Hey guys,
I recently fixed up a DT360a to try to relive some memories of an old MX ported one I used to own that felt very fast.
I built it stock with STD piston and ports, and a "sleeper pipe" and 32 mm carb to start with.
It was a very torquey bike and power fell off right around 5500rpm.
It left a bit to be desired for me, but after realizing how scarce the pistons are for these bikes, ive tempered my expectations a bit for performance improvement. But still came up with a plan.
I decided to take it all apart and measure or "blueprint" everything.
I've ported and piped quite a few 2 strokes over the years and have my own opinions on how things should be.
I took my DT360 engine apart and noticed a few things right away.
Intake port is miniscule, very small
I measured my Main transfer ports at 113 degrees, and secondary transfer ports at 108 degrees
Exhaust port was only 174 degrees.
Squishband was very large at nearly 3mm.
Exhaust port timing should be at least 180 degrees to make use of an expansion chamber most effectively. As well as the intake and transfers need to be up to the task as well
I decided to change my port timings to 185 degrees for exhaust and 118, 122 degrees for transfer ports.
I noticed that at bottom dead center, the transfer ports and bottom edge of exhaust port was still being shrouded by the piston here you can see light on the piston ring when i shine a flashlight thru the ex port at bdc.
so I decided the easiest way to achieve these timings would be to use a 2mm spacer under the cylinder

, and then mill the same off the top of the deck.


I ground a little off the bottom edge of the main transfer and now they are nice and flush with the spacer fitted

OK so with the ports raised 2mm, I went ahead and raised the exhaust port itself 1mm for a total of 3mm, giving me a bit under 185 deg of timing for exhaust. I did not widen it as I found it already to be decently wide enough for my goals.
For the intake side, I raised the intake port, but not so much that the rings were in danger of popping out, even after raising the cylinder. I took a look at the MX porting for the height of the intake port, I did not widen the exhaust or intake ports as I did not want to compromise piston life!
stock intake port :

In order to get more air thru the engine without making the intake port lower and scraficing piston life, I decided to add a pair of 3/8" boyesen case ports.
Here is close to how it ended up


I did some math on the new intake port area, and it needed a bigger carb than 32mm, there is not much reason to have a carb smaller than your intake port in my experience.
I fitted an intake manifold "510" from a SC500, and that let me fit a spare PWK38 air striker carb I had.

With the intake and port timings to my liking I went on to make my own pipe.
I wanted a torquey design that would come in early,
I based my dimensions off of concepts developed by Fritz Overmars in order to come up with the dimensions for the pipes I make. Ive had great luck in the past with this. To achieve this torquey pipe it will need to be around 1450mm long for peak power around 6500rpm. This is 200mm longer or so than my "sleeper pipe", I took inspiration from the curly header on my Honda CR250r for its design.

First I design the pipe to the dimensions needed in CAD, and then I make a paper model,
Then I get the peices laser cut from 18 ga mild steel

Mig weld it all up after some test fits


Then I made my own "slip fit" style header.
I pressure checked it with rubber plugs to test the sealing of the o rings as well as any leaks in my welding, and it holds pressure !

and all painted up in VHT satin black and mounted on the bike.


Its quite a bit louder than the double wall muffler from factory, here a short video of a cold start.
I have started with a 150 main jet (keihin) and will go from there, Ill update this thread once I have a better idea of its performance! This video was only the 2nd or 3rd start.

well today I took the bike for a ride and it felt more responsive and quicker to rev but overall the same top end power, I also noticed if I held it around 6,500 RPM to 7,000 RPM it would eventually start to four-stroke, which I thought was odd because the 150 main jet (around 270 in mikuni speak) is on the leaner side I thought.

with so many changes to the carburetion porting an exhaust and not feeling much of a difference I knew there was a problem somewhere,
I asked myself what was the only thing I haven't changed...and that is the airbox.

I disconnected the boot from the carburetor and took it for another little rip and it instantly did a big wheelie to 7,500 RPM and feels a lot angier up top and a lot louder.

I want to keep the stock air box set up for noise purposes, I'm not sure yet if the restriction is the intake boot going from carb to air box, my uni filter element being over oiled or something, or if the air box snorkel is pressing up against the tool tray too much and getting restricted.

I'm going to try to figure it out by first removing the snorkel and then riding and if that doesn't fix it I will remove the air filter element and see if that fixes it. if it still bogs up top with the air filter and snorkel removed I can assume the air tube going from carb to air box is the restriction.
I'll let you guys know what I figure out!

akara1 wrote: well today I took the bike for a ride and it felt more responsive and quicker to rev but overall the same top end power, I also noticed if I held it around 6,500 RPM to 7,000 RPM it would eventually start to four-stroke, which I thought was odd because the 150 main jet (around 270 in mikuni speak) is on the leaner side I thought.

with so many changes to the carburetion porting an exhaust and not feeling much of a difference I knew there was a problem somewhere,
I asked myself what was the only thing I haven't changed...and that is the airbox.

I disconnected the boot from the carburetor and took it for another little rip and it instantly did a big wheelie to 7,500 RPM and feels a lot angier up top and a lot louder.

I want to keep the stock air box set up for noise purposes, I'm not sure yet if the restriction is the intake boot going from carb to air box, my uni filter element being over oiled or something, or if the air box snorkel is pressing up against the tool tray too much and getting restricted.

I'm going to try to figure it out by first removing the snorkel and then riding and if that doesn't fix it I will remove the air filter element and see if that fixes it. if it still bogs up top with the air filter and snorkel removed I can assume the air tube going from carb to air box is the restriction.
I'll let you guys know what I figure out!

On My Super DT400 on which I have a MX400B top end on the original DT400 lower end, 38mm Mikuni and a 73-74 MX360 pipe. I run a Sock style Uni air filter on it. there is no way I know of to get the stock air cleaner assembly to flow well enough for the modded engine's requirements.

Many have reported the Uni element, with its double layer of foam over the cage, to be too restrictive on its own.

The Uni filter has not been a problem on my 400, It flat out Rips and it will scream to 8000 rpm with some over rev. top end is eye watering to say the least, I have confirmed it will do at least 90 mph, I was not Brave enough or really foolish enough to go beyond that.


it does a good job protecting your engine as well, better than any paper filter like a K&N for example.

To clarify... I was talking about the specific Uni element for the stock airbox, not Uni in general.

I did the same thing with my KE250. 2mm spacer under the cylinder, milled 2mm off of the deck. I might have to try that on the 360 as well. The stock exhaust timing is about 174 degrees, which is not terrible, but the transfer duration is definitely insufficient. A 2mm spacer affects the transfer duration more significantly that the exhaust duration so it's not a bad way to go.

The MX360 exhaust port is about 6mm higher than the stock DT360, about 190 degrees duration. I don't want to rev this thing that high. Shouldn't need to anyway. It's got enough displacement to make power at lower revs.

In my case I couldn't tell much difference from the Boyesen ports. I'm sure they are more beneficial as the motor makes more and more power. They were the first thing I did, before the pipe and carb.

Would you consider making those pipes for sale? Lot of work but I would pay a lot of money. I'm running an MX360 pipe as well but I'm aware of it's limitations. A longer header with a built in taper is certainly ideal.

So to update this thread, I finally got my license plates from Vermont and I've been riding the bike around jetting it and playing with it.
MarkT was right on the money, the uni air filter element with the two layers of foam is too restrictive for anything beyond a stock motor. with one of the layers of foam removed it is much better.

The bike is running great now, it has monstrous four-stroke like bottom end torque that surges into a powerful mid-range, nothing much to speak of above 6,500 RPM but thats all because of the pipe I made.
I take off in 2nd gear even with the 16 tooth front sprocket, I'm thinking of going to 17t.
It's a very responsive bike and the noise is quite something, I would like to get a video sometime if me riding it for you all.

as for making the pipe for sale I won't rule it out, I honestly it was quite time consuming to make compared to the usual low pipes I make for Yamaha Street twins because of the tight clearances against the fender and front wheel and gas tank and cylinder head and air box and your knee, as well as the custom header joiner and making it work with the original exhaust heat shield and mounting. so I am a little burned out on that but I'm making pipes for my yds5 now and then maybe afterwards I'll make another DT pipe!!

sorry to ramble on but I thought I would update everyone in this thread here to say that all the mods done in this thread have turned out well and the bike is working great on the street and I don't plan on making any more changes to it for now!!
Ended up at 155 main 55pilot (keihin jets)