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Dt 400 performance improvements

26 Jan 2020 20:48 29 Jan 2020 11:36 by FrankC.
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Dt 400 performance improvements #1
What mods have you guys done to improve the performance of your machine short of major porting. Each item must be a stand alone improvement other than timing changes or jetting to make it work. I found cutting a hole in the intake side of piston really helps with cooling which makes horse power when hot. It also feeds the boost port and creates 360 degree port timing. Mark the piston through the intake when boost is uncovered or when the piston is even with the exhaust port more accurately, with a sharpie. The hole you cut should be fully uncovered at that point. Make the hole including the chamfer the width of the port. You can cut the angle of the hole upward to the crown for better cooling but much harder to do, either way still effective. Looking forward to your comments.
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28 Jan 2020 21:06 30 Jan 2020 19:13 by FrankC.
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Dt 400 performance improvements #2
Get you a new silencer that breaths better and probably less prone to be gummed up. Many on the forum have reported some gains using an fmf turbine core 2 silencer in place of the stock silencer. There is modding of the silencer involved, so not for the faint of heart. Your power increase will depend on how gummed up the stock silencer is. If you decide to keep your stock silencer know that it's true inner dia. Is only 1/2 inch or so through the inside of the packing, maybe also in the turbine cones. Many burn the pipes to clean them out, but I have no access to this type of equipment, so I just use drain opener and hot water. Made a good difference in sound cleaning mine, but power I don't know because I did other mods at the same time, but c'mon it must have. If you decide on this you will need a rubber bung for the small end and an expansion test plug on the big end (2 inch) with a butterfly nut for easy removal. You will want to scrape any grease or carbon off the sides to seal correctly and remove any dents in the sealing area. Safety goggles very important. The stuff I use is alkaline based (sodium hydroxide) not acidic or it can wreck your pipe. Alkaline is safe for metal. Name brand I use is Liquid Plumr Clog Destroyer Gel with Pipe Guard. It will not rust your pipe even after rinsing. Mix 50/50 ratio water to cleaner than pore in pipe. Turn pipe over to remove bubbles out of turbine core. Says on the bottle to leave in 15 minutes but I have left it in 2 hour. Rinse hot water. The fun part begins how much junk in the turbine core. Fill with water cap both the ends and shake. I found the stuff won't nearly come out if just running water through it, you need to shake the pipe, then pour it out maybe 15 times. I needed to use the liquid Plumr twice my silencer was so bad. Easiest to do this near a warm water source, but try not to dump it in the bathtub. This grease is super hard to get off.
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30 Jan 2020 16:43 02 Feb 2020 13:18 by FrankC.
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Dt 400 performance improvements #3
Get a grip on that header pipe flange. Inside the flange at the exhaust port there is the collar that ends abrubtly with a square edge. Can't be good for flow or the resonate waves traveling in the pipe. Just feather that down with sanding roll on dremmel. Slivers of steel will come out as the metal thins. Keep going until all that comes out, so as not to have debree hanging around only to jiggle loose later. This does not compromise the pipe. Can't say the performance increase but what the heck.

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02 Feb 2020 12:17 03 Feb 2020 14:24 by FrankC.
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Dt 400 performance improvements #4
Clean up the Reed block. Remove the reeds. Check to see if the rubber sealing area of the reeds is flush to the aluminum back plate. If you can feel any discrepancy at all with your finger there get a flat piece of stone or other and some 600 or 1000 grit sandpaper. Make
it perfectly flat. Also round the longetudinal ribs of the cage or in other words knock down the top of the Vee. You get better flow. Also the rib where the cage comes to a point. A bullet shaped grinding stone works on the ribs. Clean up the sides of each rib with a fingernail file and deburr the rubber seal area.

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02 Feb 2020 14:36 11 Feb 2020 12:52 by FrankC.
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Dt 400 performance improvements #5
Stuff the intake. The intake volume is fairly large which reduces the signal to the carburetor. A better signal reduces the need to rejet as much for altitude changes as the carb is able to better draw what it needs. Removing the Reed valve gasket and using sealer will help. Also, or stuff the intake in a way that not only gets a better signal to the carb, but increases velocity to the intake port. Try and transition the square opening at the Reed valve to the more oval shape at the window. I just ran a long bead of filler in the corners of the intake from the Reed valve sealing surface to the ramp up to the port window. A thinner bead at the Reed valve and thicker toward the port ramp. 1/8 to 3/16 inch I think. Also, took up some volume around the top and bottom of the Reed valve in the corner of the pockets near the gasket surface. The filler I used was Por-15 Por Patch. Use at your own risk. It is good for 350 degrees continuous and like the Por-15 fuel tank sealer ethanol does not eat it away like it will with jb weld. I did my own tests with both. You must thoroughly clean the intake with alcohol or other, multiple times for a good bond. In an engine only time will tell if it lasts, so I cannot reccomend to you the Por Patch.

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02 Feb 2020 21:54
Makotosun
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Dt 400 performance improvements #6
Contact Meis here on the board and inquire about his sleeper pipes. He can convert your stock pipe, keeping all spark arrestors intact, and stock looks unchanged, but add quite a bit of useable performance. He is in California.

IF it isn't one thing, it's another.
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03 Feb 2020 05:27
sibrad
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Dt 400 performance improvements #7
Hi, iv'e recently bought a 79 monoshock dt 400 to modify into a flat tracker and i'm after a low level expansion pipe for it, any ideas where i can get one.cheers
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03 Feb 2020 14:28
FrankC
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Dt 400 performance improvements #8
I wish I knew brother.

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03 Feb 2020 16:32
monel12
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Dt 400 performance improvements #9
I agree with Mako. I had Dave work on my 75 DT 400 pipe and it runs really well!

1971 RT 1B
1971 RT 1B
1970 RT 1
1970 RT 1
1969 CT 1
1971 DT 1
1974 100MX
2016 BMW F700 GS
1982 Yamaha Towny
1971 CT 1
1975 DT400
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05 Feb 2020 12:48 05 Feb 2020 20:13 by FrankC.
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Dt 400 performance improvements #10
Intake manifold to Reed valve transition. The factory offset the carb to the right side of the bike for some reason. They did it in 2 stages. The cast intake is offset, and then the rubber carb manifold is, both put the carb to the right maybe 3/4 inch, just guessing. The left side of the rubber manifold could use a generous radius to smooth the transition into the left side of the reed valve. Just cleaned up the right side a little. The rest of the manifold not so good either, but you have to choose your battles. The ears on the manifold help as stuffers to raise velocity so I left those alone.

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