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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
oh yeah its got a 305 smallblock..
Did it backfire through the carb before you did all of the work you mentioned in your post?
Did you change plug wires?
had someone else rebuild carb ..backfires out exhaust ..changed plug wires already
doesnt backfire now
Just wondering if that is enough advance? Did you set it with the dist. vacuum unhooked and the vac line plugged?
set it with vaccum line pulled off dist and plugged
how do you adjust to use more advance..??
Try bumping the timing up to 6 or 8 degrees ... see if it helps.Quote:
Originally posted by attherain991
how do you adjust to use more advance..??
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.:whacked:Quote:
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...
Me being new to this they seem tight enough ...Yes it is a quadrajet...
Maybe something got put together wrong during the carb. rebuild? Things can happen even to experienced mechanics sometimes.
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...
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. :DQuote:
Originally posted by attherain991
Me being new to this they seem tight enough ...Yes it is a quadrajet...
Is there a way to tighten or stiffen the secondaries
Quote:
Originally posted by attherain991
Is there a way to tighten or stiffen the secondaries
yes but it hard to explane, if you think they are loose then lock them down or tie them closed, so they cant open and go drive it, wont hurt anything and if it helps then that your problem.
is there a way to tighten or stiffen the secondaries
thanks ill try tomorrow...
oh yeah you said i could put vac advance on a full vaccum to check it ..How and where???
one more thing would loose secondaries cause high gas consuption
with motor running pull vac hose off of the dist and put your finger over it, if you fill a suction its got vac. hook it back up to the vac adv and it should change the rpm. if it dont the adv is bad. if there is no vac there find a small vac port on the base of the carb and put the hose from there to the vac adv., should see some change in the rpm.:DQuote:
Originally posted by attherain991
oh yeah you said i could put vac advance on a full vaccum to check it ..How and where???
yesQuote:
Originally posted by attherain991
one more thing would loose secondaries cause high gas consuption
Look for a "Little Old Lady" car at a junkyard with a Q-jet on it. More times than not, those are good carbs. Also, there is a mixture controll module on the later q-jets. If your TPS or O2 sensor is out of whack, this will cause the conditions you describe. My personal guess is leaning towards the TPS as it is dominant towards higher RPMs. Youll find it towards the front, left side of the carb. If theres not a connector there, ignore everything i just said:P
Quote:
Originally posted by drg84
Look for a "Little Old Lady" car at a junkyard with a Q-jet on it. More times than not, those are good carbs. Also, there is a mixture controll module on the later q-jets. If your TPS or O2 sensor is out of whack, this will cause the conditions you describe. My personal guess is leaning towards the TPS as it is dominant towards higher RPMs. Youll find it towards the front, left side of the carb. If theres not a connector there, ignore everything i just said:P
drg84, i need to know what year they put TPS switches on q-jets?:confused:
Thanks to everyone that helped..Learned a lot...I bought a factory remanfactured carb...Should work fine Am putting on today..