Be careful with those early carbs as the slide likes to stick when the flange gets bent, so when you file it flat just give the throttle a few full throttle flicks before starting it. Don't run an Oring that's too fat for the job or it'll bend it again or break it off.

I'm not sure how one would keep a file flat for so many narrow, overlapping strokes. Like others above, I had a CL-175 Honda on which I lay sandpaper on a known flat surface, and went through a couple of grades of wet-or-dry. You can see if every grade is flat, and by the third, the entire surface is wearing identically.

A good file will do the job ….it does not have to be perfect as the "O" ring will allow for any minor differences....just do not tighten the studs too much as this is what causes the problem in the first place.

I use a new door window glass from a Peterbuilt 352 cabover as the flat surface I tape the wet/dry sandpaper to... It cost me $11 from Peterbuilt back in about 1988... perfectly flat.

Been using it for years... works great.

]I use a new door window glass from a Peterbuilt 352 cabover as the flat surface I tape the wet/dry sandpaper to... It cost me $11 from Peterbuilt back in about 1988... perfectly flat.

Been using it for years... works great.
And you probably haven't even used the otherside yet......GREAT value.....I like the idea.

Thanks everyone for your input, I'll let you know how I get on.

Early Bronco glass works well too! Mine was worse than that, and I run gasket only and have had no problems since blocking it down and making my own gasket. I'll watch for slide issues tho just in case.

Flange is now flat, I used a file initially to remove the bulk of material (not too much) then moved onto wet and dry taped to glass as you guys suggested. I took off the minimum I could to leave as much material as possible.
I'm pretty confident it is sealing.
I used a new gasket and left out the o-ring that was already in there as I suspect it is too large and having read the thread below I believe the o-ring was a later edition to solve the warping problem (sorry asco for bringing this one back up).
www.yamaha-enduros.com/index.php/forums/...g-68-71-dt1?start=20
I have had it running and been for a short ride, it is better but still not right. I'm not drawing any conclusion yet as I haven't replaced the jet setter o-ring yet because it hasn't arrived.
On the plus side I'm now very quick at getting the carb on and off .
I have a video of the revs "holding on" off throttle which I'll upload.
I'm confident with your help and all the experience you guys have we'll solve it

Here's the video of the revs.

On some bikes - I'm talking about a DT1 and not an RT1. As I do not have experience with the 360s. Not from a carburetor standpoint. I was told and ridiculed, that although the carburetor had a slot for the O-ring "it was not to be used." Which meant that it was going to be double gaskets. Manifold spacer and Carburetor. There is no need to apologize, I probably would not have even checked to see if the flange was flat unless there was an underlying reason to do so. I would have just used a small amount of Permatex to seal the 1/16" bow / curve.

The main thing is - is that the bike functions as it supposed to. If that is a 10/4 then life is good.

Install the jet setter 214 o ring.