I did stumble on a distributor for Boyesen reeds. they do not list the 77-78 DT400 in this page.
www.athenaparts.com/eng/models/view/4461/yamaha-dt-400

I did find one,Malcom Smith Motorsport lists #663 and it is special order from them.
www.malcolmsmith.com/boyesen-boyesen-pow...tm?productid=8701836

here is a Ebay listing for them.

Ya i ordered mine locally. Had it in stock next city over! will be in on mon-tues!

very good!

I got the seal for the clutch actuator Saturday. I did not get to it until today.

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I removed the old seal. good to know there is a roller bearing there. the bearing is good still so it is being reused.

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here is a pic comparing the new seal with the old one.

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I here is the new seal in place.

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reassembled the clutch lever into the bottom of the case.

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trying the new clutch cover gasket,it is a little tighter around the bolt holes than I would like.
means I will need to be careful when installing the clutch cover bolts.

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then I cleaned off the old gasket then cleaned and painted the clutch cover and oil pump cover.

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I am going to let the engine paint dry overnight then install the new kicker seal and do the final assembly. then I will look to put it in something and light it off for the first time.

I tried fitting a 73-74 MX360 pipe for the initial run,it did not clear the cylinder and head very well so I am going to try the Hooker DT400 pipe.

I removed the studs from the stock DT400 cylinder and cleaned the threads with a 6mm by 1.0 die.

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before fitting it with the pipe,I set the ignition timing at 2.7mm BTDC which is called for on a MX400B.

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I mounted the engine in the original frame. I should had posted this before hand.

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back to setting the timing,these two marks are the Pulsar and the flywheel need to be aligned with the dial indicator
sitting at what the timing spec is.

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I decided to check for spark and right now there is no spark. I think it might be a ground problem.

assuming the worst,I got getting the Rex's Speed shop CDI kit in my plans a bit earlier. anyway I will investigate the no spark problem and share the results. if I get desperate enough,I might even borrow the working CDI on the other DT400B.

there is also field diagnosis for checking the Pulsar and I do have 1 other CDI box to try,the box off this very DT400.

pulled the flywheel and checked the Pulsar,it was showing 75 or 79 ohms so it should be ok.

I also checked the wiring and it seems to be correct as well.

looks like I will have to borrow parts off the other DT400. it may be the coil I am trying to use or something like that.

I also checked all the grounds and they seem ok. wish my Dad's bike was not buried or else I would borrow the points mag off it to test the engine I rebuilt.

another thing I noticed,even though I got the decompression cable properly adjusted and it works ok,this thing has mondo compression,it feels like I am kicking over one of the RT1s I have. engine turns over smoothly but put a plug in it and you can really feel the compression stroke.

I might test the CDI box I am using on the other DT400 before going further,both CDI boxes I have.

I did swap the CDI box that came with this project bike and it made no difference.

Looks like fresh paint on the motor may be the ground issue,make sure to sand it off around the mount and use a bit of grease (non dielectric) to give it good purchase and corrosion protection.Good luck and let us know how it goes,those 400 MX's were pretty torquey,careful to use a boot!