Hi Lars, they tell me you are the quadrajet guru around here

I just bought a 79 Vette, 350 245 HP replacement motor (stock) no cats, true 2.5" duals with shorty headers.

The guy had just had the quadrajet totaly rebuilt by a repuable shop. Including a used carb and kit, choke pull off, loks like a new throttle base too ( not sure if that was in bill or was with used carb), and labor he paid $250.00.

Here is my problem(s)

1. the thing is extremly cold blooded, i have the choke (electric) adjusted correctly i think. it closes when cold, pull open on start about 1/4" and takes about 2 mins to open fully.

2. it has a really bad off idle stumble (bogg), you get past 2300 -2400 RPM and it starts to really run good. the stumble is right where and just past where your cracking the throttle.

3. stalls at lights. It's AT, what sould i set idle at?

I notice that the accelerator pump doesn't squirt any gas til throttle is open 1/4", you think thids is the stumble? Seems like it to me.

I have not tried setting the idle mixture yet.

Any advice will be greatly appreated, Thanks

Keith

 

Look for the thread lars posted about his Q-Jet tuning docs.

There's a web site you can grab a copy off of. They are really detailed.

Should be extremely helpful!

 

http://www.bokonon.net/corvettefaq/engine.asp

did the work for you :p:

ZD

 

Dude, save yourself some time and send your carb to Lars now. I had mine "rebuilt" by the best carb shop in town. They also did everything wrong.

 

Keith -
Sounds like the rebuild may not be quite up to snuff on your Q-Jet. This is not unusual, and i see a lot of funny things done to the Q-Jets by people...

Your accel pump should discharge fuel upon the slightest movement of the throttle. If it is not squirting fuel immediately upon cracking the throttle open, you have something defective with the accel pump, and this will certainly cause a stumble for you. I have seen a lot of defective accel pumps in kits lately where the rubber on the pump is incompatible with the oxygenated fuels. This causes the rubber to swell, and the pump gets sticky in the pump bore. I've also seen this caudse the rubber to tear, making the pump inoperable.

The choke adjustment sounds correct, but if the carb is not jetted right (too lean), it will not perform well when cold, even if the choke is working. you might want to check the jetting against the carb number and see if it's set up to the stock specs. Let me know if you need help with this.

You should be able to set the idle at about 700rpm in drive. If the metering is correct, it should not stall. You can play with the idle mixture screws a little to see what happens: Start with the mixture screws 2 full turns out from fully seated, and then turn them each 1/2 turn at a time to see if idle quality improves when you screw them out or in from the 2-turn setting.

There are other things internal to the carb that can also cause your problems, such as incorrect float setting and plugged idle air bleeds (a common "performance" mod). If you need help, please contact me. I can also take a look at the carb for you and set it up right if you'd like.

 

Thanks Lars,

I saw the guy who rebuilt it, he said to move the rod to the other hole on the acel pump.


I will check the things you mentioned , thanks you very much.

Thanks for your Q-jet paper, i Also DLed some of your other papers, q-jet, HEI etc, thanks for taking the time to write this stuff for us


Keith


[Modified by Racer78, 5:11 PM 10/8/2001]

 

Keith -
The pump will discharge fuel with the slightest throttle movement regardless of which hole the pump rod is in. If it's not discharging fuel, there is a defect. The inner hole will give you a longer, richer pump stroke, while the outer hole will lean you out just a tad. But both holes should produce an immediate pump shot with instant fuel discharge upon the slightest movement of the throttle.

 

Lars,

I snugged down the carb mounting bolts,and reajusted the choke as per the book, turn counterclockwise til just closed, then back to open 1/16".

After that it was acting real lean, spitting, and stalling then i moved pump rod to inner hole, it's a ton better.

I'm letting it cool down and try it cold again later.

You think my pump piston could be defective as you mentioned before?

I still gotta copy down the carb number and see what i have.

Thanks.

Keith


[Modified by Racer78, 11:58 PM 10/8/2001]

 

Lars,

Got the carb number 7045228

I looked it up in your paper: 75 400 CI Federal AT chevelle/Montecarlo

is federal as opposed to CA? I see the fifth diget (2) is for 49 states.

The stock main jet/rod is 71/47

My car is running a lot better now, the accel pump squirts on the slightest crack of the throttle.

Also i picked up the Haynes book on Rochester carbs, added to your paper, I sould be learning a lot

Keith


[Modified by Racer78, 12:45 AM 10/9/2001]


[Modified by Racer78, 12:46 AM 10/9/2001]

 

Keith -
You're already putting the tech info to good use - good job!
Yes, "Federal" means 49-state (non-Calif.). Also, keep in mind that GM interchanged quite a few of these carbs: Just because the actual Rochester application was for a Monte Carlo does not mean that the carb could not have been installed on a Vette.

On your choke: the choke pulloff should pull the choke open about 1/4" as measured from the forward, lower edge of the choke plate to the choke wall. If it's much less than this, you'll be running rich, and the car will sputter and choke down as you describe. You might want to play with this a little...

Keep us posted on your tuning progress, and let me know if you need assistance.

 

Thanks Lars,

The choke pull off is pulling it opena good bit, will check measurement tommorow whwn cold.

I also installed a new 195 deg thermostat, it warms up a lot faster now.

When i crack the throttle it is nice and crisp now, no sputter or spitting.

Tommorow AM when she is completely cooled off i will update you

Thanks again

Keith


[Modified by Racer78, 1:35 AM 10/9/2001]

 

Lars,

Ok the choke pull off is pulling it open about a 1/4" as you said.

It has gotten really cold here last few days (this is part of it), but this AM after warmed up good it was still stumbling and spitting back throught carb and acting really lean.

If i held my hand over the carb or closed the choke about 2/3 it was fine and real crisp.

Yet the plugs said rich (new ones today AC R45TS).

Compression was 155 worst, 170 best.

Could timing be a bit too advanced, didn't check it yet, but one time when starting it hot, itkicked back like to much advance.

The guy who did the carb said the pri jets were 71.

I fiddled with idle mixture screws, he had them like 4 turns out, i went to 2 1/2 turns. It cured it i think, if went in a hair would stumble and spit, back out a lil and good.

I turned curb idle up a tad 700 RPM in drive. Now in drive and foot on brake i can punch it, no bogg or spit and let right off and no stall.

1. If i wanted a good accel pump where and which do i buy. It does spray a good shot now and squirts even on slightest crack. The venturies are pulling equal amounts of fuel as you open up.

2. Where does one get jets and metering rods?

Being transistion from idle to off to cruise i would imagine fine tuning the m-rods would be best right?

Thanks

Keith

 

Keith -
Before messing with the carb, make sure your timing is set up right. Timing problems can seem like carb problems on occasion, and trying to cure a timing problem by playing with the carb is a difficult thing to do indeed.... Do you have copies of my timing papers and the Q-Jet Tuning paper? If not, let me know, and I'll get the stuff out to you. The Q-Jet paper has the info on jetting parts with part numbers and sources. Your 71 jets sound about right for your car, and I don't think you need a new accel pump if your current pump is giving you an immediate pump shot. The thing that concerns me is that your car was running fine initially, then suddenly something went wrong. Jets and rods don't just go bad, so you had something else happen to you that needs to be identified and fixed. Trying to fix a weird problem by doing some creative jetting is just going to mask the actual problem. You may have a vacuum leak from the sounds of it - perhaps a broken hose or a loose hose. Check everything out really well before you get into the carb...

 

Lars,

I have your paper and jet chart, thanks, very good stuff.

It had this problem before i got it, it was masked by a choke being set wrong.

The preveous owner said it was worse after he had the carb rebuilt.

Acutally it had a Holly (was leaking and had no choke) and the garge took it off and rebuiit the Q-jet that is on there now. The owner said the old Holly had more zip. I have the Holly. He paid these guys over $250 for the used carb and rebuilt, i have the bill, a brass float was included. Paid all that money and was not right!

I think i have it set about right now, it is running much better, also the weather may have effected it, getting so cold. I will check the timing. All the hoses look ok. i need to find my vaccum gauge and will do some vaccum tests then.

Appreate your help

Keith