I installed a new carb I got from Dennis Kirk, it was a Mikuni 28mm VM Series Universal Round Slide Carburetor - VM28-49.  This should have been a replacement for my ‘74 DT250.  The original carb was warped on the sides so that why I decided to go with a new one.

So I’ll get to the question, I’ve got it all in and the engine runs well, but to get this I’m now running with a high idle (at least it sounds high) and the engine revs fine when standing up about as high as it can go.  When I start driving it starts to sputter around 3/4 throttle.  I’m wondering if I should adjust the needle clip up or down, not sure which way to go as I’m guessing it’s not getting enough fuel.

maybe I should back up and check other things first?  Just a rookie when it comes to adjusting this sort of thing so I’d rather be sure of what I’m doing.

Also my tach really doesn’t work well as the cable is worn  out so I can’t really give you great numbers there.  It sounds great now when I rev it in neutral so that makes me smile, but it’s  top end performance is not where it should be.

Sounds like its too lean on the main jet which 'could let it rev easily when not under load but snuffs out when it needs that extra fuel to haul it along at revs. How does the main jet number compare to original factory specs.

It says I have a 200 vs. 140 factory specs.   I’ve also never looked at the reed valves and that I’m also finding  out would cause a similar issue.

As RT325 stated you need to check your jetting, although it might be the reverse, that Mikuni from Dennis Kirk if it is a VM28-49 comes with a main jet #200 and #60 pilot jet, stock main jet setting for a 1974 DT250A is #140, pilot jet is the same at #60 but pilot circuit only affects low speed operation. The needle clip should be set at the 3rd slot position.

Be careful of changing the main jet to match stock specs when the slide needle and needle jet are different in the new carb.

Needle and needle jet affect fuel flow at 1/4- full throttle so with different parts than stock you might also need a different main.  Always best to sneak up on the jetting...  one size at a time...  takes a lot longer but is cheaper than rebuilding the engine.

Also Mikuni Tuning Manual in the Tech Library describes operation of carb in detail and has tips on jetting...  like when at full throttle you can quickly roll back to 3/4 throttle.  How the engine responds is a clue to if it's too rich or too lean.  If you're not sure, sometimes best to go a size richer and see if it gets better or worse.

Ok, thanks everyone for the tips.  I have to do some more reading now.   Is the jet size correspond to bigger number = bigger jet = more fuel or is it the opposite.

Does anyone have a suggestion of a kit to purchase to get the more common size or is this something I could go to my local shop and buy a few jets to try?  This is all new to me.

Five words for the place to start.  Tech Library - Mikuni Tuning Manual

Just my ideas & feel free to ignore.
What i'd do with my old rat bike 360 'is' if i felt it was way too lean i'd go up in a big step to get a result then work from there. If far too rich already then you'll know straight away on full throttle because it'll just blubber & not clear. Big thing is to be able to know if it's lean or rich to start with. Or if the exhaust is strangling it through carbon buildup in the muffler which if bad enough will give rich full throttle symptoms. One things for sure, if you go rich you can't come to any harm. But if you go lean on an already lean situation then it might not end well unless you don't use full throttle for very long, just long enough to get a feel of how it reacts.
Now i go back & have a read i guess if you have a 200 jet & specs say 140 then it should in theory be way too rich. Providing original airfilter box & filter is still fitted as original is likely a bit restrictive & jetted accordingly.

So I’ve read all of the carb tuning manuals and I think I’m running lean.   Based on the reply’s I’ve gotten and the manuals I’m still not sure which direction to go with the sizes of main jet.  My new carb has 200 main and the original is 160.  Would I want to start at 210 or 190? I just see “switch to a richer jet”.  I would assume the original jet was richer, but that would mean that a lower number jet is richer (all other things assumed to be equal).  I know this isn’t the case, so that 160 jet number doesn’t really have much meaning here.  

part of me just wants to take the old jet out and put it I. The new, but I’m guessing this adjustment is to big of a leap and if it’s in the wrong direction it will potentially cause bigger issues. 

sorry I’m just intimidated by all this, I can handle changing tires, wiring and even working on brakes.  I just don’t want to blow up the engine which seems to be pretty sound at the moment.  I appreciate all the help I’ve found here.