Started & ran fine after carb rebuild, then rough with black smoke

SRBZ

MEMBER
1972 OLDSMOBILE CUTLASS
220,000 MILES

350 - 4 barrel Quadrajet. I have had car 30+ years. Original engine. Not apart except for timing chain at about 200,000. Ran fine.

The car would start up with a twist of the key when cold, or after short off period when warm. If it sat for 15 minutes or so, would have to hold throttle to floor for restart. There was gas odor after sitting too. This went on for several years. Then I started smelling more gas, so I looked under hood and could see gas dripping from between throttle body and middle part of carb near choke, after warm/hot shut-off, and gas in the depression where the accelerator pump rod comes up out of the air horn.
I thought maybe needle valve was worn and letting too much gas in after shut-off
So, I thought it was time to take apart, clean, adjust, new gaskets, and needle valve, which I have done in the past.

Rebuild was without incident. I usually put idle screws to same number of turns they were before, about 2 out, and leave idle speed adjustment alone.

Put carb back on. Air cleaner, vacuum hose going to it, and pipe from valve cover to air cleaner off for all of this. Turned key to start, cranked a bit, then started up and ran smoothly. It reached normal operating temp.
Turned on and off a few times while I checked hoses and connections. Ran fine, though slow idle was too fast.

Then I shut off and turned back on and it ran really rough and was idling so slow I could see the fan and balancer turning. Sound at tail pipe was rough too, putt putt, and some black smoke. Turned off, then re-started. Had to floor throttle to restart and good bit of smoke on restart.
Let it run some (about 10 - 15 min) and stayed running rough.
Came in the house to check troubleshooting to see what it might be.

Went back outside, started up, and it ran fine again, though a bit of rougher idle than typical, but nothing like how rough it was earlier. No black smoke. No adjustments to carb were made at all during any of this, so it is doing these changes on its own.

There is no sign of leaking gas, accel pump rod area dry. On shut off, there is not "smoke" (vaporized gas) coming out of carb. Only sign of gasoline is the air horn gasket is wet in the accelerator pump area. All gaskets were put on carb without gasket cement.

Went back out again to start up. Started up and ran ok, but still a little rough.
My 150' driveway is on an incline, so I let it roll to the bottom and drive up to check acceleration and smoothness. A little rough when first hit the gas but smooths out.

All the above occurred over about 2 hours.

What do you think is going on?

5 Replies

FIXITMR

MECHANIC
9,990 POSTS

What about power(enrichening)piston/rod? Is it stuck sticking?

Was this

answer

helpful?

Yes

No

Monday, April 30th, 2012 AT 10:08 PM

RASMATAZ

MECHANIC
75,992 POSTS

Sounds like you need another Quadra Jet

SPONSORED LINKS

Was this

answer

helpful?

Yes

No

Monday, April 30th, 2012 AT 10:11 PM

SRBZ

MEMBER
30 POSTS

Thanks.

I took apart and found the power piston retaining bushing was loose, so I put on a new one.

It now starts and fun fine, with the exception of a slightly rough idle which it had before. A kind of sputtery exhaust and mild shaking of car which evens out as engine speed increases.

Was this

answer

helpful?

Yes

No

Tuesday, May 8th, 2012 AT 11:40 PM

CJ MEDEVAC

MECHANIC
11,004 POSTS

WHAT ABOUT ACTUALLY GETTING IN THERE AND ADJUSTING STUFF WITH A DWELL/TACH METER AND TIMING LIGHT???????

REMEMBER CAN'T TIME IT UNLESS IDLE SPEED IS AT THE CORRECT SPEED (BY THE BOOK)

I GENERALLY GO FROM CARB ADJUSTING TO VERIFYING/ADJUSTING TIMING AT LEAST 2-3 CYCLES BEFORE I'M HAPPY WITH IT

I DON'T HAVE MUCH "INTERNAL EXPERIENCE" WITH A QUADRAJET---I DO HAVE A HOLLEY 390 CFM 4 BARREL ON MY 258 INLINE 6 ON MY '79 CJ 5

I OPTED FOR THAT CARB BECAUSE JETS AND OTHER PARTS ARE LOCALLY AVAILABLE....AND YOU CAN "GET INTO IT" FAST AND EZ TO ADJUST THE FLOATS....CHANGE THE JETS....AND CHANGE THE POWER VALVE---LESS THAN 15 MINUTES FOR ANY OF THOSE PROCEDURES (YOURS MAY BE EZ TOO, I JUST DON'T KNOW)

THESE WERE THINGS NEARLY IMPOSSIBLE TO DO WITHOUT TEARING THE CARB DOWN WITH THE STOCK 2 BARREL BBD----PARTS WERE IMPOSSIBLE TO FIND, SUCH AS ALTERING THE ORIGINAL JETTING

OTHER THINGS THAT FACTOR INTO THE "PERFECT" RUNNING IS NO OR WRONG PCV VALVE OR CLOGGED PASSAGES TO IT---FUEL PRESSURE---PERFECT AND EXACT FLOAT HEIGHT---VACUUM LEAKS (EVEN UNNOTICED SUBTLE ONES, LIKE INTAKE BOLT TIGHTNESS, UNCAPPED OPEN PORTS, CRACKED VACUUM LINES, A CRACKED OPEN EGR)---THEN WE GOT THE.....WAS THE TIMING DONE CORRECTLY, VACUUM HOSE REMOVED AND TEMPORARILY CAPPED, DOES ENGINE RUN BEST WITH PORTED OR MANIFOLD VACUUM ONTO THE ENGINE?---POINTS RIGHT? PLUGS GAPPED BY YOU, NOT OUT OF THE BOX, HAVE YOU PERFORMED ALL TUNE UP PROCEDURES AS OUTLINED BY THE REPAIR MANUAL?

DO YOU STILL HAVE POINTS?---MAYBE A SIMPLE UPGRADE TO ELECTRONIC IGNITION (EZily REVERSIBLE) "IGNITOR SYSTEM"--THESE ARE AVAILABLE TO FIT DIFFERENT DISTRIBUTORS AND ARE WELL WORTH IT. ONE OF MY PICS BELOW IS AN ATTEMPT TO SORTA SHOW THE BASIC SET-UP W/ EXPLANATIONS (I HAVE DONE IT BETTER IN A WALMART PHOTO SHARING ALBUM) THE OTHER PIC SHOW IT NEW IN THE BOXES, SPECIFIC TO MY 1946 JEEP'S DISTRIBUTORTHEM OTHER PICS ARE OF MY '77 CJ 5 6 CYL

https://www.2carpros.com/questions/2007-ford-taurus-tune-up

THIS LAST THING MENTIONED IS SO VERY IMPORTANT--EVERYBODY WANTS TO "WING IT"---AND EXPECT OLD STUFF TO WORK LIKE NEW FOREVER...LIKE PLUGS, DIST CAP, ROTOR BUTTON, ETC, ETC....'CAUSE THEY ARE SO CHEAP!

GET THIS----ATMOSPHERIC AND TEMPERATURE CHANGES AFFECT CARBURETORS---SOMETIMES SOME OF THE TEXTBOOK STUFF IS NOT NECESSARILY THE BEST----LIKE WITH MINE, MANIFOLD VACUUM TO MY DISTRIBUTOR WORKS BEST, NOT ONLY AT IDLE, BUT WHILE RUNNING AT HIGHWAY SPEEDS

I ALSO LEARNED A GREAT METHOD IN WHICH TO "QUICKLY AND CORRECTLY JET" MY CARB USING AN O2 SENSOR INSTALLED NEAR MY HEADERS (MORE ON THAT, IF YOU ARE INTERESTED)

BUT OUT OF ALL OF THIS THAT I HAVE BEEN SHOVELING, REMEMBER THIS:

STRAIGHT OUT OF THE BOX IS USUALLY NOT TUNED FOR YOU ENGINE

WHATEVER "JIMBOB" "DIALED IN" 5 YEARS AGO MAY NOT HAVE BEEN CORRECT EITHER, ESPECIALLY IF IMPORTANT STUFF WAS NOT DONE 1ST

AND GUESS WHAT?....IF THE IDLE SPEED IS WAY TOO HIGH (MAYBE OTHER FIXABLE FACTORS WON'T ALLOW IT TO RUN SLOWER) YOU JUST CAN'T SET THE IDLE MIXTURE SCREWS,YOU MAY NOT EVEN BE RUNNING IN THE IDLE CIRCUIT AT ALL......YOU COULD BE "ON THE MAINS"

RECENTLY I HELPED A NEW JEEP BUDDY, "HELP HIMSELF" (HE WAS USED TO PAYING AND ASSUMING STUFF WAS DONE RIGHT)

EVEN AFTER THE "PAID GUY" WAS DONE WEEKS AGO---IT RAN LIKE CRAP---AND THIS MECHANIC TOLD HIM IT WAS TUNED THE BEST IT COULD BE

I PROVIDED TOOLS, SUPERVISION, MANUALS, AND A SHOPPING LIST....SEE MY ANSWER IN THIS LINK

https://www.2carpros.com/questions/1987-jeep-wrangler-yj-4-2l-straight-6-decelerating-stop-engine-almost-stalls-ret

IF YOU NEED HELP REALLY TUNING THIS THING, AT LEAST ON THE OUTSIDE, I MAY BE ABLE TO AID YOU......YOU WILL NEED SOME BASIC TUNE-UP TOOLS AND A REPAIR MANUAL FOR YOUR RIG IS ALWAYS A PLUS FOR SPECS/ OTHER($80 OR LESS, UNLESS YOU ALREADY HAVE/ CAN BORROW THEM)

EVEN THOUGH YOU DON'T HAVE A REAL VEHICLE (JEEP) I WILL STILL AID YOU IF YOU WANT ME TO!

THE MEDIC

Images (Click to make bigger)