Just to add a little more help, I hope.
A couple of 74DT250A pic's for ref. Correct assembly & measurement for float height. Arms must be parallel+/- allowed tol to gasket surface. Then Only bend the center tab in the arm to adjust the inlet needle closed position. If your floats are correct, (they appear to be), they should have the word (UP) molded on them for correct install & function when assembled. I have attached all pic;'s you should need to get that correct.
FLOATS IN BOWL


ASSEMBLY OF ARM & INLET NEEDLE SET


PROPER LEVEL OF FLOAT ARMS IN NOMINAL POSITION


Here's a couple of more pic's just after I opened this carb up. Notice the position of the center arm tab in the 2nd pic. You should be very close to this if your set up right.



One more item, you say the float needle is not Mikuni, you mean inlet needle set ? It's possible the spring is either to stiff or to weak in the needle. I just replaced this set & was able to obtain genuine Mikuni parts, so just a thought here, the new set changeD the float arm position a little being it was new & O.E.M. Adjustment was required. Looking forward to my 1st ride with the refreshed carb.

Great post Steve F!!! Nice photos and description.

This is all great input... but SteveF, your exact pictures are worth the proverbial 1,000 words. I've looked at eBay carbs just to get an idea of what the float should look like, and the angles are never right. The service manual pictures are fax quality. But what you just provided makes SO much more sense out of what I'm dealing with. And sure enough, I THINK I'm onto the culprit. I'll be able to test it this evening, but here's what I found last night.

In short, I think my issue is the second, unnecessary red washer on my threaded brass float needle housing. This picture shows where my float was before:


Look at the angle of both the float ARMS, as well as the TAB. That's not right. The reason it's not right is because that 1mm red fiber washer is, in essence, making the needle taller, so it's necessary to bend the tab further and further.

Look at this next picture... the ONLY thing I changed was removing the second red washer:



HUGE difference, eh? I now have to bend the tab down (closer to no "bend" at all in the tab) just to make the float arms level:


I took these pictures yesterday evening and WANTED to pop the carb on the bike to try out, but I had to take my daughter to an orchestra event. But if I go back to my original post, I had the sneaking suspicion that something was "binding" internally. This plays into that theory. You see, the tab is normally perpendicular to the needle. That's how it needs to be to SMOOTHLY move the float needle. When I had my tab bent as aggressively as it was, it no longer SMOOTHLY actuates the float needle.

Here's an analogy you can probably all relate to. Open your oven door and pull the middle rack out a couple of inches. Now close the oven door. It'll smoothly push the middle rack back into the oven because the door is nearly perpendicular to the rack.
Now open the oven door again and pull the rack out halfway or 3/4 of the way. Now try to close the oven door. It's not going to close very well. The angle between the door (analogous to the tab on the float arm) and the center rack (analogous to the needle) is too great.

I THINK this is going to be the fix. And if so, I get to chalk this up 100% to user error on my part. When I bought the carb rebuild kit, none of the parts were actually labelled. I just saw the two washers and thought, "well, it looks like this is about the only place it fits" and slapped it on there.

DarinM, you were surprised that the banjo bolt for the main jet didn't have a fiber washer on it... I think I might have just found the fiber washer that's supposed to go there.

Wish me luck, and thank you again for the incredible input on this!

I think you're onto it now . As to the washer / gasket on the main jet holder .
The OEM part is an aluminium washer , not a fibre washer .
What you currently have in place looks correct . You haven't complained
about a leak there . If it ain't broken , leave it be . Good luck .
Do let us know how you get on .

The reason I first mentioned float valve not being Mikuni is because I have seen aftermarket parts be the wrong dimensions or poorly machined and cause problems. Worst was a kit valve for a large displacement MX bike... I think the OEM size was 3.0 or 3.5mm and the kit valve was maybe 1.5mm. (Wasn't marked but hole was tiny, owner wondered why it kept dying when riding like it was running out of fuel... it was!)

Niche (and others) have the Mikuni valves listed by dimensions... I don't doubt it's different than a 75 but I've sourced Mikuni float valves by the dimensions/threads from 1960's 80cc bikes up to 400's and have always found one that fit perfectly... sometimes had to settle for a slightly larger flow diameter than OEM if OEM spec'd an odd size like 2.25mm.

That said, the kit valves are the only calibrated brass from kits that I'll use... if it looks reasonably the same as OEM. All you need to do is compare the hole in the new valve with the old... close is usually good enough... and then if float arm was parallel with original valve it should need no or very little adjustment with new valve.

Hope you finally fixed it!

Thankyou, & as Swoop said if it ain't broke, leave it be.
For what it's worth,
He is correct that the main jet bolt has the aluminum washer & the fiber goes under the needle cage as shown in my pic's.
Reason is you are removing it more often to drain the bowl for cleaning & it can be reused.

As Mark was saying I also kept having overflow issues. I had a new float valve from one of those aftermarket kits. Seemed to be good, tested by blowing as air through it, but no joy. Switch to a genuine mikuni of a larger size I had laying around and bingo no more overflow issues. I think the valve was a 3.5. I needed larger anyhow because my engine is ported, but used to have it in a stock engine with no issues. Edit, mine is a 78 400.

Well, you all could very well be correct that my root cause is some dimension of the float valve that isn't spec. Although from the first chapter of "if it ain't broke, leave it be", I'll do just that. I'm a bit late with this reply, but I DID get this "fixed" a few nights ago. The quotes are important. In NO universe should this be working right, but it does. It runs better than it ever has. Last night, I took it out for more of an extended ride (under extended full throttle runs up a hill). It never starves for fuel and simply runs perfectly at every RPM.

Instead of TELLING you what I tried, I took VIDEO of what I tried, then edited it together in as short of a video as possible (about 8 minutes). You'll be able to see every single change and effect (and in the process, perhaps hear the life draining from my voice). Feel free to watch and let me know if anything comes to mind. You will be stunned when you see how UN-parallel the float arms have to be to make it work. But if i have to judge it by end results, I'm telling you, this setting is "right".

The video was a very good way to communicate the issue. The carb has the symptoms of floats that have lost some of their bouency. One thing to check for is deposits on the floats. I've had these solid (as opposed to the hollow brass "balloon") type become coated with mineral like deposits. I suspect it was metals in the fuel from a less than perfect tank that was the source. Fortunately a fine brass bristled brush cleaned them up.

I agree. The test showing the fuel shut off perfectly with arm level and to spec says to me that the floats have gone bad somehow or are the wrong parts or something like that.