Quadrajet issue

buzzinhalfdozen · 10-03-2013, 09:51 AM

Have a freind with a big block, mildly cammed with a Q-jet that will not idle. Has a rebuilt carb can turn mixture screws in all the way engine still runs, says it's very rich, has completely closed throttle blades then just cracked open, still idles rich. Since the mixture screws seem to have no effect on how it idles can we assume it's bringing in fuel elsewhere? Could the pri. metering be the issue? Been a while since I've even seen a Q-jet, but doesn't it have a piston and hanger that pulls down with vacuum to seat the metering rods? Really can't come up with any other explanation. I've not personally seen the car merely going off what the owner is saying

gsa612 · 10-03-2013, 10:16 AM

If the cam has a lot of duration it might need a lighter piston spring. what you can do is remove the pri. rods and spring and put in 5 steps leaner pri. jets 67's?, then put just the piston back in.See if that don't clean it up. If it does either nip a coil,or get a lighter PP spring. Most S/SS cars run this way,(without rods).I wouldn't drive it on the street with no rods though.You can also check if the bottom well plugs aren't leaking. gsa612

ron mattson · 10-03-2013, 11:00 AM

Could just need more idle air. There are a few
Different ways to accomplish this.

buzzinhalfdozen · 10-03-2013, 11:17 AM

Ok, yes he's gave it more idle air, that didn't solve it that merely made it idle faster, still rich. The wells have been epoxied, at idle the power piston should be drawn down correct? Pulling the rods into the jets? Looks like I may inherit this buggy so I'm looking for some things to try. Thanks guys.

ron mattson · 10-03-2013, 12:06 PM

Take a wooden match or small wood skewer
And stick down air vent and on top of primary
Metering rod hanger and mark height to top
Of vent tube, then fire car up and repeat this
Will tell you if the hanger is functioning while
Running.

Robert Simpson · 10-03-2013, 12:55 PM

After you do what Ron stated. Disconnect the accel pump, with it running. If it get crisper and idles better, you are rich and vise versa. Dean told me that one, it works. Also you might have to increase the diameter of the passages behind the fuel mixture screws in the baseplate. Example they are .079" open them up to .089". Just a thought.

Robert

Mike Taylor 3601 · 10-03-2013, 01:21 PM

A trick I have learned the hard way is needle and seat sometimes will leak slightly,but they won't run fuel out vent tube like holley,so it's not as obvious,will drop idle a few hundred rpm,and will bog few feet out because runs over. To check this I remove top and hold float pivot down with finger and put fuel pressure to it fuel level will come up shut off float,then watch for about a minute if leaking fuel level will slowly rise,I have got to where I just go ahead and test all of them I work on.

Another thing to look @ is primary butterflies there is idle circuits that have slotted opening the butterflies have to line up with these slots,adjust idle speed screw to line them up,if engine idles too slow then you have to start drilling holes in butterflies to increase idle speed.

You can also slighly choke w/hand if idles has vacc. leak or blocked idle circuits,too lean,etc
Qjets are more sensitive to vacc leaks,low vacc than holley.
I would also check timing could be retarded too much @ idle.
I would just go over basics first which you'll probaly do first anyway
You can find drinking straws that will fit down vent and push metering rods or watch them operate
Mike Taylor 3601

nhramnl · 10-03-2013, 01:33 PM

Things I would check:

Fuel pressure - it can't exceed 7psi. If it does, the pressure can blow the needle off of the seat and the carb will flood. Look for fuel dribbling from the booster clusters and listen for irregular "hissing" sound.

Float level - can't be too high, or the carb will dribble fuel into the venturis and the carb will flood.

Secondary hanger and rods - make sure the rods are fully seated in the orifices with the car at idle. The air door must be completely closed (not adjusted to provide additional idle air) because it can pick the rods up and allow secondary fuel.

Make sure the inability to achieve a stable idle is actually due to richness and not simply the result of a vacuum leak. Use the carb cleaner diagnosis method.

Alan Roehrich · 10-03-2013, 06:21 PM

Is the carburetor rebuilt by a professional, or is it a remanufactured carburetor purchased from a parts store?

tj310 · 10-04-2013, 08:55 AM

Heavy float due to absorbed fuel ---Trevor

10-03-2013, 09:51 AM	#1
buzzinhalfdozen Senior Member Join Date: Sep 2006 Location: Bellevue Ohio Posts: 984 Likes: 0 Liked 11 Times in 8 Posts	Quadrajet issue Have a freind with a big block, mildly cammed with a Q-jet that will not idle. Has a rebuilt carb can turn mixture screws in all the way engine still runs, says it's very rich, has completely closed throttle blades then just cracked open, still idles rich. Since the mixture screws seem to have no effect on how it idles can we assume it's bringing in fuel elsewhere? Could the pri. metering be the issue? Been a while since I've even seen a Q-jet, but doesn't it have a piston and hanger that pulls down with vacuum to seat the metering rods? Really can't come up with any other explanation. I've not personally seen the car merely going off what the owner is saying __________________ Joe Buchanan SS/BX 3117

10-03-2013, 10:16 AM	#2
gsa612 Senior Member Join Date: Jan 2008 Location: 100th Meridian Posts: 569 Likes: 116 Liked 280 Times in 139 Posts	Re: Quadrajet issue If the cam has a lot of duration it might need a lighter piston spring. what you can do is remove the pri. rods and spring and put in 5 steps leaner pri. jets 67's?, then put just the piston back in.See if that don't clean it up. If it does either nip a coil,or get a lighter PP spring. Most S/SS cars run this way,(without rods).I wouldn't drive it on the street with no rods though.You can also check if the bottom well plugs aren't leaking. gsa612

10-03-2013, 11:00 AM	#3
ron mattson Member Join Date: Nov 2005 Location: cloquet mn Posts: 465 Likes: 17 Liked 66 Times in 29 Posts	Re: Quadrajet issue Could just need more idle air. There are a few Different ways to accomplish this. __________________ Ron Mattson 5015 STK

10-03-2013, 11:17 AM	#4
buzzinhalfdozen Senior Member Join Date: Sep 2006 Location: Bellevue Ohio Posts: 984 Likes: 0 Liked 11 Times in 8 Posts	Re: Quadrajet issue Ok, yes he's gave it more idle air, that didn't solve it that merely made it idle faster, still rich. The wells have been epoxied, at idle the power piston should be drawn down correct? Pulling the rods into the jets? Looks like I may inherit this buggy so I'm looking for some things to try. Thanks guys. __________________ Joe Buchanan SS/BX 3117

10-03-2013, 12:06 PM	#5
ron mattson Member Join Date: Nov 2005 Location: cloquet mn Posts: 465 Likes: 17 Liked 66 Times in 29 Posts	Re: Quadrajet issue Take a wooden match or small wood skewer And stick down air vent and on top of primary Metering rod hanger and mark height to top Of vent tube, then fire car up and repeat this Will tell you if the hanger is functioning while Running. __________________ Ron Mattson 5015 STK

10-03-2013, 12:55 PM	#6
Robert Simpson Senior Member Join Date: Aug 2004 Location: Jacksonville, Florida Posts: 808 Likes: 7 Liked 20 Times in 12 Posts	Re: Quadrajet issue After you do what Ron stated. Disconnect the accel pump, with it running. If it get crisper and idles better, you are rich and vise versa. Dean told me that one, it works. Also you might have to increase the diameter of the passages behind the fuel mixture screws in the baseplate. Example they are .079" open them up to .089". Just a thought. Robert

10-03-2013, 01:21 PM	#7
Mike Taylor 3601 VIP Member Join Date: Oct 2008 Location: Somerset,Ky Posts: 1,369 Likes: 348 Liked 292 Times in 101 Posts	Re: Quadrajet issue A trick I have learned the hard way is needle and seat sometimes will leak slightly,but they won't run fuel out vent tube like holley,so it's not as obvious,will drop idle a few hundred rpm,and will bog few feet out because runs over. To check this I remove top and hold float pivot down with finger and put fuel pressure to it fuel level will come up shut off float,then watch for about a minute if leaking fuel level will slowly rise,I have got to where I just go ahead and test all of them I work on. Another thing to look @ is primary butterflies there is idle circuits that have slotted opening the butterflies have to line up with these slots,adjust idle speed screw to line them up,if engine idles too slow then you have to start drilling holes in butterflies to increase idle speed. You can also slighly choke w/hand if idles has vacc. leak or blocked idle circuits,too lean,etc Qjets are more sensitive to vacc leaks,low vacc than holley. I would also check timing could be retarded too much @ idle. I would just go over basics first which you'll probaly do first anyway You can find drinking straws that will fit down vent and push metering rods or watch them operate Mike Taylor 3601

10-03-2013, 01:33 PM	#8
nhramnl Senior Member Join Date: Feb 2010 Posts: 507 Likes: 8 Liked 358 Times in 83 Posts	Re: Quadrajet issue Things I would check: Fuel pressure - it can't exceed 7psi. If it does, the pressure can blow the needle off of the seat and the carb will flood. Look for fuel dribbling from the booster clusters and listen for irregular "hissing" sound. Float level - can't be too high, or the carb will dribble fuel into the venturis and the carb will flood. Secondary hanger and rods - make sure the rods are fully seated in the orifices with the car at idle. The air door must be completely closed (not adjusted to provide additional idle air) because it can pick the rods up and allow secondary fuel. Make sure the inability to achieve a stable idle is actually due to richness and not simply the result of a vacuum leak. Use the carb cleaner diagnosis method.

10-03-2013, 06:21 PM	#9
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,104 Likes: 1,560 Liked 1,784 Times in 407 Posts	Re: Quadrajet issue Is the carburetor rebuilt by a professional, or is it a remanufactured carburetor purchased from a parts store? __________________ Alan Roehrich 212A G/S

10-04-2013, 08:55 AM	#10
tj310 Member Join Date: Sep 2009 Location: Dresden On. Canada Posts: 460 Likes: 0 Liked 0 Times in 0 Posts	Re: Quadrajet issue Heavy float due to absorbed fuel ---Trevor