i bought a speed demon 650 vs for my 355.we have one good mechanic around here.he never tuned a barry grant but he leaned it out and it ran great.now it seems to be running rich again.if i put new plugs in its good for 300 miles or so.i have a 700r4 with 373 but with 29 inch tires my rpms are around 1500 at 60.i am wondering if my low rpms are causing this.my motor was dynod at375 but i put in a comp cam xe268 and msd vacuum advance.when it has new plugs it has great throttle responce and is a strong motor but do i have a jet problem?are my rpms to low?i havent checked where the mechanic has the four mixure screws but i really dont want to mess with them because it runs great with new plugs any replys would be appreciated

 

The first thing you need to do to tune the carburator is to take a manifold vacuum gauge reading ( to see if the power valve is opening up too soon) With a fresh set of plugs in, Warm the engine up to operating temp (180 )
then with brakes set and car in gear measure vacuum on guage. Go ahead and shut motor off and take reading and divide by 2 i.e If you get a reading of 16 then your number will be 8. The number that you come up with will be your power valve number go to a 7 instead for tuning you don't want the power valve to open too soon or you will get a fat rich condition which is what you have now !!!! the carb cfm size is ok but a 750 vacuum secondary is a bit more forgiving on the street.
I have a 86 z-28 with a Ls-6 454 turbo 400 with a 9-bolt disc rear and a stock 88 iroc 305 tpi

 

Part 2
use autolite 25 sparkplugs if stock heads also have you checked the fuel pressure electreic pump or mechanical i would not run more than 7 psi next check float level you should be able to see that there picture windows!!! set the level in the center of the glass what kind of distributor are you using a weak coil could lead to fouled plugs

 

i havent checked vacuum yet but i will i have a edelbrock 110 which was recomended from the company that makes these motors.i will buy a fuel presure gauge and see where i am at.

 

i also have msd and a blaster 2 coil

 

i am putting new plugs in today,r43t they seem to work good,i checked my four mixure screws and the ones up front were one and a quarter out and the rear ones were only out three quarters.i no they are suppose to be equal so i am going to try one turn out.should i get my rpms up to 2000 turn one screw at a time to the right until it stumbles then to the left till it stumbles then set it in the middle?then i will have to change the idle right?