Quadrajet sputtering

[Eighty5ov]

Any ideas what would cause my carb to sputter when I am in park just revving it up ?

 

[SirRobyn0]

Start with the basics. Shut the engine off look down the throat of the carb, operate the throttle butterfly but hand and make sure each side is getting a good shot of fuel off the accelerator pump.

Then check the ignition timing. Set base timing to specification which should be on the underhood sticker. If no sticker is present shoot for 8 - 12BTDC. Check to make sure the vacuum advance is hooked up and functional. Check to make sure the mechanical advance in the distributor is functional, you can do that with the timing light but watching the light and the marks as you rev the engine looking for a total advance of around 30 - 35 or just pop the cap of the distributor and make sure the weights and springs are in place and move freely.

That's the main stuff. How does it run otherwise and how is the idle? When was the last time the carb was rebuilt? Is there a lot of slop in the throttle shaft? Generally throttle shaft looseness will cause other issues to but that or any other vacuum leak like from a rotted or broken hose can cause sputtering but would generally be accompanied by other issues.

When was the last tune up, a plug wiring that only breaks down when your stepping on the gas is technically a misfire but can feel like a sputter to some folks.

Feel free to post underhood pics, but your testing is what will find the problem in the end.

 

[Eighty5ov]

I’m very new to the truck so I’m just figuring it all out as I go. Driveway mechanic with no real experience so this all seems like a lot ha ha. So start with the carb then ignition?

 

[Paladin]

It's not a Holley!!!!! Get a Holley!!!!!

 

[SirRobyn0]

I’m very new to the truck so I’m just figuring it all out as I go. Driveway mechanic with no real experience so this all seems like a lot ha ha. So start with the carb then ignition?

Well I wouldn't go tearing in to the carb. but checking to make sure the accellerator pump is working is a good first step. If it's not getting that it will sputter for sure.

I didn't mean to throw an over whelming amount of testing at you in one post, but I figure you can use it sort of as a list when you get to working on it.

It would help me to help you if you could answer the questions I posted though, as it would help me give you some direction. Mainly how does it run? Does it idle ok? Does it only stutter on acceleration only? Does it stutter at cruse on the road?

 

[Eighty5ov]

She runs and idles well. When I bought it the high idle and idle screws were out of whack, but when I do give it a kick it sputters.

 

[Eighty5ov]

If I ease into the gas it doesn’t sputter

 

[Bextreme04]

Post pics and maybe a video of the issue happening if you can. Sputtering on initial idle kick could be from a few things. Did you adjust the idle speed, high idle speed, and idle mixture screws at all? What do the plugs look like?

 

[SirRobyn0]

She runs and idles well. When I bought it the high idle and idle screws were out of whack, but when I do give it a kick it sputters.

If I ease into the gas it doesn’t sputter

Post pics and maybe a video of the issue happening if you can. Sputtering on initial idle kick could be from a few things. Did you adjust the idle speed, high idle speed, and idle mixture screws at all? What do the plugs look like?

I agree with all of that. A video would be helpful, because it could be a few things, but in general your description makes me thing it is more likely to be in the accelerator pump.

 

[Eighty5ov]

I adjusted the idle and high idle. High idle was set to 4000 and base idle 1700 it was crazy. I have to tune it to vacuum was my next project as well with changing the spark plugs. The truck was sitting 12 years in a barn.

 

[Bextreme04]

I adjusted the idle and high idle. High idle was set to 4000 and base idle 1700 it was crazy. I have to tune it to vacuum was my next project as well with changing the spark plugs. The truck was sitting 12 years in a barn.

Like @SirRobyn0 said, it does sound like an accelerator pump issue. I would adjust the idle mixture using the lean drop method, no vacuum gauge needed. Make sure timing is set right first. Then get it nice and warmed up and verify that the choke has pulled off. Then set base idle speed to about 650-750. Then adjust one idle screw 1/4 turn at a time until you hit max RPM. Adjust your idle rpm as you go so that it stays within 50 rpm of your start RPM. Once you hit the peak RPM, screw in the idle mix screw until the idle comes down 50-100rpm. That will be a lean-of-peak idle mixture. Then repeat the procedure for the other side. Once you have that set, you will need to wait for the engine to fully cool, then give a single pump of the pedal to the floor and slowly release. This will set the choke and put the throttle on the high idle cam. Start the truck and set the initial high idle speed to about 950rpm. This will slowly climb to about 1200-1300rpm as the engine warms and the choke pulls off.

 

[Eighty5ov]

Like @SirRobyn0 said, it does sound like an accelerator pump issue. I would adjust the idle mixture using the lean drop method, no vacuum gauge needed. Make sure timing is set right first. Then get it nice and warmed up and verify that the choke has pulled off. Then set base idle speed to about 650-750. Then adjust one idle screw 1/4 turn at a time until you hit max RPM. Adjust your idle rpm as you go so that it stays within 50 rpm of your start RPM. Once you hit the peak RPM, screw in the idle mix screw until the idle comes down 50-100rpm. That will be a lean-of-peak idle mixture. Then repeat the procedure for the other side. Once you have that set, you will need to wait for the engine to fully cool, then give a single pump of the pedal to the floor and slowly release. This will set the choke and put the throttle on the high idle cam. Start the truck and set the initial high idle speed to about 950rpm. This will slowly climb to about 1200-1300rpm as the engine warms and the choke pulls off.

I will give that a try and let you know with video.

 

[Eighty5ov]

And I don’t know how to do the timing. I’ll have to find someone.

 

[Bextreme04]

And I don’t know how to do the timing. I’ll have to find someone.

Buy a timing light with tach. You'll need it for timing and for doing your carb adjustments. Timing a SBC is easy to do, but you will need the light. Do a google search and watch a youtube video on it. It is a 5 minute job once you have the light

 

[Bextreme04]

Here's a good one:

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