rough idle has me stumped

[bumpster]

Aquired a 1970 GMC 350 auto,quad, automatic. Truck runs great as soon as it comes off idle, but at idle the whole truck just shudders. Thought it was a vacuum leak, but could not find one. replaced all hoses just for good measure. Then I found Champion plugs and off-brand points, so they went away. I then went into my fit to be tied mode and changed the entire ignition system over to HEI,(don't tell shuttermutt where I got the parts).With timing at 8 degrees and idle set at 950 (no load), it becomes almost bearable, but still not right. Exhaust sounds like a steady miss, but when I pull plug wires,none of them fail to drop rpm's about 75 to 100. Compression is a little low at 90, but no more than 5 to 7 lbs difference between cylinders and that was on opposite banks. Any ideas before the rest of my hair comes out?

[68C15]

is this a qjet?
sounds like the idle ckt is not working correct.

[walker]

Had a similar thing happen to me when I first got my truck. It wouldn't idle well at all. I finally found out that the idle screws had beed turned in too far and the points had broken off and plugged the idle circuit holes. Just back out your mixture screws all the way and look at the ehd for a nice sharp point. I have also had the idle circuit plug up with old gas in another car, I just pulled the mixture screws out and blew compressed air in the holes to clear them.

[9footfan]

sounds almost certainly like a fuel delivery issue-good luck!

[71Longbed]

could also be the carb throws too much fuel in to the engine turn her on and accelerate it while watching how much gas goes in

[shuttermutt]

Yep, my guess is your woes center around that damn Quadrajet. You've got an adapter plate, why not throw the 1406 that was on the Nova over on it and see if that clears it up. If not, I'd start tracing the fuel delivery system, but that's a distant second, IMHO.

By the way, I got that PS bracket put on the truck (thanks again, Mike!) and everything's quiet and smooth enough now to make me want to fix the fooking exhaust leaks. I'm thinking real hard about having the pipes welded up and a crossover installed. That 1406/Performer RPM combo is a pretty fair match to the cam even with my advance curve out of whack, but I think a crossover will help beef up the bottom end.

Oh, and what's the going rate to have a diff rebuilt? Anyone? My 3.73 Posi howls when my foot's in the gas (goes away when I lift my foot off the pedal).

PS: Normally, I'd send Tony and Vinnie to break your legs for robbing the Nova for parts, but since you've pulled me out of some jambs lately, I'll let it slide.

[Longhorn Man]

Is ther a chance that you have water in the fuel? Maybe the gas cap is leaking, or a wall mart locking gas cap that leaks.
Toss a 99 cent bottle of fuel dry in it and drive a few days untoll almost empty and fill it back up.
It probably is the Q-Jet...but water in the fuel does exactly what you describe.
When the wife and I got together, we got her an 84 Plymouth Reliant. (It was actually a pretty dam good car). this sucker had what I was calling a 'variable miss'. the miss would travel from cylinder to cylinder. I would unplug one wire, and it'd drop and have a steady miss, but then it would kick and sputter as another one would miss and the engine could barly keep the flywheel spinning. The exact same thing would happen when I would unplug any single wire. (Be carefull, this was also the day I discovered that a car's ignition system is WAY stronger than the brigs and stratton that I was suckered into holding the wire while the other guy pulled on the cord)
I was about to have the holley 2 bbl rebuilt and the old man at NAPA refused to sell it to me untill i ran a bottle of dryer in it.
I'll be damed if that didn't fix it. How is it the old parts guy always knows whats goin on? How did they get so smart?

[shuttermutt]

Originally Posted by Longhorn Man

Toss a 99 cent bottle of fuel dry in it and drive a few days untoll almost empty and fill it back up.

Sage advice (and cheap, too!)

[Longhorn Man]

I like trouble shooting with the dirt cheap methods...and the lazy man methods.

I'm the laziest and cheapest guy you'll ever meet. (that's not quite true)

[cdowns]

i hope when you changed to HEI ignition that you didn't use the original resistor wire if so you'll be burning up the module in short order

i to think use fuel cleaner will help i prefer OUTLAW brand it's the best i've found ,also change the filter in the carb and you might check the fuel strainer in the tank on the fuel pickup assy

[bumpster]

OK, I took the adjuster screws out, looks like nice sharp ends and no crud. I then sprayed carb cleaner up the holes. It's amazing how the cleaner can blow back in your eyes faster than it left the can in the first place. Didn't fix it, but it did change it, truck now surges at idle and has an occasional moment of sounding right. Pretty much locks in the carb as the culprit.Thanks for the help, as soon as I can see again I'll pull carb and continue the battle. By the way shuttermutt, I didn't want to steal parts from the Nova, so remember the 350 horse 350 we pulled from your Chevelle? see-ya

[shuttermutt]

Originally Posted by bumpster

OK, I took the adjuster screws out, looks like nice sharp ends and no crud. I then sprayed carb cleaner up the holes. It's amazing how the cleaner can blow back in your eyes faster than it left the can in the first place. Didn't fix it, but it did change it, truck now surges at idle and has an occasional moment of sounding right. Pretty much locks in the carb as the culprit.Thanks for the help, as soon as I can see again I'll pull carb and continue the battle. By the way shuttermutt, I didn't want to steal parts from the Nova, so remember the 350 horse 350 we pulled from your Chevelle? see-ya

How many times have you put carb cleaner in your eyes now? I can't remember a time when you didn't pick up a can and get 1/2 of it in your face. Sheesh!

I'm tellin' ya, ditch the Quads and move to an Edelbrock. You'd be shocked at how well the 68 is running after I put that spacer under it and tweaked the mixture screws per the Edelbrock manual. I'll never put a Quad on another vehicle as long as I can help it.

PS: Vinnie and Tony will pencil you in for a leg breaking next Tuesday for robbing that 350. Word has it that they may be appeased with a rearend for my truck. The howl from my Posi is getting worse and the clutches are grabbing even with fresh oil and a bottle of GM additive.

[68C15]

I'll have to disagree with you on the Qjet-Edelbreak issue. bring your truck in the spring & we can run them before we tear mine apart.
Bumpster, send me the Qjet & I can-will set it up for you.
Todd

[shuttermutt]

Hey, I'm not saying they can't be set up to run and I'm not saying they can't be tuned to perform better than a 'Brock, but I've gotta stick with my own personal experience. I should've sent that Q-jet back with you, Todd. I'm sending it to Bumpster!

[68C15]

and to think I was looking at it wondering if it was for sale. I tell you what, send it to me & at my spring meet I will feed you brats until you cant eat any more. not to mention I will promise to let you sleep in on the weekends.

[68C15]

& I can fit about 30 of them on my grill!!!!!

[Brainchild]

Don't dis a Qjet till you've had a good one.I'm running one on my truck right now and it's a great carb.But whenb they go bad,

[shuttermutt]

Well, I'm not gonna jump in the middle of yet another carb brand flame war. All I know is that my 'Brock is working great. To each his own, freedom of choice and so forth.

PS: Those brats would almost be worth it!