My exhaust smells rich...I ordered a vacuum gauge and an inductive tachometer....never done this before. Any words of wisdom on this process?
CK5
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Air / Fuel Mixture on Quadrajet
- Thread starter Chief Brody
- Start date
If you have a carb that has idle mixture adjusters that are accessible, get it idling and turn them inward( clockwise) about 1/4 turn alternating from one to the other until the idle starts dropping. Then turn back the other way (clockwise) 1/4 turns until it stablizes.
You want to get the highest and steadiest reading on the vacuum gauge possible with the engine fully warmed up...you may need to turn the idle mixture screws out a slight amount equally so the engine will idle smoothly cold,depending on how cool it gets where you are and the humidity levels..
Ignition timing affects vacuum a lot too,I'd set the timing first,then adjust the carb..also ensure there are no vacuum leaks first too..
I spray carb cleaner on suspected areas and listen for the idle to change,its the easiest method..
Ignition timing affects vacuum a lot too,I'd set the timing first,then adjust the carb..also ensure there are no vacuum leaks first too..
I spray carb cleaner on suspected areas and listen for the idle to change,its the easiest method..
Is this something I need to do for winter and then summer? It is 70 today but will be in the 30's by the weekend....then in the 50's....until March or April at which time it will be in the 90's again...
It runs fine right now...I just took it to the custom muffler shop down the street and the guy said "you are running rich"...but it was only 1/4 mile down the road and hadn't warmed up. I guess I need to get it really warm and then see
I'm going to get him to make my exhaust to look like factory...but he's going to weld it. If I want to get crazy with it I could put the claps on for "effect".
Once it is set it "should" stay in tune,but you may need to tweak it now and then...air temps and humidity do affect vacuum,altitudes much more so,that can change the air/fuel ratio too..
It is not uncommon for carbed engines to smell rich after a cold start,and until they warm up fully..the choke being closed more,cold engine cylinders makes for a lot of un-burned fuel going out the exhaust..
Keep in mind the idle mixture screws only affect idle and a bit past idle,the rest of the time the mixture is set by the jetting of the carb,the float setting,metering rods,it may need those things altered or changed.
Leaky well plugs are common on Q-jets and will make an engine run rich and also start harder in hot weather...
It is not uncommon for carbed engines to smell rich after a cold start,and until they warm up fully..the choke being closed more,cold engine cylinders makes for a lot of un-burned fuel going out the exhaust..
Keep in mind the idle mixture screws only affect idle and a bit past idle,the rest of the time the mixture is set by the jetting of the carb,the float setting,metering rods,it may need those things altered or changed.
Leaky well plugs are common on Q-jets and will make an engine run rich and also start harder in hot weather...
I read that a 350 with QJ should be set to 750 RPM for idle?
I wouldn't worry so much about the actual idle speed just get it to idle where is seems happy and doesn't slam into gear and doesn't die if you blip the throttle in gear.
I've had 350s that idle down to 3-400 rpm if you would let it.. qjets are amazing when working properly
There should be a sticker on the radiator support with the idle speed setting and timing specs --if its missing its no big deal,as sreidmx said you can do it by ear and judge for yourself if its too fast or slow..
You'll want to have it warmed up fully,off choke completely,and idling slow,like 500-600 rpm while you adjust the idle mixture screws--if its idling faster it will be running off the main jet circuit some too ,and it'll mess with the adjustment..
You'll want to have it warmed up fully,off choke completely,and idling slow,like 500-600 rpm while you adjust the idle mixture screws--if its idling faster it will be running off the main jet circuit some too ,and it'll mess with the adjustment..
If your Q-jet is the original carb, then it is probably worn out. The throttle shafts wear out and create a vacuum leak. Also, the well plugs carrode out a dump excessive fuel into engine.
One of the greatest screws on a qjet is the cruise fuel screw. It allows the change of depth the meaturing rods go into the jets.
Some don't have this opened up but just a plug in the top air horn. It is a D shaped screw, I counted the turns out then cut a slot for a screw driver.
Some don't have this opened up but just a plug in the top air horn. It is a D shaped screw, I counted the turns out then cut a slot for a screw driver.
That's called the APT adjustment and some qjets done have it
Couldn't remember what it was called but man it sure made a difference in the way the engine ran.
Oh heck yea that should!
The adjustable part throttle (APT) at the front of the float bowl accessed from the top on 76 and later quads except for light truck carbs until around 79.
He should have an early quad that also can be made adjustable to some degree. The adjustable part is in the baseplate and sealed with a cap. Not intended to be messed with.
Assuming his carb is the correct one he shouldn't have to worry about that for his situation.
He should have an early quad that also can be made adjustable to some degree. The adjustable part is in the baseplate and sealed with a cap. Not intended to be messed with.
Assuming his carb is the correct one he shouldn't have to worry about that for his situation.
I think around 1975 is when they added the APT screw..when emissions became a big deal,and they started using catalitic converters,EGR valves,and air pumps on most cars..
I had a '75 K5 2wd Blazer with a 250 six and a one barrel carb,that engine had a bad lean surge when cruising around 30-35 mph..it also spark knocked like crazy with the timing set to specs,I had to retard it by 4 degrees just to get it to a minimum and that only aggravated the lean surging condition..
I tried rebuilding the carb,plugged off the EGR valve,nothing seemed to help--until I read about that APT screw hidden under a lead plug near the bottom of the bowl..
The carb kit instructions said in BOLD print "Do NOT attempt to remove or adjust this screw--it is factory adjusted for lowest emissions and attempting to remove or change its setting will result in higher emissions,and possible damage to the carb casting"..
That was all I needed to read--I immediately drilled the plug out...
and saw the screw had a slotted head--I unscrewed it a bit at a time,until several test rides showed after 1-1/2 turns out,the engine ran smooth and no more surging & bucking..being a manual transmission that was very annoying..
Only trouble I had after that was trying to seal up the port where that plug was originally..I tried epoxy paste,a tiny freeze plug,but it always weeped,and the screw pointed down at the intake & exhaust,so a fire hazard was a possibility..I ended up putting several coats of Indian Head Gasket Shellac over the area and it finally sealed up..
I had a '75 K5 2wd Blazer with a 250 six and a one barrel carb,that engine had a bad lean surge when cruising around 30-35 mph..it also spark knocked like crazy with the timing set to specs,I had to retard it by 4 degrees just to get it to a minimum and that only aggravated the lean surging condition..
I tried rebuilding the carb,plugged off the EGR valve,nothing seemed to help--until I read about that APT screw hidden under a lead plug near the bottom of the bowl..
The carb kit instructions said in BOLD print "Do NOT attempt to remove or adjust this screw--it is factory adjusted for lowest emissions and attempting to remove or change its setting will result in higher emissions,and possible damage to the carb casting"..
That was all I needed to read--I immediately drilled the plug out...
and saw the screw had a slotted head--I unscrewed it a bit at a time,until several test rides showed after 1-1/2 turns out,the engine ran smooth and no more surging & bucking..being a manual transmission that was very annoying..Only trouble I had after that was trying to seal up the port where that plug was originally..I tried epoxy paste,a tiny freeze plug,but it always weeped,and the screw pointed down at the intake & exhaust,so a fire hazard was a possibility..I ended up putting several coats of Indian Head Gasket Shellac over the area and it finally sealed up..
The 75 quads were a transition year for the ADP and changed again in 76 for cars and 78/79 for trucks, shouldn't be an issue here if he has the correct carb..
