Dies at Idle

[Classics Fan]

Good NFL Sunday! I have a 1971 Jimmy with 35,000 miles on the rebuilt original engine. What I assume to be the original Quadrajet is on the truck. It doesn’t have a choke but does have factory manual throttle that I use for cold start warm-up. It’s been converted to HEI ignition.

My problem is with idle. The vast majority of the time, like 9 times out of 10, even after warm-up it will idle nicely at around 850 RPM (IIRC) for around a minute then slowly develop a stumble, and if there is no addition throttle input it will eventually die. It also has a slight off-idle stumble on acceleration. I have replaced the fuel filter, checked and set timing, replaced HT leads and olugs, and gone over the engine a number of times and am confident there aren’t any vacuum leaks in the plumbing. I sprayed the throttle shaft and carb base plate with Berrymans, but there was no noticeable increase in RPM to indicate vacuum leaks in those areas. The only possible thing that I can think of is a problem in the carb idle circuit. I have never worked on any type of carburetor beyond setting idle mixture, which, by the way I have done on this vehicle as well. Does anybody have any thoughts or ideas on where the problem could lie besides what I’m thinking? TIA for any input!
Posted via Mobile Device

[Dead Parrot]

850 for idle speed is a bit high. Try slowing to 700 or so. What is your timing set at with the distributor vacuum advance disconnected?

Was a non-resistor 12V wire ran to the HEI distributor?

Check the suction lines on the fuel system. A small hole from rot or mice can let air in and mess with fuel flow.

[Classics Fan]

Originally Posted by Dead Parrot

850 for idle speed is a bit high. Try slowing to 700 or so. What is your timing set at with the distributor vacuum advance disconnected?

Was a non-resistor 12V wire ran to the HEI distributor?

Check the suction lines on the fuel system. A small hole from rot or mice can let air in and mess with fuel flow.

I will look at these things. The HEI was already installed when I bought the truck in late 2016 and has always had an idle problem that got better after chasing vacuum leaks and tune-up but has never been perfect. I never thought to check for non-resisted power.
Posted via Mobile Device

[geezer#99]

Is this a recent problem or has it plagued you all summer?

[Classics Fan]

Originally Posted by geezer#99

Is this a recent problem or has it plagued you all summer?

This problem has persisted at different degrees since I’ve owned the rig (late 2016) but it’s inconsistent. Yesterday it idled beautifully when I took it out for errands. Today it wouldn’t run below what I would guess to be between 1200 and 1300 RPM (a guess since there’s no tach in the panel and I didn’t hook up my digital timing light today). One time for some unknown reason the misfire/stumble cleared, engine speed increased considerably to where it was idling beautifully for a few moments, then the misfire returned and it wouldn’t run without dying unless I fed it throttle. I can’t detect a misfire at higher engine speeds. In the past I have had a timing light on it when it’s idling poorly and ignition timing doesn’t change as it slowly dies, leading me to believe it’s fuel-related and not ignition.

I traced the fuel line back from the inlet of the fuel pump to where it disappears above the fuel tank. Along that span there are three short rubber sections that connect between the steel line. Two of those rubber connectors are cracked and need to be replaced, but there is no evidence of dampness, so I don’t believe they are a source of air being introduced. Nonetheless, they need to and will be replaced in the next few days.

Power at the HEI is 12.1V, so unresisted.

I’m stumped. A thought that occurred to me today is maybe it’s the fuel pump so unless someone has any better ideas I’m considering getting a fuel pressure gauge and checking outlet pump pressure.
Posted via Mobile Device

[geezer#99]

What’s your initial timing?
And where’s the vac advance hooked to?

[Classics Fan]

Originally Posted by geezer#99

What’s your initial timing?
And where’s the vac advance hooked to?

I will have to check my notes to confirm initial but IIRC it’s 10*. Has an RV cam so a hair above spec. Vac advance is connected to a manifold vac source on the carb.
Posted via Mobile Device

[Classics Fan]

Correction on initial: 13*
Posted via Mobile Device

[geezer#99]

Timing seems right.
Could be the needle/seat is the problem.
Sometimes people hang the clip on the needle onto the float wrong.
You’d need to pull the top off the carb to check it.
Or you could be getting something in the needle/seat causing erratic idling.
I had one that I found a tiny piece of wire that moved in and out of the needle and changed the idle like yours.

When was the last time you had the carb apart?

[Dead Parrot]

One possible vacuum leak is the power brake booster if you have power brakes. I had a booster on my C10 that leaked enough to cause idle issues but still worked well enough the problem wasn't obvious. Another often missed place is the small bit of hose on the modulator on automatic transmissions.

If you have an old style vacuum gauge, they often included a fuel pressure gauge as well.

[Classics Fan]

Originally Posted by Dead Parrot

One possible vacuum leak is the power brake booster if you have power brakes. I had a booster on my C10 that leaked enough to cause idle issues but still worked well enough the problem wasn't obvious. Another often missed place is the small bit of hose on the modulator on automatic transmissions.

If you have an old style vacuum gauge, they often included a fuel pressure gauge as well.

I checked the power brake booster hose assembly but totally missed the transmission modulator. Good tip. My vacuum gauge doubles as a pressure gauge, I will see if I can make that work. Thanks!
Posted via Mobile Device

[Classics Fan]

Originally Posted by geezer#99

Timing seems right.
Could be the needle/seat is the problem.
Sometimes people hang the clip on the needle onto the float wrong.
You’d need to pull the top off the carb to check it.
Or you could be getting something in the needle/seat causing erratic idling.
I had one that I found a tiny piece of wire that moved in and out of the needle and changed the idle like yours.

When was the last time you had the carb apart?

My carb skill is limited to adjusting the idle AFR so I’ve never had it apart. I recently bought a manual on rebuilding Quadrajets when I was suspecting it was the cause of my problem. Sounds like it may be time to pull the carb and crack the manual open. Any advice for a newby?
Posted via Mobile Device

[geezer#99]

Get the best carb kit available from Cliff Ruggles. A bit more money but the best out there.
Don’t forget the two screws buried inside below the choke horn. Long needle nose pliers needed to lift them out.
Go slow and easy.
The tiny screw for the secondary rods likes to go awhol.
Watch for the check ball for the accelerator pump well too.


https://cliffshighperformance.com/

[Classics Fan]

Originally Posted by geezer#99

Get the best carb kit available from Cliff Ruggles. A bit more money but the best out there.
Don’t forget the two screws buried inside below the choke horn. Long needle nose pliers needed to lift them out.
Go slow and easy.
The tiny screw for the secondary rods likes to go awhol.
Watch for the check ball for the accelerator pump well too.


https://cliffshighperformance.com/

Cliff’s manual is the one I bought. I’ll get the kit from him too. Your help here is appreciated!
Posted via Mobile Device

[Dead Parrot]

Originally Posted by Classics Fan

I checked the power brake booster hose assembly but totally missed the transmission modulator. Good tip. My vacuum gauge doubles as a pressure gauge, I will see if I can make that work. Thanks!
Posted via Mobile Device

Its not just the booster hose. The booster proper can develop an internal leak bad enough to cause idle issues. At the same time, it can still work well enough that you assume it is good. Quick test: With the motor running, push the brakes a couple of times to get the feel. Turn motor off. Leave for a couple of minutes. Then with the motor still off, press and release the brakes just like you did with the motor running. The feel should be the same and you should hear a hiss from under the dash. A harder feel and/or no hiss indicate a possible booster problem. A 2nd test but more work is pull the booster hose and cap the carb/manifold fitting. If your idle gets better, probably a leaky booster.

But with your initial symptom list, a fuel flow problem was one of the things I thought of first.

[Classics Fan]

Originally Posted by Dead Parrot

Its not just the booster hose. The booster proper can develop an internal leak bad enough to cause idle issues. At the same time, it can still work well enough that you assume it is good. Quick test: With the motor running, push the brakes a couple of times to get the feel. Turn motor off. Leave for a couple of minutes. Then with the motor still off, press and release the brakes just like you did with the motor running. The feel should be the same and you should hear a hiss from under the dash. A harder feel and/or no hiss indicate a possible booster problem. A 2nd test but more work is pull the booster hose and cap the carb/manifold fitting. If your idle gets better, probably a leaky booster.

But with your initial symptom list, a fuel flow problem was one of the things I thought of first.

That’s what I like about forums like these: I always learn something new. The challenge is remembering what I learned later when I need it again. Sometimes for me it’s use it or lose it especially as “maturity” creeps up. I’ll check to booster can to eliminate that possibility. Thanks for the education.
Posted via Mobile Device