q-jet help

[cudaman]

I have a quadrajet in a 69 corvette that has sat idle for maybe a year. I just bought it, but ostensibly, it ran fine before storage. It now starts hard, idles rough, and has a small backfire when the throttle is blipped. It won't rev over approximately 2500 rpm and runs roughly at all rpm.

I checked the ignition and valve timing-both appear to be correct. Compression is consistently ~125 psi, no vacuum reading yet (lent gauge to a friend).

The thing acts like plug wires are switched (those check okay). It also has classic lean backfire symptoms. However, the plugs get fouled (black and wet-like it's rich).

I've been out of the carburetor game for a long time, so, is this just a case of a carb that is gummed up from sitting too long?

[cudaman]

In addition, I watched the intake valves through the cycle and they don't appear to be sticking.

[MrQuick]

a year isn't very long but that would depend on the fuel used.

with the car not running, remove the air filter, open the choke (you will need to hold open or do it with a hot choke) cycle the throttle and obeserve the accelerater pump stream. Should be an instant, nice and steady stream.

Pumping the torque pedal with a dry carb can kill the pump diaphram (especially plunger type) and will cause a lean accel issue aka off idle stumble. When we start a stored car I like to either prime the carb with a little fuel down the vent tube and then slowly cycle the throttle a few times till I get a fuel stream or start it on ether then cycle the throttle after it fills ensuring that it gets well lubricated.

Also look for flow. A sticking needle seat can cause a hard start from too much flow or not enough fuel. At idle you can use a mirror and flashlight to pear down the carb venturies to look for puddling or large droplets forming around the venturies. You should see no flow at idle.
The floats get saturated on those carbs so inspect it if you can.


If the plugs are wet and dark I alway like to replace them. Its a cheap reassurance. I have seen alot of guys chasing their tails only to find they had fowled plugs. This is after they swore that they have cleaned them and 4 + hours of failed trouble shooting.


Vince

[cudaman]

I get nice, steady squirts of fuel when cycling the throttle. I cleaned the plugs and got it started, after which I warmed it up for about five minutes. The plugs are black and sooty already. I have a new Edelbrock performer carb that's never been used, so I'm just going to pick up a new matching intake and replace the stock induction along with the plugs. I don't have the time right now to chase down carb problems, so well just see how that goes. A new intake is cheaper than a new carb. I'll update later. Thanks for the reply.

[HILROD]

I had this exact problem with a Q-jet. I won't tell you everything I changed as the car ran perfect when parked, but it was a lot! It was all in the carb. There was tiny corrosion flakes that clogged up the metering holes. I ended up taking it apart a few times. When I got mean with a blow gun it finally blew some junk out of the tiny holes. Good luck.

[4mul8ion]

It's been a while myself for QJet diagnosis, but IIRC, the Qjet metering holes were prone to clogging and varnishing after long periods of sitting as Hilrod mentioned. I believe the quick fix to this was to run the carb dry and let the engine stall so that the metering holes would fully unseat. Then reattach the fuel supply and have the fuel pressure clear out the metering holes. Its been a while so I'm trying to find the source of this.

[David Pozzi]

Does this car have ignition points? If the dwell is off it will run like that. Q jets are also known to have the float absorb fuel and get heavy which raises the fuel level, causing rich running. If you see fuel dripping from the booster nozzles at idle, the float level is too high.

Is the choke staying on? the choke unloader can leak and fail to pull the choke open. An easy test is to pull the hose off, compress the arm sticking out of the diaphragm, then cover the vac inlet with your finger. Release the arm and it should only push out a little, if it pushes out fully, the diaphragm has a hole in it.
David

[MonzaRacer]

Well if its stock for car look at jet well plugs beginning to leak. pull it off, get a can of carb cleaner flip it over and pull the base off, give it a spray a cleaning then let dry.
Use some 320 grit wet/dry paper and scuff the jet well plugs, mix up some JB weld and seal them up also look at the smaller plugs closer to the front on the idle channels.
It doesnt take much, , while your there if you have the cash buy an Echlin kit from NAPA and a good float, spray it out good and pay attention to the movement of the power piston and make sure the metering rods are ok.
Now if you can find the plug over the mid range adjustment screw, pull that, and make sure its free.
if its free, then you can adjust the midrange mixture as it moves the power piston stop up and down.
Its not hard just time consuming, either the newer gas has degraded the float causing it to sink lower now or the jet well plugs are leaking.
Good Luck

[cudaman]

I have to say I'm appreciative of all the help offered. However, I guess I can't really try any of the suggestions at this point, as I've already removed the entire induction and replaced it with new Edelbrock stuff. I still have to hook up cooling and vacuum lines tomorrow, so I'll advise on the results. Thanks again for the replies, fellas.

[MrQuick]

that a boy!!

vince

[cudaman]

Problem solved. Runs like a sewing machine now.

The fortuitous thing is that my brother gave me a new 600cfm Edelbrock AFB a couple of years ago and I thought I would never use that thing because I've fuel injected all of my other cars. So for $150 I got a whole new fuel delivery setup, instead of whatever a new q-jet costs with the stock iron intake. I remember thinking to myself, "You dope, I'll never carburete anything again, except for maybe a BBC."

There is lesson there somewhere, grasshoppers.

Thanks again.

[79PonchoUK]

The Q-Jet is probably a better carb though...and it was probably just a bit of dirt in the jets.

If your edelbrock stuff is new though, it's probably a better bet anyway, though I'd consider rebuiding that Q-Jet.

[xxxturbo6]

Don't mean to highjack this thread but If anyone needs some QJ carbs, I have two sitting here and would be happy to send pic's or tell ya the numbers off them.

Simply PM me if your intrested. Hope you get it squared away.


Scot W.