Quadrajet help

[steveimpala]

Just an update... last month while driving to work, had the idling issue come back again.... I was going up a hill, accelerated to get up over the hill, came to a intersection and the truck wanted to stall out. Talked with my mechanic about it and he mention that it could be the accelerator pump - so at that point, decided to go ahead and rebuild the carburetor ( the truck has 88000 miles and the carb appear to be untouched) . The carb is back on the truck, no vacuum leaks, timing is around 12. The truck starts up and runs great for the first 10 to 15 mins into the drive, then I noticed the idle has come down again....not sure to look at next.... it does have a HEI that was installed by the previous owner - could this be an issue ? Very frustrating... like I mention, she runs great at the start, but something is happening once the engine get's hot.

Thanks guys.... I appreciate any input !

Steve

[dmjlambert]

You are supposed to do the tuning such as setting the idle speed when the engine is fully warmed up. So go ahead and get it warmed up, make sure the choke is fully open, and then adjust your idle speed and mixture screws if necessary, for smooth idle. Idle should be about 600 to 700 while in drive with parking brake set and wheels chocked and you standing to the side making the adjustment. Then see how it does the next day. When the engine is cold and choke is active it should idle faster, there is a fast idle adjustment screw tied to the choke.

[dmjlambert]

And by the way, accelerator pump would not be used at idle.

And it is not possible to tell what your initial timing should be set to, that is nobody could tell you it belongs at 12 BTDC, without first knowing from you how much advance your vacuum advance gives. My vacuum advance gives 23 degrees, so 12 initial is way to high for my truck. Timing settings are something you have to study as a whole picture of what's going on. When the day comes I want to hot rod the distributor, I will start by adding a starter brace, then limit the vacuum advance to smaller than 23 degrees, perhaps 10, then set the initial timing to around 12. Right now I am good with the initial timing set lower at 7.

Then every thread about timing and tuning has to deteriorate to an old argument about whether to use ported vacuum or manifold vacuum for your vacuum advance. So that will be the next part of the discussion, probably. Some engines run better and it is easier to get a good tuning when you use manifold vacuum for the distributor. My engine runs well either way, and I currently have it running on manifold vacuum. When you alter the timing or the source of vacuum for the vacuum advance, it will affect the idle speed, so you have to go back and re-adjust idle speed after doing something that affects timing.

Have you checked that the vacuum advance works? That capped port on the passenger side of your carburetor is manifold vacuum. If you connect the vacuum advance of the distributor to that, and play with connecting and disconnecting it (while blocking the carburetor port with your finger) while the engine is running, you should notice a change in idle speed and if you have a timing light on it you should also notice timing changes. It may be a good idea to check to make sure the vacuum advance works, because it if doesn't it could be the diaphragm is broken and giving you a vacuum leak.

[steveimpala]

Ok.... we'll give it a try.....

Thanks

[Greasey Harley]

What is your fuel pressure?

Have you replaced all the rubber fuel lines?

Did you check for a vacuum leak on the line the runs down to the trans?

When was the last time your truck had a complete ignition tune-up?: Plugs, wires, cap, rotor, coil pack.

90% of "carburetor problems" are actually ignition problems.

[steveimpala]

Good points.... thanks

[mike16]

Rochester Carburetors written by Doug Roe and first published by HP Books in 1981 and is the difinitive text on Q-jets. It has gone through several editions and is currently being republished.

Your truck may be low miles but its 50 years old. The service manual will also help is setting up your fuel system properly but there are problems that occure when a truck is that old.

I suspect rust or garbage in the fuel system. Go through it thoroughly to eliminate those conditions as variables. if you dont; any condition will create an infinite number of variables to your trouble shooting attempts.

replace rubber fuel lines, blow out hard lines and or replace them if in doubt, rebuild your carb, replace the tank. check carefully any and all vacuum lines for leaks or cracks that could become leaks. any rubber lines that are 50 years old may be a novelty to admire but they can be a source of frustration or even dangerous if they are no good. Rubber lines on aircraft usually must be replaced after two years of service so there's the bench mark for if its any good, Others can argue this point but the evidence that your truck does not run suggests they ought to be replaced. If they are origonal, then fine, keep them but put them aside and replace them with fresh new rubber if you intend to drive the truck.

[steveimpala]

Ok, thanks for the information Mike !

[jamyers]

Whoever suggested "Accelerator pump" either didn't understand you, or doesn't know beans about carburetors. Acc pumps (at least on a Q-jet) ONLY come into play when you open the throttle quickly - they have zero to do with idle, slow off-idle transition, cruise, main, or power circuits

I'm with Mike16, and my money is on you having some trash in the idle passages. I love Q-jets, but their idle passages are a labyrinth of ups/down, air bleeds, restriction tubes, and corners.

If it idles fine when cold, but once warm has issues at idle or *just* coming off idle, look at the idle circuits or maybe the transition slots just above the throttle plates.

If it's been more than 5 years since the carb has been opened up, you won't go wrong pulling of off, opening it up, and giving it a good spraying-out with carb cleaner, especially the tiny passages that lead down to the idle mixture screws.

Taking a Q-jet apart looks like rocket surgery, there are a ton of videos and other online resources that'll walk you through it.

Clear off a workbench, Go slowly, take pics of each part before you remove it, and lay them out carefully in order. I've been messing with them for years and can tear one down in 10 minutes, but if you can run a screwdriver and needle-nose pliers, it shouldn't take more than a nice Saturday afternoon to get the airhorn (Carb top) off, get the float/needles/jets out, pull the idle mix screws, spray everything out, and put it back together again. Then reinstall on the truck, set the idle mix screws and be done.

[Steeveedee]

Originally Posted by jamyers

Whoever suggested "Accelerator pump" either didn't understand you, or doesn't know beans about carburetors. Acc pumps (at least on a Q-jet) ONLY come into play when you open the throttle quickly - they have zero to do with idle, slow off-idle transition, cruise, main, or power circuits

I'm with Mike16, and my money is on you having some trash in the idle passages. I love Q-jets, but their idle passages are a labyrinth of ups/down, air bleeds, restriction tubes, and corners.

If it idles fine when cold, but once warm has issues at idle or *just* coming off idle, look at the idle circuits or maybe the transition slots just above the throttle plates.

If it's been more than 5 years since the carb has been opened up, you won't go wrong pulling of off, opening it up, and giving it a good spraying-out with carb cleaner, especially the tiny passages that lead down to the idle mixture screws.

Taking a Q-jet apart looks like rocket surgery, there are a ton of videos and other online resources that'll walk you through it.

Clear off a workbench, Go slowly, take pics of each part before you remove it, and lay them out carefully in order. I've been messing with them for years and can tear one down in 10 minutes, but if you can run a screwdriver and needle-nose pliers, it shouldn't take more than a nice Saturday afternoon to get the airhorn (Carb top) off, get the float/needles/jets out, pull the idle mix screws, spray everything out, and put it back together again. Then reinstall on the truck, set the idle mix screws and be done.

True enough, but I'd recommend getting one of the recommended books, either from Ruggles or Roe. Reason being, people on youtube still put out all kinds of baloney. Ruggles and Roe have been doing it for decades. If you pay $20 for the book, you got what you paid for. If you watch a free youtube video, you also got what you paid for.

[oldgold70c10]

If you are having idle speed trouble AFTER the engine is warmed up, adjust the curb idle speed screw on the driver side of the carb. The Q-jet has 2 idle speed adjustments, a fast idle adjustment that operates only when the choke is not full open. The fast idle adjustment screw is on the throttle linkage on the driver's side of the carb. The fast idle will slow down as the choke opens because of the shape of the fast idle cam which has 2 or 3 steps on it. Once the choke is fully open, the curb idle screw and the idle mixture screws will be what determines idle speed.