Likely all your carb problems are just a lack of a correct timing curve!
Where do you live in the world? Elevation and climate are tuning factors.
What's your initial timing?
How much mechanical?

 

I'm in Missouri. Elevation is about 1100 feet.


I haven't messed with initial timing, all I did was set the total timing. Just disconnected the vacuum advance, held the engine at ~3000 and set my dial back timing light to 35 BTDC and adjust the distributor. I just let the initial fall where it will.


I've never done much with distributors, do you set the timing curve using springs and weights?


What initial timing should I be at?

 

Dig out your timing light and see what your initial is.
Then adjust from there.
You should be able to run at least 14 initial.
If it runs better then try a couple more degrees until you find the spot the motor likes. That spot would be when it starts good when hot and doesn't ping.
Once you figure out your optimum initial then you can modify your distributor timing. Leave the vac advance plugged until you're done your timing mods.
Here's a link or two. First on timing.
Where should I set the timing on my performance engine?
Second on modding your hei.
http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor

 

Dig out your timing light and see what your initial is.
Then adjust from there.
You should be able to run at least 14 initial.
If it runs better then try a couple more degrees until you find the spot the motor likes. That spot would be when it starts good when hot and doesn't ping.
Once you figure out your optimum initial then you can modify your distributor timing. Leave the vac advance plugged until you're done your timing mods.
Here's a link or two. First on timing.
Where should I set the timing on my performance engine?
Second on modding your hei.
http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor

Thanks for the help. I'll check out my timing tonight or tomorrow and see if that helps. I'll post back with the results.

 

What psi is your fuel pump putting out? Qjets don't like much more than 5psi. I just put a regulator on mine because the parts store mechanical pump was pegging my gauge at 10psi. Helped alot!

Also, once you have your timing right, trying gently turning in your idle adjustment screws until it starts to want to die and back them off a hair. Do them the same, one at a time.


I'd also suggest, Cliffs High Performance Quadrajets :: Qjet Carburetor Rebuild Kits, Parts, Quadrajet Rebuilding, Quadrajet Parts, Bushing Kits, Carb Tuning , if you havent already been there. Cliff Ruggles is THE authority on quadrajet carbs. He has a very detailed book you can buy or you can visit the forums over there and get some help.

 

i had this problem with a q-jet that i just rebuilt...


if i left it in gear or idle it would load up after 45---90seconds ( very very gradual )

the truck would smell rich and fuel mileage wasnt what it had always been.

i took the carb apart and it was something " grit/paste/tank crud " that wouldnt let the seat close off the fuel system...

1 can of carb cleaner and a few blasts of 120psi air, all has been well ever since


truck is daily driver ( fwiw )

 

Be sure you are idling on the idle circuit and not on the primary.Also needle valve adjustment. Here is a link that may be educating.Rochester Quadrajet Tech Info - The Carburetor Doctor

 

I have a 1979 Corvette with a Quadrajet carb. SBC 350. I also changed the gaskets in the carb. I was on 74 main jets before the rebuild.


When I started it up and hooked up my Wideband 02 sensor, it showed I was running very rich (10:1 A/F ratio), plugs were black and sooty.


I have my choke disabled and the choke flap is locked open.


I have 15" of vacuum at idle. My float height is set at .25" (1/4").


My idle has been acting weird, it will idle really high even with the idle screw turned all the way out. But then when I'm driving it and clutch in and step on the brakes to stop, it tries to die.

Any thoughts?

yes.. carb number please..

at what engine speed is it 10 to 1. at idle... slightly above idle to 1200 or
1300 RPMs.. 1400 to 3000... before the secondaries throttle plates start to open in the base..

at idle.. the system is running on the idle circuit ONLY not on the primary main circuit so jet changes do nothing. to any engine speed below 1200 or 1300 rpm this is all dependent on engine displacement.

mid primary throttle .. note that the primary rod holder is still seated on the APT adjusting screw.. this is cruising speed..

when you tip into the throttle.. the primary rod holder will come up off its stop as the manifold vacuum drops.. enriching the primary main circuits..

if the primary rod holder is adjusted too far down.. you will have a lean or a rich off idle as the vacuum pulling the rod holder down against the stop will either be too far up or too far down. this is why i ask about what speed you are having 10 to 1 on your wide band ..


this is wide open throttle with the primaries and the secondaries open and the engine speed enough to pull the secondary air doors wide open. note the primary rod holder is all the way up.. enriching the primary circuit..

are you sure you got the proper gaskets installed there are several very easy to put the Wrong ones in..

you could also have slightly too much float level.. you could have leaking plugs in the bottom of the main body.. in either the primary wells or the secondary wells.

if your idle speed is set too fast.. you could be exposing too much of the idle transition slots. you may have this because of your cam... or a vacuum leak. you could also have a lack of designed in vacuum leaks. or bypass air.. wrong PCV valve.. out of place throttle blades.. they do get banged off center when the carb is off. i usually have to spend 5 or 8 minutes when i have a qjet apart just on adjusting the throttle blades to recenter them.


edit... i would take the carb off.. take out the long 2 screws from back of the top cover.. flip it over and take off the 2 or 3 base plate screws.. you can flip the base plate around to unhook the throttle rod easily... spend 10 minutes recentering the throttle blades.. do not loosen the screws more than 1/4 turn they will break off.. check the throttle shafts for play also..

 

yes.. carb number please..


are you sure you got the proper gaskets installed there are several very easy to put the Wrong ones in..

you could also have slightly too much float level.. you could have leaking plugs in the bottom of the main body.. in either the primary wells or the secondary wells.

if your idle speed is set too fast.. you could be exposing too much of the idle transition slots. you may have this because of your cam... or a vacuum leak. you could also have a lack of designed in vacuum leaks. or bypass air.. wrong PCV valve.. out of place throttle blades.. they do get banged off center when the carb is off. i usually have to spend 5 or 8 minutes when i have a qjet apart just on adjusting the throttle blades to recenter them.


edit... i would take the carb off.. take out the long 2 screws from back of the top cover.. flip it over and take off the 2 or 3 base plate screws.. you can flip the base plate around to unhook the throttle rod easily... spend 10 minutes recentering the throttle blades.. do not loosen the screws more than 1/4 turn they will break off.. check the throttle shafts for play also..

Thanks for the info. Very helpful. My carb # is 17059216 . I don't have the date handy at the moment, but I looked up the carb # and it's from a 1979.


I'll double check my gaskets tonight and take a look at the throttle blades.


At idle (currently like 1200-1300 RPM) it has about 13.5:1 A/F. As soon as I start driving it in gear it drops down to under 12:1 and if I give it a little throttle it will peg at 10:1 (the lowest my gauge will read)

 

Should have went with the smaller camshaft. Now you need to tune, tune, and tune some more to get it right.

 

Ok so I fixed the rich condition, I DID use the wrong gasket. Switched to the correct one and now the car has about 17:1 A/F at cruise and light throttle and 14.5:1 at WOT. So I'm assuming I should jet it up a bit?

Also, I set my total timing with vac advance plugged to 36 degrees which gave me 19 degrees of initial timing. However now the engine feels slow. It was definitely faster before, when it was running rich. It accelerates smoothly, but not very urgently haha. Something tells me that just jetting it up a bit to get the A/F ratio down to the 13.5 range isn't going to be that big of a difference. THoughts?

 

Ok so I fixed the rich condition, I DID use the wrong gasket. Switched to the correct one and now the car has about 17:1 A/F at cruise and light throttle and 14.5:1 at WOT. So I'm assuming I should jet it up a bit?

huge hint..

check the air fuel ratio at idle... and each rpm held steady for 15 or 20 seconds so you can tell which part of the carb to tune..

EDIT .. try to do this with a video camera running and a vacuum gauge next to the air fuel meter.. so you can record both as you call out the numbers... you will still need to deal with the APT to set the primary only air fuel ratio between 1800 and 2500 rpms with enough manifold vacuum to pull the primary rod holder down..


below 700 RPMs..

at 1000 RPMs..

at 1400 rpms

at 1800 rpms

at 2200 rpms..

at 2500...

at 3000..

continue edit..

once you get these as stable. then you can play with apt adjustment.. that is crazy easy on some quadrajets..

 

after you get the steady throttle air fuel ratios in...

perhaps just going back to close to the stock jetting..

do you have the APT plug out of the top of the carb.. you only need to reinstall the plug or put in a 1/8 pipe plug to return to normal service so you don't splash fuel out of the carb top..

you can use one of the larger valve core tools for farm tractor and off road equipment tires to do the adjustment..

you will also want to create a Dipstick out of plastic.. Zero of the 1/32 scale starts 39mm from the tip..

you will want to find the minimum spot where while you are pushing down lightly on the dipstick that the APT screw starts lifting the primary rod holder. you may want to start the additional adjustments about 1.5 turns up from just touching..

have you ever seen a drag racer after the burn out blip the throttle and do a part throttle take off that only moves the car 6 or 8 feet.. this is what you are going to do many times...

keep turning it upward 1/4 turn at a time.. back up.. drop it in drive and blip the throttle.. you will find that it will backfire and spit up thru the carb.. that is expected.. back up.. give it another 1/4 turn up.. blip the throttle again... still spits.. back up give it another 1/4 turn up.

what is happening is the manifold vacuum pulls the primary metering rod holder down.. if the APT screw is too far down.. as the carb moves thru the idle transition/low speed circuit and the primary circuit starts to flow.. too low a setting leans out the air fuel ratio and causes it to spit and backfire thru the carb..

keep doing this 1/4 turn at a time..you only need 40 feet of room to do this..

you will be totally amazed when you get it right. you will get to the spot where it does not improve... you may want to lower that a 1/4 turn.. you can fine tune the APT adjustment at 2200 RPMS steady throttle on a level road.

this can make or break your love affair with the quadrajet.

 

Hey guys I'm back, was on vacation for the last week so I didn't get to work on the carburetor much.


What I've discovered though is that the mixture slowly gets richer and richer. Even if I use the 69 jets, the first couple times I put my foot down the A/F ratio will be around 14:1. After a few times of doing this it will be down to 11:1 and even my idle A/F will drop to 13:1.

 

do you have compressed air. and an air pressure regulator on the line so you can turn the pressure down to about 2 to 4 PSI. . even a fish tank pump will do this..

take the carb off... take the carb apart.. find the main well plugs in the bottom of the main body... spray some bubble soap or just dish soap on them.. blow air up into the main wells.. see if you get bubbles .. you should not.

tiny leaks have plagued quadrajets and others.. the entire bottom of the main body on some is exposed to manifold vacuum. these leaks will not be a vacuum leak.. they will be UNCOMMANDED FUEL directly into the intake..

please verify by placing the main body to base gasket on both sides to verify it covers everything .. this is just a verification..

please with the base plate off.. hold it up to the light.. make sure that your throttle blades are properly centered.. not hanging open. i am hoping you are aware that you can loosen the throttle blade screws 1/4 to 1/2 turn but not more without breaking them off in the throttle shaft.

this might take you 5 or 10 minutes.. just keep playing with it.. it gets easier and you will realize what an improvement it makes.

by the way... open the primary throttle slightly.. wiggle the throttle shaft with the blades off their seats.. you should not have much if any movement ( O) or (O ).. it should be tight.. (O).. worn throttle shaft bores are an issue. as the throttle blades will never come back to the exact same position.

remember... the idle mixture is controlled by the IDLE MIXTURE SCREWS.. unless you have the idle speed up high enough to over expose the idle transition slots.. then depending on the heat built up in the carb.. will make micro variations to the throttle position.

please also verify that your throttle cable or linkage is NOT holding the throttle open part way.. there must be a little slack..

these are all issues i have run into..

when you get it all back together..

AF at idle.. at what idle speed..

AF at 1200 RPMs STEADY THROTTLE after 15 seconds at that fixed speed..

AF at 2000 RPMs.. steady throttle after 15 seconds at that fixed speed..

AF at 2500 RPMS.. steady throttle after 15 seconds at that fixed speed..

Get the primary jetting done first.. secondary tuning is tricky on quadrajets.. there are many sizes of motors that you might want to use that secondary metering rod.. and try various hangers.. various secondary air door spring tensions to allow the secondary to come in earlier or later..

there are some things that happen with the secondary... and it requires load on the engine to get any results in the secondary..

have you a APT screw like in some of the images i posted..

 

Is it possible that you are sucking something into the air cleaner or carb body and cutting off air flow when you floor the accelerator? Do you have something like a foam filter that might be pulled up against the carburetor top?

Bruce

 

You need a fuel pressure gauge plumbed into the fuel line, preferably near the carb inlet, like one of these.https://www.summitracing.com/search...rtOrder=Ascending&tw=fuel pressure guage&sw=Summit Racing� Fuel Pressure Gauges

 

with the harborfreight fuel pump /vacuum tester .. those read from 15 pounds of pressure to 30 inches of vacuum.. you can just dead head that over the end of the hose with some creative adapters, hose and hose clamps.

i am expecting that you have a hard line between the pump and the carb inlet..

while you have the carb off.. could you do me a favor..

disable the ignition distributor so NO SPARKS can be created..

with the fuel line to the carb disconnected, extend the fuel line with some rubber fuel hose... blow into the fuel hose... you should NOT be able to blow backwards thru the pump..

the next test is to stick the extended hose into a clean clear soda bottle.. have a friend crank the motor for 15 seconds while you are observing the pulses into the bottle. each pulse should be equal to the others.. not diminishing.. this tests the fuel system ability to deliver .. use some kind of timer..

you can install the fuel pressure tester in the end of the hose.. crank the engine again. you should not loose much fuel pressure when you stop cranking..

i have run into massive issues with sediment in the fuel system.. holding open the fuel pump check valves... the sediment making it into the needle and seat in the carb and holding that open also.

i have installed so 1990 ford ranger fuel filters in a longer piece of fuel hose directly before the fuel pump inlet... then done the 15 second full flow cranking test THREE TIMES to flush the fuel pump check valves with clean fuel.. i hook the carb back up.. start the engine... with a long pair of needle nose pliers.. i squeeze the fuel hose shut between the new filter and the fuel pump inlet. with my other hand.. i bring the RPMS up to 2500 and hold it there... when the carb and fuel pump start to run out of fuel. .. i release the pliers.. allowing another fast surge of fuel thru the wide open needle and seat in the carb.. this i do twice..

i have been fighting fuel system sediment issues since 1983. the sediment can cause the fuel level to rise in the carb and trickle over causing issues..

this will also cure vapor lock issues.. and cold start.. run for a few seconds and needing to crank the engine for a minute before it will run..

i put a clear vinyl hose on the disconnected fuel line at the carb and ran it up to the hood and back down.. i cranked to fill the clear hose full of fuel.. when i stopped cranking. the fuel vanished back down thru the pump.. i could blow bubbles in the fuel tank... the 10 micron fuel filter cured it..

Vapor lock. when the fuel pump check valves are held open by sediment. there is reduced pressure to the carb as the fuel in that line is not held at max pressure of the fuel pump.. during hot idle.. the lower pressure in that fuel line and the heat cause the fuel to boil and the expanding bubbles push the liquid out back thru the pump and to the tank.. fuel pumps cannot pump expanding gas.. the fuel level in the carb drops.. leaning out the mixture causing the combustion temps to increase.. expanding the pistons till there is almost no clearance.. making it really hard on the starter.

its all complicated. magnets inside the fuel tank.. really help..

10 micron fuel filters before the pump really help..

the Fram HPG-1 is a 10 micron fuel filter.. there are other fuel injection type fuel filters that are also 10 micron.. i only choose the 1990 ranger as it will fit 5/16 or 3/8 hose.. and it has a massive element and is available in ever parts store and even big box stores automotive sections. only a few hose clamps .. there are also brackets on many ford ranger pickups for it.

sorry for the wall of text..

 

High fuel pressure will absolutely make it idle rich. I just went through this on my truck. Put a new stock replacement fuel pump on it when i rebuilt the engine and it was flooding the carb. I used the HF vacuum/fuel pump tester and found that fuel pump was pegging my gauge at 10psi. Qjets only want about 5psi so I put a regulator on it and it runs great now.


This is the one I used. https://www.summitracing.com/parts/sum-g3032b

 

High fuel pressure will absolutely make it idle rich. I just went through this on my truck. Put a new stock replacement fuel pump on it when i rebuilt the engine and it was flooding the carb. I used the HF vacuum/fuel pump tester and found that fuel pump was pegging my gauge at 10psi. Qjets only want about 5psi so I put a regulator on it and it runs great now.


This is the one I used. https://www.summitracing.com/parts/sum-g3032b

Cool, I'll rent one of those fuel pressure testers from a local auto parts store so I can check the fuel pressure and see if that's part of the issue. Thanks for the input!

 

Couple updates. I didn't have much time this weekend so I tried to perform a couple quick tests based on responses in this thread.


I rented a fuel pressure tester. My fuel pressure stays right around 5 PSI (which I believe is just fine for a Quadrajet from what I've read).


I took some A/F readings at different RPMs. All of these readings are taken with the car in neutral. I got to each set RPM by screwing in the idle screw.


A/F RPM
~17 <1000
~16 1000
~15 1200
~14.9 1500
~14.3 2000
~13.8 2500


I also went for a drive and paid careful attention to where the A/F ratio goes rich. Turns out, it goes crazy rich at partial throttle while cruising. The car will run well at idle and decent under hard acceleration, but at partial throttle (just to maintain speed while cruising) the gauge pegs at 10:1 A/F. Would that be my APT screw out of adjustment?


Basically it seems like it's getting WAY too much fuel when I lightly press the gas.

 

When you "lightly press on the gas" what is your manifold vacuum?

It sounds to me like your power valve is opening and making the mixture rich.

Checking the air/fuel ratio in neutral won't give you proper readings because the engine isn't under load. The carburetor will stay in the idle and idle transfer circuits in neutral past 3000 rpm without a load.

 

I know at idle I have about 15" of vacuum. I haven't checked what it is with a little throttle, but I can.


So are you saying I have a bad power piston? If so, what one would you recommend? I'm still using the stock power piston.

 

I had a float in a Q Jet absorb fuel once and it acted just like this one. The float was not metal and you could not tell by feel that the float was heavy but a new float fixed it. Also be sure that none of the plugs in the underside of the float bowl are leaking.

 

Ok I just figured out something else interesting. I noticed when I took out my power piston, it was slightly bent (see picture below). I bent it back to shape and put the carb top back on and screwed it down as normal, and then took it off again to check, and the power piston was bent again?! Anyone seen this before?

 

the piston has a little plastic ring around it just above the largest section.. that is supposed to latch the entire assembly down after you have aligned the needles in their holes.. always a pain..

if something in the main jet is blocking the back side of the hole.. you could have an issue.. usually the rod holder tries to jump up and let the metering rods out of the jets then it bends the rod holder.


there is a chance there is an issue with the top of the carb also. more pictures... will the metering rod holder fully drop with the metering rods in the holes with the gasket and plastic insert removed..