Will a blocked jet reduce performance on my dt 50

Bigger jet is not the problem. More fuel would be put in and you said that it runs well at the top end. So it is not a fuel shortage issue.

As a general rule, add more fuel and more air
The tricky part is keeping it balanced at all the rev ranges especially at idle.
You could try finding a larger main jet, freeing up the air box and experimenting with your mixture screw.
Better still, find some other DT owners to see what performance mods they have had success with.

Hey Anon,
Proper and correct jetting is something that can only be achieved in person by a REAL MECHANIC that KNOWS how to "read" a spark plug,,,With the mods you mentioned.. you need to know the stock jet sizes..a close guess would be reduce the main jet by two or three steps and lower the needle one clip position... oil mix should be at least 32:1....or better yet 28:1.. KEEP the oil mixture and jet the carb for performance....( I see TOO MANY IDIOTS that instead of jetting a carb correctly just reduce the oil ..some as low as 50:1(INSANE) just to keep from fouling plugs.).. then WONDER why their engine doesn't last long... hahaha????
YOU > BE SMART!!

problem could very well be that stock exhaust system as it was designed for the 50cc cylinder and not a 70cc cylinder.

i sounds like when you sit on your bike your traping a fule line and stoping it getting fule as for it not reving up thats also short of fule it could be a blocked pipe or a blocked jet in the carb as you say it starts ok if you hold your hand over the carb this is the choke not working but as soon as its warmed up it dont need a choke, so its back to the carb jets blocked or fule shortage, also look at the air filter is it clean also cheak the exhaust pipe,,,if its slightly blocked it wont go very well, if at all,,,