HJ, I'm confuzed, and would like to help you...
Would u splain?
'downjetted to 68 then 65 and still four stroking'
....if you are referring to a number drill bit size, the number 65 bit is LARGER than the #68 and would allow MORE fuel to pass, making the 'four-stroking' over-rich condition even worse.
...Now if you are referring to some part # stamped on the jet, well that's a different story...
If it were me, and my engine was running fat and four-stroking with a #68 drill bit size, I would go to #71 and see what happened.
Here's the reason.
It would get better, or starve for fuel.
If starved for fuel, I could then pass a #70 bit through the jet and ream it open just a little bit.
Once again, it would get better, or worse.
If your power and speed improve at WOT, give it a try with the #69 drill bit.
Again, better, or worse.
Always, and I mean ALWAYS, adjustment will be too far, and you will need to go at least one step back the way you came to get the proper mix of air/fuel.
That's how we do it.
Quite possibly this will require soldering up the jet so you can make a smaller hole than the one you just did in the test.
(It's good to have a couple jets on hand, but most of us do NOT buy a buncha jets hoping to find one that works. We custom drill/ream them. At my local discount tool outlet, the little tiny number drills are sold for a buck to 2 bucks each. It's most likely all you would ever need for this class of carbs is #65, the largest, thru #75, which is the smallest.)
To ramble on even further...
Another member just made a post saying how he went on his WOT run and shut off his fuel petcock to see what happened at the moment the fuel was expended... a valid test. If you get one burst of speed just as you run out of fuel, it's an indication that your jet is too big.
On the other hand, when on a WOT run, you can activate the choke to dump more fuel in... before it floods, if you get a burst of speed, that is an indication you are running too lean, and should open your main jet.
NONE of this applies to anything but WOT, even though the end result will require you begin the process of resetting the needle in the slide for suitable mid-range operation.
THEN, you will need to get your idle set to a reasonable speed, and make adjustment to the fuel mixture at idle, if your carby has an air/fuel adjustment for the idle circuit. (I know the nt carby does NOT, the cns DOES.)
Idle circuit adjustment MAY affect the setting of the mid-range needle.
Design shortcomings and poor quality in darn near ALL these china motors have left us with a significant probability of intake leaks, either in the crankcase, or the intake tract, which make proper tuning difficult.
Good luck, I hope this helps
rc
