eboula24

iTrader: (1)

so im getting my rochester carb rebuilt by a guy by the name of lee howie hes apparently a reputable man who builds outlaw class cars here in ohio (got a 6.9 sec twin turbo'd vette cough..2500hp..cough) hes doing it for free also. im planning on getting an exhaust from another member here on tgo..headman 1 5/8 longtubes with 3 in h-pipe dumped off after 2 ft.. Its a bit much exhaust maybe for my POS lg4 but dont really care. lee says he will put in larger jets to compensate for the exhaust..does this sound like a good idea to anyone? i know the guy knows his stuff but i wanna hear feedback from the best forum out there. thanks!

Sonix

iTrader: (1)

Is this a stock Quadrajet - ie, computer controlled?

Does this guy Lee have much experience with a Quadrajets? Most "hot-rodders" don't. Generally mechanics (old ones) may have some experience with Q-jets, but most hot rodders don't. Swapping jets because you upgrade your exhaust sounds like something a "hot rodder" would say - ie not someone experienced with Q-jets.

1) It's highly unlikely you'd need bigger jets on your LG4 just because of exhaust.
2) If this is computer controlled, upsizing your jets is something you need to be careful of. You generally don't need to do this.
3) If he says he's going to upgrade your jets on your primaries and secondaries, go get your carb back, he doesn't have a clue.

FWIW - I believe my 350 (with heavily ported stock heads), and very aggressive camshaft still uses 74 primary jets.

InfernalVortex

iTrader: (4)

When I put a very similar exhaust on my 305 TBI car (getting ready for a 350 swap that happened a month later) the 305 was a little slower and never ran quite right afterwards. It was drivable and all, but it wasnt running the way it shouldve been. Very high idle for one.

Anyway, the difference between the L03 and the LG4 is that the TBI is speed density. You change the volumetric efficiency and the computers pre-determined correct amounts of fuel are no longer accurate. The LG4 is carbureted... it will, to an extent, adjust for you. More airflow = more fuel.

Will it need an adjustment? I dont know, I'd trust Sonix's opinion. But I just wanted to point out that there are cases where mods like that will upset the induction, but that doesn't mean yours is necessarily one of them.

eboula24

iTrader: (1)

yeah it is the comp controlled quad. the guy said he knows how to work on it, but ill have to check with him about it being comp controlled, he may not know possibly. but if it can be tuned properly would it hurt to upgrade the jets? would it benefit performance also

eboula24

iTrader: (1)

i dont actually know if it is computer controlled actually i assumed it was but it doesnt have mass airflow sensor, the only wire i have for the carb is for the electric choke...what does the computer supposed to control for the carb? my friend seems to think with how mines set up it cant control the a/f ratio..but hes not claiming to be an expert either

eboula24

iTrader: (1)

nevermind...i had a rare bright idea to look at the shop manual...

five7kid

iTrader: (14)

If the computer can maintain control with bigger jets, then it wouldn't "hurt" anything. But, if the only wire going to your carb is for the electric choke, you don't have a computer carb. A computer carb has a connector on the top right front corner for the mixture control solenoid (MCS), and a connector on the front left corner for the throttle position sensor (TPS). If your carb doesn't have those connectors, then it should have a distributor with vacuum advance - if it doesn't have vacuum advance on it, then someone has done an incomplete job of changing the car from computer carb to non-computer carb.

Damon

Don't touch the primary jets on a computer controlled QJet. Leave them alone. Just removing them takes something a bit more than an old screwdriver.

The only part you touch is the secondary rods and hangers. In that area a slight change can make a noticable difference in WOT performance, even on a lowly LG4. For example, replacing the stock CH secondary rods with a set of DR rods from an L69 HO carb and installing them on a G hanger is a pretty well documented performance-improver and is very easy to do.

But don't touch the primary jets or rods in a CC-QJet. You will regret it.

eboula24

iTrader: (1)

the good thing about this guy is if he doesnt do it right he will make it right or replace it. he was gonna do some things already with the carb and is confident so crossing my fingers (not that it helps anything)