I don't really know how successful you would be using the adjustable main jet. I'm also not so sure that the issue you are having would be helped if it did. That being said, if there doesn't seem to be any physical interference up in the stem, I can't think it would hurt to interchange them. You may want to review the different carb series in the service manual.
I'll assume your engine has a rubber primer- correct? I can't recall an early 521. Hopefully you had cleaned the tank and replaced the fuel line as well. Tecumseh carbs defy logic more times than they have a right to. If the needle and seat kit was not oem, they have a reputation for not seating. Even getting Tec parts direct from the distributor, bunches of the stuff is in different packaging- but it seems be be oem. Sometimes it is also best to just replace the float and pin as well. That way binding is eliminated as a cause. Welsh plugs we were always told to seal w/ nail polish on the these carbs too. Maybe you used the wrong color? ;) We were also told only to dip them for 15 minutes as not to remove the sealant that was applied after they built the carbs. Gives you a real warm feeling about Tecumseh carbs, doesn't it? Maybe your not starting with the recoil will improve after running it for a bit. One could pop the shroud and clean the coil, flywheel and adjust the gap to see if that gives you a more solid fire initially.

@Tomplum
Thank you for your reply.
The carb does have a rubber primer, not on the carb itself though. The primer is functioning as should.

Today I spent considerable time playing with it. Let it run for a while.
It now starts with the recoil starter, must pull very hard and use the full length of the rope. It runs very well so I've decided to scrap the idea of changing the mixture control screws. Usually these fixed jet carbs run a bit rough at idle due to lean mixture, this one idles smoothly.
If it ain't broke, don't fix it is appropriate in this case.

Oddly enough, the rebuild kit I bought from a local dealer had Tecumseh on the bag, go figure.

I usually don't replace welch plugs;carb cleaner with strong blast of air pressure cleans very well.......you didn't mention anything about the fuel inlet fitting;IF float is good, needle and seat are new ,and needle is not scratchted,or some other defect,then needle must be binding in bore or take a look on the float tang to see if it has a step,or maybe corrosion........it has to work IF everything is good,AND you're fuel supply is clean.........

@jammer1
The fuel supply is clean, removed the tank, cleaned out both tank & line. If there was a problem with fuel supply it would be starving for gas not suppling too much I would imagine.

What also baffles me is why it was leaking from the bowl seal at the top. When I bought the blower and transporting it there were no leaks. Replaced the seal from the kit and it leaks, Changed to old seal and still leaked. Even if the float permitted too much fuel to be delivered the bowl should not leak from the top if properly sealed, that's what a seal is supposed to do.

Needle is not scratched and the float tang is clean.

No;what i meant was that if there was foriegn matter (Solids) in the fuel,it could get between the needle and seat,and prevent the needle from closing all the way,and in that case,it would obviously fiil the carb as high as the nearest point of exit,be that a vent,a jet;..ARE YOU sure THE LINE FOR THE PRIMER IS NOT CRACKED?....was the carb kit original equipt.? I've heard issues from aftermarket parts etc. ; How bad does it leak? Is it continuous..........if you changed the seal from new to old and it still leaked,chances are good it's not the seal that's leaking........

jammer1.
Fuel and line is clear. Had there been any foreign matter I would have seen it with the numerous times I took it apart and made sure everything was clean.
I did use a Tecumseh repair kit, dealer charged me over 18.00 with tax which I thought was a bit high.

There is another thread here that baymee started with the same carb and symptoms. It appears he fixed his with the third carb kit he tried.
http://forums2.gardenweb.com/forums/load/lmower/msg1119382820256.html?13

As far as leaking from the seal at the top of the bowl, I may have caused that by tightening the bowl nut too tight and caused it to bow. Doesn't matter any more since the new carb works fine and I don't have to deal with these poorly designed Tecumseh carbs for awhile I hope.

The tightening of the main jet on the bottom of the bowl seals that area as well as the bowl O ring. It's a very small space between the bottom of the bowl and the main jet holder and almost impossible to overtighten the bowl nut and bow the bowl.

You can overtighten it to the point where you damage the threads or break the small fiber washer, but you can wrench them pretty good.

baymee,
Thanks for the info, if nothing else it's an education for me.

I'm just wondering now if the dealer sold me the wrong Tecumseh repair kit. Gave him the carb # and he sold me the kit. Threw the bag out after the rebuild so I can't check.

Last night I blew throug the fuel inlet fitting and I could feel the air coming out of the top bowl seal, that's why I thought it was bowed.
Still don't think it should cause the leak out of the throttle plate though.

Blowing air through the fuel inlet can blow the seat out at the same time. I would expect air to pass by the bowl seal since it's only designed for gravity gas pressure and really only to stop the gas from sloshing around in the bowl and leaking out there.

Many of the carbs use the same kit, since it contains no jet.

By the way, the air vent was open on my carb, which now seems to be cured of its woes.

@baymee
Makes me happy to know we both have working carbs now.
So I see you found that small vent hole, never knew about it and difficult to see if you don't know it's there.

This should wrap up this thread now since the problems are solved.

I have the same problem with a Tecumseh series VI carb. on a Murray lawn mower. I have cleaned carb. and put in new carb. kit. I did not remove the venturi but I blew Carb. cleaner thru it. Also adjusted float. I have opened the carb, several times and cleaned. But it still leaks gas thru the throat of the carb. I have cleaned Carburetors before and have never had this problem before. Any kind of help would be appreciated very much. Thanks

Check the float for any liquid inside; shake it. Also, is the rubber seat installed properly? Not sure what a VI carb is.