Quadrajet High idle issues

[davidh86]

Well i have a late 70's quadrajet which had a gasket leak on the base gasket which when i removed noticed was just a gasket meant to be used inside the carb it self which the last owner just threw on the bottom instead of buying the two dollar gasket. So when i installed the proper gasket the engine wouldnt start until i re adjusted the mixture screws, the only issue im having now is the engine idles at 1500 rpm. I checked for vacuum leaks around the carb/manafold and found nothing. the choke opens properly so im considering a carb rebuild, but im not sure if it will help since the carb looks pretty clean. Any thoughts this thing has me stumped, is it possible their is a leak within the carb? oh and i tried the idle screw, its not even touching the plate at this point lol.

 

[achris]

Re: Quadrajet High idle issues

Are the primary and secondary throttle plates fully closed?

Chris.........

 

[davidh86]

Re: Quadrajet High idle issues

From what i seen earlier yes. IM worried the throttle plates arnt sealing properly, they seemed fully closed but when i sprayed some carb cleaner on the edges of the secondary plate it sucked it rite up. Im going to take the carb off the intake and let you know for sure when i get off work, should the plates be completely air tight to the point where no liquid should flow passed them when their closed (ill test it when its off the manifold)?

 

[achris]

Re: Quadrajet High idle issues

Not air-tight, but closed enough so you would have difficulty seeing any light passed the edges. The usual set up is when the plates are loose on the shaft, the shaft is jiggled so the plates are as tight as possible, then the screws tightened.

 

[davidh86]

Re: Quadrajet High idle issues

just did the light test and i can see alot of light passing one of the secondary plates like a few mm of gap along a 1/4 of the circle.

 

[makonnen]

Re: Quadrajet High idle issues

That will be a problem ... the secondary plates (the ones that are mechanically opened) should be completely closed. Otherwise, they will expose the secondary air valves to manifold vacuum and cause them to open letting more air and fuel (air valves linked to metering rods, as air valve opens, metering rods get pulled out of jets letting in more fuel).

But if this is a problem, you should be able to see the secondary air valves crack open if the engine is idling at 1500 RPM.

 

[Robert D]

Re: Quadrajet High idle issues

The quadra-jets are really finicky carbs. When tuned correctly, they are awesome....but people who are really good at quadra-jets are a rarity. I would suggest going to a Holley marine carb. Much simpler, easier for anyone to rebuild and tune. They are not as efficient as a quadra-jet, and the transition from 2 to 4 barrels is better in a quadra-jet....but that only really matters in your car, right? For the time and hassle of keeping a quadra-jet tuned, I personally don't think it's worth it. I'm running a Holley on my 5.7....bought it direct from Holley. (My quadra-jet was too corroded to even think about.) Holley told me they flow test each carb and adjust it for the application, and said 90% of the time you won't need to make any adjustment. (Like I really believed that? No.) Well, I bolted it on, dragged boat to the water, turned the key and it started. And it idled at 650 rpm, so smooth! 'Wait...it's cold?' So I took off the flame suppressor/air filter to verify the choke was closed and watched it slowly opening. Okay, the idle is too slow, it'll die when it comes off choke. Nope, it stayed around 650 rpm, choke was wide open, I blipped the throttle to idle down, and it settled at 625 rpm. Just purrs. I was impressed. I'll never fight with a quadra-jet ever again.

 

[achris]

Re: Quadrajet High idle issues

Errr, you really need to AVOID Holleys in a boat... Merc stopped using them in the late 70's and for good reason. If you want to replace, rather than repair, your carb, use an Edelbrock 1409...

 

[davidh86]

Re: Quadrajet High idle issues

Well thanks for your input guys, i just rebuilt the carb so i can at least say i tried, ill know if it even helped when i start it up in the morning. But ill more than likely just end up buying a new one since the old carb was pretty clean and i cant see how anything i did would have made a difference.

 

[makonnen]

Re: Quadrajet High idle issues

The quadra-jets are really finicky carbs. When tuned correctly, they are awesome....but people who are really good at quadra-jets are a rarity. I would suggest going to a Holley marine carb. Much simpler, easier for anyone to rebuild and tune. They are not as efficient as a quadra-jet, and the transition from 2 to 4 barrels is better in a quadra-jet....but that only really matters in your car, right? For the time and hassle of keeping a quadra-jet tuned, I personally don't think it's worth it. I'm running a Holley on my 5.7....bought it direct from Holley. (My quadra-jet was too corroded to even think about.) Holley told me they flow test each carb and adjust it for the application, and said 90% of the time you won't need to make any adjustment. (Like I really believed that? No.) Well, I bolted it on, dragged boat to the water, turned the key and it started. And it idled at 650 rpm, so smooth! 'Wait...it's cold?' So I took off the flame suppressor/air filter to verify the choke was closed and watched it slowly opening. Okay, the idle is too slow, it'll die when it comes off choke. Nope, it stayed around 650 rpm, choke was wide open, I blipped the throttle to idle down, and it settled at 625 rpm. Just purrs. I was impressed. I'll never fight with a quadra-jet ever again.

I keep hearing this over and over about Quadrajets, I'm not so sure why people feel they are so hard to tune. I have a feeling folks are just repeating what they heard from others without first hand experience.

Assuming the carb is clean and in decent shape which is a requirement for all carbs, tuning a Quadrajet requires using a small flat blade screwdriver on the idle ports (2), choke spring / cover, idle screw, secondary air valves. The latter three adjustments you set once and leave it.

I mean if people think holleys are better because they don't have a secondary air valve ... well QJets have mechanically opened secondary plates and an air valve that opens depending on air flow. Holleys have secondary plates that open from vacuum (or lack of vacuum) and no secondary air plates. Both require adjustments.

 

[achris]

Re: Quadrajet High idle issues

I agree. I've set up a few Q-jets, and as long as you take your time and do it step by step, there's no drama. I guess they have a few more parts then a Holley or a Weber, but that's about all...

Chris.

 

[Don S]

Re: Quadrajet High idle issues

You might want to disconnect the throttle cable from the carb and see if it then will slow down if you hold the lever to idle.

 

[Robert D]

Re: Quadrajet High idle issues

Wait till you have to try different metering needles...what a pain in the a** to deal with. Setting up a quadra-jet is easy to get it 'acceptable', but to get it where it really should be is out of the scope of most people. Once you get it where it should be, they really are wonderful. Nothing wrong with the newer Holleys in a boat. In fact, it should be considered a safer alternative, as there is less to go wrong, which equals less chance of your motor dying while you're entering a tight channel with jetties on each side, or the Columbia River bar!

 

[davidh86]

Re: Quadrajet High idle issues

well the carb rebuild worked out perfect the rpms went way down for some reason and its running great. Now i just have a crazy overheat issue that keeps randomly occurring, i opened up all the riser and manifolds and they were all clear checked all the hoses and they were fine and changed the impeller and housing and made sure the gaskets were all on the rite way. Im thinking its worth trying a new temp sender.

 

[achris]

Re: Quadrajet High idle issues

Use an infrared heat gun to check and see what the temperature actually is...