newbie needs help with misfire at high rpms

[attherain991]

Frst of all i'm new to working on anything under the hood. I bought a 85 chevy silverado that needed some work. Bought the haynes manuel for it. Started by checking all the basics. Had the carb rebuilt with mixture screws set at 2and ahalf turns out. Replaced all vaccum hoses,dist cap,rotor,spark plug and wires.set timing to 4 degrees BTDC . Starts and idles fine but when driving i cant accelerate fast or up hill, hesitates (i think misfires) and blows black smoke.... HELP

[attherain991]

oh yeah its got a 305 smallblock..

[73RS]

Did it backfire through the carb before you did all of the work you mentioned in your post?
Did you change plug wires?

[attherain991]

had someone else rebuild carb ..backfires out exhaust ..changed plug wires already

[attherain991]

doesnt backfire now

[pro70z28]

Just wondering if that is enough advance? Did you set it with the dist. vacuum unhooked and the vac line plugged?

[attherain991]

set it with vaccum line pulled off dist and plugged

[attherain991]

how do you adjust to use more advance..??

[pro70z28]

Originally posted by attherain991
how do you adjust to use more advance..??

Try bumping the timing up to 6 or 8 degrees ... see if it helps.

[attherain991]

tried 6, 8, even as high as ten. still same problem..would a bad diaphram in the vaccum advance cause this or does vaccum advancing only pertain to low rpm's ..also only getting about 8 miles to the gallon...

[lt1s10]

Originally posted by attherain991
tried 6, 8, even as high as ten. still same problem..would a bad diaphram in the vaccum advance cause this or does vaccum advancing only pertain to low rpm's ..also only getting about 8 miles to the gallon...

if its a q-jet carb. tap on the top secondery betterflys and see how tight they are, shouldnt be loose, could be dumping to quick.

[attherain991]

Me being new to this they seem tight enough ...Yes it is a quadrajet...

[pro70z28]

Maybe something got put together wrong during the carb. rebuild? Things can happen even to experienced mechanics sometimes.

[attherain991]

I hate to bother the guy again. I took it up to him a second time 4 days ago and he took it apart and checked over it again...Said it was ok... Oh looking back over this I saw I said mixture screws were at 2 and a half. When he looked at it the second time He set both screws to 4 turns out...

[lt1s10]

Originally posted by attherain991
Me being new to this they seem tight enough ...Yes it is a quadrajet...

the reason i said that is if the seconderys get loose on a q-jet (and the do all the time) they will open to soon and dump to much fuel making the motor bogg down. if it does all right until you accelerate fast then it coulb be the seconderes, but the rest of the carb sounds ok. you can check the vac adv. by taking a vac. hose with full vac. on it and hook it to the vac adv. and it should change the rpms. if nothing happens the adv. could be bad. be sure the choke is open when you check the seconderes. when you push them open they should have a lot of tension on them. on the mixture screw they would effect the idle only.