black smoke

[GREASEMONKEY72]

first off trucks a 72 with a stock 350sb with a quadrajet and when i rev it not on the first rev but the second and if i do it the third to, big puffs of black smoke come out and i know black smoke is fuel but is there more to it maybe that im missing?

FWIW-i just put on a new pcv last thursday after not having one for awhile

[cstm68]

it could be runnig rich
im not shure but its a possibility

[GREASEMONKEY72]

thats what im thinking but its kinda weird becuase its the stock quadra and i havnt done anything to it exept rebuild it like 6 or more months ago, but it seems like more smoke now than it used to be

[RACINJASON22]

I don't think you are having a problem. Here's why on a rev specialy with the q-jets big secodaries they dump fuel more and then when you let off it chokes the air down quike putting unburned fuel out. second and third revs do the same but compounding things each time. Not to say your not running a little rich but your best bet is to run the truck under load fairly hard and shut down before you let the truck come back down to idle and check plugs right then and there. ON THE SPOT. You will get the correct reading this way. Hope it helps. RJ22

[GREASEMONKEY72]

never thought about the big secondaries dumpin the fuel

i did check a couple plugs the other day just for the hell of it and they were black not brown like they should be idealy, but it was cold when i checked

[bryanw1968]

Originally Posted by RACINJASON22

I don't think you are having a problem. Here's why on a rev specialy with the q-jets big secodaries they dump fuel more and then when you let off it chokes the air down quike putting unburned fuel out. second and third revs do the same but compounding things each time. Not to say your not running a little rich but your best bet is to run the truck under load fairly hard and shut down before you let the truck come back down to idle and check plugs right then and there. ON THE SPOT. You will get the correct reading this way. Hope it helps. RJ22

But, a vacuum secondary carb should NEVER open up the secondaries in neutral. The secondaries should only open up under a load while in gear. If you have them opening up in park then there is a problem for sure.

[RACINJASON22]

Vacume secondary carbs work on vacume so there still is a possibility of the secondaries opening on a quick high rev when the engine pulls hard vacume under quick throttle stab. I have seen it more than once when every thing else was in tune and working GR8. This happen alot on some of the older pontiac engines (first hand knowledge)

[GREASEMONKEY72]

they can and will still open, i also have first hand experience with that(not on my truck) but yea im just a tard and didnt think about the secondaries before i posted that and i just got home and did it again to check and there wasnt as much smoke and i also didnt rev it as fast so yea

[bryanw1968]

From what I know they're never sposda do that. Unless you have a super light spring in it or took the ball bearing out(holley). That's weird that you've both seen them do that. Hmm.....

[72 CustomCamper]

Every Q jet I've ever owned works like gresemonkey is saying. The carb is vacuum it doesn't know if it's in gear or not...


Chuck

[bryanw1968]

But that's the whole point of having a vac sec carb. It works off of vacuum. When reving in neutral the motor will see a drop in vacuum but not like when it's in drive. Once in drive the motor sees that load and slows down and the vacuum drops to basically nothing. That allows the secondaries to open to feed the demand. That's why vac sec's are more fuel effecient.

[bryanw1968]

But that's the whole point of having a vac sec carb. It works off of vacuum. When reving in neutral the motor will see a drop in vacuum but not like when it's in drive. Once in drive the motor sees that load and slows down and the vacuum drops to basically nothing. That allows the secondaries to open to feed the demand. That's why vac sec's are more fuel effecient.