Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Guadzilla on January 30, 2021, 02:18:06 PM
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Working on a 70 trans am q-jet 7040270. Poor idle and lots of movement on vacuum guages. While running if I close off the bowl vent for an instant the engine shuts down like I tuned off the key. Should there be that much vacuum in the bowl.
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Which "bowl vent". The bowl on that carb vents in at least 4 places.......
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Working on a 70 trans am q-jet 7040270. Poor idle and lots of movement on vacuum guages. While running if I close off the bowl vent for an instant the engine shuts down like I tuned off the key. Should there be that much vacuum in the bowl.
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The 3/4" vent on the front that pokes through a hole on the lower air cleaner plate. Plug it and it kills the engine instantly
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Is this a 7040570 carb?
No such vent on a 7040270 unit.
Either way, no vacuum in float bowl, only atmospheric pressure.
Pictures?
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You're right it's a 7040570 and there shouldn't be any vacuum in the float bowl. What could be wrong to create vacuum in the float bowl. It has evap system.
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The only other place it has to vent on that airhorn is past the gasket where the secondary metering rods go thru it. There isn't a vent in the airhorn behind the choke flap and POE wells vent internally back to the float bowl.
I've never seen one that would stall the engine instantly when you covered the large vent in the front but I've never really tried that move very often.
In any case it wouldn't be "vacuum" in the bowl it should be at standard pressure and pressure would only build up if you blocked off all the venting so the incoming fuel would raise the pressure some or quit flowing into the bowl as venting is required for fuel to be able to come in under low pressure.
Anyhow, "rough" idle can be caused by many things.
Big cam with tight LSA is one. Not enough compression for the cam being used is another.
Not enough initial timing.
Not enough fuel available from the carburetors idle system.
Vacuum leaks like sucked in intake gasket.
Bad spark plug(s), plug wires, broken valve spring(s), worn cam lobe(s), etc, etc, etc.
I would try several things here.
First try advancing the timing. I'd also gently "tip-in" the choke flap to see if it smooths out some.
Eliminate any vacuum leaks.
If it's missing on one or more cylinders try to figure out why?
Compression test, pull one plug wire off the cap at a time and see which ones are firing well and which ones aren't...etc....etc.....
Also no mention of engine specs, what is the engine and what has been done to it?
More information is better than less..........Cliff
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Cliff Thanks for the checklist. The pontiac 400 RA III engine
has 160 psi compression in all 8, a Ram air 4 spec cam and all new ignition parts. No missing at speed. Vacuum guages at idle alternate from 8 to 15. I'm thinking the intake manifold gasket isn't sealing but no indication of hissing. Should I pull the manifold.
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The RAIV cam in a 400 will need 15° initial timing.
And your carb will need at least .037" idle tubes and .055"-.059" idle down channel restrictions.
Without this you can expect nothing but poor idle.
A/T or M/T?
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The RAIV cam is a very poor choice in one of those engines. I get more complaints from folks who have tried that combo than any other. The real RAIV cam has over 80 degrees overlap, so kills signal to the carburetor making for poor idle quality.
You can "crutch" the scenario with a lot of timing and opening up the carburetor some, but that cam really needs high compression, 4 speed or at least a 3200 stall speed converter, and at least 3.90 rear gears........
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Cliff, Keith, Thanks I was afraid of that, so I can't expect a nice even 15 inches of vacuum at idle. I have the timing as high as it can be without the idle falling off again at around 15. Would a ram air IV speced quadrajet help. It has the original turbo 400 trans. Carb bogs when trans kicks down.
Thanks for your help.
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The real RAII and RAIV engines had high compression with very small combustion chambers in the heads. Even with higher compression the real power curve really doesn't start coming on till past 3000rpm's or so.
Using a RAIV cam in a RAIII or lower compression build with "D" port heads just doesn't work well. They don't have the good flowing round port heads and most of the combustion chambers, especially on the auto trans cars are quite a bit larger (lower compression) than what was used on a real RAII/RAIV engines.
I've had a good many brought here for custom tuning over the years and never once impressed with them anyplace. With the auto trans engines you'd pretty much have to drop it off a floor jack at full throttle to do a decent burnout! They fair a LOT better with a 4 speed application and pretty tall gearing but never on par with a real RAIV anyplace.
As far as tuning you can push the timing up and add idle fuel to get them to idle decent, but it doesn't change the laws of physics here, the real cure is a smaller cam or a lot more compression, higher stall converter (or manual transmission) and at least 3.90 rear gears........FWIW......Cliff
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Thanks Cliff for your honesty. I wish it had a 4 spd which goes in easy but it's a numbers car, so putting the right cam in is the logical approach. So the cam is cuasing the rough vacuum osculation from 7 to 15 inches. It will do burn outs if it doesn't bog and go 60 to 80 mph in a flash. 3.51 stock auto gears. Try covering up the bowl vent on one these carbs. I have a 7042262 that I'm going to put on it to see if it acts the same.
Thanks
Tom
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The distributor could be adding to the issues. The timing may be fluctuating some at idle speed.
Adding timing and idle fuel will help improve vacuum and idle quality.....
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Timing light strobe is steady at idle. Turning my idle mixture screws in and out barely has an effect on idle speed.
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Take a look inside the carb at idle speed and see if fuel is dribbling in off the boosters.
Gently place your hand over the carb or "tip-in" the choke flap to see if it idles any better?...
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Choking it doesn't help. It isn't running rich. Plugs are on the white side.
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I didn't say to "choke it", just tip in the choke flap or gently place your hand over the primary side of the carb to richen it up just a tad and see if it helps?
If it's lean at idle adding a little fuel shown show some improvement....
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Ok I'll try that to get it to run richer. I also have a 7028266 carb with rebuilt by Holley sticker on it. I may put it on and see if it idles better with it.
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Save yourself lots of headaces, new gray hair and vocabulary NOT using the Holley "Frankencarb", they usually are a pile of mismatching and hacked up parts.
It may idle due to the stupid drilled out idle circuit and plugged idle bypass air but it will never run right at speed with decent fuel usage.
I repeat, the RAIV cam in a 400 will need 15° initial timing.
And your RamAir III, IV, 455 RamAir carb #7040570 will need at least .037" idle tubes and .055"-.059" idle down channel restrictions.
Also, #73 jets with #43 rods will make your car fly.
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Cliff Can I buy the 037 idle tubes and other suggested items from you or what is your rate to go over the carb if I sent it out to you, hoping that it doesn't get lost in the mail.
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I can supply parts for it and precision drill bits. Shipping would be very expensive back and forth....
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Probably ought to pull your idle tubes (like an idle jet in a motorcycle). They have tiny holes and get easily get plugged. Most likely, your tube size is smaller than .037, so good time to drill it and the DCR out.
Easy way to remove them is to use a drill bit the size of the tube, pointy end up and just lightly tap it in. The collar will stay in place. Don't worry if the tube drops out of the collar, once the collar is removed, turn the carb upside down and the tube will fall out.
To remove the collar, use a small screw. I don't like drywall screws, use a fine thread small screw and thread it in an 1/8" or so. Then use your needle nose pliers to pry up and it should pop right out. If it doesn't, warm the carb body with a butane/propane/map torch just for 5 secs or so and try again. Be sure to place a piece of sheet metal or wood under the pliers to protect the little ridges on top of the body. Those ridges are used to keep the airhorn gasket sealed and in place.
Once removed, tap the collar back on by hammering on the small tube end and driving it into the collar. THEN drill the small end .037".