10-02-2022, 04:01 PM | #1 |
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Dies at Idle
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 |
10-03-2022, 12:26 AM | #2 |
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Re: Dies at Idle
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. |
10-03-2022, 09:29 AM | #3 | |
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10-03-2022, 10:10 AM | #4 |
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Re: Dies at Idle
Is this a recent problem or has it plagued you all summer?
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10-03-2022, 08:36 PM | #5 |
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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 |
10-03-2022, 08:58 PM | #6 |
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Re: Dies at Idle
What’s your initial timing?
And where’s the vac advance hooked to? |
10-03-2022, 09:09 PM | #7 |
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10-03-2022, 09:19 PM | #8 |
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Re: Dies at Idle
Correction on initial: 13*
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10-03-2022, 11:23 PM | #9 |
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Re: Dies at Idle
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? |
10-03-2022, 11:26 PM | #10 |
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Re: Dies at Idle
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. |
10-04-2022, 12:10 AM | #11 | |
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10-04-2022, 12:17 AM | #12 | |
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10-04-2022, 01:05 AM | #13 |
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Re: Dies at Idle
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/ |
10-04-2022, 01:19 AM | #14 | |
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10-04-2022, 11:19 AM | #15 | |
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Re: Dies at Idle
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But with your initial symptom list, a fuel flow problem was one of the things I thought of first. |
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10-04-2022, 12:22 PM | #16 | |
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