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Anybody tried boyesen ports on the reed valve motor?

  • turbodan
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I presume they had not thought of these yet in 1974. Seems to me that the DT360 is pitifully short on intake port area so these should help.

I'm not looking for a screamer, just more power everywhere. The stock powerband is fine, there's just not enough of it.
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11 Jul 2020 13:21 #1

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Have you tried it out yet?
11 Jul 2020 17:49 #2

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This jug needs a fresh bore. I've got a top end coming but it will probably be a week or two before I can slap it on.
11 Jul 2020 18:00 #3

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Installed Boyesen reeds in the RT2 engine. Have not run it yet. Primary reason was saving the engine if a stock reed failed.

Not expecting any huge change, if that is needed it will be via modification to the cylinder head.
30 Sep 2020 05:54 #4

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Im not a fan of thse reeds . Too stiff . Better of with single stage carbon fiber and better yet V force conversion

Rich
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30 Sep 2020 09:01 #5

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I ported mine to mx250 specs and it now has 360 degree intake timing as well as a 30mm wide boost port. Didn't bother adding the 2 boyeson ports wanted to see what it was like just as mx250. It's a tractor down low in the Bush in 2nd and 3rd but gets up and bogies whem you crack open the 35mm keihin. Still has stainless reeds and stock chamber with all the guts cut out of it
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30 Sep 2020 12:09 #6

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The stainless reeds do suck, and the stock reed block is not good for much either . I stuffed a KX125 reed block in this thing, carbon fiber reeds included, and it was a significant improvement.

Regarding the original topic, these ports did not make a big difference with otherwise stock porting and a stock pipe. The transfer porting is so conservative that the stock intake is probably not a significant restriction. I would actually like to try going another way with a stock jug, using a piston without windows to close off the intake port sooner and boost crankcase compression. These boyesen ports are open to the intake at all times so there is no way to try my little experiment unless I epoxy the ports closed again.

With a good expansion chamber, suitable porting and a big carb, these ports would certainly be beneficial. The stock DT360 cylinder closes the crankcase from the intake port at BDC which diminishes the ability of the pipe to draw more mixture through during scavenging.
30 Sep 2020 12:11 #7

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I read the KX 125 bit. I agree with Rich about Boysens but have no complaints about running them in my TDR 250.
They do have a tendency to run weak lowdown and rich up top. You would have to try them .
I think you would see better results just re cutting the reed block.
In other words, where its square at the top near the mount screws for the reeds, cut it back and make it into a half circle.
Knife edge it . Knife edge the other end where the end of the reeds rest, on the inside though.
Makes the bottom end smoother.
Then you have started something. Get the casting marks out the inlet tract, clean the transfers up, make sure the gaskets don't hang over the crank case or transfers. Match the piston ports up a little. You could put a flat slide Keihin on too.
Everything there , out with the carb, is free drivability not necessarily power but it is noticeable .
Regards Ted
30 Sep 2020 13:47 #8

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shyted wrote: I read the KX 125 bit. I agree with Rich about Boysens but have no complaints about running them in my TDR 250.
They do have a tendency to run weak lowdown and rich up top. You would have to try them .
I think you would see better results just re cutting the reed block.
In other words, where its square at the top near the mount screws for the reeds, cut it back and make it into a half circle.
Knife edge it . Knife edge the other end where the end of the reeds rest, on the inside though.
Makes the bottom end smoother.
Then you have started something. Get the casting marks out the inlet tract, clean the transfers up, make sure the gaskets don't hang over the crank case or transfers. Match the piston ports up a little. You could put a flat slide Keihin on too.
Everything there , out with the carb, is free drivability not necessarily power but it is noticeable .
Regards Ted


I installed the entire KX reed block. Replaced the stock unit completely, didn't bother trying to modify it. I've got some Boyesen reeds I was going to install but I remembered I had already modified this cylinder to fit the larger reed block.

The KX reed block is a six petal unit like the original but it benefits from 30 years of development. All I had to do was open up the cylinder a little bit as the KX reed block is slightly wider than the DT reed block.

I am running a 35mm Keihin PWK and an MX360 pipe as well. Its running pretty strong but I did just send the original cylinder and head off for professional help. The main transfer ports only have around 100 degrees of duration, I expect a significant improvement from proper porting.
Last edit: 30 Sep 2020 14:21 by turbodan.
30 Sep 2020 14:19 #9

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If you think the DT360 is short on inlet--check out an RT2 or RT3. They only ever used a reed block straight from a 1980 YZ80 or a bit earlier.
30 Sep 2020 14:28 #10

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