Sorry for confusion....lost in interpretation . I wanted him to make a video of the bike running and place the recorder near the cylinder. The piston should NOT have rattle. If it rattles it means the cylinder/rings have been worn out, and the piston is no longer tight, the rattling sound is caused by the piston slamming around in the cylinder. Once this happens....the cylinder will need to be serviced by someone with precision tools and knowledge on these Yamaha 2 strokes.

The bike will run without a battery. though it is not recommended as you may blow bulbs, if it does have a 12 volt battery change it out for a 6 volt. Modified bikes can run off a 12 volt battery (Powerdyno ignition system). Your problem is most likely air leaks from bad crank seals. I would check for air leaks before diving into carb tunning. The bike will never run properly if you have air leaks.

What elevation are you at? you said you are running a 50 pilot jet? that sounds way to low to me. We have the Mikuni tuning manual in the downloads section, lots of good info. Here is another good source to read to get the carb dialed in www.jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm (once you know the engine is air tight with no leaks).

CRAP! That is what I was noticing the other day when riding it around. It has power but a rattle when it idles.

Dammit cant find that mikuni manual now.
I am at 2100ft anyone know base jetting?

Post a video and the engine experts will weigh in.

#190 Main Jet, #80 pilot Jet

Here is the mikuni tunning manual, lots of other manuals in the Tech Library including the 77 DT 400 manual with the 78 Supplement.

www.yamaha-enduros.com/index.php/other-m...ni-carb-tuning-guide

What Psi should the compression test have on the Dt400?

A healthy stock DT400 should be around 125 PSI.

to test this:

Clean dirt/sand around spark plug so it wont drop into the cylinder when you remove it. remove spark plug.
Insert compression gauge
Remove de-compression cable from the front of engine. If you leave it connected you will not get an accurate reading.
Turn fuel pet cock off
Make sure you have a clean air filter
Hold throttle wide open
Rapidly kick (like you were starting) 5-6 times or until reading stops climbing.

More detailed info on compression tests: www.klemmvintage.com/squish&comp.htm

Does anyone plug the decomp and run with out it?

Not that I know of. It could be done, but it would be much harder to start and kick over. They don't really go bad just gunk up over time and the 0-ring might need to be replaced, or the spring wears out. I think DEET has some of these o-rings. They are high heat gas/oil resistant (regular o-ring wont work).