hyprr1

Hey guys, I wonder if anybody can help me out with my problem?
From the day I picked up my car (last year Aug. 2016) it’s had an inconsistent idle problem.
I’ve done a complete tune up;
Wires, plugs, cap, rotor, ignition module, air filter…
I also replaced most of the vacuum hoses & tightened some loose bolts on the intake and a few on the carb.
(sprayed carb cleaner all over and no increase in idle)
I had a friend help me out with adjusting the idle at 600-650 in drive and about 800 at idle. Within a couple of minutes, the rpms are jumping up and down
inconsistently & the engine sometime violently shakes…..
Also hooked up a vacuum gauge and the needle bounces up and down between 15-20 psi

(my headlights work very well)

Can anybody help?
Thank you

speedreed8

And the year of your car is ???? What engine? Carb. Brand size?

hyprr1

It's a 1980 L48
Quadrajet completely stock with all smog and emissions crap still
hooked up


(the spark plug wires have been replaced by original equip. not accel)

speedreed8

I would start with looking for a vacuum leak. Check your timing. You may have a faulty throttle position sensor. And or O2 sensor.

hyprr1

my Vette's an 80.
I don't have O2 sensor or throttle pos sensor

NewbVetteGuy

I have the same problem but all 40 year old vacuum lines and a very obvious leak in the headlight circuit.

I'd definitely look for vacuum leaks and as they're so dang pesky- take it to a place that's got a smoke machine. Just save yourself lots of hours of trying to track this one down and get someone to use the professional smoke machine to find the leak / leaks so you can fix whatever $5 part is causing this aggravating issue and spend your time on something more fun.


Adam

NewbVetteGuy

I don't mean to hijack your thread, but I've had a question for a while and your picture is perfect to ask the question: What is the silver duct going from what looks like your exhaust manifolds to your cold air intake? That's part of the exhaust gas recirculation, right?

(Mine is missing and that bottom "pipe" on the snorkel is just open to the engine bay- I had headers installed- should I get some sort of plug to plug that up or is it ok to just leave it open to the air?)


Adam

hyprr1

I'd just leave it open!
Or
Maybe remove the pipe and seal it underneath to give it a cleaner look from above....
Just make sure the flaps stay open in your air filter housing.

Gunfighter13

Did you check your EGR Valve and/or controller? Fuel plugs on bottom of Qjet may be leaking?

midigike

Or check to see that the fuel pump is still good. When you start car runs ok but hit the gas and it sputters?

raydog9379

It's the carb pre-heater hose. You can buy them at a parts store or Amazon. I went the Amazon route because I wanted a stock black look vs. the silver shiny look. Expect to spent a whopping $8-$15. Only could find shiny at the parts stores here. There are 2 small hose clamps that hold it as well. You can find those clamps at most of the vendors here.

I stole this info and stored it from someone from a while back:

"That is called the Thermac system in GM nomenclature. It's purpose is to ALWAYS deliver at least 100*F air into the carb by mixing hot air from the heat stove and cold air from the regular snorkel opening. There is a thermal vacuum switch built into the base of factory GM air cleaners to modify the amount of vacuum sent to the "flapper door servo" on the snorkel for this reason- to get the proper mix of hot and cold air to make it 100*F inside the air cleaner.

It serves TWO purposes, actually, neither of which probably matters to you, but definitely matters on factory vehicles.....

1. It is to help with emissions. Carbs do a better job supplying a consistent A/F ratio if the intake temp is held constant. Carbs can't compensate for air temp changes very well. It's not a big difference, but when you have to pass Gov't emissions testing like the OEMs do, every little bit helps.

2. The REAL reason. Preventing carburetor icing. This is a phenomenon that happens between about 30* and 50*F. Any moisture in the air (humidity) will condense on cold parts, much like fogging a mirror. This is not a problem unless the part they condense onto is COLDER than 32*F. Then that condensation turns instantly to ice. The throttle plates on a carburetor get VERY cold due to the fuel being atomized and evaporated in their vicinity (evaporation is a cooling process, if you remember high school physics). When there is sufficient moisture in the air and temps are within spitting distance of freezing the throttle plates can start to get a buildup of ice on them! This buildup slowly starts to choke off the airflow past them, especially at idle when the throttle opening is very very small. Idle speed goes lower and lower and lower as more and more ice builds up. The engine can actually stall in extreme situations! And this phenomenon will persist until either the air going through the carb gets warm enough to stop it or engine heat penetrates into the carb and makes the throttle plates too warm to allow ice to form.

It takes a LONG time for engine heat to conduct up into the carb. Heating the incoming air can be done MUCH faster with the Thermac (intake air heating) system. Which is the main reason why it existed even before the "emissions era" of the 1970s.

Do you need this on your modified vehicle? No. Not unless you daily drive in cooler weather. Then it can be handy. But even on a daily driver, you can just feather the throttle a little on cold mornings until the carb and intake heat up enough to prevent carburetor icing. In short, it's no biggie."