Carb Adjustment 1992 Mariah 4.3 Litre

seapoo · 06-29-2009, 06:03 PM

What are the adjustments for the two screws on front of carb? Rochester Quadrajet.

***WetWilly*** · 06-30-2009, 12:26 AM

Hi seapoo,

They are the idle mixture screws. I suggest you leave then alone unless you're familiar with the procedure for adjusting them, To perform the adjustments correctly, you need to have the ignition in "Base Timing Mode" first. The procedure is explained in section 5C starting on page 12 of the Mercruiser 4.3L service manual.

WetWilly

seapoo · 06-30-2009, 12:42 AM

Is there a default adjustment to start off with? Motor was smoking at idle after Marine adjusted both screws. They had them 3 turns out. I put them to 1.5 turns out and now it idles great with no smoke. Just want to ensure I have them at factory settings. Please advise.

**SEMIJim** · 07-01-2009, 12:45 AM

Quote:

Originally Posted by seapoo

What are the adjustments for the two screws on front of carb? Rochester Quadrajet.

Holy smokes! They were still making those in the '90s?!?! First thing we used to do with those, back in my late-60s, early-70s Woodward Ave. street-racing days, was either remove the vacuum controls on them (and then carefully adjust the linkages) or replace them with Holly carbs. We used to call them "Rochester QuadraJunks." I hope they're not as much trouble as they used to be. They worked well when they were working right. Problem was keeping them working right.

If I recall correctly: One screw would be the idle mixture and the other the low-speed mixture.

I tore-down and rebuilt those a couple times, back in the day.

Jim

***WetWilly*** · 07-01-2009, 08:14 PM

Hi seapoo,

Wow, three turns, huh... that way too much. I would seek another mechanic who knows what their doing! The manual says to start with the screws 1.25 turns out to start, so your right in the ballpark! To adjust them as best as possible, follow the manual's procedure.

BTW, Per the manual, the carb is a Weber, not a Quadrajet four barrel, although they look the same. Oh, and SEMIJim, on the four barrel Webs and Quads, the two screws in front are the idle mixture screws for the front two bores (see carb exploded view, Pages 5C-18 and 19) and I was Holly man too except for some reason, I've got an Edelbrock Quad on my '70 Mustang, go figure...

WetWilly

**SEMIJim** · 07-01-2009, 08:59 PM

Quote:

Originally Posted by WetWilly

Oh, and SEMIJim, on the four barrel Webs and Quads, the two screws in front are the idle mixture screws for the front two bores (see carb exploded view, Pages 5C-18 and 19) ...

I stand corrected. It's been a long time

Jim