New Quadrajet Setup

[kpurser]

I have a 78 K20 with a 6.6liter, 400ci engine. Automatic trans. Everything on the engine is stock. I just pulled this out of storage and started working on it. First I made sure it stops. I redid all the brakes. Now I am trying to make it go. The engine had a Quadrajet. The truck ran at low RPM well. If i gunned it the engine would bog and almost die. The fuel filter on the carb was cross threaded and JB welded together so I opted to replace the carb instead of rebuilding it. I purchased an overhauled Quadrajet from National Carburetors. This is supposed to be flow tested and ready to install with some small adjustments. I installed the carb and now can't get the engine to run. When I turn over the ignition it fires and runs for about 2 or 3 seconds and then back fires out the carb and dies. I have played with the idle mixture screws from 1.5 turns out to 6 turns out with the same result. I received the overhauled carb with both idle mixture screws 4 turns out. The removed carb had them 2 turns out. I tried feathering the pedal to give a little squirt from the accelerator pump but it still dies. I am an aircraft mechanic but have no experience with vehicles.  I understand I may need to look at timing or fuel pressure, but it did run with the old carb so I thought it would at least start with the new carb. Can anyone help me with a setup process for a new Quadrajet?

[blazer74]

Call them up and tell National carb what’s going on.
There are mixed results with remanufactured carbs.
Hang on to the carb that was on it if you still have it at least until you resolve your issues.

Ran before with other carb indicates first thought is the carb or open vacuum line.

Assuming timing, valve train, fuel pump are in order.

Fresh fuel is first considering just out of storage.
What’s in the tank, was it running prior to storage?
Dirty fuel is going to cause havoc.

All vacuum hoses connected, in good shape or plugged if not used.

Correct base gasket.

Correct choke setting for initial start a must. Depending on outside temp  choke flap should just close on a cold engine.
Choke should open 1/8 to 3/16 on initial start via the pull off diaphragm.

Pics of carb would help to  identify what they sent you.
Is the carb the same as the original?
How is the choke connected to the engine.

The carb that should be on there would require 2-3 turns out mixture if set up correctly. Later model Qjets can be double that due to design.
Should run either way 3-4 turns out initial.

After cranking or failed start.
With the engine (OFF) look down the front barrels and actuate the throttle linkage.
You should see 2 streams of fuel from the accelerator pump.
If not your not getting proper fuel to the carb or there’s is a problem with the carb.
I assume the carbs not flooding.

Backfiring thru the carb is generally lean fuel mix, timing or valve train issue

I recommend Cliff Ruggles book on Quadrajets, covers everything.
There’s lots of info on the web on Quadrajets with detailed set up instructions as well.

Basics are the same in the aviation world, recip or turbine.
Fuel, air,ignition source, control.
You need that Maint Manual.

[Henry]

Hi K:
Before you changed out the carb, did you change or move the distributor? Did you disconnect or change anything else? What model Q-jet do you have?

Assuming everything else on the engine is OK, a rebuilt carb should allow the engine to run even if the mixture screws and idle screws are not set quite right. 2 turns out on the Q-jet is the documented starting point.

Make sure you have all the vacuum ports either connected or plugged on the front and pass side of the carb. Inspect your vac advance hose for leaks.

Regards,
Henry

[kpurser]

Fresh fuel is first considering just out of storage.
What’s in the tank, was it running prior to storage?

Fuel is old but has stabilizer in it. I will drain the tank and refill.

All vacuum hoses connected, in good shape or plugged if not used.

I have one vac line with nothing to connect to. Tried it with the line open and then plugged it with same result. the lined comes from a T-fitting. The other ends go to the thermostat and a connection on the passenger side of the carb. adjacent to the choke linkage.

Correct base gasket.

I used the gasket sent with the carb from National Carburetors. The gasket I removed was very thick, almost 1/4". The new one is a thin paper type but fits.

Correct choke setting for initial start a must. Depending on outside temp  choke flap should just close on a cold engine.
Choke should open 1/8 to 3/16 on initial start via the pull off diaphragm.

It was 60deg outside. I am working by myself so I cant see if the choke pulls open when I crank.

Pics of carb would help to  identify what they sent you.
Is the carb the same as the original?
How is the choke connected to the engine.

It is the same as the original. It is a 4MV
The choke is the coil type mounted to the manifold with a linkage rod to the choke assembly linkage.

After cranking or failed start.
With the engine (OFF) look down the front barrels and actuate the throttle linkage.
You should see 2 streams of fuel from the accelerator pump.

I did that and I do have fuel from the accelerator pump.

did you change or move the distributor?

no

Did you disconnect or change anything else?

no

Also, I had noticed the vac lines are in poor condition. I planned on replacing them before I did a final tune on the carb.

[Captain Swampy]

1/4" in height definitely changed your choke setting. Fresh gas will surely help.

[Henry]

Hi K:
From the information you have provided, it sounds like you had a problem with you old carb in that it ran, but hesitated on acceleration which indicates to me you had a problem with the power sub-system in the carb like the power valve or acc pump not working correctly...or maybe the float chamber not having enough gas. Anyway, assume for now that problem is solved as you have a new carb.

Your new problem is different and you have not changed anything else on the engine other than installing a new carb. So, I would assume that since the carb is new it is functioning OK and it may just be the set-up and adjustments that need to be done. Did those guys at National supply you with a adjustments and set-up sheets for your 4MV? They should have and here is what you need to set up for 1978 4MV with divorced choke on 400 engine:
1. Pump rod adjustment: 9/32 inch
2. Choke rod: 17/64 inch
3. Vacuum break: 7/64 to 9/64 inch
4. Unloader: 1/4 to 17/64 inch
5. Air valve spring adjustment: 3/4 to 7/8 inch
6. Air valve dashpot adjustment: 1/64 inch (yours may not have this configuration).
7. Mixture screws: 2 turns out initial setting
8. Idle screw: Back off till screw hits stop then turn in 2 turns to just open the throttle blades (initial setting, fairly fast idle). Does your carb have the idle solenoid attached to the idle screw?

The sheets should have illustrations as to where all the above adjustments are to be made. There will be a lot of illustrations and adjustments that do not apply to your engine so you need to make notes on all of the illustrations as to which ones you do and dont do after comparing to your carb. Even the 4MV series of carbs had some differences based on what engine they went on. As Blazer74 said, your divorced choke rod setting may be way off as your carb gasket went from the oem thick one to the supplied thin one, so this needs to be checked for sure. All the above mentioned adjustments are done on a cold engine and lets assume the internal adjustments done on the carb at National are good (for now). Insure all the linkage move freely without any binding.

Vacuum hoses: sounds like all your hoses are plugged or connected. Attached are how the hoses are connected on 1976 and earlier 400 engines...should be similar in 1978. Just make sure they are not cracked and leaking.

After all this, the engine should run and be ready for final tuning. If not, there is something else wrong. Give it a try and let us know what happens.

Good luck,
Henry

[kpurser]

The information you guys are giving me is really great, thank you. Last night I finally got time to work on it again. I adjusted everything out as described in your earlier post. The engine started and ran for about 8 seconds and died. I scrutinized over every detail I had done, got pissed and pulled the carb off. Put the old carb on and it started right up. Idles fine with the old carb. I'm gonna call National Carb today and say WTH.

Henry, thank you for the pics of the vac hoses. I wanted to make sure my hoses were connected correctly. I was looking for a diagram yesterday with no luck. The first pic you sent is exactly what I needed.

[Henry]

Hi K:
Well your test certainly seems to indicate there is something internally wrong with your rebuilt carb from National. Sounds like the float may be sticking down in the float chamber. Like someone else mentioned, dont let your old carb go yet...it may still be an option to rebuild it...being a aircraft mechanic you have the skills to do it.
Regards,
Henry

[kpurser]

So last night I was looking over both carbs and realized the gasket they sent is the wrong one. It covers an air passage in the bottom front of the carb. I should have heeded more attention to your comments on checking for the correct gasket.  So I put the new carb on with the old gasket and it ran. The gasket just leaks horribly. After work today I'm gonna go find a new gasket. I appreciate everyone's input and help. Should be able to get it on the road this weekend and see how it does. Get this adjusted in and it's on to the next project, a new radiator.

[blazer74]

If they can’t send you the right gasket for the carb, what else?

Always use the thick gasket.
4mv’s use its own style gasket not the same as the later model modified carbs.

What are they getting for a core charge on the old carb?

[Henry]

Hi K:
Assuming you get it to run and finish your tune up procedures, be aware you need to check leakage from the carb gaskets after a couple of heat cycles. With the low torque of the bolts and screws on the carb and all the gasket stack-ups, it is typical to leak soon after running. Re-check the 4 bolts and 9 screws on the air horn(throttle plate). The 4 bolts are torqued to 75 inch-pounds but typically need to be snugged a little tighter after a few heat cycles. The 9 screws are pretty much a tighten by hand with a straight screw driver as tight as you can go without damaging the screw head and carb body-keep in mind you are threading into aluminum threads here. After each heat cycle look and see if you have leakage; if not, you are done. If you do continue to have leakage, then snug everything up again (more than 75 inch-pounds) and recheck after each heat cycle. I got my 4MV to stop leaking after two snug-ups.
Regards,
Henry

[kpurser]

What are they getting for a core charge on the old carb?
They are not getting a core back because the carb I removed has the threads on the filter stripped and the JB welded together. That's the main reason I chose to replace rather than rebuild. I don't think it has enough meat to HeliCoil. Their core charge is 50 dollars but when I ordered on the phone I was talking to the guy about why I was replacing the carb and he took off 30 dollars.

Thank you for the advise on re-torque. I drove the truck around about 10 miles this weekend. I think I need to check my timing too. I will defiantly check the torque and check for air leaks too.

[Henry]

Hi K:
I think I remember quadrajetparts.com having a thread repair kit for the fuel filter nut.
Regards,
Henry