Quadrajet Flutter off of idle Stock 305 automatic

[SkidRo]

All,

Sorry for the long read but I want to give as much info as possible.  I have this weird issue that I can't narrow down. My truck has about 87000 original miles and it is all stock.  It has A/C (still works), automatic, power brakes and steering.

Up until my battery died a few months back the carb ran (acted) fine except for a slight dead spot right off of idle.  Then the original battery, which was  about 9 years, old died. After I replaced the battery the carb started to act up to a point where I could drive it but only if I keep the throttle open/rpms higher (like cruising around in second gear). However, once at a stop light etc. it might idle but I typically have to put it in neutral and press the accelerator fast enough to get it past the dead spot. It also has a hard time idling once the choke opens.  I know this generally means the idle circuit is dirty/bad BUT the other day I started the truck and it ran fantastic.  Like so good that I decided not to buy a new carb.  I drove it around for a while to charge the new battery. It was acting very weak which I attributed to not driving it that much and having to start it over and over when it died on me.  I also had the battery and charging system checked by my local FLAPs a few weeks prior and they claimed that everything was good.  I will take that assessment with a grain of salt though. 

On my drive when it was running like a champ, I stopped at the local Home Depot.  I went to start the truck and the battery barely manage to turn the engine over and start.  Immediately the carb went back to the same issue.  it has a flat/continuous dead spot off of idle and a rough idle. Once through the dead spot the truck runs great. I didn't think I would make it home due to the stop and go traffic. 

Once home I put the battery on my charger. The next morning it was fine and cranked like new.  However the carb was acting the same. But once it clears that flat spot it runs great.  The only thing that I have done in regard to the carb is add some Techron in the tank on the last fill up and shoot some carb cleaner into it. Again, except for the other day that treatment didn't appear to have helped. I have almost ran through that tank of gas and I was thinking of doing the same treatment.

So, am I looking at an electrical and carb issue or just an old Quadrajet that is dirty and doesn't like the gas in the DC/Maryland area.

Many thanks,
Rory

[boulky12]

When was the last time the carb was rebuilt

[SkidRo]

I have owned the truck for three years.  It should be presumed the carb had never been rebuilt by the previous owners (2).  If these can be rebuilt with success I have the means to do so once I buy the proper rebuild kit.  I have rebuilt several motorcycle carbs in the recent past. I also have no issue with buying a rebuilt one from National if that is the end suggestion. 

[AZ4X4SQBDY]

What year is this truck?

[bd]

Please include Year, Make, Model etc.. when posting

...Up until my battery died a few months back the carb ran (acted) fine except for a slight dead spot right off of idle.  The original battery was about 9 years old and dead. I got a new battery, which seemed to take care of that issue....

What do you mean by this?  What issue was resolved by replacing the battery ...the 'dead spot' off of idle or the 'dead battery' won't crank?

...It also has a hard time idling once the choke opens....

Does it run and idle correctly with the choke closed?  If you hover the carburetor with a shop rag or mist carb cleaner down the carburetor throat, will it idle?  Have you sprayed around the base of the carburetor, intake manifold and vacuum lines for vacuum leaks?

[Stewart G Griffin]

What year is this truck?

X2

[zieg85]

Sure sounds like a lean miss to me, cracked vacuum lines.

[SkidRo]

Sorry all, I thought the year and make was in my avatar.  The answers to your questions are as follows.

It's a 1986 C10 Scottsdale. 

"What do you mean by this?  What issue was resolved by replacing the battery ...the 'dead spot' off of idle or the 'dead battery' won't crank?"
That was a terribly stated sentence on my part.  The cranking caused by the dead battery. The dead spot is still present in the carb off of idle just magnified.  I was trying to ask can a weak battery cause this type of carb issue. I have reworded my original post to make more sense - hopefully.

"Does it run and idle correctly with the choke closed?  If you hover the carburetor with a shop rag or mist carb cleaner down the carburetor throat, will it idle?  Have you sprayed around the base of the carburetor, intake manifold and vacuum lines for vacuum leaks?"

When the choke is set the engine will fast idle fine.  However, when the choke opens it tends to stall out.  I have sprayed carb cleaner in the carb and it will stall if I do not give it gas. I have not tried spraying around the base or any vacuum lines yet.  I will do this when I get home tonight.

"Sure sounds like a lean miss to me, cracked vacuum lines."

I am leaning toward this as a potential cause.  Now it's a matter of figuring out which line/what's generating the leak . 

[zieg85]

I am leaning toward this as a potential cause.  Now it's a matter of figuring out which line/what's generating the leak .

One by one disconnect the vacuum lines to the carb and plug them, see if any makes and improvement.  If the ESC connection on the distributor is still there that also may make the timing go whacky off of idle.

[Stewart G Griffin]

1) Was this a California truck?   i'm trying to determine if the carb is a CCC carb.

2) If not, then being an 86 it probably has that dual accelerator pump thing and this could be contributing to the problem.

3)......Or the regular accelerator pump may need to be checked.....

4) Please DO NOT buy a carb from national.

[SkidRo]

1) It is not a California truck.  It was bought new in Damascus, MD and it has lived its life here.

2&3) Now that you mentioned it, this issue has some commonality on an issue I had with the carbs on my Corvair.  The two primary carbs had bad/incorrect accelerator pumps. They had a dead spot off of idle and then when cruising the car would jerk. They also had many more issues that have since been resolved.  I agree that the pumps could be an issue on my truck.  This ties into an earlier post about when the carb was last rebuilt, which was never.

4) Please let me know the issue with National.  Or, if you prefer, PM me.

I did not get to the truck last night to do any troubleshooting.  I will work on it on Saturday and report back any finding in regard to vacuum leaks, etc.

Thanks for all the suggestions so far. 

[bd]

When the choke is set the engine will fast idle fine.  However, when the choke opens it tends to stall out.  I have sprayed carb cleaner in the carb and it will stall if I do not give it gas. I have not tried spraying around the base or any vacuum lines yet.

An accelerator pump failure will only cause a 'flat spot' when opening the throttle quickly.  It will not affect idle with constant throttle.  You need to figure out the idle issue first.

Count the exact number of turns in to lightly seat the idle mixture screws, record and post the counts.  Remove the screws and thoroughly spray the idle passages with Berrymans B12 carburetor cleaner followed by compressed air.  Reinstall the screws, adjusting to the same number of counts, out.  Spray and check for vacuum leaks as previously recommended.  Have you made any changes to the timing?

[SkidRo]

BD, I will perform the tests and procedures as suggested and post the results.  The timing has not been adjusted since I gave it a tune up 2 years ago.

[bd]

Good!  For now, leave the timing as is so a new complication isn't introduced into your diagnosis.

[79 Royal Sierra]

agreed, start with what you know is likely wrong, dont add to the mix