and reduces thermal loads on the piston and head.

The sensitivity to pinging on mine is certainly correlates to throttle position so much so that I have not noticed an RPM range that it will not occur. But frankly, I will need to do a test run and look for a correlation. I do know that at higher RPM and at very small throttle openings I feel I am damaging the engine. So I quickly close or open the throttle more depending on what conditions allow.

I did do a timing check as well with at timing light after setting it by the manual. The timing is steady hot and cold albeit 1 or 2 degrees retarded from what the static setting would suggest. I passed that off to a delay in the Craftsman timing light and left the timing alone. If I dropped the idle speed very low, to the point that it would eventually stall, I could see the timing change to the retarded-starting setting. I did both a hot and cold timing check.

One of my first thoughts after assembling the operating the newly built engine (top and bottom end) was that there was an air leak that developed as the engine heated up. Thinking that the engine was pulling in air under light throttle causing a lean condition and possibly the source of the pinging. So I performed a leak down (pressure and vacuum) test on the engine while it was still warm. It was impossible for me to do a test at running temp but I was prepared with all the bits and can get the exhaust, carb and oil line off pretty quick because I had all the correct tools ready. No leaks were found.

I ordered 2 size larger pilot jets to richen up the mixture at that temperamental throttle opening. Rich to the point that the engine would 4 stroke or "blubber", yet still ping.

One thing that I haven't tried is blocking off the compression release valve or installing a stronger hold-close spring. The o-ring on the shaft is new and the valve was lapped to the seat and the pressure/vaccum test did not show leakage, even while operating the valve.

So, another question comes to mind. Even though this valve sits in an exhaust filled cavity, is it possible that exhaust pulled in through the decomp hole in the cylinder under conditions of a vacuum (like during a 4 stroking event where there is no power stroke and hence exhaust pressure) could be a contributor to the pinging?

Has anyone tried blocking off the decompression valve and testing?

Or tried a stronger hold-close spring?

www.dragonfly75.com/moto/SquishVelocity.html

I thought about that too at one point. The thing with that is that at no point is there a vacuum in the cylinder bore. The exhaust port is open to atmosphere at all times and even while coasting down with the throttle closed, at a minimum there will be no less than atmospheric pressure present in the cylinder bore when the piston closes the port on the way up to TDC. By the time the decomp port is closed off by the piston I imagine pressure on that side of the valve is over 100 PSI.

With a good pipe, there is a brief vacuum present in the cylinder while the exhaust pulse runs through the diverging portion of the chamber. I suppose a small air leak through the decompressor valve stem could cause some air to be drawn in. I've never taken one of these valves apart so I am not sure how likely this is.

In any case, you could block it off pretty easily to see what it does. You can remove the decompressor without removing the cylinder, only the head needs to come off. Block it off with a wad of chewing gum or maybe something else a little more suitable and take it for a ride.

I am at peace with my pinging ring-a-dinging DT360A.
Hot, cold or anywhere in between its as you guys describe
I messed with pilot jets, needle position, etc...short of stuffing my crankcase, I have come to believe that this is just how things stack up for this bike so I ride and enjoy it. I am using really good 2 cycle injector oil and stock timing and carb settings and it screams like hell right up to 9000 rpm and behaves just as the reviews others have shared from 1974 describe.

I didn't quite understand the comment about the squish chamber....is the assertion here that if I mill the head - which increases the compression ratio - that the weirdness goes away? A bit counter-intuitive, but perhaps more efficient combustion offsets the increased effects on detonation of a higher CR? If this worked for someone, then please provide specifics and I will buy a used head and give it a try.

Until then I am going to enjoy my crazy bike!

thanks
Martin

Jeez I must have really gotten a lemon. Mine damn sure doesn't do that.

TurboDan - I misspoke, it screams up to 7000 RPM redline, NOT 9,000
sorry about that

"I thought about that too at one point. The thing with that is that at no point is there a vacuum in the cylinder bore. The exhaust port is open to atmosphere at all times"

Good point, so pulling in air or exhaust via the decompression valve is improbable.

It's my interpretation that pinging and detonation are two different phenomenons. Although I don't understand the technical differences deeply enough to choose either one as a category on Jeopardy.

From my limited understanding is that both are a fuel/air ignition event caused by something other than the spark of the sparkplug.

I've jacked the octane rating of the fuel up to a calculated 100 and the condition still existed.

So some questions to those who have some "brainstorming" thoughts on this:

So, is it reasonable to think the cause is from heat or a hot spot in the combustion chamber causing the unintended ignition event?

Or is the sound not generated from what is traditionally known as "pinging" but from something else that creates a similar sound?

Could it be something mechanical in the cylinder?

In the meantime I'll be shopping for that high temp chewing gum.

I like this line of thinking. Pinging under light throttle/no load WITH good fuel just doesn't make sense.

Could the noise be the compression release valve rattling?