Can anyone help me trouble shoot a 4MV Quadra Jet carb on my 350 engine. Problem is that I can't get it to idle (warm) at less than 1000 rpm.
When driving, the idle speed wants to hang up around 1300 to 1400 rpm when I push in the clutch for a stop. then it will slowly drift back down to around 1000 again is the car remains idling for a minute or two at a stoplight.
The engine is mildly warmed up with .294/.455 hydraulic cam, high comp pistons, headers.
I'm not sure if this signifies air leaks, sticking float or what. I'm not strong on carb tuning or repair so could use some help.
Thanks for any help anyone can offer.
:yesnod:

 

Could be a vacuum leak. Check the gasket between the carb and intake manifold. I had exactly the same problem when I put a QuadraJet on a small block, and it turned out that the gasket was incorrect and leaked at one spot.

Another source of air leak might be the vacuum lines that come into the carb and intake manifold.

Double check the settings of both the normal idle adjustment screw and and also the fast idle adjustment screw setting on the choke cam when the engine is warm and the choke is wide open. Note: Be sure the choke goes wide open and the fast idle cam rotates with it when the engine is warm.

You might also check the vacuum advance system on your distributor. With the engine idling, disconnect the vacuum hose from the distributor.....you should notice a definite downtick in engine speed when you pull it off (timing retard), and a definite uptick when you reconnect the hose (timing advance). If you don't, it could mean a faulty advance diaphragm or leaking hose.

Also, engine timing advanced too far can sometimes make it difficult to get proper idle speed.



[Modified by RestoVette, 12:17 AM 1/30/2004]

 

"You might also check the vacuum advance system on your distributor. With the engine idling, disconnect the vacuum hose from the distributor.....you should notice a definite uptick in engine speed when you pull it off (timing advance), and a definite downtick when you reconnect the hose (timing retard). If you don't, it could mean a faulty advance diaphragm or leaking hose."
The engine RPM should DROP off with the hose discconected and should INCREASE with the hose connected.
I would check to see if there is any binding of the secondary throttle plates.
As stated above check for any vacuum leaks.
I would also check the centrifcal advance mechanism. Check the springs to make sure they are not too light and that the weights are not binding.

 

Plaidside wrote: The engine RPM should DROP off with the hose discconected and should INCREASE with the hose connected.


Correct.....my bad...typo....I've corrected the post. Thanks.

 

You might also make sure that the throttle linkage is free at all times with both a warm and cold engine. Another thing to check is the operation of the hot idle compensator valve on the front top of the carb. It will open prematurely if its bimetal element is bent out of position.

Dyna

 

Thanks for the suggestions. I'll check these this weekend to see if I can isolate the problem. If not, it may be time for a rebuilt or a new unit. According to the numbers this carb is the original one on this '69 block.
It may be time for it to take a permanent vacation.

 

Q-Jet on a C-2 ? :nono: :)

Stick a Holley on that baby !

 

Red -
There are several Q-Jet issues that will cause the symptoms you describe. When properly set up, a Q-Jet will run very strong on your 350 - I've been running one on my 350 in my '64 for some time, and even the hard-core CCC guys are impressed with the throttle response and drivability of the car.

First, as the guys above suggested, make darned sure your timing and timing advance systems are set up right. I always tell people that "90% of all carb problems are ignition problems." Make sure you're set up for 36 degrees total advance, and make sure this is slamming all in by 2500-2800 rpm. On top of that, make sure your vacuum advance system is working and that you're getting an additional 16 degrees-or-so out of the vac advance (for a combined total maximum advance of 52-54 degrees). Make sure you're using a vacuum advance control unit that pulls its full advance all the way in at your idle vacuum. You can play with ported versus manifold vacuum depending on what your engine needs: my 350 idles best when using ported vacuum. Most engines will idle best using manifold, but it all depends on your setup.

Once you have the timing right, start playing with the carb. If you're still having idle problems, check your jetting/rod combination. Make sure you have the right jets & rods for the carb number and tune from there (I can give you the specs if you don't have them). Make sure your float level is set right at 3/8" (not 1/4" as the specs show). With this set, run your idle mixture screws out 2 turns from fully seated. Now, with the engine fully warmed up, drop the idle rpm down as low as it will idle and hold your hand over the choke air horn. If your idle speed improves and smooths out with a restriction, you're running lean (very common with a slightly warmed-over engine). Check your power piston lower limit height to make sure it's not dropping down too far (I can give you a spec on this) and then drop your primary rod size by 2 sizes.
Tag me back if you need the jetting specs on your carb. Also, I have a complete "Q-Jet Problems" tech paper that you may find interesting. Drop me an e-mail if you want a copy and I'll send it to you.
V8FastCars@msn.com

 

:iagree: stick a holly on that baby-and can the quadra :U , really, think about it. The quadra jet has small primaries for economy, large for power. Hey you're driving a performance machine. Besides quadra pukes are to tempermental. But there is a place for them-I have two on my boat, where not mpg's but gallons per hour are a factor and economy is a must. I'll probably get fried for this-but oh well.

 

lars, thanks for the info ! Other than the idle issue this carb runs strong on this engine. It'll still snap your butt back in the seat when the secondaries kick in ! I think it's tired and needs a rebuild though. If I deciphered the number correctly (7043208) it's a '73 4mv from a Chevy passenger car.
I've pulled the carb and have it at a rebuild shop here in the area that is supposed to specialize in fuel systems on earlier iron.
I'll pass your info on to them to help in adjusting the carb after rebuild.
Thanks again
Red '66