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36mm carb modded 78 Dt400

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Replied by MarkT on topic 36mm carb modded 78 Dt400

Check the chamfer on the ports...  it looks like the rings were nearly catching in the exhaust port from the bore scuffing above the port and on the opposite side above the rear transfer?

That's a wide port...  might need a piston with thin rings like a Wiseco?

Maybe Rich or one of the other porting experts can comment?  What I see concerns me but maybe it's nothing to worry about.
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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22 Feb 2021 18:07 #131

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Replied by FrankC on topic 36mm carb modded 78 Dt400

Thanks for catching that. I thought those marks seemed a little weird. Might be catching as you said. I dont have experience in this area of expertise at all. I got the transfers window profiles much better this go around particularly on the rear transfers. They were fairly ugly before. Hopefully that will help with that. I will give a closer look in the cylinder and examine the piston rings. What was real strange when I lifted the cylinder off the piston on tear down the piston rings stuck in the bore at the last 1 inch or less on the bottom of the skirt. Thanks Mark

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Last edit: 23 Feb 2021 12:30 by FrankC.
22 Feb 2021 19:21 #132

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Replied by turbodan on topic 36mm carb modded 78 Dt400

Those carbide bits do like to bite, have to be very careful.  I do like how they cut the iron sleeve, very cleanly.  The small diameter of the bit allows small radius corners in the transfers too.  Maximum port window area for any given timing.

It looks like they narrowed down the main transfer port window quite a bit compared to the earlier 360's.  It looks like you could move the leading edge forward some more, you still have room before you get too close to the exhaust.  You have to do a good amount of grinding back into the tunnel for this though, it can be tricky.

I like the transfer port directional aim a lot better than what they did on the 360.  I have use a lot of epoxy to get to where the 400 is stock.
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23 Feb 2021 08:32 #133

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Replied by turbodan on topic 36mm carb modded 78 Dt400

 This is the one I'm working on now, with the epoxy set up but before grinding it back to the desired shape. You can really see the difference in size of the transfer port windows. 

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23 Feb 2021 08:44 #134

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Replied by FrankC on topic 36mm carb modded 78 Dt400

Those port windows are hugh. Good tips on porting. I did also switch back and fourth with carbide and sanding rolls. I only had 2 sand rolls that would fit in the transfers. For porting this i wanted a wide and low exhaust port and only slightly wider transfer ports.. Most of the transfer flow from raising them in the hopes a new pipe would find them open for a extended draw through them. I enlarged the tunnels but in the opposite direction, not side to side but inside to outside.. That was at least my reasoning for this build. Thanks
Last edit: 25 Feb 2021 09:57 by FrankC.
23 Feb 2021 12:02 #135

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Replied by FrankC on topic 36mm carb modded 78 Dt400

Got some work done on those chamfers. Took a closer look in there and Mark was correct there I found quite a few rough spots that could snag a ring so I smoothed those out by hand using sand paper. Also i added more chamfer to the bottom of the exhaust port where it was lacking some.Supposed to be a 30 degree chamfer angle. I think I got a little muddy on that. The shape to the transfers windows could be better. Maybe next time. I may try my hand at welding up a pipe for it later on.
Last edit: 23 Feb 2021 12:26 by FrankC.
23 Feb 2021 12:15 #136

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Replied by FrankC on topic 36mm carb modded 78 Dt400

So the bike will definitely loop it in second wacking the throttle. Slightly less down low more in the mid. I probably not next time drop the intake so far or widen the exhaust as much. 67 percent is good.
Last edit: 23 Feb 2021 15:10 by FrankC.
23 Feb 2021 15:06 #137

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Replied by FrankC on topic 36mm carb modded 78 Dt400

Hole in piston i cut. Looks ugly but it works.
Last edit: 25 Feb 2021 09:48 by FrankC.
24 Feb 2021 15:58 #138

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Replied by FrankC on topic 36mm carb modded 78 Dt400

I believe it washes and cools the underside of piston crown, lubricates the small end, and overall cooling. Almost 500 miles. I dont know if there is supposed to be carbon build up under there.
Last edit: 25 Feb 2021 09:47 by FrankC.
24 Feb 2021 16:02 #139

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Replied by FrankC on topic 36mm carb modded 78 Dt400

The bike rips without the pipe. 3rd gear will pull the front wheel up 2 inches. You can feel the power band in action
 
24 Feb 2021 16:05 #140

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