Q-Jet and Electric Choke

[fine69]

You could adjust the fast idle screw located on the passenger side of the carb. It's down low tucked in under the choke/fast idle linkage. But 1000 rpm is pretty close to where I might leave it.

Bill

Hey Bill,

THANK YOU for the info... I never knew that a fast idle screw was located there! My car running like crap, and I'm having a tough time setting the idle.

I've been using the screw that's under the throttle linkage in an attempt to adjust the idle.

Ralph

 

[Evolution1980]

"Would there be any benefit to removing the stock dual snorkel air cleaner"?

After thinking about the pros/cons of the dual-snorkel setup versus an open element filter...I'm really starting to think that if you want cold air induction ("CAI") keeping the dual snorkel setup is the way to go. As it is now with my car (open element), all the air it pulls in is the hot air under the hood.
As always, the only way to be sure of performance difference is back to back dyno runs, one with the snorkel setup and the other without.

 

[fine69]

Jeff,

Sorry for the late reply.

The only way to adjust how high the fast idle gets is like Bill explained. And the easiest way I know is to use an Lisle carburetor adjustment tool. The one I have has a flexible shank, so it will get around piping or other obstacles. It comes with adapters (sort-of-like a tool socket concept) that adjust fast idle, idle mixture, and even throttle position sensor.

Also remember that the turning of the choke cap adjusts how long the choke will remain "ON" for the spring mechanism to release the choke lock-out. That is why when you kick the accelerator, and the spring has not expanded long enough, then the choke won't come-off and the engine could still remain at the fast idle speed (but there are also other "steps" carved into the lever to allow a somewhat lower rpm in between a cold choke spring and a hot choke spring). :crazy

Anyway, seems like you have a handle on the issue.

Have you ever experienced an "a-ha, that makes sense" moment? I just did --- thanks Gerry - very helpful.

 

[Vette79]

EVO, I know we touched base on this before but I heard again that open air element will provide increased air flow which might be better. But I agree that the dual snorkel setup is a CAI which is good and what you would want.

 

[Vette79]

OK guys. I got the choke working but now after the Vette is warm I have to floor the gas pedal for the choke to kick out or disengage. Anotherwords, when cold starting, the Vette will idle high at 1400 RPM's then when warm I'll hit the gas pedal to kick the choke down to the next level at about 1000 RPM's. After driving for a bit and when fully warm it will idle at arounf 800 RPM's. For some reason the choke seems to want to stay engaged at 1000 RPM's. I have to hit the gas pedal a few times in order to bring it back down. What do I need to adjust to resolve this issue?

 

[Evolution1980]

When cold starting, the Vette will idle high at 1400 RPM's then when warm I'll hit the gas pedal to kick the choke down to the next level at about 1000 RPM's. After driving for a bit and when fully warm it will idle at arounf 800 RPM's.

That's exactly the way my car normally is. The reason, so I've been told, that mine idles aroun 1000 RPM is that my TV cable is putting enough stress on the throttle geometry that it doesn't fully release (800 RPM). The difference of 200 RPM hasn't been enough to cause me to bother trying to get it absolutely perfect.

You might also simply have a tach that reads high. If you can, get an external tach monitor and see if it jives with with your dash tach reads. Mine reads about 150-200 RPM higher than actual. If it's the same on yours, that could be your 200 RPM difference that you're noticing.

 

[Vette79]

EVO, the tach is fine. It's definately related to the adjustments that I am making to get the choke to function again. Think I will make some adjustments later and see what I get. I'll let you know. Thanks...

 

[Bill75]

Jeff,

I've found that the choke mechanisms must operate very smooth without binding or obstruction of any kind to work properly. I've had mine apart several times until I've gotten everything to work freely. With it working properly now, I experience the same thing you do BUT I don't have to wait very long for the choke to totally disengage, sounds like yours is either not able to work freely causing it to stick OR it might need to be leaned out just a little.

I would make sure the linkage that sets the fast idle operates properly. It's located where you set the fast idle speed, just above it. I used some carb cleaner on all the pivot points making sure they were clean. Don't put any lubricant on them, it just collects dirt. Look at everything that moves, clean and operate it by hand to see that it's free.

You could also lean it out just a little to see if that allows it to drop out sooner. Mark where you have the circular plastic piece set now, loosen those 3 screws and turn it to the lean direction a little. It's more critical than you think as to where that choke butterfly is set so the carb will operate properly.

Bill

 

[GerryLP]

OK guys. I got the choke working but now after the Vette is warm I have to floor the gas pedal for the choke to kick out or disengage. Anotherwords, when cold starting, the Vette will idle high at 1400 RPM's then when warm I'll hit the gas pedal to kick the choke down to the next level at about 1000 RPM's. After driving for a bit and when fully warm it will idle at arounf 800 RPM's. For some reason the choke seems to want to stay engaged at 1000 RPM's. I have to hit the gas pedal a few times in order to bring it back down. What do I need to adjust to resolve this issue?

Jeff,

From what you have described about your carburetor and Bill's picture of his set-up, it appears that you have a 4MC Q-Jet. Being a Corvette, then it could be a M4MC Q-Jet.

You might want to check your adjustment again. I am suspecting that perhaps you are not on the highest step when adjusting the fast idle screw. If, for example, you adjust the fast idle on the wrong step of the fast idle cam follower, then you won't reach the 2,000 - 2,200 rpm fast idle speed that is "typical" for these carburetors. Also, being on the wrong step will prevent the engine fast idle rpm to drop to curb idle. So, it could be as simple as adjusting your fast idle speed again, but this time ensuring that you are on the highest step for the cam follower.

I apologize, but I could not find the fast idle setting details for your 79 Vette. My AC-Delco book only goes back as early as 1981. But when you find them on your shop manual, it should say on which step of the lever the adjusting screw should be and what the fast idle speed is set at.


While opening the throttle with one hand (engine off), reach under the choke housing, and carefully feel the travel of the cam follower. You should be able to feel the lever bumping or following the lever steps, so make sure that it is on the highest (last) step. Now, take a look at your throttle and notice how far it is from the curb idle adjusting screw. If it is a 3/8 " or more away from it, then the fast idle screw can be back-out a bit to get this reference gap a little closer (turned counter-clockwise from the screw head's perspective). The relatively large gap between the curb idle screw and the throttle could equate to fast idle speeds above the 2,200 rpm setting. Its nothing to worry about. We're just trying to avoid unnecessary rpm levels.


Take notice of the gap between the choke plate and the air horn. At the highest lever step, there should be a small gap between the choke plate and the air horn. The air gap affects how rich your fast idle will run. Right now, the choke plate should NOT be touching the air horn. The shop manual will tell you what the rod gage size to use. Typically, it is 0.120". A drill bit this size or near this size will work too. So to adjust, turn the choke cap to accommodate the rod/drill bit shank. Some shop manuals will have you bend the rod for the choke plate, but since your are still in troubleshooting mode, I would recommend not bending anything until you get a feel of what the choke mechanism is doing. Just f.y.i..

MAKE SURE TO NOT DROP THE ROD/DRILL BIT IN THE CARB. AFTER GAP IS CHECK, KEEP ROD/DRILL BIT AWAY FROM ENGINE.


If the gap is good and with the throttle still set with the choke engaged, start the engine. Notice RPM. If higher than 2,000 - 2,200 rpm, then back fast idle screw until you get the speed you need. Turn the screw clockwise if you need to increase the fast idle speed.


Be cautious about where your laying the tools, and keep your hands away from the moving pulleys, belts, and fan. Get in the good habit of blocking the Vette's wheels in at least two locations (moderate engine horsepower) or all 4 wheels (higher output engines). Wearing eye protection will prevent injury should the fan blow small objects towards your face.


If the speed is good, you'll notice also the choke plate opening-up. When it is completely open, then this is the warm setting. Move the throttle as if you were just kicking the gas pedal to release the choke. The engine should return to curb idle or near it.

Sometimes it takes more than one adjustment to get it right.

 

[glen242]

After thinking about the pros/cons of the dual-snorkel setup versus an open element filter...I'm really starting to think that if you want cold air induction ("CAI") keeping the dual snorkel setup is the way to go. As it is now with my car (open element), all the air it pulls in is the hot air under the hood.
As always, the only way to be sure of performance difference is back to back dyno runs, one with the snorkel setup and the other without.

Most/all dyno runs I have seen are with the hood open and a LARGE fan blowing through the front (radiator) of the car. Would that not screw up the performance difference of the dual cold air intake vs the open element when run on a dyno?

 

[Evolution1980]

Most/all dyno runs I have seen are with the hood open and a LARGE fan blowing through the front (radiator) of the car. Would that not screw up the performance difference of the dual cold air intake vs the open element when run on a dyno?

The hood is up because they have the cables that read spark and whatnot coming out of the engine bay. There's nothing to say they couldn't gently set the hood down to get a more accurate comparison. If anything, having the hood up probably helps the numbers on an open-element because it's not getting as much hot-engine air.
It's a good question...

 

[TMPJoe]

Most/all dyno runs I have seen are with the hood open and a LARGE fan blowing through the front (radiator) of the car. Would that not screw up the performance difference of the dual cold air intake vs the open element when run on a dyno?

No not really

in both cases the fan simulates the car moving at speed and getting air flow.

The cold air is an issue. It seems most people think that having more surface area to ingest air means more air will be flowing. But CFM is what we are talking... and that is speed. So ducting from the front where the air is moving faster is important. Same as having a cowl hood and ducting air where it hits the windshield and can be drawn in! (Hood scoops cut into the hood suck hot air out... they do not flow air in another often misunderstood item. A raised scoop, like pro-stock racers does allow air to be sucked in)

If you look at the stock air intake it truly is a cold air system. Youwould imagine there is a restriction over having an open element cleaner under the hood, but open elements suck in nothing but hot air and the carb can only draw at a certain rate no matter what. (600cfm carb only flows 600cfm, etc.)

There is just not a lot of air flow in the engine compartment. How could there be? have you seen your motor after a hard run in the rain? was it soaked? of course not as the air flow into the radiator, some sucked through by fans and the rest hits the hood and is almost nil to the back part of where the engine is sitting.

I unfortunately am running an open element due to the intake and the carb set up I have. That is definitely costing me HP... I am working on an air intake that would be similar to stock but the tubes would be bigger and the inlets would be positioned better. One difference is I run the BECOOL 700hp radiator and twin electric fans so I actually have some other possibilities for the inlets....

I would love to see a set up work so all of us can benefit from cold air...I will update as I progress... i am coming off my achilles surgery and should be able to work on this in September finally!