Quadrajet question

[Guest zthang43]

Actually 2 quadrajet questions. First one: I have a 4.3 in my Z, mostly stock, very mild cam. Using a stock 4 barrel manifold from a astro van. (The motor is from an 88 S10) It ran great the very first time I started it up, except at full throttle it bogs a little. No amount of rpm (safely anyway) makes it pick up again, but if I ease my foot out just a bit, the power picks up again. I have adjusted the secondaries as tight as I dare, and they don't seem like they would be opening, but the bog is still there. Any ideas?

Second question. Just the last week or 2 my car has picked up the mid-temp blues something fierce. (I was only driving it about 1 week before that, but it seemed fine then) It runs real rough anywhere under 1k, and wants to die. There is a definite lag in the throttle when it gets down this low (slow cruising speeds, or trying to idle). Once I rev it up it seems ok except a very faint surging until it warms up.

I'd appreciate any help. I've had this mid temp blues problem on my Chevelle and other sbc motors too, never have been able to get rid of it.

[denny411]

you have a fuel starvation problem ie.bad accelerator pump.are the vacume secondairies open when it bogs? It also could be a plugged fuel passage.or posssibly just a fuel filter.the pump is easy to change.but I`d do the filter first,it`s easiest.you could also check your fuel volume(could be a weak pump)Sometimes you can clear a plugged passage by reving the engine and while holding the throttle open w/ one hand, cover the choke horn w/ the other.Hold it there until the engine almost dies then remove it while still holding the throttle open. This will use manifold vacume to pull fuel through the primary fuel passages.If it`s a minor blockage this will usually clear it out

[georgiaz]

A possible cure fo your stumbling problem is to drill a 1/6" (one sixteenth) hole near the leading edge of the secondary butterflies. I can't promise this will work, but I have seen it done many times to resolve the same problem. I have done it on the quadrajet on the 327 in my 67 Nova a few years ago and it resolved the problem. Of course take the carb off first.

[Guest zthang43]

I should add that for the bogging issue, it does not act like fuel starvation (ie, running rough and sputtering); it is a very gentle bog, like it is over-carbureted or something. That doesn't seem right though since it does it even though the secondaries are adjusted so that they open very slowly.

About the mid-temp stumbling issue: It *only* does this when it is in the process of warming up. When the engine is cold or completely warmed up, idle is smooth and throttle response is excellent.

Anyone else have these problems?

[dmanzo57]

Your "mid temp blues" could be caused by the choke releasing before the engine is warm enough. As for your bog, check the choke pull-off. It regulates the opening of the secondary air valve.

 

Happy tuning,

Don

[Guest Anonymous]

Lean or rich could be causing your problem. Get the car to start the stumbling then put the car in neutral and shut it off. Don't let the motor return to a smooth running condition. You need to look at the plugs to see if your rich or lean. Coast off to the side of the road and pull a plug. White is lean, and black is rich.

[DAW]

Sounds like a lean sag/throttle tip-in condition rather than something as gross as an accelerator pump volume mismatch. I think there are various power valve springs, jets, etc., for tuning this out of a Quadrajet. You've got to get the primary set up right before doing anything to the secondary.

DAW

[John Scott]

I used to experience this with my old NA 4.3. I kept trying to get more gas to it, to no avail. The fact that it cleans up when you let up sounds like too much fuel to me. Here's what I found and it might help. Find what secondary metering rods you have and go LEANER, that is a fatter tip on the rod. If you can't locate any, I thought I spied some in the bottom of my toy box in the garage. I couldn't believe how much the WOT cleaned up after this. I had some needle size tips I was sure would add hp, but the big ones really sped things up. Also, I'd adjust the secondary spring for off line performance. I start loose, then go in 1/4 turns until bog was just eliminated. Then I had a major traction problem. If this doesn't help you might trygoing richer, with the thinner tips, but I'm betting on the other way. I'd look for a vacuum leak around the carb, pcv and lines, as well as accel pump, for you other problem. If you always have a part throttle surge, then jet up 2 numbers.

[Guest zthang43]

Thanks for the tips yall. I was able to cure a large part of the mid-temp blues; heres what I found out:

In some fit of insanity I plumbed the vacuum advance into a vacuum port below the throttle valves, so it was getting vaccuum all the time, so it was always advanced. I remember retarding the timing because it was pinging at low rpm, large throttle conditions. I think my motor must experience less vaccuum during the mid-temp warm up period, so the vaccuum advance was falling off, and most motors don't run well with really low timing anyway, so that makes sense.

Once I plumbed the vaccuum advance in the right place and set the timing properly (with vac. adv. disconnected) it runs great.

Still have the full throttle bog but I'm going to try John Scott's suggestion of secondary metering rods and see what happens.