As you probably already know that I have had one heck of a time with my Q-jet. The question is this- I finally got the motor to smooth out but the air/fuel mixture scews are turned out 7 turns know. I had them out 4 turns are the motor seemed to sound like it didn't want to run and it seemed to develope a missfire. I also had to turn in the idle set screw almost till the spring was compressed. With the air/fuel mixture screws out 7 turns I was able to back the screw out to a reasonable position. Could I have a problem running the carb at this setting. Or should I pull it off and get it reworked. The carb was just rebuilt. Pleasa advise on what I should do as you (they say) are the one that knows. :cheers:

 

Something to think about. I have on many occassions counted half turns as full turns when doing things like carb adjustments, especially with standard flat blade screwdriver. Maybe, just maybe, something like this is going on?

Anyway, TTT, Lar's?

 

Joe -
7 full turns out on the idle mixture screws is too much. This is indicative that the carb is set up too lean, and you are not getting enough metering range on your idle circuit. This can be caused by incorrect main jets, incorrect primary rods for the jets in use, incorrect float level, engine mods inducing a lean idle condition (jetting not optimized for the engine mods), or an internal obstruction in the carb's idle metering circuit.

To start, can you give me just a little bit of info? I assume you're running an '80. Can you post the carb number off your carb and let me know what type (if any) mods you have on your car? Did this problem just start, or have you been striuggling with this for a while? Did it start before or after the carb rebuild? (Is this a carb that I rebuilt...?). The Q-Jet is very tunable when done in a methodical manner, and I can run you through the process if you provide me a litle info.

As a side note, make absolutely certain that your ignition system/timing/timing advance curve is set up properly before you start tuning the carb. Make sure your total timing is set up for 36 degrees total, and that this is giving you an initial timing somewhere in the 12-16 degree range. Make sure your vacuum advance is working and that it's hooked up to a ported vacuum source. Once you are certain of these parameters, and you still have a problem, we can start tweaking the carb.

 

okay, here are my simple questions, if the screws are turned out is it leaner? i have a 77 vette with a crate 454, duel exhaust with hedders and no cats, and k&n filter. i have a edelbrock preformer rpm ( the tag says #1910) the car runs at 600 rpm at idle, (1500 when cold) the mixture screws are set a 2 turns out and when you floor the car it bogs for a split second then takes off, also the car seems really sluggish when it has sat over night, am i runing to rich, could the choke be working incorectly? im really confused? thanks for any help :confused:


[Modified by ykp53, 12:30 PM 9/20/2001]

 

The idle mixture screws are like metering rods in a fuel metering oriface: When they are screwed INTO the oriface, they restrict and choke down the fuel flow; when they're screwed OUT, they open up the fuel bleed holes. Out is rich; in is lean. If you're set up at about 2 turns out, this sounds correct. I usually do my base settings anywhere from 1-1/2 turns to 2-1/2 turns as a good starting point. I have found that if you need to run the screws much further out than 4-5 turns that they are at their limit in terms of flow (once they're more than 4 turns out, the screws are no longer impeding the size of the metering oriface, and are therefore ineffective). Fuel for the idle curcuit is taken from the main metering circuit, so a change in main metering (primary rod/main jet combination) will affect the idle circuit. If the idle circuit needs more than 4-5 turns out on the screws, I usually recommend trying a jet change or a rod change to get the idle mixture screws into the 2-turn-range.

The installation of headers and a free-flowing filter tends to lean a carb out. If your carb is stock with the stock jetting, you can generally increase main jet size by 2 sizes as a good starting point any time you reduce exhaust backpressure or improve intake flow.

If you get a bog upon light, normal acceleration out from a stop light (not when you floor it), it is usually attributed to a lean primary system or a lean accel pump shot. If you only get the bog when you mash the pedal to the metal, it is usually caused by incorrect secondary airvalve windup spring adjustment as noted above.

I'm not sure what you mean by the car being sluggish after sitting overnight.... do you mean that it's hard to start? Is the car sluggish once started while it's cold? Or does it remain sluggish even after warm-up...? It is not uncommon for the fuel bowl on a Q-Jet to dry up overnight, making the car hard to start. Incorrectly set up choke systems are also quite common, making the car possibly run lean and sluggish while the engine is cold. If you give me a little info on this I may be able to point you in the right direction.

 

well the car starts after pumping the gas once or twice. but when you are driving down the street you can feel a surging feeling every few seconds. after about 5 min. the idle drops down and the feeling goes away.

 

What you're describing is indicative of a lean condition when the engine is cold. This is typical of an inoperable choke or a mis-adjusted choke. So first verify that your choke is working: When the engin is cold and off, depress the gas pedal. The choke plate in the air horn of the carb should snap closed. Completely. Now, pull the vacuum hose off the choke pulloff diaphragm (the "can" in the forward passenger side of the carb) and install a long piece of hose to it. Suck on the hose to seat the diaphragm. This should open the choke plate just a little. With the choke plate cracked open like this, softly push on the lower edge of the plate, just like the air passing over the plate would obviously push on it. With the plate cracked open like this, the distance between the lower (forward) edge of the plate and the forward wall of the air horn should be 1/4". You can easily measure it by slipping a 1/4" drill bit down between the choke plate and the air horn. If it's more than 1/4", you're going to be running too lean, and the condition you describe will occur. If your car is a mid-to-late '70s car, you can adjust this by screwing the adjustment screw at the top of the choke vacuum break linkage - there is a screw with the screw head facing back. If you have an early C3, you adjust this by bending the linkage where the secondary airvalve rod contacts the choke break tang on the linkage.

Contact me if you need further info.

 

Lars- The problems has occured ever since I rebuilt the motor. Heres what I have- 350c.i. bored .40 over, 9-1 pistons, comp cam XE262H, Late model vortec heads, Edelbrock performer vortec intake, flowtech ceramic coat headers into 2 1/2' flowmaster deltaflow mufflers. The carb number that I gave the shop that built it was 17080228. They inturn gave me this carb. Buy the way, this is the second carb I got from them, the first one I had the same problem. They told me to bring it back and they would exchange it. Oh and you didn't rebuild this one. I think if you did it I wouldn't be having this problem. I talked to a friend up here in Canada and he is willing to rework the idle circuit for me. Let me know what you think. P.S. The seven turns is not 1/2 turns. :chevy

 

Joe -
The carb number 17080228 is a 1980 Carb, and these carbs are normally jetted a bit lean on the cruise/idle side. This lean mixture is achieved by installing a main jet in the 71-72 size range and plugging it up with a running about 48 to 50. This gives you a very lean mixture at idle and cruise (for good economy and low emissions), but does not give you much idle adjustment range, especially with the type of engine mods you're running.

First, you need to check to see how the carb has been jetted. If you have jets in the 72-74 range, you should be in pretty good shape. If it's jetted anything lower than 72, you need to get some other jets in it. With the 72-74 main jet size, install a pair of main rods in the 43-46 range. This will richen you up a bit at idle and cruise, which is what you need with the headers and the other mods. This will give you much better adjustment range on your idle mixture screws, and should make things come alive for you.

Be aware on the rods: The late-70's & '80 carbs use different rods than the '60s and early '70s carbs. You can get the right rods from Edelbrock through your local NAPA dealer or through Summit. Make sure you get the "late" style.

 

Lars- Forgot to mention, the distributor is new and I have the timing set at 14 degrees. The funny thing is that when I first put this carb on the car would idle with air fuel mixture screws set at 2 1/2. It has got gone down hill from there. :cheers: