Hi guys, some of you may know I've been doing a bit of a restoration on my 76 DT400, it's not a total restoration but here's what I've done for the most part.  All parts that were painted from factory have been repainted, new seat pan, foam and cover, I had a new rod with bearings and seals put in, bought but not yet on the bike are new stainless steel rims and spokes.  Today I started it for the second time and took it for a quick ride up and down the block, it's running pretty good but figure I need to play with the carb to get it dialed in and see if I can get rid of the dreaded surging at steady speed.  I do have one issue, ok two, that is bothering me, 1) the top end is making more noise than I think it should and 2) when I shifted it into 5th it sounded like something was vibrating badly but this is only in 5th.  Also (ok so there's three issues), revving past 6k is it was missing so I'm wondering if that is carburation and needs a bigger main jet. So, here's what I'm thinking of doing - thinking of sending the motor off to Klemm Vintage to have them blueprint the engine and sleeve the cylinder then port it.  This way I know the engine is 100% and my bike will be 99% brand new.  I didn't want to spend the cash, still trying to outfit my new T7 but want this DT400 running tip top.Here's some pics I took today: 

photos.app.goo.gl/PCB9tjkMM3WtGtn27
photos.app.goo.gl/R155AQEKetpDR2Ki9
photos.app.goo.gl/WTPTdyfSkHhseXk47
photos.app.goo.gl/ssyCL7Y81fR3JHAY8

Tony it’s looking good 
    

Thank you Wes

Gosh, lot of questions or ideas you have. Not sure where to start. Video of how it runs to hear its issues. Bike looks very nice.

Thank you Morley, I'm going to try and get a video of it soon.  Will need to figure out the best place to connect the camera so you can hear the vibration in 5th gear but it almost sounded like it was coming from the air cleaner but I didn't try to investigate too much after I heard it, got kinda paranoid that something would go wrong, seems to be a theme for this bike

wow nice work, that's a pretty bike!
my '78 dt400 had a bad 5th gear that had a burned out bushing that I missed when I rebuilt so I had to buy a really nice used transmission on eBay for $40.  My symptoms were a very terrible noise all the time that got even worse with clutch pulled in.  I ahd to take it apart and that was a drag after getting the paint and yamabond on nice.  If the bore is lose you might be hearing the piston slapping around?  I had mine rebored by a good dude who did it within Yamaha spec which I have determined to be important the hard way
if you have stock jetting then maybe the exhaust needs to be BBQed?  My 400 is no fun over 6K anyhow because it makes my periferals numb

Vibration only in 5th gear i can only think is related to the teeth condition--but maybe not.
5th is connected by 5 sliding into 7 which locks it to the shaft then teeth on 7 drive to 19 which is permanently splined to 'that shaft' therefore drives the sprocket on the end.
So just saying check teeth on 7 & 19 for through the hardening.
I might add that it's pretty unlikely but just going by the fact that it's 5th gear only.
www.partzilla.com/catalog/yamaha/motorcy...ission-250b-c-400b-c

MSAVITT thank you! I did burn out the pipe before I painted it so it being clogged is not an issue but that is a good thought. As for the transmission, I recently had a new rod with bearings installed and the person whom did it told me everything else looked good but that's not to say he didn't miss something.

Morley, thanks for your thoughts and as above it certainly could be the transmission and specifically 5th since that's the only gear the issue appears.

I think I've pretty well made up my mind that I'm going to send the motor off to Klemm Vintage to have the engine blueprinted and cylinder sleeved (it's on it's last bore). This way I will know that all is as perfect as can be. I didn't want to spend the money but I have a small windfall (really small), coming from a lawsuit so I will have the cash now and don't know when I would be able to in the future.

On another note, if someone with a DT400 or any other model that has the same seat latch could post some pictures of it from different angles and with it opened up I'd REALLY appreciate it. I did find two pictures I took before taking it apart but it doesn't show me everything.

My 75 was "breaking up" at the higher revs. I have stock bore, stock carb, intake and Yamaha filter. I was using UNI foam wrapped over the cage for a filter and was having issues so I bought a genuine Yamaha filter so there would be no question as far as restriction and using the stock size 160 main. The Yamaha filter left me with the same result. I dropped to 150 main and then ended up with a 140! Revs out great and pulls the wheel with no problem.
I still have some of the typical surging at cruise speeds but I'm at 2 down on clip and going to try a 50 pilot.

When I am at the shop today I will take pics of the seat latch.

Your bike is looking nice.

Thank you Clegg, Alan (Gr8uncleal), e-mailed me a picture so I was able to get it back together and installed. The bike ran fine (well before the rod big end bearing decided it had enough of my crap), so I'm not sure why it's breaking up at high RPM's, maybe it's just the main jet needs some cleaning. As for the dreaded surge, I am going to try a bigger pilot jet, I've played with the air jet with no luck there. Like I've said before I'm going to send this motor off to Klemm to have them go completely through it and do their magic so I know I will have a better than factory new motor. I had hoped to have it on the road well before this but I guess that's the way it goes and well spending all my free money on my newest toy certainly didn't help LOL