Q-Jet conversion

[Mertz]

I have a 72 3/4 ton with 350 that I have been running with an Edelbrock EPS manifold and Edelbrock 1406 carb. I wanted to get better gas mileage so I switched to a stock aluminum EGR intake manifold and a Jet stage II Q-jet. It idles fine and I had it at 20" Hg. The problem was I had no power when I hit the gas from idle or a slow roll. I could get it up to speed and it would work better. The last time I drove it it would act like it died when I floored it but after I pumped it it would run again. I was told I might have a internal leak at the intake manifold but found no oil in the manifold. I am using a 1" phenolic spacer.

I bought the carb used and the secondary linkage was sticking but I freed it up. Could this be to much carb for the motor or does this sound like the secondaries aren't working? The guy said he had it on a 318 but took it off before selling the car. It looks brand new.

[oldcarfart]

Originally Posted by Mertz

I have a 72 3/4 ton with 350 that I have been running with an Edelbrock EPS manifold and Edelbrock 1406 carb. I wanted to get better gas mileage so I switched to a stock aluminum EGR intake manifold and a Jet stage II Q-jet. It idles fine and I had it at 20" Hg. The problem was I had no power when I hit the gas from idle or a slow roll. I could get it up to speed and it would work better. The last time I drove it it would act like it died when I floored it but after I pumped it it would run again. I was told I might have a internal leak at the intake manifold but found no oil in the manifold. I am using a 1" phenolic spacer.

I bought the carb used and the secondary linkage was sticking but I freed it up. Could this be to much carb for the motor or does this sound like the secondaries aren't working? The guy said he had it on a 318 but took it off before selling the car. It looks brand new.

it sounds like a bad accelerator pump, cheap part, easy fix.

[Mertz]

It sounds like it is working properly but i will check it out.

[BCOWANWHEELS]

X2 on the accel. Pump in the carb. Hard to get a good used carb......

[truckster]

Accelerator pump or the vacuum adjustment on the secondaries.

[ChevLoRay]

The original accelerator pumps for these carbs had a leather piston. Later replacement parts are composite material for longer life. You can go to a NAPA store and buy a pump all by itself....or you used to be able to do it. A person may have to buy a complete kit nowadays.

[mclairmo]

Convert to a Holley. Then, you will have something.

[BCOWANWHEELS]

if it was me i,d trade it in for a rebuilt q-jet then you know what you have. OREILLY'S has genuine holley remaned q-jets.

[Mertz]

The carb is from Jet Performance and has already been gone thru so I thought it was ok. I will pull the pump and see what it is like. I think I might also loosen the secondary spring. I forgot to mention that when I let off the gas it has a pop out the exhaust. Could that be a lean afterfire?

[mclairmo]

Problem with a used carb is that many times owners will try to "tune" their carb and will mess it up. Also, even a new carb can get gunked up real fast if no fuel filter is used or if the gas tank is filled with crap or if it sits unused for a long period of time. Since your carb is used, all these factors are unknown. You may want to buy a Q-Jet rebuild and tuning guide and tear it apart and clean it up and verify that all adjustments are correct and that all is healthy inside.

[Sport/Truck]

Another option would be to take it to a shop that can tune it on your car or a test engine.
Or call Jet and see what they can do. Note when calling Jet: act like it's the greatest and you really want it running the best.
Nobody wants to hear complaints and you'll get better service.
s/t

[Mertz]

I have turned out the APT 2.5 turns and it made a big difference. It no longer falls on it face but still has a bog between 35 and 45 mph. I adjusted the secondary air valve to 1/2 turn and it made a small difference. I reinstalled the AU secondary rods. I come off the line great and above 50 it accelerates great. It is just very sluggish at 35 to 45.

I also did a test with the accelerator pump disconnected and it ran about the same as with it. I also noticed some fuel at the top of the pump rod. I am going to change the pump. It must still be running rich if it doesn't need the pump to get off the line. The plugs are dark brown.

Lars Grimsrud has told me that with a modified carb there is no baseline to start from and I need an O2 sensor to get it right. I might have to get a bung put in so I can install a sensor.

[Hard Luck]

If you step in it and the thing wants to die, it's too much fuel. With Q-jets that can be very likely because those secondarys are so large, look at rear rods and jets sizing. Also, it came off of a 318 so the vacuum signal is a bit different. Always recheck the float setting, could be too much fuel in the bowls leaking into the venturis. You would be supprised how many cars are over-carbed and run ok but once you dial it in right it is like you did a performance cam upgrade.

What we do not know is the size of the engine, any mods to it, and the size of the carb.

Always pull a used carb apart if it has been run. Even if you just put in a gasket kit. It may have been gone through but if there is junk in the there and it gets in between the rods and jets, same problem. Gum and varnish build up if it has been sitting for a while and don't nessesarly come off from running it.

I got a truck with a NEW carb on it. He had put it on two years ago and only put about 500-1000 miles on the truck since then. I dialed it all in but was having intermittent problems but decent drivablity. I pulled it apart to find out that some junk from the old tanks and lines had passed the filter and got everywhere. A $17 gasket kit, some cleaner, compressed air, and a few hours, and I was back in business ready to move on to the next project! Even though it was new, once it has been run, you need to check it out.

[mclairmo]

Yeah, these carbs get gunked up easily with all the little orifices. I always tear em down and clean em up and add new gaskets if they have been sitting a long time without being used or if they are just not running right. Cheap and easy to do and usually helps a bunch.

[Mertz]

The guy at Jet told me the jets were 74 and the rods are 41 but it turned out that the jets are 73 and the rods 40. I checked the APT adjustment he told me to make and found that the rods were all the way up in the jet and could not move down. He told me base was 5 turns out and I could go 3 more. I talked to a carb rebuilder and he told me the base is 3 out. He also told me that the phillips screw in the APT is wrong and should have a brass adjuster. He also noticed that the secondary lock out was missing. I found a few other things wrong. To say the least I am very disappointed in a carb that I have been told is one of the best I could buy.

At this point I am not sure what to do next. Hopefully some of the cleaning I did will work and I will be able to get it running right. The carb is a 17058204 off a 78 Vette with 350 AC AT that would have come with 77 and 52 jet and rod combination. I read on another forum that going to a 48 with the set up I have would work great. I am not so sure that going to a lean rod will work since I am running very rich with the rods all the way out of the jet. It runs great except for from 35 to 45 mph but picks up again after 50. The accelerator pump works fine. I am getting good squirt and it should be better after cleaning the ports. With the current setup the pump doesn't really do anything.

I think I have ruled out any manifold leaks but I am still not totally sure. That could be the reason I need to much fuel at the jets.

Any help would be greatly appreciated.

[Mertz]

I cleaned the Jet Performance stage II and got it running better but is now has a new problem.
Driving the truck until it gets warm it is fine but after it is warmed up it goes slightly warm from what it has been running. If I stay in light cruise it is fine but a little warmer than normal. If I stop and do a fairly aggressive takeoff it gets really hot. If I do a slow takeoff it is fine. I changed t-stats to one I know is working right and it made no difference. I took out my 1" spacer and it made no difference. The carb number is a 17058204 that originally came with 77 jets and 52 rods and a CH secondary rod. It now has 73 jets and 40 rods. Am I right in thinking the excess heat is a lean condition?

I am getting very good at taking apart the q-jet and putting it back together. I changed the jets and rods to 76 and 52. I also installed the taller spring in the power valve and set the APT to where is just touches the power valve with it in the down position. It ran good and had the best power and off the line take off that it ever had with the q-jet. It ran cool like it did with the Edelbrock for most of the trip. I thought I had it resolved then just when I was about finished with my loop it started to get hot when stepping on it. It also had a little afterfire when I let of the gas. A small adjustment fixed the afterfire but worsened the heating problem. After a short fly fishing trip I came back and did a few more adjustments and the truck ran cooler but still had a few warm points but not near as hot. The problem is that it ran like crap. It wasn't bad but it was not near as good as it was in the morning when I first installed the carb. I ran out of time to make more changes and test them so it will have to wait until tomorrow evening.

Some thoughts: I got more overheating when I raised the APT to keep it from leaning out. Doesn't make sense to me.

The overheating doesn't happen until I run it for a number of miles and the t-stat has gone through its cycles.

I have adjusted the mixture and the APT and both seem to have an affect on the heating problem.

It is possible that once the car is running good and making max power there is something restricting the water flow. The upper hose is soft but I don't think it is collapsing. The part of the hose near the t-stat is bulging. Bottom hose is hard and feels like it has a spring in it.

The carb didn’t come with the secondary lock out so I put one back on. I am not hearing the secondaries since I added the lock out. I looked at it after it was warm and it is disengaged.

I had none of these problems with the EPS manifold and the Edelbrock carb.

[Dan-K]

I had Jet rebuild a quadrajet for me and I had to send it back twice because it did not perform properly. Also the gentleman that runs the Carb. dept. was not real easy to deal with either. I will not give them anymore of my business or refer anyone to them.

[Mertz]

I wanted to give you all an update on my progress with the quadrajet. First I found a way to adjust the APT that is very simple and accurate. I use a black marker and mark the depth of the shoulder of the jet on the rod just above the top step then when I put it in (without the spring) I can turn out the adjusting screw until I see the black mark even with the top of the jet. This method works with whatever length rod you have. My final adjustments were to use the 73 jets and 43B rods with a taller power valve spring. The truck runs great. I will be fine tuning it with the O2 sensor when I get it installed.

The truck now runs better than it ever has with the Edelbrock and the sound of the secondaries kicking in is a real rush. I was doing 75 on my test run before I knew it. I got the wheels spinning on some gravel and they kept spinning when I hit the pavement. Shifts are smooth even with my shift kit. It idles great and I haven't even adjusted the mixtures screws yet. I set them at 3 turns out and have 18" of vacuum. I know I can get 20".

[Jim_PA]

Be careful with the wideband, you'll get addicted