Mechanics

WHY WON'T MY TRUCK IDLE WHEN HOT?

1972 GMC Acadia • 71,000 miles

I have a 1972 GMC truck with a problem. It runs fine when the engine is cold, but when the engine warms up the engine starts to surge when I stop. It runs fine as long as I'm on the gas, but when I let off the throttle, the rpms go really low and it tries to die, then it picks back up to a very fast idle, then back down again, etc. It does this until the engine finally dies. I was told that I have a vacuum leak, but I can't find one. I drove it until it got hot and started acting up, then I unhooked and blocked off the power brakes, modulator valve, and vacuum advance. No help. The engine is the stock 307 with a Holley alum. Intake and 600 cfm vacuum secondary carb. I was told maybe the intake was cracked so I changed to a new polished intake and tried a new quick fuel carb and a used edelbrock carb. No help. The truck had a 3 speed trans when I bought it and I changed it to a turbo 350 with a B&M 2000 stall convertor that I bought from a friend. I thought maybe the convertor was bad, so I bought a new 1800-2000 stall TCI one. Again, no help.
The engine has 71000 original miles so I thought maybe the cam had a bad lobe. I pulled it out and the cam and lifters look great. While I had it out, I installed a new timing chain. When I had the engine out to install the trans and a new chrome steering column, I took a look at the rod and main bearings (they looked fine), and installed a new std. Volume melling oil pump. I don't know if it had this problem before I bought it because it didn't have any brakes so I hauled it home and never tried to drive it until after I installed the new trans and new power brakes. It also has a HEI distributor with a flame thrower coil, cap and module. I don't see anything wrong with them and I just installed new AC plugs ans new plug wires. NO HELP AGAIN ! A friend says it may be the exhaust doesn't have enough back pressure. It has stock manifolds and the pipes are stepped up to 3" into dual flowmaster 40 mufflers then 3" tail pipes out the sides in front of the rear tires. I don't like this set up but it was like this when I bought it. I've done everything that I know to do. Any help would be great.
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Dragtruck
September 22, 2012.




Idle circuit in the carb. You need to find someone familiar with carbs from the old days to set it up for you?

Roy
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ASEMaster6371
Sep 22, 2012.
I'M GOING WITH CHECKING THE FOLLOWING

FLOAT HEIGHT

MIXTURE ADJUSTMENT

WHAT SORTA FUEL PRESSURE YOU GOT GOING IN?

HOW MANY TURNS "OUT" ARE YOUR MIX SCREWS?

SURLY YOU DON'T HAVE MANY VACUUM LINES/APPLIANCES....THIS IS AN OPTION YOU MIGHT TRY, I'VE SEEN IT REALLY WORK A COUPLE OF TIMES

http://www.youtube.com/watch?v=zMok2y05jNE

WHEN YOU'RE RIDING, YOU ARE ON THE MAINS

WHEN YOU ARE SITTIN' STILL, YOU ARE ON THE IDLE CIRCUIT (LIKE ASEMaster6371 EXPRESSED) SO JETTING IS NOT THE PROBLEM

WHAT HAVE YOU OR YOUR EVIL PALS DONE TO THE CARB THUS FAR....GENERALLY PEOPLE START ADJUSTING STUFF, OR TAKING IT APART W/ NO CLUE AS TO WHAT THEY ARE REALLY ACCOMPLISHING, OTHER THAN TRIAL AND ERROR

LET ME KNOW WHAT YOU GOT GOING WITH THE CARB

I HAVE A HOLLEY 390 ON A OFFENHAUSER INTAKE. ON MY JEEP CJ5 258 6 CYLINDER

AFTER A LOT OF TALKIN'/ WATCHIN'/ READIN'/ DOIN'....TRYING TO SEPARATE FACT FROM FICTION I FINALLY GOT MY CARB ABOUT WHERE I WANT IT.

I'M NO CARB EXPERT...BUT I STRIVE TO KNOW MORE AND MORE...I DO PRETTY GOOD TOO!

THE DRAG STRIP FELLERS "MAY" KNOW ABOUT SPEED....I FOUND THAT MOST, CANNOT COMPREHEND THE "ECONOMY AND MPG" END OF CARBURATION

AFTER PICKING UP A FEW POINTERS, CONTINUED MY VENTURE

I FOUND THAT I CAN "SEE" A RICH PLUG, I FOUND I CAN "SEE" A LEAN PLUG......ANYTHING IN BETWEEN, ALL LOOK THE SAME TO ME!......I NEEDED SOMETHING RELIABLE AND MORE ACCURATE THAN "PLUG CUTS"

DOING THE NORMAL RUN OF THE MILL ADJUSTMENTS AND FINALLY FINDING "CORRECT JETTING", I NOW GET 18 MPG BEING NORMAL AND GENTLE, ON THE FRONT TWO(THAT'S 2-3 MORE THAN MY FEW/BEST MPG CHECKS THAT I GOT WITH THE STOCK CARB--OR THE WEBER DGEV I TRIED)...ON TOP OF THAT, IF I "GET ON IT"...THE POWER IS THERE!

THE BIGGEST MPG TRICK I FOUND WAS NOTHING MORE THAN USING AN OXYGEN SENSOR AND A VOLTMETER FOR THE BEST JET SELECTION IN MY CARB.......LATER I GOT THE PERMANENT AIR/FUEL GAUGE (IN PLACE OF TEMPORARY VOLTMETER)

I CAN GO ON MORE.....I'LL LET YOU SOAK IN SOME OF THIS...PLEASE RETURN W/ ANSWERS...IF YOU WANT MORE FROM ME, OR LINKS TO FIND STUFF I FOUND, LET ME KNOW.

THE MEDIC



Tiny
CJ MEDEVAC
Sep 23, 2012.

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