Quadrajet Flutter off of idle Stock 305 automatic

[bd]

Rory, in addition to the repair steps already outlined, lift the distributor cap to inspect the rotor and carbon button inside the cap.  Look for any chalky discoloration on the plastic, carbon tracking or excessive burn-back of the button.

[SkidRo]

This is what I managed to do this past weekend.  Despite my best effort and using images of the carb I unfortunately could not locate the idle mixture screws. They may/must have a cap over them. So any work on the carb was minimal.  I did find one vacuum hose that was broken and someone had wrapped it in electrical tape.  It is a thin hard plastic line that runs to the back of the carb.  After removing the tape I managed to slip some shrink wrap over the two ends, and then slip another piece of tubing over that so make sure the line did not collapse or separate. I also removed the distributor cap and noticed that the metal contacts on the cap and rotor button had arc marks on them.  I cleaned them with 1000 grit sand paper and electrical cleaning spray.  The two front nuts holding the carb onto the manifold were loose. I tightened them and checked the rears and they were fine.  The only other thing that I was able to do with the carb was spray the Berrymans into the primary and secondary sides and around the outside. I also made sure all of the electrical connections were properly seated.  Spraying clean cleaner around the base of the carb, the carb itself and the vacuum lines did not have any affect on the way the truck ran. 

I started the truck without the air cleaner and gave it gas.  It still had a dead spot, backfired out of the carb (nice flame), and then idled fine.  I turned it off and installed the air cleaner, etc.  Then I took it for a drive. It started and idled fine.  It still had the dead spot off of idle and it wanted to cut out unless I accelerated though it.   While driving about 25 mph it backfired one more time and then it ran perfect.  No issues at all.  I took it out on a main road and was able to cruise, open it up, and then come to an idle and repeat numerous times.  I drove back home and turned it off and on a few times without issue. 

I am not stating that the issue is resolved until I test it further and can get into the carb itself.

[bd]

Replace the spark plugs, check the plug wires and install a new fuel filter.  Inspect for arcing between the wires and ground (this is most easily accomplished at night in the dark) and measure the wire resistance with an ohmmeter (should measure anywhere in the range of 1,000 - 3,000 ohms per foot of individual wire length).  Look inside each of the plug wire boots for traces of black soot and/or any greenish discoloration.

With the engine off and choke held open, look down the primary throats of the carb using a bright flashlight and open the throttle.  There should be two strong streams of fuel exiting the accelerator pump nozzles each time the throttle opens.  There should be zero lag between throttle opening and accelerator pump squirt.  In order to remove the idle mixture screw caps for adjustment, the carburetor will need to be removed.  I would postpone that step for the time being.

Does the engine pop-pop-pop-pop back through the carburetor whenever the throttle is open rapidly?

[Stewart G Griffin]

As a side note, 85-86 trucks had a dual capacity accelerator pump (both 4.3 v-6 and 305,350 v-8s).  i'm not sure why only 85-86 trucks had this?   But just pointing it out.

[SkidRo]

I will check the wires and boots (and fix if needed) and replace the plugs. I use the same technique for the Corvair to check for arcing.   Ditto for the checking of the carb for immediate squirts of fuel.  I'll do these tonight.

If I recall correctly, if the throttle is opened rapidly it acts fine. No popping or backfiring at all.  But I will check this too. It's when it comes off idle slowly that the dead spot is felt.  Then it seems like the too much fuel is being delivered, it bogs down and it will get a back fire.  Also, if I am at cruising speed whether 35 or 60 it has that flutter or dead spot. When I give it more gas to get past that point it appears that it is loaded up with too much fuel, will back fire and then run okay until I get back to a steady cruise again. 

I have not driven since the last troubleshooting so I do not know how it is running right now.  When I parked it it was running good. 

[blazer74]

Popping thru the carb would indicate lean. Along with the rough idle could have picked up some blockage in the idle tubes
When you get on it hard your accelerator pump shot  covers up your lean spot and your into your off idle and main circuit.

[bd]

...It's when it comes off idle slowly that the dead spot is felt.  Then it seems like too much fuel is being delivered, it bogs down and it will get a back fire.  Also, if I am at cruising speed whether 35 or 60 it has that flutter or dead spot. When I give it more gas to get past that point it appears that it is loaded up with too much fuel, will back fire and then run okay until I get back to a steady cruise again. 

Rory, your write-ups hint at a few possible causes, but lean fuel delivery seems to dominate.  Perform the actions recommended to date to eliminate them as variables.  Adding to your list of things to check, disconnect at both ends the 3/8" fuel bowl vent hose that runs between the carburetor fuel bowl and the vapor canister.  Wash it out with carb cleaner into a clean rag.  Inspect the rag for any trace of black carbon granules washed out of the hose.

[SkidRo]

I didn't get the opportunity to work on the truck this weekend.  I will fire the truck up when the sun drops to check for any arcing or grounding of the wires.  I also bought a new fuel filter which I will replace as soon as possible as well as check the canister line.  I did move the truck and it acted the same with that dead spot off idle.  It was also back firing out of the carb quite a bit.  Once I got past the dead spot and moving it appeared to run fine.  However, if I was cruising at a moderate speed (in that dead spot zone) the truck, at times, would cut off or just sputter.

[SkidRo]

I managed to put some time into the truck this weekend.  I did not spot any grounding of the spark plug wires when tested while it was dark outside. I was able to replace the fuel filter in the carb. The old one did not look dirty or clogged but it may have been.  I could not get the line that runs from the carb to the canister disconnected.  I was concerned that I would break it so i did not force it off.   Starting the engine produced several results all random.  It may start and everything works perfect - idles, very minimal flat spot (like it always had), and accelerates fine.  Or it may sputter, backfire out of the carb and have a huge flat spot off of idle. These happen while in park and/or driving. 

The battery continues to perplex me too.  It was low on the charge so I put it on the charger. Remember, I bought this battery new in June 2016. When fully charged and cranking the engine turns over slow/sluggish.  The volt meter also reads around 8 volts.  Once it starts the volt meter goes to around 14 volts - like it is charging.  What I think I have observed is that when the battery is fully charged the carb works better.  There are a few electrical connections on the carb - a two prong connector right above where the float bowl is (and the fuel filter) and the choke.  I removed the two prong connector and it was filthy inside like a gummy substance.  I cleaned the connections and plugged it back in.  As for the battery, why isn't holding a charge?  I have turned off all of the electrical components - radio, heater/A/C, etc. so there is no drain on the battery.  When I take readings when it's running or not it reads 12-13 volts.  I have not taken a reading when the engine is turning over but I suspect it will read low volts.  I noticed the two prong connector at the alternator was dirty so I unplugged it and determined it was not making good contact with the two prongs in the alternator.  Maybe this is why the battery is not charging.  I can't describe what was in the plug except that it was hard to remove.  It looked and felt like tar although I doubt it was.  Carb and brake cleaner would not remove it. I manage to get the connector and the prongs clean and plugged it back into the alternator.  I don't know if this was contributing to the battery issue or if the battery is bad out of the box.  But the original battery was dead as a door nail suggesting it was not being charged by the alternator. 

At this point I had to stop work. However, I am beginning to think that a low charge on the battery MAY have an effect on the engine and carb. It seems when I charged the battery the issue goes away until the battery gets to low to even start the truck.  This is speculation but sometimes fuel and carb issues are really electrical issues.

[bd]

...The battery continues to perplex me too.  It was low on the charge so I put it on the charger. Remember, I bought this battery new in June 2016. When fully charged and cranking the engine turns over slow/sluggish.  The volt meter also reads around 8 volts.  Once it starts the volt meter goes to around 14 volts - like it is charging.  What I think I have observed is that when the battery is fully charged the carb works better.  There are a few electrical connections on the carb - a two prong connector right above where the float bowl is (and the fuel filter) and the choke.  I removed the two prong connector and it was filthy inside like a gummy substance.  I cleaned the connections and plugged it back in.  As for the battery, why isn't [it] holding a charge?  I have turned off all of the electrical components - radio, heater/A/C, etc. so there is no drain on the battery.  When I take readings when it's running or not it reads 12-13 volts.  I have not taken a reading when the engine is turning over but I suspect it will read low volts.  I noticed the two prong connector at the alternator was dirty so I unplugged it and determined it was not making good contact with the two prongs in the alternator.  Maybe this is why the battery is not charging.  I can't describe what was in the plug except that it was hard to remove.  It looked and felt like tar although I doubt it was.  Carb and brake cleaner would not remove it. I manage to get the connector and the prongs clean and plugged it back into the alternator.  I don't know if this was contributing to the battery issue or if the battery is bad out of the box.  But the original battery was dead as a door nail suggesting it was not being charged by the alternator. 

At this point I had to stop work. However, I am beginning to think that a low charge on the battery MAY have an effect on the engine and carb. It seems when I charged the battery the issue goes away until the battery gets to low to even start the truck.  This is speculation but sometimes fuel and carb issues are really electrical issues.

The sticky brown accumulation in the gray plastic tower protruding from the top of the fuel bowl is condensed solids from fuel that has wicked up out of the bowl along the two wire leads from the dual capacity fuel pump solenoid.  It is recurring maintenance to clean periodically.  The brown-black crud in the alternator connector is desiccated antioxidant used to control corrosion of the electrical connections to the regulator.  It is not detrimental and should be replenished with fresh antioxidant compound, such as Truck-Lite NYK-77 Anti-Corrosion Compound available from many commercial truck dealerships and truck stops.  Alternatively, you can use Gardner Bender Ox-Gard or similar compounds available from the electrical departments of most hardware stores.

Charge the battery then disconnect the charger.  The engine should remain off for the following checks.  Connect an analog (filament type) test light between the battery terminals and notice how bright the test light illuminates; this in "normally bright" illumination.  With every electrical appliance in the truck shut off and the doors closed, disconnect the negative battery cable and insert the test light between the battery negative post and the ground cable.  How brightly does the test light illuminate - 'normally bright' or less than normally bright?  When you open the door or pull the headlamp switch, does the test light get brighter?

[SkidRo]

BD,

I will run these tests in the next day or so.  To be clear, you want me to test between the negative post and the ground cable (negative battery cable) while it is disconnected. Is this correct? 

Rory

Charge the battery then disconnect the charger.  The engine should remain off for the following checks.  Connect an analog (filament type) test light between the battery terminals and notice how bright the test light illuminates; this in "normally bright" illumination.  With every electrical appliance in the truck shut off and the doors closed, disconnect the negative battery cable and insert the test light between the battery negative post and the ground cable.  How brightly does the test light illuminate - 'normally bright' or less than normally bright?  When you open the door or pull the headlamp switch, does the test light get brighter?
[/quote]

[bd]

Correct.  The test light should be inserted between B- and the ground cable, such that the only connection to the battery's negative terminal is the test light.  Inserting the test light in series between the battery and cable will tell you whether there is sufficient parasitic draw on the battery to discharge it.  Analog test lights illuminate "normally bright" at ~40ma of current flow, depending on the bulb used in the test light.  If the test light is dim when connected in series with all appliances switched off (including engine compartment lamp, dome lamp, glovebox light, etc), parasitic draw isn't an issue.  If the test light illuminates 'normally bright,' further diagnosis is indicated.

[SkidRo]

That is exactly what I thought you were getting at.  Thanks.

[SkidRo]

Update - I charged the battery.  I disconnected the positive cable while charging so I could get a good charge. The battery reading is 12.85 volts. Next with my test light I connected it between the positive and negative posts.  It was bright.  I connected the positive cable to the battery and ran the same test with the same results. Next I disconnected the negative cable and place the test light between the negative post and the ground cable.  The light did not come on ( I will do this test again).  However, when I connected the light between the negative post and frame (or a bolt) the light came on. It appeared to be the same brightness.  That was all I could do last night.  I did disconnect the positive cable again and I will take a volt reading tonight to see if the battery dropped significantly from the 12.85 volt reading. 

[bd]

...when I connected the light between the negative post and frame (or a bolt) the light came on. It appeared to be the same brightness....

Which did you use - a filament type test light or an LED test light?  Was everything shut off, all doors closed, underhood light disconnected, etc?  Do you have a sound system installed that uses a direct connection to the battery?  Does the truck have a factory clock?

Start removing fuses one-at-a-time until the test light dims or extinguishes.