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Makotosun

DT360 Carbon Pattern

  • Snglsmkr
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DT360 Carbon Pattern was created by Snglsmkr

I'm looking for comments on whether this pattern and rate of deposit of carbon is typical for 200 miles of operation?

The engine had 600 miles on it when the head received the Harry Klemm 91 Octane mod.

The piston crown was cleaned up at that time and the head was machined-aluminum clean.

A trip of 200 miles after the head installation was a combination of town and country with much high speed (55mph and higher) opportunities.

The bike is stock (including jetting) with the exception of an overbore and a sleeper pipe.

The oil used in the autolube system is Pennzoil Outdoor Premium 2 stroke oil.
22 Nov 2020 15:43 #1

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Replied by msavitt on topic DT360 Carbon Pattern

hey man

if it runs good and the autolube is oiling correctly, I wouldn't worry cause there's nothing left to do.

how does yours run? I restored one and absolutely love riding it. It is very fast and ornery nasty at midrange which at first was a concern, but others provided me old reviews and write-ups that spoke of the behavior back in 1974 which meant that I fully restored it!

what is a "Harry Klemm 91 Octane mod"? did you mill the head? did you go faster?
22 Nov 2020 16:46 #2

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Replied by DaveHunter on topic DT360 Carbon Pattern

My understanding is that there should be fingernail sized sections of clean piston where each intake transfer port “washes” away the carbon on the top of a piston. As the fuel mixture gets too rich the black area of the piston becomes smaller and smaller. A completely black piston top is like yours would therefore be too lean a mixture
22 Nov 2020 16:51 #3

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Replied by turbodan on topic DT360 Carbon Pattern

The piston and head look fine to me. You can form carbon deposits in a half hour, they are normal.

The uniform carbon on the piston dome doesn't worry me. If it runs properly, no pinging, and the plug looks good I would leave the jetting alone.

That head looks better than the one I got back from TMR. How does it run?
22 Nov 2020 17:48 #4

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Replied by JayB on topic DT360 Carbon Pattern

It looks fine to me. I just wonder two things on the head:
Is the exhaust/forwards side up in the picture?
What is the orientation of the spark plug side electrode?
22 Nov 2020 17:49 #5

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  • Snglsmkr
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Replied by Snglsmkr on topic DT360 Carbon Pattern

The bike runs strong and runs best at more than 3/8 throttle once completely warmed up.

While not evident when warming up, it will ping lightly around 1/8 - 1/4 throttle under very light (think easy cruising) load once warm.

The head modification did not effect the compression but changed the squish clearance and shape. It made a minor but noticeable change in the smoothness of the motor. Difficult to describe but it effected even the stability of the idle. In a positive way.

I was hopeful the mod would eliminate the light load/light throttle pinging once warmed up. It has dampened it but not completely eliminated it.

The head is oriented with the exhaust side facing up in the pic. I pulled the plug before I loosened the head nuts and did not check the ground electrode orientation. It was burning a dark (milk) chocolate brown color on the ceramic.
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22 Nov 2020 18:24 #6

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Replied by turbodan on topic DT360 Carbon Pattern

The only effective treatment for that pinging with my 74 360 has been stuffing the crankcase.

The stock motor has a large crankcase volume and stiff reeds. This results in a very weak intake signal and low pumping efficiency at low throttle openings. I epoxied the hell out of the main transfer tunnels and it runs better everywhere. Better idle, cleaner running, more low end and midrange power. The proper place to stuff would be down in the cases, there is plenty of room down there to fill in, but I had success with filling the transfer tunnels for the experiment.

I couldn't get the stock carb to jet properly, it was always lean, pinging and surging at low throttle with a hanging idle or rich, four stroking and still pinging. It also liked to stall coming to a stop unless the idle was set high, then it would idle at 2500 RPM when warm. I initially blamed the carb, so I installed a Keihin 35mm PWK. This ran better and was more responsive to jetting changes but it still didn't run right. Still liked to ping and stall, still wouldn't jet crisply. Low tension carbon fiber reeds helped but it still wasn't where I wanted it. I'm used to modern reed valve two strokes and the stock DT isn't close.

With the crankcase volume reduced it runs cleaner down to very low throttle openings. It idles more consistently at all engine temperatures. I'm back over 50 MPG on my commute, probably because it runs properly at low throttle openings and I don't have to open it up to get anything out of it.

When you get tired of endless jetting attempts, checking and rechecking for air leaks, replacing perfectly good parts to no effect and generally chasing your tail, consider some epoxy. I was actually enjoying riding this bike until the ignition quit last week. I'm sure I'll be enjoying it again when the new CDI shows up.
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23 Nov 2020 08:50 #7

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Replied by msavitt on topic DT360 Carbon Pattern

Turbodan - we've chatted on this topic, which is very interesting to me having grown up fixing and tuning many types and sizes of engines. I got my '74 starting cold on 3 kicks (a fart on 2nd) and running with a very smooth (never dies) 1800 to 2000 rpm idle that hauls ass and bucks and chugs and pings and ring a dings at that low load in-between point which I now love since it is precisely how the literature described the bike back in '74!! This bike harkens me back to the days when a kids had to know how to fix and tune bikes and cars in order to get around. I love this bike!
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23 Nov 2020 09:17 #8

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Replied by turbodan on topic DT360 Carbon Pattern

I could put up with some quirks on a high performance machine. A pokey old smoker that makes something like 22 HP at the wheel, on the other hand, had better exhibit a charming personality.

I wanted to leave it stock but with the pinging, irregular carburetion and lethargic power output I really had no choice. Even back in the 70's I think these issues would have been hard to overlook. The older Yamaha 360's ran better, comparable bikes from other brands ran better. The DT360 seems like an experiment that didn't quite work out and they didn't have time to remedy until the next model year. By all accounts, the 75 DT400 is a better running and more refined machine, though still down on power considering the displacement.
23 Nov 2020 09:49 #9

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Replied by MarkT on topic DT360 Carbon Pattern

That's a bit of carbon for the low miles... carbon sticks and builds up where it gets hot.

Pennzoil/Shell used to make one of the best mineral two stroke oils for these bikes... formula changed a few years ago... It seems to carbon up a lot more now. Previous stuff burned very clean... I loved it.

I switched from the Pennzoil/Shell stuff to Legend Lubricants mineral oil and found it burns much cleaner than the reformulated Pennzoil... works good in injection system too.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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23 Nov 2020 15:22 #10

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