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DT360A tuning advice

  • Boomflamingo
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DT360A tuning advice was created by Boomflamingo

Hello

I am building a DT360 flat tracker and would like some tuning advice please

From research I understand that the exhaust is very restrictive as is the carb

Plan is to fit a 34mm Mikuni, have a custom exhaust made, and maybe a head from an mx360?

I have a Rex speed shop electronic ignition to fit.

Does the above seem right? Anything else I should be aware of, and any idea where to start with jetting?

Thanks

Mark
20 Jan 2021 11:07 #1

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Replied by akara1 on topic DT360A tuning advice

hey there

I had my old mx ported dt360 on a flat track a couple times and it was a handful.

if I was you I would try to build a torquey engine that revs to about 7,500 RPM
here's a thread where I did something similar
www.yamaha-enduros.com/index.php/forums/...t-pipe-and-carb-work

I recommend shimming the cylinder 2mm up, decking the head and cylinder head to set squish, and build a pipe that is about 1350mm long from the piston face to the end of the last cone. This will make an engine with strong mid-range.

I use a 150 keihin round main jet on a flatslide. which is similar in size to a 250 mikuni hex main jet.
Last edit: 20 Jan 2021 11:21 by akara1.
20 Jan 2021 11:20 #2

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Replied by turbodan on topic DT360A tuning advice

It depends on the kind of power you want to make.

An MX360 cylinder and head drop right on, doesn't make much power until 6000 RPM and then pulls reasonably well until about 8500. The porting is a bit wild but it is what it is. You can take advantage of gearing being able to turn that kind of RPM. I don't know about longevity though. Seems like its going to be short lived.

The stock DT360 is ported much more conservatively. I have been stubbornly committed to the stock exhaust timing with mine, which is good for low end and midrange power. This is a somewhat large displacement two stroke, you shouldn't need to wind it way out to build power. It peaks around 7000 as is, which is not bad. The stock port can be widened considerably before reliability is a concern.

A spacer under the cylinder and machining off the top is a good way to go about improving port timing. The transfer timing is very conservative and if you only raise the exhaust by the same amount you are still on the torquey side of timing. The transfer ports are aimed rather sharply toward the rear of the cylinder, I have found that this generally favors top end power at the expense of low-mid. Wouldn't hurt to adjust this, though some epoxy is required.

The intake port is the most restrictive aspect of the stock cylinder. It is not nearly sufficient for a single cylinder of this displacement. The port can be widened, lowered and squared off up top and if you feel adventurous you can add some boyesen ports through to the transfer tunnels. In the MX360 Yamaha made the intake much taller but not much wider. Boyesen ports help tremendously by increasing intake area without eating away cylinder wall on the thrust load side of the bore.

The stock reed cage sucks pretty bad too. With some grinding and fabrication you can fit a more modern four or six petal reed block in. The stock block is grossly undersized and even with modern resin or carbon reeds it will restrict power anywhere above mid RPM. I have a KX125 reed block in mine and it makes a significant difference.

The MX360 head by itself will raise compression by a lot. I like the dome and squish area a lot better than the DT360 head though. If you use it with stock port timing you would need to remove a significant amount of material from the dome to lower your cranking PSI to a reasonable level. If you raise the exhaust by two or three millimeters you might be able to run it as is or with only a small amount of material removed.

Lastly, you will notice that the bottom end has a very large amount of volume in the cases and transfer tunnels. Yamaha ran this design for exactly one year and went right back to a small crankcase volume in 1975. The best thing for it is to stuff the crank and epoxy the transfer tunnels and cases to get case volume into the acceptable range. This improves carburetion significantly and bumps up your low mid power as well. While I was at it with the epoxy I filled the main transfer port to the extent that I could re-angle the leading edge of the port to aim slightly ahead of the center of the bore. This is comparable to what Yamaha did with the MX360 and also other big two stroke singles of the era, such as the TS400 and Kawasaki Bighorn.

What I would really love is a proper low-mid pipe. I am running the MX360 pipe at the moment which is good enough for the 1970's but leaves quite a bit to be desired. It just doesn't have the length to make good low-mid. You need a long, tapered header, a long belly and a slow, gradual taper in the baffle.
The following user(s) Liked this Post: Snglsmkr, Pedalcrazy
20 Jan 2021 13:41 #3

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Replied by Boomflamingo on topic DT360A tuning advice

Thanks very much for the detailed response

I don’t have anywhere near the knowledge you guys have and so apologies for the stupid questions.

I would ideally like a mix of some torque and a nice powerband.

INTAKE
34mm Mikuni with pod filter
Intake rubber from an MX360 to fit the larger carb
Intake port - I would need to get in there with a dremel?
Make it big enough to fit a KX125 Reed block and fit boyesen reeds

CYLINDER
Raise height by extra base gasket, or should that be 2 extra?

HEAD
Source an MX360 head, and get machined - by how much, what am I measuring?

Custom exhaust- going to run exhaust along right side of bike, not over the top and out on left, so can make longer as required

Thanks again for all advice
21 Jan 2021 02:01 #4

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Replied by turbodan on topic DT360A tuning advice

You don't have to use the KX reed block, I just had that laying around. It doesn't bolt right on, you have to make it work. There are hundreds of other options out there, I'm sure there is an easier one to fit. Next one I do will be a four petal block, probably from a modern 250. That would be preferable to the six petal, though just about anything is preferable to the stock unit.

I'm actually not a big fan of Boyesen reeds. I'm running the stock carbon fiber KX reeds. The Boyesen two stage reeds wear out quickly if you run them without reed stops like the suggest. I think they just want to sell more reeds.

Boyesen ports go from the intake, through the casting and into the transfer tunnels, aimed downward into the case. These are what you need to increase intake area.

Raising the cylinder by 2mm would be a good start. It raises the exhaust port too but 2mm won't hurt much. To do this you will need a 1.5mm or .060" spacer and one additional gasket. The stock deck height includes one base gasket, so on top of this you are adding 1.5mm of spacer and an extra .5mm of gasket, 2mm total. You must then machine 2mm off of the top of the cylinder to correct the deck height.

What you are measuring on the cylinder head is volume. The MX360 exhaust port is 6mm higher so the trapped volume is smaller. Yamaha used a smaller combustion chamber volume to bring the compression back up. On the DT, the exhaust port is lower and the trapped volume is larger. If you raise your exhaust port 2mm you still have an extra 20cc's of trapped volume. Accounting for compression, I think you could expect to remove around 3cc's from the MX head to get cranking compression back to stock. You probably don't want to go that far though, stock compression is on the low side. You might try removing 1.5cc's from the head and see how it does.

The best way to fit a long pipe through this thing is to follow the stock route. Going diagonally through the frame the way it does give you a couple more inches of room. You still need to snake the head pipe around a bit though if you want good low-mid power.
The following user(s) Liked this Post: Jack
21 Jan 2021 07:52 #5

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