Sudden Idle Issues with Quadrajet

[dz1087]

So last night after packing up the kiddo from baseball practice, I cranked the truck, let it sit in fast idle for about 60 seconds or so and gave the throttle a tap to take it out of fast idle. Drove it to the end of the drive for the baseball field (about 70 yards). When I came to the stop sign, the engine sounded like it was about to die. Idle was horrible, and RPMs were low enough that my lights were dimming. Gave it some throttle and it picked up and ran fine on the way home, except at stop lights where I had to keep some throttle in and brake with my left foot.

Drove perfectly fine on the way to practice.

I immediately thought vacuum leak, so I looked at all my hoses once I got home. Did not notice anything amiss. This carb was rebuilt by QuadraJet Power just 1500 miles ago. I had it adjusted to run fine in the truck and was happy with the performance of it. Then this happens all of a sudden. Any ideas?

Specs:
stock rebuilt 350
stock rebuilt quadrajet
stock replacement fuel pump
fuel filter before fuel pump

[Mts67C10K20]

Double check the carb mounting bolts
Could be sucking air at tha base
Also brake boster hose

[68Gold/white]

Could be some trash in the needle/seat. It can happen, even with a filter on it...

[fatbass]

It may not be the carb. Verify your ignition is working properly first.

[dz1087]

Originally Posted by Mts67C10K20

Double check the carb mounting bolts
Could be sucking air at tha base
Also brake boster hose

I'll spray it down with some carb cleaner after work today and see if I get a hit on a vacuum leak. None of the usual suspects were there when I looked last night.

[dz1087]

Originally Posted by 68Gold/white

Could be some trash in the needle/seat. It can happen, even with a filter on it...

Which needle are you referring to? The idle mixture screws?

[dz1087]

Originally Posted by fatbass

It may not be the carb. Verify your ignition is working properly first.

My initial thought when it started acting up was that my vac advance hose had come loose again. It was in place though. Engine acts normal (power/response/sound) once it is off the idle circuit though. I'll definitely check the lightning hoses and the lightning whirler though to make sure they are working appropriately.

[Indyuke]

I had the same issue with my 1970 q-jet. The problem came on gradually over several weeks. I would stop at a light and the engine would start to "load up", bog down and run slower... sounded like it was struggling harder and harder to keep running. Turned out the float was fuel logged and heavy.

[Corts60]

Sounds like it is most likely a vacuum leak if it came out of nowhere....

[geezer#99]

Check the vac advance pot.
Could be blown.

[Steeveedee]

Probably ignition, unless you have an HEI, which would reduce the chances of ignition being the problem. Mark uses quality parts resistant to alcohol in the fuel, so the float being ate up by alcohol is probably off the table.

'68 Gold/White is referring to the needle and seat assembly that are controlled by the float. It is possible that some small piece of crud got in there. With the elastomeric seal on the needle, crud (chunks of dirt, oxidized gasoline, etc.) could get stuck there. It wouldn't show up during cruise, but would affect the idle by loading (not necessarily flooding) the engine.

Back in the day, if crud got stuck on the needle (unless it was all metal- yes, I'm that old) we would give it a "six-second tune-up" by revving the engine while holding a shop rag over the carburetor. That would cause a lot of fuel to be drawn through the carburetor, potentially stirring up any crud in the float bowl and running it into the engine, to be burned. It didn't always work, but when it did, you were a hero to the customer, having saved them some money. These trucks are in the era where that kind of thing actually worked. With fuel injection, that kind of thing is completely off the table.

[Getter-Done]

Do you have HEI ignition?
Rotor Button bad?






.

[lil hoodlum]

My guess a piece of debris or trash has lodged into the needle and seat area also.

[HO455]

To help troubleshoot the problem I suggest hooking up a timing light and viewing the spark. Is it steady? Or is it erratic, or jumping around? This should help determine if you have a fuel or spark problem.
Does the exhaust smell rich? Then the float may have sunk.

Good luck.

[dz1087]

Originally Posted by Steeveedee

Probably ignition, unless you have an HEI, which would reduce the chances of ignition being the problem. Mark uses quality parts resistant to alcohol in the fuel, so the float being ate up by alcohol is probably off the table.

'68 Gold/White is referring to the needle and seat assembly that are controlled by the float. It is possible that some small piece of crud got in there. With the elastomeric seal on the needle, crud (chunks of dirt, oxidized gasoline, etc.) could get stuck there. It wouldn't show up during cruise, but would affect the idle by loading (not necessarily flooding) the engine.

Back in the day, if crud got stuck on the needle (unless it was all metal- yes, I'm that old) we would give it a "six-second tune-up" by revving the engine while holding a shop rag over the carburetor. That would cause a lot of fuel to be drawn through the carburetor, potentially stirring up any crud in the float bowl and running it into the engine, to be burned. It didn't always work, but when it did, you were a hero to the customer, having saved them some money. These trucks are in the era where that kind of thing actually worked. With fuel injection, that kind of thing is completely off the table.

So I went out and cranked it up last night after I got back from work. Went in to fast idle fine, sounded normal. However, it also sounded normal right before this problem manifested so I wasn't holding my breath. Let it sit in fast idle for about a minute, then tapped the accelerator to take it out of fast idle. I fully expected to have it die almost immediately. Nope. Idled fine.

I think what you mentioned about the six second tune up may have been inadvertently caused by the fast idle circuit. Is that even a thing - could the fast idle circuit have cleared a bit of debris from the needle seat?

Anyway, drove it in to work this morning with no issues whatsoever. Idled fine the whole time, even in some stop and go traffic.

[dz1087]

Originally Posted by Getter-Done

Do you have HEI ignition?
Rotor Button bad?
.

Yes, HEI. Rotor, Distro, and cap are all new with under 1600 miles on them.

[dz1087]

Originally Posted by HO455

To help troubleshoot the problem I suggest hooking up a timing light and viewing the spark. Is it steady? Or is it erratic, or jumping around? This should help determine if you have a fuel or spark problem.
Does the exhaust smell rich? Then the float may have sunk.

Good luck.

I did hook up my timing light after I got it to running heat and the idle was where it was supposed to be. Ignition looked fine. Very steady. I did not unhook the vacuum advance, but even with it on, it was very steady.